ARTICLE Download: 83| View: 185| Comments: 0
Subject: Chemistry, Physical Chemistry Keywords: heat capacity; molecular volume; force-field geometry optimization; hydrocarbons; ionic liquids; siloxanes; metal complexes
Online: 29 March 2019 (08:54:23 CET)
A universally applicable method for the prediction of the isobaric heat capacities of the liquid and solid phase of molecules at 298.15 K is presented, derived from their “true” volume. The molecules’ “true” volume in A3 is calculated on the basis of their geometry-optimized structure and the Van-der-Waals radii of their constituting atoms by means of a fast numerical algorithm. Good linear correlations of the “true” volume of a large number of compounds encompassing all classes and sizes with their experimental liquid and solid heat capacities over a large range have been found, although noticeably distorted by intermolecular hydrogen-bond effects. To take account of these effects, the total amount of 1303 compounds with known experimental liquid heat capacities has been subdivided into three subsets consisting of 1102 hydroxy-group-free compounds, 164 monoalcohols/monoacids, and 36 polyalcohols/polyacids. The standard deviations for Cp(liq,298) were 20.7 J/mol/K for the OH-free compunds, 22.91 J/mol/K for the monoalcohols/monoacids and 16.03 J/mol/K for the polyols/polyacids. Analogously, 797 compounds with known solid heat capacities have been separated into a subset of 555 OH-free compounds, 123 monoalcohols/monoacids and 119 polyols/polyacids. The standard deviations for Cp(sol,298) were calculated to 23.14 J/mol/K for the first, 21.62 J/mol/K for the second, and 19.75 J/mol/K for the last subset. A discussion of structural and intermolecular effects influencing the heat capacities as well as of some special classes, in particular hydrocarbons, ionic liquids, siloxanes and metallocenes, has been given. In addition, the present method has successfully been extended to enable the prediction of the temperature dependence of the solid and liquid heat capacities in the range between 250 and 350 K.
Thu, 28 March 2019
ARTICLE Download: 37| View: 162| Comments: 0
Subject: Chemistry, Physical Chemistry Keywords: diperoxides; monomolecular layers; quantum-chemical calculation; conformation
Online: 28 March 2019 (09:33:41 CET)
The monomolecular films of diacylic diperoxides on the water/air interface were studied. Their general formula: CH3-(CH2)m-C(O)-O-O-C(O)-(CH2)n-C(O)-O-O-C(O)-(CH2)m-CH3. The behavior of monomolecular films of diperoxides are affected by the structure of their molecule. The numerical values of the areas of molecules that are extrapolated to zero pressure are different. This indicates a different conformation of the molecules in the monolayer. The optimal geometric structure of the molecule of diperoxide, the total area (S), the volume (V), the heat of formation (∆fH298), the energy of higher occupied (EHOMO) and the lower vacant (ELUMO) molecular orbitals were obtain in the calculations. The optimal geometric structures of peroxides and their electronic properties were calculated by the quantum-chemical method. Calculations of conformational states of the molecule of diperoxides are carried out. Experimental data and quantum-chemical calculations are consistent with each other.
Wed, 16 January 2019
ARTICLE Download: 117| View: 641| Comments: 0 | doi:10.20944/preprints201901.0118.v2
Online: 16 January 2019 (13:01:55 CET)
The purpose of this study is to check out the involvement of entropy in Mpemba effect. Several water samples were cooled down to frozen in order to probe if preheat affects the cooling duration time. We found out that preheating of the water sample the cooling duration was reduced. Given this, we theoretically show that water gains more entropy when warmed and re-cooled to the original temperature.
Mon, 10 December 2018
ARTICLE Download: 123| View: 181| Comments: 0 | doi:10.20944/preprints201812.0107.v1
Subject: Chemistry, Physical Chemistry Keywords: clay; diazonium salt; ion imprinted polymers; radical photopolymerization; visible light; adsorption; copper ions
Online: 10 December 2018 (14:27:52 CET)
There is an urgent demand worldwide for the development of highly selective adsorbents and sensors of heavy metal ions and other organic pollutants. Within these environmental and public health frameworks, we are combining the salient features of clays and chelatant polymers to design selective metal ion adsorbents. Towards this end, the ion imprinting approach has been used to develop a novel nanohybrid material for the selective separation of Cu2+ ions in aqueous solution. The Cu2+-imprinted polymer/ montmorillonite nanocomposite (IIP/Mt) and non-imprinted polymer/montmorillonite nanocomposite (NIP/Mt) were prepared by radical photopolymerization process in the visible light. Ion imprinting was indeed important as the recognition of copper ions by IIP/Mt was significantly superior to that of NIP/Mt that is the nanocomposite synthesized in the same way but in the absence of Cu2+ ions. The adsorption process as batch study was investigated under the experimental condition affecting same parameters such as contact time, concentration of ions metals and pH. The adsorption capacity of Cu2+ ions is maximized at pH 5. Removal of Cu2+ ion achieved equilibrium within 15 minutes; the results obtained were found to be fitted by the pseudo-second order kinetics model. The equilibrium process was well described by the Langmuir isothermal model and the maximum adsorption capacity was found to be 23.6 mg/g.
Mon, 3 December 2018
ARTICLE Download: 75| View: 110| Comments: 0 | doi:10.20944/preprints201812.0019.v1
Subject: Chemistry, Physical Chemistry Keywords: defective ZSM-5; hydroxyl nests; Si(OH)Al; Zn(C2H5)2, chemical liquid deposition; operando dual beam FT-IR spectroscopy
Online: 3 December 2018 (09:41:48 CET)
A series of defective ZSM-5 zeolites (~300 nm, SiO2/Al2O3 ratio of 55, 100, 400 and 950) were intentionally prepared and systematically studied by XRD, SEM, 29Si MAS NMR, argon physisorption, NH3-TPD and FT-IR technologies. The nature, the amount and the accessibility of the acid sites of defective ZSM-5 zeolites are greatly different from reported ZSM-5 zeolites with perfect crystal structure. The co-existed strong Brønsted acid sites (Si(OH)Al) and weak Brønsted acid sites (hydroxyl nests) over defective ZSM-5 zeolites might lead to unique catalytic function. Zn(C2H5)2 was grafted onto defective ZSM-5 zeolites through chemical liquid deposition (CLD) method. Interestingly, FT-IR spectroscopy studies find that Zn(C2H5)2 was preferentially grafted on the hydroxyl nests with weak acidity rather than the Si(OH)Al groups with strong acidity over different defective ZSM-5 zeolites. Particularly, home-built operando dual beam FTIR-MS was applied to study the catalytic performance of Zn species locating at different sites of defective ZSM-5 zeolites under n-hexane transformation. Results show that Zn species grafted over hydroxyl nests obtain better dehydrogenation performance than Zn species over framework aluminum. This study provides guidance for the rational design of highly efficient alkane dehydrogenative aromatization catalysts.
Mon, 26 November 2018
ARTICLE Download: 97| View: 233| Comments: 0 | doi:10.20944/preprints201811.0587.v1
Subject: Chemistry, Physical Chemistry Keywords: Enhanced Oil Recovery (EOR); Polymer flooding; Injectivity; Rheology; Viscoelasticity; Non-Newtonian flow; Mechanical degradation; HPAM
Online: 26 November 2018 (11:27:13 CET)
Water soluble polymers have gained an increasing interest in enhanced oil recovery (EOR) processes, especially as polymer flooding. Despite the fact that the flow of polymer in porous medium has been a research subject for many decades with numerous publications, there are still some research areas that need progress. The prediction of polymer injectivity remains elusive. Polymers with similar shear viscosity might have different in-situ rheological behaviors and may be exposed to different extent of mechanical degradation. Hence, determining polymer in-situ rheological behavior is of great significance for defining its utility. In this study, an investigation of rheological properties and mechanical degradation of different HPAM (partially hydrolyzed polyacrylamide) polymers was performed using Bentheimer sandstone outcrop cores. Results show that, HPAM in-situ rheology is different from bulk rheology measured in rheometer. Specifically, shear thickening behavior occurs at high rates, and near-Newtonian behavior is measured at low rates in porous media. This deviates strongly from measurements in the rheometer. Polymer molecular weight and concentration influence its viscoelasticity and subsequently its flow characteristics in porous media. Exposure to mechanical degradation by flow at high rate through porous media leads to significant reduction in shear thickening and thereby improved injectivity. More importantly, the degraded polymer maintained in-situ viscosity at low flow rates indicating that improved injectivity can be achieved without compromising viscosity at reservoir flow rates. This is explained by reduction in viscoelasticity. Mechanical degradation also leads to reduced residual resistance factor (RRF), especially for high polymer concentrations. For some of the polymer injections, successive degradation (increased degradation with transport length in porous media) was observed. The results presented here may be used to optimize polymer injectivity.
Wed, 21 November 2018
REVIEW Download: 137| View: 162| Comments: 0 | doi:10.20944/preprints201811.0523.v1
Subject: Chemistry, Physical Chemistry Keywords: photocatalysis; plasmon; metal/semiconductor; electron transfer; energy conversion
Online: 21 November 2018 (11:18:11 CET)
Hetero-nanomaterials constructed by plasmonic metals and functional semiconductors show enormous potential in photocatalytic applications, such as water splitting, hydrogen production, CO2 reduction, pollutants treatment. Their photocatalytic performances can be better regulated through adjusting structure, ingredient, and components arrangement. Therefore, the reasonable design and synthesis of metal/semiconductor hetero-nanostructures is of vital significance. In this article, we briefly review the recent progress in efficiently establishing metal/semiconductor nanomaterials for improved photocatalysis. The defined photocatalysts mainly include traditional binary hybrids, ternary multi-metals/semiconductor and metal/multi-semiconductors heterojunctions. The underlying physical mechanism for the improved photocatalytic activity of the established photocatalysts are highlighted. At the end of this article, a brief summary and possible future perspectives for further development in this field are demonstrated.
Fri, 16 November 2018
ARTICLE Download: 53| View: 100| Comments: 0 | doi:10.20944/preprints201811.0404.v1
Subject: Chemistry, Physical Chemistry Keywords: silica, europium complexes, luminescence, sol-gel, zirconia
Online: 16 November 2018 (11:11:07 CET)
The present contribution is focused on the relation preparation – structure – optical properties of hybrid silica and zirconia powders containing Eu(III) complexes: [Eu(ntac)3][pphendcn] and [Eu(phen)2](NO3)3. Methods for functionalization of silica and zirconia microparticles, based on adsorption of 1,10-phenantroline and [Eu(ntac)3][pphendcn], and doping with [Eu(phen)2](NO3)3 nanoparticles are proposed to obtain of red emitting hybrid composites with quantum efficiency 20 – 50 %. The optical properties of rare earth ions incorporated in silica and zirconia composites are used to explore the site symmetry (D2, C2v or lower) of the Eu3+ optical centers in the hybrid composites. Quantum yields of the investigated sol-gel micropowders and quantification of their optical spectra are presented.
ARTICLE Download: 112| View: 99| Comments: 0 | doi:10.20944/preprints201811.0401.v1
Subject: Chemistry, Physical Chemistry Keywords: gas separation; lattice dynamic; mixed gas hydrates, greenhouse gases, computer modeling
Online: 16 November 2018 (10:51:20 CET)
In this contribution, a method based on a solid solution theory of clathrate hydrate for multiple cage occupancy, host lattice relaxation and guest-guest interactions has been presented to estimate hydrate formation conditions of binary and ternary gas mixtures. We have performed molecular modeling of structure, guest distribution, and hydrate formation conditions for the CO2 + CH4, and CO2 + CH4 + N2 gas hydrates. In all considered systems with and without N2, at high and medium content of CO2 in the gas phase we have found that CO2 is more favorable to occupy clathrate hydrate cavities than CH4 or N2. Addition of N2 to the gas phase increases ratio concentration CO2 in compressing with concentration CH4 in clathrate hydrates and makes gas replacement more effective. The mole fractions of CO2 in CO2 + CH4 + N2 gas hydrate rapidly increases with the growth of its content in the gas phase. And the formation pressure of CO2 + CH4 + N2 gas hydrate rises in comparison with the formation pressure of CO2 + CH4 gas hydrate. Obtained results agree with the known experimental data for simple CH4, CO2 gas hydrates and mixed CO2 + CH4 gas hydrate.
Mon, 5 November 2018
ARTICLE Download: 136| View: 141| Comments: 0 | doi:10.20944/preprints201811.0124.v1
Subject: Chemistry, Physical Chemistry Keywords: Lignin, nanoparticle, protein, nanocellulose, fibril, enzyme, heat, self-assembly, cross-link
Online: 5 November 2018 (15:18:23 CET)
Lignin has interesting functionalities to be exploited in adhesives for medicine, foods and textiles. Nanoparticles (NPs) <100 nm coated with poly(L-lysine), PL and poly(L-glutamic acid) PGA were prepared from the laccase treated lignin to coat nanocellulose fibrils (CNF) with heat. NPs ca. 300 nm were prepared, β-casein coated and cross-linked with transglutaminase (Tgase) to agglutinate chamois specimens. Size exclusion chromatography (SEC) and Fourier-transform infrared (FTIR) spectroscopy were used to characterize polymerized lignin, zetapotential and dynamic light scattering (DLS) to ensure coating of colloidal lignin particles (CLPs). Protein adsorption on lignin was studied by quartz crystal microbalance (QCM). Atomic force microscopy (AFM) was exploited to examine interactions between different polymers and to image NPs with transmission electron microscopy (TEM). Tensile testing showed, when using CLPs for the adhesion, the stress improved ca. 10 and strain ca. 6 times compared to polymeric lignin. For the β-casein NPs the values were 20 and 8, respectively, and for the β-casein coated CLPs between these two cases. When NPs were dispersed in adhesive formulation, the Young's moduli confirmed significant improvement in the elasticity of the joints over the adhesive alone. Exploitation lignin in nanoparticulate morphology is a potential method to prepare bionanomaterials for advanced applications.
Fri, 2 November 2018
ARTICLE Download: 52| View: 119| Comments: 0 | doi:10.20944/preprints201811.0012.v1
Online: 2 November 2018 (04:39:35 CET)
In this work, silver nanoparticles (AgNPs) were synthesized rapidly and eco-friendlily using the extract of Mulberry leaves and aqueous solution of silver nitrate without any toxic chemical [1,2]. The Mulberry leaves extract acts as both reducing agent and stabilizing agent. The UV-Vis spectrum shows peak at 430 nm. The TEM image of synthesized AgNPs sample shows spherical shaped particles whose size range from 15 to 20 nm. TEM image of nano silver solution sample synthesized by microwave assisted method shows nearly spherical particles with an average particle size of 10 nm. The absorption UV-vis spectrum of silver nanoparticles synthesized by microwave assisted method (AgNPsmw) shows a sharp absorption band around 415 nm. After two month storage of AgNPsmw, the absorption spectrum of AgNPsmw was taken again. The UV-Vis spectrum shows negligible peak changes of silver nanoparticles have occurred after two months of storage. The synthesized AgNPs material could be used as an antimicrobial, used in the field of textile and in wastewater treatment.
Wed, 31 October 2018
ARTICLE Download: 51| View: 134| Comments: 0 | doi:10.20944/preprints201810.0750.v1
Subject: Chemistry, Physical Chemistry Keywords: fast gas-liquid reactions; online electrospray ionization mass spectrometry; heterogeneous atmospheric chemistry
Online: 31 October 2018 (10:54:59 CET)
One of the research priorities in atmospheric chemistry is to advance our understanding of heterogeneous reactions and their effect on the composition of the troposphere. Chemistry on aqueous surfaces is particularly important in this regard because of their ubiquity and expanse. They range from the surfaces of oceans (360 million km2), cloud and aerosol drops (~10 trillion km2) to the fluids lining the human lung (~200 m2). Typically, ambient air contains reactive gases that may affect human health, influence climate and participate in biogeochemical cycles. Despite their importance, reactions between gases and solutes at air-aqueous interfaces are not well understood. New, surface-specific techniques are required that detect and identify the intermediates and products of such reactions as they happen on liquids. This is a tall order because genuine interfacial reactions are faster than mass diffusion into bulk liquids, and may produce novel species at low concentrations. Herein, we review evidence that validates online pneumatic ionization mass spectrometry of liquid microjets dosed by reactive gases as a technique meeting such requirements. Next, we call attention to results obtained by this approach on reactions of ozone, nitrogen dioxide and hydroxyl radicals with various solutes on aqueous surfaces. The overarching conclusion is that the outermost layers of aqueous solutions are unique media, where equilibria shift and reactions proceed faster than, in some cases along different pathways from the bulk liquids. The fact that the rates and mechanisms of reactions at air-aqueous interfaces may not be deduced from experiments in bulk liquids opens new conceptual frameworks and lines of research, and adds an overlooked dimension to atmospheric chemistry.
Fri, 19 October 2018
ARTICLE Download: 110| View: 298| Comments: 0 | doi:10.20944/preprints201810.0434.v1
Online: 19 October 2018 (05:29:19 CEST)
Theoretical approaches to calculate pKa values for Brønsted acids is a challenging task that, most of the time, involves sophisticated and time-consuming methods. Therefore, heuristic approaches are efficient and appealing methodologies to approximate these values. Herein, by considering the electrostatic potential on acidic hydrogen atoms in a similar fashion that a σ–hole is defined, we calculated the maximum surface potential, VS,max, and used it as a descriptor to correlate it with experimental acidity constants. These values were calculated using the CPCM implicit solvent model (water) with six different methods: five density functionals and the Møller–Plesset second order perturbation theory. Six different basis sets were combined with each method in order to benchmark a total of thirty-six levels of theory. Overall, 1080 calculations were performed and found to correlate with experimental data. The ωB97X-D/6-31+G(d,p) level of theory stands as the best one for consistently reproduce the reported pKa values.
Tue, 9 October 2018
ARTICLE Download: 92| View: 180| Comments: 0 | doi:10.20944/preprints201810.0190.v1
Subject: Chemistry, Physical Chemistry Keywords: Biomarkers, Dye , Fluorescence, acetate detection, FLIM
Online: 9 October 2018 (15:39:16 CEST)
Development of biomarkers of analytes with interest in clinic is an important field of study. In this work, we synthesized and analyzed the new fluorescent acetate-biomarker, Iso-PG. The mechanism of detection is the acetate buffer mediated proton transfer reaction. The rate constants involved were obtained, and we measured the change in the fluorescence lifetime produced as a consequence of the presence of acetate in the medium. Finally, we checked its potential use as acetate biomarker in synthetic serum
Mon, 8 October 2018
ARTICLE Download: 65| View: 119| Comments: 0 | doi:10.20944/preprints201810.0137.v1
Subject: Chemistry, Physical Chemistry Keywords: anoxic iron oxidation; abiogenic ferric iron; high-subcritical water; alkaline; ferric oxides; ferric silicates; amorphous silica; origin of life; fluid inclusions; geobiotropy
Online: 8 October 2018 (10:42:53 CEST)
The expression "Follow the water" is used in order to recognize inside the universe, life as it exists on Earth. It is shown here that the expression "Follow the water in its high-subcritical state" can be used in order to recognize the components of life which form prior to the emergence of life. This specific state of water leaves signatures inside the minerals which are produced during high-subcritical water/rock interaction.
Thu, 27 September 2018
ARTICLE Download: 75| View: 123| Comments: 0 | doi:10.20944/preprints201809.0547.v1
Subject: Chemistry, Physical Chemistry Keywords: ice X; vibrational spectrum; first-principles DFT; Raman scattering; IR absorption
Online: 27 September 2018 (13:25:06 CEST)
A typical vibrational spectrum in the ice phase has four separate bands: translation, libration, bending and stretching. Ice X, the final ice phase under high pressure, shows an exotic vibrational spectrum. Theoretically, an ideal crystal of ice X only has one peak at 998 cm-1 for Raman scattering and two peaks at 450 cm-1 and 1507 cm-1 for infrared absorption in this work. These three characteristic peaks are indicators of the phase transition between ice VII/VIII and ice X. Despite much experimental and theoretical work on ice X, only this study has clearly indicated these characteristic peaks in the region of the IR band. The phonon density of states shows quite different features than ice VIII, which could be verified by inelastic neutron scattering in the future. The dynamic processes of 15 vibrational normal modes are discussed and the typical hydrogen bonds are missing.
Wed, 26 September 2018
ARTICLE Download: 80| View: 82| Comments: 0 | doi:10.20944/preprints201809.0110.v2
Subject: Chemistry, Physical Chemistry Keywords: polyoxometalates; hexatantalate; tantalum; countercations; ion-pairing; calorimetry; solubility
Online: 26 September 2018 (05:24:51 CEST)
Ion association is an important process in aqueous dissolution, precipitation, and~crystallization of ionic inorganic, organic, and biological materials. Polyoxometalates (POMs) are good model compounds for understanding the complex relationships between lattice energy, ion-pairing in solution, and salt solubility. Here we perform calorimetric measurements to elucidate trends in cluster stability, lattice energy, and ion-pairing behavior studies of simple hexatantalate salts in neat water, parent hydroxide solutions, and molybdate melts, extending previous studies on the isostructural hexaniobates. High temperature calorimetry of alkali salts of hexatantalate reveals that the enthalpies of formation from oxides of the K, Rb, and Cs salts are more similar to each other than they are for their niobate analogues and that the tantalate cluster is energetically less stable than hexaniobate. Aqueous dissolution calorimetry reveals that the cesium salt of hexatantalate has a similar concentration dependence on its dissolution enthalpy to that of hexaniobate. However, unlike~rubidium hexaniobate, rubidium hexatantalate also exhibits increased concentration dependence, indicating that hextantalate can undergo increased ion-pairing with alkali salts other than cesium, despite the dilute environments studied. Dissolution enthalpies of POM salts in the parent alkali hydroxides shows that protonation of clusters stabilizes lattices even more than the strongly associating heavy alkali cations do. Additionally, neither weak nor strong lattice ion associations necessarily correlates with respectively high or low aqueous solubility. These studies illuminate the importance of considering ion-pairing among the interrelated processes in the aqueous dissolution of ionic salts that can be extended to serving as a model of cation association to metal oxide surfaces.
Mon, 24 September 2018
HYPOTHESIS Download: 67| View: 230| Comments: 0 | doi:10.20944/preprints201809.0456.v1
Subject: Chemistry, Physical Chemistry Keywords: glass transitions, universality, Bell-Evans-Polanyi principle
Online: 24 September 2018 (12:44:06 CEST)
The Vogel-Fulcher-Tammann equation is exposed as a particular example of the mean field theory. It is generalized by taking into account an arbitrary critical exponent of susceptibility, discriminating between different classes of universality. The Bell-Evans-Polanyi principle is employed to estimate the difference between the activation energies of flows in crystals and glasses, which appears to coincide with the excess Gibbs energy of the glass compared to the crystal.
Thu, 13 September 2018
ARTICLE Download: 139| View: 154| Comments: 0 | doi:10.20944/preprints201809.0228.v1
Subject: Chemistry, Physical Chemistry Keywords: carbon bond; three centre-two electron bond; protonated methane
Online: 13 September 2018 (05:01:54 CEST)
Atoms in Molecules (AIM), Natural Bond Orbital (NBO), and normal coordinate analysis have been carried out at the global minimum structures of TH5+ (T = C/Si/Ge). All these analyses lead to a consistent structure for these three protonated TH4 molecules. The CH5+ has a structure with three short and two long C-H covalent bonds and no H-H bond. Hence, the popular characterization of protonated methane as a weakly bound CH3+ and H2 is inconsistent with these results. However, SiH5+ and GeH5+ are both indeed a complex formed between TH3+ and H2 stabilized by a tetrel bond, with the H2 being the tetrel bond acceptor. The three-center-two-electron bond (3c-2e) in CH5+ has an open structure, which can be characterized as a V-type 3c-2e bond and that found in SiH5+ and GeH5+ is a T-type 3c-2e bond. This difference could be understood based on the typical C-H, Si-H, Ge-H and H-H bond energies. Moreover, this structural difference observed in TH5+ can explain the trend in proton affinity of TH4. Carbon is selective in forming a ‘tetrel bond’ and when it does, it might be worthwhile to highlight it as a ‘carbon bond’.
Mon, 10 September 2018
ARTICLE Download: 111| View: 132| Comments: 0 | doi:10.20944/preprints201809.0176.v1
Online: 10 September 2018 (14:36:02 CEST)
Mineral surfaces have been demonstrated to play a central role in prebiotic reaction, which are supposed to be at the basis of the origin of life. Among the various molecules proposed as precursors for these reactions, one of the most interesting is formamide. Formamide has been shown to be a pluripotent molecule, generating a wide distribution of relevant pre-biotic products. In particular, the outcomes of its reactivity are strongly related to the presence of mineral phases, acting as catalysts toward specific reaction pathways. Even if the mineral-products relationship has been deeply studied for a large pool of materials, the fundamental description of the formamide reactivity over the mineral surface at a microscopic level is missing in the literature. In particular, a key step of formamide chemistry at surfaces is its adsorption on the available interaction sites. This report aims to investigate the adsorption of formamide over a well-defined amorphous silica, chosen as model mineral surface. An experimental IR investigation of formamide adsorption has been carried out and its outcomes have been interpreted on the basis of first principles simulation of the process adopting a realistic model of the amorphous silica.
Thu, 6 September 2018
ARTICLE Download: 107| View: 171| Comments: 0 | doi:10.20944/preprints201809.0110.v1
Subject: Chemistry, Physical Chemistry Keywords: polyoxometalates; hexatantalate; tantalum; countercations; ion-pairing; calorimetry; solubility
Online: 6 September 2018 (05:41:22 CEST)
Ion association is an important process in aqueous dissolution, precipitation, and crystallization of ionic inorganic, organic, and biological materials. Polyoxometalates (POMs) are good model compounds for understanding the complex relationships between lattice energy, ion-pairing in solution, and salt solubility. Here we perform calorimetric measurements to elucidate trends in cluster stability, lattice energy, and ion-pairing behavior studies of simple hexatantalate salts in neat water, parent hydroxide solutions, and molybdate melts, extending previous studies on the isostructural hexaniobates. High temperature calorimetry of alkali salts of hexatantalate reveals that the enthalpies of formation from oxides of the K, Rb, and Cs salts are more similar to each other than they are for their niobate analogues and that the tantalate cluster is energetically less stable than hexaniobate. Aqueous dissolution calorimetry reveals that the cesium salt of hexatantalate has a similar concentration dependence on its dissolution enthalpy to that of hexaniobate. However, unlike rubidium hexaniobate, rubidium hexatantalate also exhibits increased concentration dependence, indicating that hextantalate can undergo increased ion-pairing with alkali salts other than cesium, despite the dilute environments studied. Dissolution enthalpies of POM salts in the parent alkali hydroxides shows that protonation of clusters stabilizes lattices even more than the strongly associating heavy alkalis do. Additionally, neither weak nor strong lattice ion associations necessarily correlates with respectively high or low aqueous solubility. These studies illuminate the importance of considering ion-pairing among the interrelated processes in the aqueous dissolution of ionic salts, that can be extended to serving as a model of cation association to metal oxide surfaces.
Fri, 10 August 2018
ARTICLE Download: 128| View: 186| Comments: 0 | doi:10.20944/preprints201808.0128.v2
Subject: Chemistry, Physical Chemistry Keywords: bentonite, N2 physisorption, BET, thermogravimetry, clay
Online: 10 August 2018 (09:51:31 CEST)
The employment of clays in industry and on laboratory scale is still of interest, despite their long history. The chemical-physical characteristics of such material are strongly related to their geographical origin and the availability of proper characterization techniques is of great importance in order to gain as more information about their behavior when utilized as filling materials. In the present contribute a physical characterization by meaning of Thermogravimetry and N2 physisorption, including a thermal stability evaluation, of samples of Sardinian bentonite collected from the Alghero area (Italy) is reported.
Wed, 8 August 2018
ARTICLE Download: 180| View: 181| Comments: 0 | doi:10.20944/preprints201808.0166.v1
Subject: Chemistry, Physical Chemistry Keywords: Ferrocenium conformers, electron structure of organometallic complexes, excess orbital energy spectrum (EOES), α- and β-electrons of Fc+, energy decomposition analysis (EDA).
Online: 8 August 2018 (11:16:59 CEST)
Ferrocenium (Fc+) inherits a number of molecular/electronic properties from the neutral counterparts’ ferrocene (Fc) including the high symmetry. Both Fc+ and Fc prefer the eclipsed structure (D5h) over the staggered structure (D5d) by an energy of 0.36 kcal·mol-1. The present study using the recently developed excess orbital energy spectrum (EOES) shows that the open shell Fc+ cation exhibits similar conformer dependent configurational changes to the neutral Fc conformer pair. A further energy decomposition analysis (EDA) discloses that the reasons for the preferred structures are different between Fc+ and Fc. The dominant differentiating energy between the Fc+ conformers is the electrostatic energy (EEstat), whereas in neutral Fc, it is the quantum mechanical Pauli repulsive energy (EPauli). Within the D5h conformer of Fc+, the EOES reveals that the -electrons of Fc+ experience more substantial conformer dependent energy changes than the -electrons (assumed the hole is in a β orbital).
Mon, 6 August 2018
ARTICLE Download: 76| View: 184| Comments: 0 | doi:10.20944/preprints201808.0128.v1
Subject: Chemistry, Physical Chemistry Keywords: bentonite, N2 physisorption, BET, thermogravimetry, clay
Online: 6 August 2018 (16:16:19 CEST)
The employment of clays in industry and on laboratory scale is still of interest, despite their long history. The chemical-physical characteristics of such material are strongly related to their geographical origin and the availability of proper characterization techniques is of great importance in order to gain as more information about their behavior when utilized as filling materials. In the present contribute a physical characterization by meaning of Thermogravimetry, N2 physisorption, , XRD and SEM analyses of samples of commercial bentonite collected from the north of Sardinia (Italy) is reported.
Fri, 3 August 2018
REVIEW Download: 118| View: 137| Comments: 0 | doi:10.20944/preprints201808.0055.v1
Subject: Chemistry, Physical Chemistry Keywords: sonochemistry; cavitation chemistry; biomolecules; prebiotic chemistry
Online: 3 August 2018 (00:03:12 CEST)
This review considers the generation of relatively simple biomolecules, e.g., primary amino acids, resulting from the rapid collapse of bubbles in aqueous solutions containing various, low molar mass, gases. Some of the possible primary reactions occurring to produce the biomolecules are introduced and discussed. A brief discussion is included on the possible involvement of hydrodynamic cavitation, at the primordial seashore, as the initiating processes for the formation of the first organic molecules on prebiotic earth and the origins of life on Earth some 4 billion years ago.
Mon, 30 July 2018
ARTICLE Download: 140| View: 149| Comments: 0 | doi:10.20944/preprints201807.0559.v1
Subject: Chemistry, Physical Chemistry Keywords: magadiite; organo-layered silicate; removal; eosin; thermal stability; regeneration
Online: 30 July 2018 (05:41:11 CEST)
Na-magadiite exchanged with cetyl-trimethylammonium cations provided organophilic silicate materials that allowed the effective removal of the acidic dye "eosin". The organic cations were intercalated into the interlayer spacing of the layered silicate via an exchange reaction between the organic cations from their bromide salt and the solid Na-magadiite at room temperature. Different techniques were used to characterize the effect of the initial concentration of the surfactant on the structure of the organo-magadiites. The C, H, N analysis indicated that a maximum of organic cations of 1.03 mmol/g was achieved at initial concentrations higher than 0.25 mmol/g and was accompanied by an expansion of the basal spacing of 3.15 nm, with a tilted angle of 64.5° to the silicate layers. The conformation of the organic surfactants was probed using solid-state 13C, finding mainly the trans conformation similar to that of the starting cetyl trimethylammonium bromide salt (C16TMABr). Thermal gravimetric analysis was carried out to study the thermal stability of the resulting organo-magadiites. The intercalated surfactants started to decompose at 200 °C, with a mass loss percentage of 8 % to 25 %, depending on the initial loading of the surfactant, and was accompanied by a decrease of the basal spacing from 3.20 nm to 2.51 nm, as deduced from the in situ X-ray diffraction studies. At temperatures below 220 °C, an expansion of the basal spacing from 3.15 to 3.35 nm occurred. These materials were used as a removal agent for the anionic dye "eosin". The maximum amount of the dye removed was related to the organic cation content and to the initial concentration of eosin, with an improvement from 2.5 mg/g to 98 mg/g. This value decreased when the organo-magadiite was preheated at temperatures above 200 °C. The regeneration tests indicated that an 85 % removal efficiency was maintained after 6 cycles of use for the organo-magadiites.
Wed, 25 July 2018
ARTICLE Download: 163| View: 186| Comments: 1 | doi:10.20944/preprints201807.0489.v1
Subject: Chemistry, Physical Chemistry Keywords: CO2 adsorption, CH4 adsorption, biomass, activated carbon.
Online: 25 July 2018 (15:29:22 CEST)
The aim of the present study is to provide new insights into the CO2 and CH4 adsorption using a set of biomass-based activated carbons obtained by physical and chemical activation of olive-stones. The adsorption behavior is analyzed by means of pure gas adsorption isotherms up to 3.2 MPa at two temperatures (303.15 and 323.15 K).The influence of the activation method on the adsorption uptake is studied in terms of both textural properties and surface chemistry. For three activated carbons the CO2 adsorption was more important than that of CH4. The chemically activation resulted in higher BET surface area and micropore volume that lead to higher adsorption for both CO2 and CH4. For methane the presence of mesopores seems to facilitate the access of the gas molecules into the micropores while for carbon dioxide, the presence of oxygen groups enhanced the adsorption capacity.
Fri, 13 July 2018
REVIEW Download: 438| View: 254| Comments: 0 | doi:10.20944/preprints201807.0233.v1
Subject: Chemistry, Physical Chemistry Keywords: nanocrystals; poorly soluble drug; nanotechnology; stability
Online: 13 July 2018 (11:16:15 CEST)
Many approaches have been developed over time to counter the bioavailability limitations of poorly soluble drugs. With advances in nanotechnology in recent decades, science and industry have been approaching this issue through the formulation of drugs as nanocrystals, which consist of pure drugs and a minimum of surface active agents required for stabilization. They are carrier-free submicron colloidal drug delivery systems with a mean particle size in the nanometer range, typically between 10 and 800 nm. By reducing particle size to nanoscale, the particle surface area available for the molecule dissolution in the direction of dissolution medium is increased, and thus bioavailability is enhanced. This approach has proven successful, as demonstrated by the number of such drug products on the market. R&D and industry have offered many technological solutions to reduce the particle size to nanoscale, and also devised solutions for the handling of particle of nanodimensions, such as methods to accurately measure nanoparticle size and techniques to prevent physicochemical and stability related problems, such as aggregation. The present work provides an overview of the more recent achievements in improving the bioavailability of poorly soluble drugs according to their administration route, and describes the methods developed to overcome physicochemical and stability related problems.
COMMUNICATION Download: 327| View: 239| Comments: 0 | doi:10.20944/preprints201807.0221.v1
Subject: Chemistry, Physical Chemistry Keywords: N14; N18; density functional theory (DFT); high energy; stability
Online: 13 July 2018 (03:35:47 CEST)
Novel high energy density materials N14 (1,6-dihydro-1,2,3,3a,4,5,5a,6,7,8,8a,9,10,10a-tetradecazapyrene) and N18 (1,2,2a,3,4,4a,5,6,6a,7,8,8a,9,10,10a,11,12a-octadecazacoronene) were designed, and their structures, detonation performance and stabilities were calculated employing density functional theory (DFT). Calculations reveals that they have a good balance between high energy and stability. Their energy gaps between LUMO and HOMO are all lower than that of TATB, while their impact sensitivity h50% is estimated close to that of RDX. Concerning energy, detonation performance of the N14 (P = 43.6 GPa, D = 10040 m/s, Q = 2214 cal/g) and the N18 (P = 37.4 GPa, D = 9400 m/s, Q = 2114 cal/g) are comparable to CL-20.
Tue, 26 June 2018
ARTICLE Download: 174| View: 215| Comments: 0 | doi:10.20944/preprints201806.0418.v1
Subject: Chemistry, Physical Chemistry Keywords: ice XIV; vibrational spectrum; hydrogen bond; CASTEP, first-principles; DFT
Online: 26 June 2018 (12:44:03 CEST)
It is always a difficult task to assign the peaks recorded from vibrational spectrum. Herein we explored a new pathway of density functional theory (DFT) simulation to present three kinds of spectra of ice XIV and can be referenced as inelastic neutron scattering (INS), IR, and Raman experimental spectrum. The INS spectrum is proportional to phonon density of states (PDOS) while the photon scattering signals reflect the normal vibration frequencies near the Brillouin zone (BZ) center. Based on good agreements with experimental data, we identified the relative frequency and made scientific assignments by normal vibration modes analysis. The debating two hydrogen bond (H-bond) peaks among ice phases from INS have been discussed and the dynamic process of H-bond vibrations were found to be classified into two basic modes. we deduce that two H-bond modes are a general rule among ice family and more works are ongoing to investigate this subject.
Tue, 19 June 2018
ARTICLE Download: 180| View: 178| Comments: 0 | doi:10.20944/preprints201806.0300.v1
Subject: Chemistry, Physical Chemistry Keywords: polyelectrolytes; charge regulation; long-range interactions; Debye-Hückel interactions; transfer matrix; Ising models; semi-grand canonical ensemble; Monte Carlo simulations; conformational equilibria; variational methods
Online: 19 June 2018 (12:33:38 CEST)
The classical Rotational Isomeric State (RIS) model, originally proposed by Flory, has been used to rationalize a wide range of physicochemical properties of neutral polymers. However, many weak polyelectrolytes of interest are able to regulate their charge depending on the conformational state of the bonds. Recently, it has been shown that the RIS model can be coupled with the Site Binding (SB) model, for which the ionizable sites can adopt two states: protonated or deprotonated. The resulting combined scheme, the SBRIS model, allows to analyse ionization and conformational equilibria on the same foot. In the present work this approach is extended to include pH-dependent electrostatic Long Range (LR) interactions, ubiquitous in weak polyelectrolytes at moderate and low ionic strengths. With this aim the original LR interactions are taken into account by defining effective Short Range (SR) and pH-dependent parameters, such as effective microscopic protonation constants and rotational bond energies. The new parameters are systematically calculated using variational methods. The machinery of statistical mechanics for SR interactions, including the powerful and fast transfer matrix methods, can then be applied. The resulting technique, to which we will refer as Local Effective Interaction Parameters (LEIP) method, is illustrated with a minimal model of a flexible linear polyelectrolyte containing only one type of rotating bonds. LEIP reproduces very well the pH dependence of the degree of protonation and bond probabilities obtained by semi-grand canonical Monte Carlo simulations, where LR interactions are taken explicitly into account. The reduction in the computational time in several orders of magnitude suggests that the LEIP technique could be useful in a range of areas involving linear weak polyelectrolytes, allowing direct fitting of the relevant physical parameters to the experimental quantities.
ARTICLE Download: 142| View: 162| Comments: 0 | doi:10.20944/preprints201806.0298.v1
Subject: Chemistry, Physical Chemistry Keywords: B-DNA; Z-DNA; circular dichroism; calorimetry; enthalpy; conformational transitions; heat capacity
Online: 19 June 2018 (11:37:10 CEST)
High concentrations of Na+ or [Co(NH3)6]3+ can induce the B to Z conformational transition in alternating (dC-dG) oligo and polynucleotides. The use of short DNA oligomers (dC-dG)4 and (dm5C-dG)4 as models can allow a thermodynamic characterization of the transition. Both form right handed double helical structures (B-DNA) in standard phosphate buffer with 115 mM Na+ at 25 oC. However, at 2.0 M Na+ or 200 mM [Co(NH3)6]3+, (dm5C-dG)4 assumes a left handed double helical structure (Z-DNA) while the unmethylated (dC-dG)4 analogue remains right handed under those conditions. We have previously demonstrated that the enthalpy of the transition at 25 oC for either inducer can be determined using isothermal titration calorimetry (ITC) [Ferreira, J. M. & Sheardy, R. D., Biophys. J. 2006, 91, 1–7]. Here, ITC is used to investigate the linkages between temperature, water activity and DNA conformation. We found that the determined enthalpy for each titration varied linearly with temperature allowing determination of the heat capacity change (DCp) between the initial and final states. As expected, the DCp values were dependent upon the cation (i.e. Na+ vs [Co(NH3)6]3+) as well as the sequence of the DNA oligomer (i. e., methylated vs unmethylated). Osmotic stress experiments were carried out to determine the gain or loss of water by the oligomer induced by the titration. The results are discussed in terms of solvent accessible surface areas, electrostatic interactions and the role of water.
Fri, 15 June 2018
ARTICLE Download: 234| View: 261| Comments: 0 | doi:10.20944/preprints201806.0260.v1
Subject: Chemistry, Physical Chemistry Keywords: ATRP; chitosan grafted poly acrylic acid; heavy metals; adsorption; water treatment.
Online: 15 June 2018 (14:50:41 CEST)
In this work, we synthesized chitosan grafted-poly acrylic acid (CS-g-PA) through surface-initiated atom transfer radical polymerization (SI-ATRP). We also studied the adsorption process of copper and lead ions onto CS-g-PA surface. Adsorption equilibrium studies indicated pH 4.0 as the best pH for the adsorption process, while the maximum adsorption capacity for Pb2+ ions was 98 mg*g-1 and for Cu2+ was 164 mg*g-1, higher adsorption capacities than chitosan alone (CS), where Pb2+ was only 14.8 mg*g-1and Cu2+ was 140 mg*g-1, respectively. Furthermore, the adsorption studies indicated that Langmuir model describes all the experimental data. All these results suggest that the new CS-g-PA polymers had potential as adsorbent for hazardous and toxic metal ions produced by different industries.
Thu, 14 June 2018
ARTICLE Download: 114| View: 217| Comments: 0 | doi:10.20944/preprints201806.0228.v1
Subject: Chemistry, Physical Chemistry Keywords: femtochemistry; VMI (velocity map imaging); multidimensional analysis
Online: 14 June 2018 (09:44:38 CEST)
We present a tutorial to realize a multidimensional fitting procedure capable of extracting all the relevant information contained in a sequence of charged particle images acquired as a function of time in femtosecond pump-probe experiments. The images are reproduced using a 3D fitting method, which provides the velocity (or center-of-mass kinetic energy) and angular distributions contained in the images and their time evolution. A detailed example of the method is shown through the analysis of the time-resolved predissociation dynamics of CH3I on the B-band origin [Gitzinger et al., J. Chem. Phys. 133, 234313 (2010)]. We show that the multidimensional approach is essential for the analysis of complex images that contain several overlapping contributions where reduced dimensionality analyses cannot provide a reliable description of the features present in the image sequence. This methodology can be generalized to many types of multidimensional data analysis.
Tue, 22 May 2018
ARTICLE Download: 402| View: 408| Comments: 0 | doi:10.20944/preprints201805.0291.v1
Subject: Chemistry, Physical Chemistry Keywords: HPAM polymer; rheology; viscosity; injectivity; mechanical degradation; polymer flooding
Online: 22 May 2018 (06:21:35 CEST)
Polymer flooding is an established enhanced oil recovery (EOR) method, still many aspects of polymer flooding are not well understood. This study investigates the influence of mechanical degradation on flow properties of polymers in porous media. Mechanical degradation due to high shear forces may occur in the injection well and at the entrance to the porous media. The polymers that give high viscosity yields at a sustainable economic cost are typically large, MW > 10 MDa, and have wide molecular weight distributions. Both MW and the distributions are altered by mechanical degradation, leading to changes in the flow rheology of the polymer. The polymer solutions were subjected to different degrees of pre-shearing and pre-filtering before injected into Bentheimer outcrop sandstone cores. Rheology studies of injected and produced polymer solutions were performed and interpreted together with in-situ rheology data. The core floods showed a predominant shear thickening behavior at high flow velocities which is due to successive contraction/expansion flow in pores. When pre-sheared, shear thickening was reduced but with no significant reduction in in-situ viscosity at lower flow rates. This may be explained by reduction in the extensional viscosity. Furthermore, the results show that successive degradation occurred which suggests that the assumption of the highest point of shear which determines mechanical degradation in a porous media does not hold for all field relevant conditions.
Wed, 11 April 2018
ARTICLE Download: 595| View: 460| Comments: 0 | doi:10.20944/preprints201804.0155.v1
Subject: Chemistry, Physical Chemistry Keywords: g-C3N4; photocatalysis; NaOH; exfoliation; Cr(VI) reduction
Online: 11 April 2018 (14:29:45 CEST)
A simple, effective and environmental-friendly method was adopted for enhancing the photocatalytic activity of g-C3N4 in the reduction of aqueous Cr(Ⅵ) under visible-light irradiation. The enhancement was achieved via treatment of g-C3N4 in organic solvent with addition of NaOH particles by ultrasonic process for two hours. The results demonstrated that the treated g-C3N4 exhibited much higher photocatalytic activity than pristine g-C3N4 in the reduction of Cr(VI) . Under visible light irradiation for 120 min, the reduced ratios of Cr(VI) with the initial concentration of 50 mg/L in the presence of the treated g-C3N4and pristine g-C3N4 were 100% and 37.1%, respectively. With the addition of fulvic acid, Cr(VI) was efficiently removed at 40 min. Based on the characterization results of the structures and other physiochemical properties of the treated g-C3N4 and pristine g-C3N4 by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and UV Vis diffuse reflectance, the possible reasons responsible for the enhanced photocatalytic activity of the treated g-C3N4 were proposed. The yield and mechanism of different exfoliation methods were compared by semi-quantitative method.
Thu, 22 March 2018
REVIEW Download: 215| View: 277| Comments: 0 | doi:10.20944/preprints201803.0193.v1
Subject: Chemistry, Physical Chemistry Keywords: polymer thermoreversible gels; self-assembled systems; functional materials
Online: 22 March 2018 (13:01:15 CET)
The synthesis these past few years of novel organic molecules that spontaneously self-assemble into a large variety of molecular architecture, particularly generating organogels, has opened up new horizons for preparing functional materials. Here, we present an original preparation path of such materials through the making of hybrid gels of these molecules together with covalent polymers. Three types of systems are described: i) intermingled gels where a polymer gel and an organogel pervade one another; ii) encapsulation of self-assembled filaments in polymer fibrils, which provides a system with unusual magnetic properties; iii) the reverse situation where self-assembled nanotubes sheathe polymer fibrils. Here two covalent polymers are considered: a neutral polymer, namely stereoregular polystyrene (isotactic or syndiotactic), and a semi-conducting polymer, P3BT. In the latter case, semi-conducting nanowires are obtained.
Mon, 5 March 2018
ARTICLE Download: 205| View: 244| Comments: 0 | doi:10.20944/preprints201802.0074.v2
Subject: Chemistry, Physical Chemistry Keywords: statistical mechanics; irreversibility; arrow of time; chemical dynamics
Online: 5 March 2018 (05:54:00 CET)
The second law of thermodynamics states the increase of entropy, ΔS > 0, for real processes from state A to state B at constant energy from chemistry over biological life and engines to cosmic events. The connection of entropy to information, phase-space and heat is helpful, but does not immediately convince observers of the validity and basis of the second law. This gave grounds for finding a rigorous, but more easily acceptable reformulation. Here we show using statistical mechanics that this principle is equivalent to a force law 〈〈f〉〉> 0 in systems where mass centres and forces can be identified. The sign of this net force - the average mean force along a path from A to B - determines the direction of the process. The force law applies to a wide range of processes from machines to chemical reactions. The explanation of irreversibility by a driving force appears more plausible than the traditional formulation as it emphasizes the cause instead of the effect of motions.
Thu, 8 February 2018
ARTICLE Download: 168| View: 270| Comments: 0 | doi:10.20944/preprints201802.0074.v1
Subject: Chemistry, Physical Chemistry Keywords: statistical mechanics; irreversibility; arrow of time; chemical dynamics
Online: 8 February 2018 (18:14:41 CET)
The second law of thermodynamics states the increase of entropy, Delta S > 0, for real processes from state A to state B at constant energy from chemistry over biological life and engines to cosmic events. The connection of entropy to information, phase-space and heat is helpful, but does not immediately convince observers of the validity and basis of the second law. This gave grounds for finding a rigorous, but more easily acceptable reformulation. Here we show using statistical mechanics that this principle is equivalent to a force law 〈〈f〉〉> 0 in systems where mass centres and forces can be identified. The sign of this net force - the average mean force along a path from A to B - determines the direction of the process. The force law applies to a wide range of processes from machines to chemical reactions. The explanation of irreversibility by a driving force appears more plausible than the traditional formulation as it emphasizes the cause instead of the effect of motions.
Thu, 18 January 2018
ARTICLE Download: 537| View: 331| Comments: 0 | doi:10.20944/preprints201801.0168.v1
Subject: Chemistry, Physical Chemistry Keywords: phosphor; optical sensor; temperature sensor; manganese, luminescence
Online: 18 January 2018 (05:37:15 CET)
Phosphors based on magnesium titanate activated with Mn4+ ions are of great interest because, when excited with blue light, they display a strong red-emitting luminescence. They are characterized by a luminescence decay which is strongly temperature dependent in the range from 0 to 80 °C, making these materials very promising for temperature sensing in the biochemical field. In this work the optical and thermal properties of the luminescence of Mg2TiO4 are investigated for different Mn4+ doping concentrations. The potential of this material for temperature sensing is demonstrated by fabricating a fiber optic temperature microsensor and by comparing its performance against a standard resistance thermometer. The response of the fiber optic sensor is exceptionally fast, enabling monitoring of temperature fluctuation in subsecond time domain.
Tue, 12 December 2017
ARTICLE Download: 342| View: 297| Comments: 0 | doi:10.20944/preprints201712.0070.v1
Subject: Chemistry, Physical Chemistry Keywords: 6Li and 7Li nuclear magnetic moments; NMR liquid-phase studies; nuclear magnetic shielding constants
Online: 12 December 2017 (06:55:05 CET)
The LiCl and LiNO3 water solutions in the presence of small amounts of 3-helium have been investigated by means of multinuclear resonance spectroscopy. The resulting concentration dependences of the 3He, 6,7Li+, 14NO3¯ and 35Cl¯ resonance radiofrequencies are reported in the infinite limit. This data along with new theoretical corrections of shielding lithium ions was analyzed by a known NMR relationship method. Consequently, the nuclear magnetic moments of 6Li and 7Li were established against that of the helium-3 dipole moment: μ(6Li)=+0.822046(5)μN and μ(7Li)=+3.256418(20)μN. The new results were shown to be very close to the previously obtained values of the (ABMR) atomic beam magnetic resonance method. This experiment proves that our helium method is well suited for establishing dipole moments from NMR measurements performed in water solutions. This technique is especially valuable when gaseous substances of the needed element are not available. All shielding constants of species present in water solutions are consistent with new nuclear magnetic moments and these taken as a reference. Both techniques – NMR and ABMR – give practically the same results providing that all shielding corrections are properly made.
Fri, 8 September 2017
REVIEW Download: 366| View: 405| Comments: 0 | doi:10.20944/preprints201709.0023.v1
Subject: Chemistry, Physical Chemistry Keywords: Halogen bonding; supramolecular chemistry; NMR spectroscopy
Online: 8 September 2017 (04:22:09 CEST)
In the last 20 years, a huge amount of experimental results about halogen bonding (XB) has been produced. Most of the systems have been characterized by solid state X-ray crystallography, whereas in solution the only routine technique is the titration (by using 1H and 19F NMR, IR, UV-Vis or Raman spectroscopies, depending on the nature of the system), with the aim of characterizing the strength of the XB interaction. Unfortunately, the titration techniques have many intrinsic limitations and they should be coupled with other, more sophisticated techniques to have an accurate and detailed description of the geometry and stoichiometry of the XB adduct in solution. In this review, it will be shown how crucial information about XB adducts can be obtained by advanced NMR techniques, as Nuclear Overhauser Effect-based Spectroscopies (NOESY, ROESY, HOESY…) and diffusion NMR techniques (PGSE or DOSY).
Fri, 21 July 2017
ARTICLE Download: 486| View: 524| Comments: 0 | doi:10.20944/preprints201707.0059.v1
Subject: Chemistry, Physical Chemistry Keywords: open-cyclic tautomers; molecular switching; decay-associated spectra; warfarin; excited-state lifetime; cyclodextrins
Online: 21 July 2017 (04:56:54 CEST)
The excited-state lifetimes of the anticoagulant drug warfarin (W) in water and in the absence and presence of methyl-β-cyclodextrins (Me-β-CD) were recorded using time-resolved fluorescence measurements. Selective excitation of the open and cyclic protonated isomers of W were acquired with laser emitting diodes (LED) producing 320 and 280 nm excitation pulses, respectively. Formation of the inclusion complex was checked by UV–visible absorption spectroscopy, and the values of binding constants (2.9 × 103 M–1 and 4.2 × 102 M–1 for protonated and deprotonated forms, respectively) were extracted from the spectrophotometric data. Both absorption and time-resolved fluorescence results established that the interior of the macromolecular host binds preferentially the open protonated form, red shifts the maximum of its absorption of light at ~305 nm, extends its excited-state lifetime, and decreases its emission quantum yield (ФF). Collectively, sequestration of the open guest molecules decreases markedly their radiative rate constants (kr), likely due to formation of hydrogen-bonded complexes in both the ground and excited states. Due to lack of interactions, no change was observed in the excited-state lifetime of the cyclic form in the presence of Me-β-CD. The host also increases the excited-state lifetime and ФF of the drug deprotonated form (W¯). These later findings could be attributed to the increased rigidity inside the cavity of Me-β-CD. The pKa values extracted from the variations of the UV–visible absorption spectra of W versus the pH of aqueous solution showed that the open isomer is more acidic in both ground and excited states. The positive shifts in pKa values induced by three derivatives of cyclodextrins: HE-β-CD, Ac-β-CD, and Me-β-CD supported the preferential binding of these hosts to open isomers over cyclic.
Mon, 15 May 2017
ARTICLE Download: 728| View: 668| Comments: 3 | doi:10.20944/preprints201705.0112.v1
Subject: Chemistry, Physical Chemistry Keywords: adsorption; magadiite; cationic clay; methylene blue; remediation
Online: 15 May 2017 (12:14:05 CEST)
The removal of methylene blue (MB) dye from water was investigated using synthetic nano-clay magadiite (SNCM). SNCM was synthesized by hydrothermal treatment under autogenous pressure. A rosette-shaped single mesoporous magadiite phase with 16.63 nm average crystallite size and 33 m2.g-1 BET-surface area was recorded. The adsorption results indicated the pronounced affinity of the SNCM to the MB dye molecules which reached adsorption uptake of 20.0 mg MB dye/g of SNCM. The elimination of MB dye by the SNCM was kinetically and thermodynamically considered; a pseudo second order kinetic model was attained, and a spontaneous, chemical, and exothermic in nature was verified.
Tue, 11 April 2017
ARTICLE Download: 529| View: 537| Comments: 0 | doi:10.20944/preprints201704.0058.v1
Subject: Chemistry, Physical Chemistry Keywords: Fe (III) sensor; Poly(azomethine-urethane); Chemosensor; Hydrophobic property
Online: 11 April 2017 (05:53:15 CEST)
A new and easy-to-make polymer film sensor was prepared by dip-coating technique and this film used as fluorescent chemosensor for the detection of metal ions in aqueous solution. The proposed film sensor exhibited dual emissions at 540 and 582 nm under a single excitation wavelength. The fluorescence behavior of this film sensor toward metal ions has been investigated using these two wavelengths. The proposed sensor was found to show selectivity and sensitive to Fe3+ ion when compared the other metal ions in deionized water with excellent photostability. Detection limit of the polymeric film sensor was calculated as 86.15 and 28.90 μM. The results showed that the sensor can be successfully applied to the determination of Fe3+ ions in aqueous media. Contact angle measurement of PAZU film probe was also investigated and the result showed that the proposed film probe has hydrophobic property.
Wed, 8 March 2017
ARTICLE Download: 815| View: 803| Comments: 0 | doi:10.20944/preprints201703.0050.v1
Subject: Chemistry, Physical Chemistry Keywords: mefenamic acid; diamond anvil cell; high-pressure; polymorphism; high-pressure crystallization; non-steroidal anti-inflammatory drugs
Online: 8 March 2017 (08:58:38 CET)
High pressure crystallization technique has been successfully used to prepare an elusive form II of a non-steroidal anti-inflammatory drug, Mefenamic acid. Single crystal of form II was grown at 0.3 GPa from an 4:1 methanol/ ethanol mixture as a solvent using Diamond Anvil Cell. Comparison of crystal structures show that the efficient packing of MA molecules in Form II results from the structural flexibility of MA acid molecules. Compression studies performed on the single crystal of form I resulted in 14 % decrease of unit cell volume up to 2.5 GPa. No phase transition was observed up to this pressure. A reconstructive phase transition is required to induce conformational changes in the structure, which is confirmed by crystallization at high pressure results.
Tue, 15 November 2016
ARTICLE Download: 998| View: 1374| Comments: 0 | doi:10.20944/preprints201611.0077.v1
Subject: Chemistry, Physical Chemistry Keywords: computational theoretical chemistry; photochemistry; nonadiabatic dynamics; ultrafast processes; surface hopping; nucleobases; thymine
Online: 15 November 2016 (11:06:06 CET)
After UV excitation, gas phase thymine returns to ground state in 5 to 7 ps, showing multiple time constants. There is no consensus on the assignment of these processes, with a dispute between models claiming that thymine is trapped either in the first (S1) or in the second (S2) excited states. In the present study, nonadiabatic dynamics simulation of thymine is performed on the basis of ADC(2) surfaces, to understand the role of dynamic electron correlation on the deactivation pathways. The results show that trapping in S2 is strongly reduced in comparison to previous simulations considering only non-dynamic electron correlation on CASSCF surfaces. The reason for the difference is traced back to the energetic cost for formation of a CO p bond in S2.
Fri, 4 November 2016
ARTICLE Download: 749| View: 958| Comments: 0 | doi:10.20944/preprints201611.0030.v1
Subject: Chemistry, Physical Chemistry Keywords: neutral organic radical; perchlorotriphenylmethyl radical; tetrathiafulvalene; donor-acceptor
Online: 4 November 2016 (14:19:09 CET)
During the last years there has been a high interest in the development of new purely-organic single-component conductors. Very recently, we have reported a new neutral radical conductor based on the perchlorotriphenylmethyl (PTM) radical moiety linked to a monopyrrolo-tetrathiafulvalene (MPTTF) unit by a -conjugated bridge (1). Interestingly, this system behaves as a semiconductor with high conductivity and small energy gap under high pressure. With the aim of developing a new material with improved conducting properties, we have designed and synthesized the radical dyad 2 which was functionalized with an ethylenedithio (EDT) group in order to improve the intermolecular interactions of the TTF subunits. The physical properties of the new radical dyad 2 were studied in detail in solution to further analyze its electronic structure as well as its potential use as radical conductor in the solid state.
Mon, 22 August 2016
ARTICLE Download: 1160| View: 927| Comments: 0 | doi:10.20944/preprints201608.0189.v1
Subject: Chemistry, Physical Chemistry Keywords: amine-modified MCM-41; CO2 adsorption; inorganic alkali; alkali amounts
Online: 22 August 2016 (05:47:02 CEST)
Three kinds of inorganic alkali are introduced into tetraethylenepentamine (TPEA) and polyethyleneimine (PEI) modified MCM-41 as the CO2 adsorbents. FT-IR and TGA are used to characterize the surface groups and the thermal stability of adsorbents. Chemical titration method is used to measure the alkali amounts of adsorbents. Thermo-gravimetric analysis with 10%CO2/N2 as the simulated flue gas is used to test the CO2 adsorption performance of adsorbents. The results show that all three kinds of inorganic alkali containing adsorbents exhibit promoted CO2 adsorption capability than traditional TPEA and PEI modified samples. Ca(OH)2 and PEI modified samples exhibit the highest adsorption capacity and stable regeneration property. The introduction of inorganic alkali changes the chemical adsorption mechanism between CO2 and increases the effective alkali amounts due to its hydroxyl groups. The CO2 adsorption capacities have a linear dependence relation with the alkali amounts of adsorbents, indicating that alkali amount is a critical factor for novel adsorbents exploring.