Thermodynamic and Dynamic Transitions and Interaction Aspects in Reorientation Dynamics of Molecular Probe in Organic Compounds: A Series 1-Alkanols with TEMPO

1. Introduction

In general, the dynamics of glass-forming liquids, i.e., organics and inorganics forming the supercooled liquid by their cooling below the melting temperature, T_m, and finished by a liquid-to-glass transition to a glass below the glass temperature, T_g, is non-monotoneous and exhibits a change at the so-called dynamic crossover temperature, T_cross = T_B or T_X lying between T_m and T_g [1,2,3,4,5,6,7,8,9,10,11,12]. This dynamic crossover phenomenon between the relatively weakly and strongly changing supercooled liquid dynamics is observed using experimental techniques such as viscosity (VISC) at T_B,η or T_X [1,2] and dielectric spectroscopy (DS) at T_B,DS^ST [3,4] or T_B,DS^MG [5] and T_B,DS^KWW [6] as well as T_B,DS^SCH [7]. Standardly, the crossover temperatures are determined by fitting the supercooled liquid dynamics using a combination of classic phenomenological expression for viscosity, η, or structural relaxation time, τ, such as the Vogel-Fulcher-Tamman-Hesse (VFTH) equation [1] or using the power law (PL) equation [2]. Lately, a special evaluation method via Stickel‘s temperature-derivative type of analysis or via more general Martinez-Garcia’s apparent enthalpy analysis of the relevant dynamic quantities giving T_B,DS^ST and T_B,DS^MG, respectively, were proposed [3,4,5]. Another ways of determination of the crossover temperature are based on the onset of increasing broadening of the frequency dispersion of the structural relaxation time distribution and on the change of structural relaxation strenght, Δε_α, leading to T_B,DS^KWW or T_B,DS^SCH, respectively [6,7]. In the case of the PL eq. with T_X [2,9], this expression is rationalized theoretically within the idealized mode coupling theory (I-MCT) of liquid dynamics [11] by derivation of the same form of temperature dependence for viscosity and relaxation time with the so-called critical temperature, T_c ≈ T_X.

The crossover transition is very significant feature of the supercoled liquid behavior as it is demonstrated by findings of several empirical correlations of T_BorT_X with various characteristic temperatures of a variety structural-dynamic phenomena, such as the decoupling or bifurcation of the primary α relaxation and the secondary β process, T_αβ, from DS or dynamic light scattering (DLS) [8]. Moreover, the crossover phenomenon is also reflected by various extrinsic probe techniques, such as FS, ESR and PALS. They revealed the decoupling of translation from rotation of molecular probes and the medium dynamics, at T_decoup, either for the relatively large fluorescence probes via fluorescence spectroscopy (FS) [13] or decoupling of rotation of the spin probes from the medium dynamics using electron spin resonance (ESR) [14]. Finally, crossover in the supercooled liquid state is also manifested by a bend effect in ortho-positronium lifetime τ₃ vs. T dependence as detected by positron annihilation lifetime (PALS). This slope change reflects a change in the free volume expansion at the characteristic PALS temperature T_b1^L above T_g in amorphous glass-formers [15]. Evidently, it is increasingly recognised that the dynamic crossover before glass transition temperature plays an essential if not fundamental role in our understanding glass transition phenomenon [9].

In contrast to the afore-mentioned cases of amorphous glass-formers, observations of the crossover transition in strongly crystallizing, i.e., relatively hardly supercooled apolar and polar organics is substantially more difficult. This is connected with the problem of formation of sufficiently large amorphous domains in the otherwise dominantly ordered material and subsequently, with their characterization by suitable experimental technique. Recently, we have proposed one special method of creation of such amorphous domains in crystalline materials consisting in an introduction of appropriate molecular probe disordering its immediate surroundings of the otherwise ordered medium. This included spin probe (2,2,6,6-tetramethyl piperidin-1-yl)oxyl (TEMPO) with V_TEMPO^W = 170 ų in a series of apolar n-alkanes ranging from n-hexane to n-nonadecane using ESR technique [16]. On the basis of a close correlation between one of the characteristic ESR temperatures, namely, T_X1^fast, lying a bit above the main slow-to-fast transition at T_50G and marking the onset of pure fast motion regime of TEMPO and the crossover temperatures, T_X^VISC, as obtained from fittings the corresponding viscosity data for a series of n-alkanes using the PL eq., the dynamic crossovers in the local disordered regions around the probe molecules in the otherwise dominantly crystalline organics were detected.

One of the most important aspects of the extrinsic probe techniques, such as ESR is a potential interaction between the used probe and the medium‘s constituents which can in more or less extent influence the corresponding probe response from the investigated organic matrix. In our previous work on a series of apolar crystallizing n-alkanes we used one of the smallest polar spin probe TEMPO where this interaction aspect is supposed to be small [16]. The aim of this work is to test another types of strongly crystallizing organic media consisting of protic polar compounds, such as 1-alkanols with a potential of the intermolecular H-bonding interaction not only between the own polar molecules but also between these polar molecules and polar spin probe TEMPO. The obtained spectral and dynamic data for the TEMPO on the family of aliphatic monohydroxyalcohols or 1-alkanols H(CH₂)_nOH with N_C = 1-10, i.e., ranging from methanol to 1-decanol are interpreted using the newly analyzed viscosity data from the literature in order to reveal roles of the thermodynamic and dynamic transitions as well as of the interaction aspect in the main slow to fast transition behavior of the the used spin probe TEMPO.

2. Results and Discussion

2.1. Thermodynamic and crossover transition behaviors in 1-alkanols

It is well known that 1-alkanols similarly as n-alkanes belong to the class of relatively easily crystalizing organic compounds. For this strong ordering tendency they require special ways of preparation of the totally or partially amorphous samples with one exception of 1-propanol (C3OH) which is a very good glass-former [17].

2.1.1. Thermodynamic transitions in 1-alkanols

Figure 1 and Figure 2 and Table 1 summarize the literary data about three basic thermodynamic transitions of condensed materials, i.e., glass-to-liquid (devitrification) transition of the amorphous phase and solid-to-liquid (melting) transition of the crystalline phase to liquid one as well as liquid-to-gas (evaporation) transition of the liquid phase to gas one. In general, the corresponding transition temperatures, i.e., glass temperature T_g and melting temperature T_m for 1-alkanols exhibit the non-monotoneous character as a function of their molecular size expressed by the number of carbon atoms in the chain, N_c, or molecular weight, M, while the boiling temperature, T_b, shows up the monotoneous type of dependence over the whole molecular size interval. In contrast to the melting with the well-defined T_m values [18], the T_g values measured so far exhibit a scatter up to 10 K which depend on both preparation procedure and measuring technique, such as dynamic-mechanical spectroscopy (DMS), differential thermal analysis (DTA) or differential scanning calorimetry (DSC) and dielectric spectroscopy (DS) [19,20,21,22,23,24,25,26,27,28], apparently diminishing with increasing molecular size. In spite of this fact, the T_g value for the shortest 1-alkanols decreases from methanol (C1OH) to ethanol (C2OH) followed by monotoneous increasing trend starting from C2OH for DMS data [26] or from 1-propanol (C3OH) for CAL ones [17]. As originally proposed by Faucher & Koleske [26], their DMS results could be described by the power law (PL)–type expression as a function of molecular weight M:

T_g = AM ^α

(1)

where A, α are empirical parameters for a given homologous series of compounds. As seen from Figure 1A, they exhibit similar trend depending relatively strongly on the way of generation of amorphous material and the used set up in DMS or DTA, respectively. The latter values of T_g^DTA coinciding with T_g^CAL in cases of lower 1-alkanols, such as ethanol, 1-propanol and 1-butanol studied by special CAL technique, i.e., quasiadiabatic calorimetry (QADC) [17,28] are considered to be more reliable ones mainly because of the experimental complexity of DMS.

As for the melting transition of 1-alkanols, the same expression can be approximately used for the melting points:

T_m = CM ^γ

(2)

where C = 8.41 and γ = 0.705 are empirical parameters for the melting of a given homologeous series of compounds as determined mainly from the calorimetric data [18]–Figure 2. Similar approach has been recently applied for n-alkanes and monoalcohols in spite of the very pronounced zig-zag effect for the formers with the similar γ value of 0.7 by Novikov & Rössler [29].

Finally, in Figure 1B the estimated values of glass transition, T_g*, calculated according to the well-known empirical rule for many organic and inorganic glass formers: T_g* = (2/3)T_m [see e.g. Ref. 29–32] are also listed. From their comparison with the measured T_g data from DTA or DMS it follows that this rule is not valid for our series of the first ten 1-alkanols with one exception for C3OH. Alternatively, the measured T_m^CAL/T_g^DTA ratios fullfil rather another empirical rule of ~ 1.70 instead of T_m/T_g*= (3/2) = 1.50 valid again for C3OH only–Figure 3.

2.1.2. Dynamic crossovers in 1-alkanols

As mentioned in Introduction, the crossover temperature in the supercooled liquid state of many organic compounds can be obtained using the power law (PL) equation connecting the viscosity of liquids, η, with temperature T in the normal liquid and weakly supercooled liquid states [2,9,16]. Figure 4 and Figure 5 and Table 1 as well as Figure 6 give the results of such a way of determination of the crossover temperature, T_X. Thus, Figure 4 presents compilations of the viscosity data for a series of ten 1-alkanols ranging from methanol (C1OH) up to 1-decanol (C10OH) as a function of temperature mostly from the two large summarizing literature sources [33,34]. Most of the viscosity data of 1-alkanols falls into the normal liquid state between the melting temperature, T_m, and the boiling temperature, T_b. For two lower members of this series, namely, ethanol (C2OH) and 1-propanol (C3OH), the viscosities were also measured in the supercooled liquid state below the corresponding T_m’s; sligthly for the former [36] but even almost down to the corresponding T_g for the latter because of its very good glass-forming ability [36,37,38]. All the viscosity data for a serie of the first ten 1-alkanols can be described by the power law (PL) equation:

$${\eta }_{}\left(T\right)={\eta }_{\infty }{\left[\frac{\left(T-T_X\right)}{T_X}\right]}^{-\mu }$$

(3)

where, η_∞ is the pre-exponential factor, T_X is the empirical characteristic dynamic PL temperature or the theoretical critical MCT temperature T_c and μ is a non-universal coefficient. The corresponding fitting curves are plotted in Figure 4 and the obtained crossover temperatures T_X are listed in Table 1 and Figure 6. In the afore-mentioned case of 1-alkanols for which the viscosity data also in the supercooled liquid state exist [43], the extracted T_X‘s for, e.g., C2OH from fitting over the usual normal liquid state range T_b - T_m = 192 K and over the whole accessible temperature range of 227 K [36] = T_b –T_min = 227 K, change by 4 K only. Similarly, for very good glass-former C3OH this difference reaches 3 K towards the lower value by including also the strongly supercooled liquid range data from Ref. 36 - see Figure 5A,B. Thus, the PL equation. fit over the normal liquid state only appears to be a very good approximation to obtain the T_X‘s lying in the supercooled liquid state below the corresponding T_m’s in strongly crystallizing organics, such as 1-alkanols. These are also listed in Table 1 together with scarce determinations on the first three members, namely, C1OH, C2OH and C3OH by other authors [2,5,9].

It is shown that the PL equation is valid for a large number of organic molecular glass formers over rather higher temperature range [2,5,9] It is also known, the I-MCT works very well also for the relatively lower viscosity regime [11]. In reality, although the viscosity does not diverge at T_X ≈ T_c, several analyses of the slightly supercooled and normal liquid dynamics in various organic glass formers in terms of the extended mode coupling theory (E-MCT) which removes this singularity, provide the same crossover temperature in the supercooled liquid phase [11,40]. Thus, the T_X parameter marks two distinct regimes of the strongly and weakly supercooled liquid dynamics [11]. In particular, it corresponds to the onset of dynamic heterogeneities, i.e., regions with slower dynamics embedded into regions of higher dynamics, when the decoupling of translation from rotation of the molecular tracers and the decoupling of rotation of the molecular tracer from that of the medium constituents occur and the classic Stokes-Einstein law or Debye-Stokes-Einstein law, respectively, are violated [9,14].

Figure 6 displays the molecular size dependence of the extracted dynamic crossover temperature TX for 1-alkanols together with its fitting curve of a similar form as for T_g and T_m. Similarly to the T_g and T_m quantities, after the initial decrease to the second lowest member the series of nine 1-alkanols follows the power law formula:

T_X = BM ^β

(4)

with the β exponent equals 0.656 lying in between those for the glass temperature, T_g(α = 0.503) and melting point, T_m(γ = 0.705).

Finally, returning to Figure 3, a comparison of T_X/T_g vs. T_m/T_g dependencies as a function of molecular mass M starting from C2OH shows diametrally different trend for the former quantity with respect to the latter one, i.e., increasing distance of the particular T_X from the respective T_g with the increasing molecular size of 1-alkanol. This finding together with the almost same relative distance of T_X from T_m being ca. 0.81xT_m indicates that the larger molecule of 1-alkanol the larger temperature range of the existence of the strongly (or deeply) and correspondingly the shorter weakly (slightly) supercooled liquid state. On the other hand, on cooling of the smaller 1-alkanols, the weakly supercooled liquid state persists for longer T range with the correspondingly shorter deeply supercooled liquid range.

2.2. ESR data

2.2.1. General spectral and dynamic features

Figure 7 presents the 2A_zz‘ vs. T dependencies for our series of the spin systems: TEMPO/1-alkanols. In all cases, a quasi-sigmoidal courses of these plots are found with the higher 2A_zz‘ values in a slow motion regime at relatively lower temperatures and the lower 2A_zz‘ ones in a fast motion one in relatively higher temperature region.The main feature of 2A_zz‘ vs. T dependencies is a more or less sharp change at the main characteristic ESR temperature T_50G , at which the 2A_zz‘(T_50G) value reaches just 50 Gauss corresponding to the correlation time of the TEMPO in a typical organic medium around a few ns. Note that the detailed spectral simulations of the TEMPO dynamics in several organics, including one of the investigated 1-alkanols, namely, 1-propanol [41], revealed that the spin probe population even at T_50G is not completely in the fast motion regime which occurs at a bit higher temperature T_X1^fast. In addition to these main characteristic ESR temperatures T_50G and T_X1^fast other effects appear at T_X1^slow,T_X2^fast a discussion of which goes beyond the scope of this work and therefore it will addressed elsewhere. All the 2A_zz’ vs. T plots include also the afore-mentioned thermodynamic and dynamic temperatures: T_g, T_m or T_X, respectively. Then the mutual relationships of these three basic characteristic thermodynamic and dynamic temperatures to T_50G and T_X1^fast in a series of 1-alkanols will be discussed below in Section 2.2.2.

In principle, the main slow-to-fast motion transition of the TEMPO in any organics is related not only to these thermodynamic and dynamic transitions but it may also be influenced by further factors, such a potential mutual interaction of a polar spin probe with organic media, especially, polar ones. The values of anisotropic hyperfine constants A_zz‘(100 K) at the lowest measured temperature of 100 K and of isotropic ones A_iso(RT) at room temperature are summarized in Table 1. Their dependencies on N_C as well as on some relevant bulk property of media such as bulk polarity of media through their dielectric constant, ε_r, will be discussed in the Sect.2.2.3.

Finally, the mutual connection between temperature parameters of the slow-to-fast transition and the thermodynamic as well as dynamic ones in relation to the polarisation interaction of the polar TEMPO probe with a series of polar 1-alkanols are discussed in Sect.2.2.4.

2.2.2. The mutual relationships of T_50G and T_X1^fast with thermodynamic and dynamic transitions

In Figure 8 global comparisons of the characteristic ESR temperatures T_50G and T_X1^fast with the afore-mentioned thermodynamic and dynamic temperatures T_g, T_X and T_m are presented. In all the cases, the slow-to-fast transition in all 1-alkanols occur above the corresponding glass-to-liquid transition T_g’s, i.e., in the amorphous phase liquid sample or in the local amorphous liquid zones of partially crystalline matrices, at least. Figure 9 expresses these comparisons in terms of the corresponding ratios: T_50G/T_m, T_50G/T_X and T_X1^fast/T_m, T_X1^fast/T_X. We can approximately distinguish two distinct regions of these ratios with a boundary at around C5OH:

low M region: C1OH-C5OH with T_50G/T_m ≈1

(5)

high M region: C6OH-C10OH with T_50G/T_X ≈ 1

(6)

So, for higher members starting at C6OH to C10OH with a relatively longer aliphatic part we observe a quite plausible closeness between the characteristic ESR temperatures and the T_X‘s indicating that the main ESR transition is related to the dynamic crossover between the deeply and slightly supercooled liquid state. This basic finding is similar to the previous one for a series of n-alkanes [16] with the fact that the T_X‘s for n-alkanols are higher than the T_X‘ ones for the corresponding n-alkanes with the same number of C atoms in the molecule. This difference indicates that the spin probe TEMPO is not fully surrounded by the apolar aliphatic parts of the n-alkanol molecules and that the polar -OH groups influence its dynamics as it will be discussed later in Section 2.2.3. Anyway this indicates that the immediate environment of the molecular-sized spin probe TEMPO is locally disordered and subsequently, sensitive to the crossover transition in this local amorphous phase. On the other hand, for low M members from C1OH to C5OH T_50G and T_X1^fast lie significantly above the corresponding T_X values and they are situated rather in the vicinity of the corresponding melting temperatures, T_m. This indicates that the slow- to-fast transition of TEMPO appears to be related to the global disordering process connected with the solid–to-liquid state phase transition in the otherwise partially crystallized samples.

2.2.3. Isotropic and anisotropic hyperfine constants A_iso(RT), A_zz‘(100 K) as a function of N_C and polarity and proticity of 1-alkanols

Figure 10 displays the anisotropic hyperfine constant, A_zz‘(100K) and the isotropic. hyperfine constant, A_iso(RT), of the TEMPO as a function of the chain length in a series of 1-alkanols studied. Our values of A_iso(RT) for TEMPO are quite consistent with the scarce ones obtained for lower members of our series, namely, C1OH-C4OH [42,43,44]. Although both the quantities are decreasing with increasing chain size, a significant difference does exist in the corresponding trends. The former quantity has two clearly regions of distinct behavior: a sharper decreasing trend for low-M members and the weaker one for higher-M ones above N_C ~ 4. On the other hand, the A_iso(RT) parameter shows up rather slighter reduction with the number of C atoms in the molecule, N_C.

These basic empirical findings can be discussed in relation to polarity of a set of the polar media with the dissolved polar spin probe TEMPO μ_TEMPO ~ 3D [45] from both the phenomenological and theoretical viewpoints. First, the A_iso(RT) values can be related to various measures of polarity of the medium, e.g., the dipole moment of the medium‘s molecule, μ_entity,X, as a measure of the polarity of individual entity in a given phase state X = g,l or the static dielectric constant of the medium, ε_r(RT), as a measure of the polarity of the bulk liquid medium as listed in Table 1. In the first case evidently no any relationship does exist due to the quasi-constant values of the gas-phase μ_g = 1.66±0.05 D or the liquid-phase μ_l = 2.84 ± 0.15 D dipole moments [34,35]. On the other hand, Figure 11 displays the mutual relationship between the isotropic hyperfine constant, A_iso(RT), and the dielectric constant, ε_r(RT), of 1-alkanols [34] together with of the latter quantity at RT as a function of the number of C atoms in chain in insert. Both the quantities decrease with N_C resulting into A_iso (RT) vs. ε_r (RT) relationship with approximately two regions of a distinct behavior: i) for lower polar 1-alkanols (C10OH-C5OH) with ε_r < ~ 17 with a strong sensitivity of A_iso(RT) to polarity and a weak one to proticity and ii) for higher polar 1-alkanols (C3OH-C1OH) with ε_r > ~ 17 with weak sensitivity of A_iso(RT) to polarity and stronger to proticity due to the increased population of HO-groups potentially interacting with the spin probe TEMPO molecule. The apparent boundary between both regions occurs at N_C = 4-5, i.e., for 1-butanol or 1-pentanol, where the ε_r(RT) vs. N_C(RT) plot changes rather pronouncedly from the sharply decreasing dependence to the slightly decreasing one and where, at the same time, conformational degrees of freedom and related enhanced alignement of the apolar parts of the molecules start to occur. Interestingly, in spite of the absence of ε_r(100K) data for their direct comparison with A_zz‘(100K), the boundary for this quantity seems to be consistent with that for A_iso(RT) suggesting a significant role of polarity and proticity in both the mobility states of the spin probe TEMPO. These findings of a solvent dependence of the different ESR parameters are consistent with the previous ones for A_iso(RT) [46,47] as well as for A_zz‘(77 K) [48,49].

This our basic finding is similar to that for another larger nitroxide spin probe 1-oxyl- 2,2,5,5-tetramethyl pyrroline-3-methyl)methanethiosulfonate (MTSSL) in a series of 17 solvents ranging from apolar methylbenzene (toluene) [ε_r(RT) = 2.4] to highly polar water [ε_r(RT) = 80.4] and even more polar formamid [ε_r(RT) = 109] including most of the members of our 1-alkanol series with one expection for 1-pentanol [50]. These authors similarly distinguished the following two regions, i.e., „apolar“ region for ε_r(RT) < 25, where the sensitivity of A_iso(RT) and A_zz‘(77K) to the polarity expressed by ε_r(RT) is large and „polar“ region for ε_r(RT) > 25, where the sensitivity of A_iso(RT) and A_zz‘(77 K) to the polarity is small and the change is ascribed to the medium proticity.

However, it is evident that this their dividing is rather arbitrary and very rough because the former „apolar“ region include also many of our polar 1-alkanols. In connection with the afore-mention empirical relation between spectral parameters and bulk polarity, more elaborate theoretical approaches based on models of the medium as a dielectric continuum with dielectric constant, ε_r, and the molecular solute, e.g., polar spin probe as a molecular entity localized in a spherical cavity [47,51] can be discussed. Within the reaction field concept of the polarization of the continuum medium by the polar solute, one obtains for the Onsager‘s reaction field [52] and Böttcher‘s reaction field [53] the following functional relations: A_iso = f[(ε_r–1)/(ε_r + 1)] [47] or A_iso = f [(2ε_r + 1)/(2ε_r + n_D²)], where n_D is the refraction index of the pure nitroxide [51], respectively. Figure 12 displays test of the validity of the first functional dependence for two basic groups of organic compounds at RT doped by TEMPO. The first is represented by a series of apolar and aprotic polar solvents which range from apolar benzene (BZ) with ε_r(RT) = 2.3 to highly polar but aprotic dimethylsulphoxide (DMSO) with ε_r(RT) = 48.9 as taken from Ref.50. The other group including our series of ten 1-alkanols from methanol with ε_r(RT) = 33 to 1-decanol with ε_r(RT) = 7.9 differs significantly from the predicted linear trend due to the specific protic character of the molecules allowing for the H-bond formation between the polar spin probe TEMPO molecule and the alkanol’s one(s). This is quite consistent with the maximal value of A_iso (RT) = 17 Gauss [44] for highly polar and protic water ε_r(RT) = 80.4 [50]. The relative large difference between water and the first member of alkanol family is on the basis of theoretical calculations using density functional theory (DFT) interpreted in terms of the complexation of nitroxide with two water or one methanol molecules, respectively [45,50]. Moreover, a closer inspection of this group of protic polar compounds confirms a distinguishing a series of 1-alkanols into two subgroups with the distinct slopes of A_iso(RT) as a function of the corresponding dielectric function: i) weaker for the higher members from C10OH to C6OH and ii) stronger for the shorter ones from C5OH to C1OH with the approximate boundary between C5OH and C6OH, i.e., for ε_r(RT) ~ 16.5. Similar situation can be found for the Böttcher type of reaction field due to a linearity between the respective functional forms. Both these findings appear to be consistent with the purely empirically found boundary at C4OH - C5OH as seen from the A_iso(RT) vs. ε_r(RT) plot without inclusion of the polarization interaction between the polar solute and the solvent in Figure 11.

2.2.4. Connection of the main slow to fast motion transition of the spin probe TEMPO with the polarity and proticity and the thermodynamic and dynamic transition behavior of 1-alkanols.

In Figure 8 and Figure 9 in Section 2.2.2 we have compared the characteristic ESR temperatures T_50G and T_ini^fast of the slow to fast transition of TEMPO in a series of 1-alkanols with the dynamic crossover T_X and thermodynamic transition temperatures T_m, and their mutual ratios as a function of molecular size, N_C, of the media. In particular, we revealed rather a step-like change in the main spin probe TEMPO transition from that related by dynamic crossovers at around T_X for the longer chains to that related to thermodynamic transitions around T_m for the shorter molecules at N_C ~ 5.

Next, in Figure 10, Figure 11 and Figure 12 in Section 2.2.3 we presented the relations of spectral parameters A_iso(RT) and A_zz‘(100 K) to N_C as well as their phenomenological and theoretical relationships of especially A_iso(RT) to polarity properties for a set of 1-alkanols. Here, we observed a change in the trend of hyperfine interactions with polarity and proticity of 1-alkanol media at N_C ~ 4. Now, a combination of these findings indicates that the slow to fast transition in mobility of TEMPO in a series of 1-alkanols is relatively strongly dependent on the strenght of intermolecular interactions between the polar constituents of the polar media as well as between the polar spin probe and the polarity and proticity of the 1-alkanols investigated. In the longer members of the 1-alkanol family with the relatively higher population of the apolar aliphatic methylene groups related to a weakly changing polarity, the slow to fast transition is related mainly to the dynamic crossover process around T_X similarly as in the apolar n-alkanes [16]. On the other hand, in the shorter members with the relatively higher dielectric constants and proticity due to the relatively higher population of the polar hydroxyl groups, the larger-scale disorder process connected with the solid-to-liquid phase transition around T_m is needed for destroying not only the dense H-bonding network between the medium’d molecules, but also of clusters of the polar TEMPO molecules with them and subsequently, to the appearance of slow-to-fast transition in mobility of TEMPO. The critical molecular size of 1-alkanol for this step-like change in the slow to fast transition of TEMPO lies at N_C ~ 5 below which the polarity and proticity aspects of the media become to be dominanting factors.

3. Experimental

3.1. Materials and Methods

A series of 1-alkanols ranging from methanol to 1-decanol from Sigma-Aldrich, Inc was used as model protic polar media. The reason for our choice of this series stems from the fact that all of the used 1-alkanols are in the liquid state at room temperature because of easy spin probe system preparation. As an extrinsic particle, the spin probe 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) of the quasi-spherical shape and the relative high dipole moment of μ_TEMPO = 3 D [45] was applied in the deoxygenated 1-alkanols with a very low concentration of ~ 5x10^-4 M.

3.2. ESR

ESR measurements of the very dilute spin systems 1-alkanol/TEMPO were performed on the X-band Bruker–ER 200 SRL spectrometer operating at 9.4 GHz with a Bruker BVT 100 temperature variation controller unit. ESR spectra were recorded after cooling with a rate ~–4 K/min in a heating mode over a wide temperature range from 100 K up to 300 K with steps of 5 - 10 K. To reach the thermal equilibrium, the sample was kept at a given temperature for 10 minutes before starting three spectra accumulations. The temperature stability was ± 0.5 K. The microwave power and the amplitude of the field modulation were optimized to avoid the signal distortion. The ESR spectra were evaluated in terms of the spectral parameter of mobility, 2A_zz’(T), i.e.,z-component of the anisotropic tensor of hyperfine interaction A(T), corresponding to the outermost peaks separation of the triplet spectra of the spin probe of nitroxide type in a given medium, as a function of temperature and the subsequent determination of the spectral T_50G parameter [50,54]. This is the characteristic ESR temperature at which 2A_zz’ reaches the conventional value of 50 Gauss (G). Furthermore, additional characteristic ESR temperatures can be obtained describing in detail the slow to fast regime transition zone over more or less wide temperature interval around T_50G as well as in both slow and fast motion regimes: T_Xi^slow, T_Xi^fast [55,56,57]. In addition to this anisotropic hyperfine splittting parameter, A_zz’(100 K), the isotropic hyperfine constants, A_iso(RT) of the spin probe TEMPO as a measure of its interaction with a given medium were determined at room temperature under the fast motion condition in low-viscosity media [50].

4. Conclusions

The spectral and dynamic behaviours of the spin probe TEMPO in a series of 1-alkanols ranging from methanol to 1-decanol over a wide temperature range from 100 K up to 300 K using electron spin resonance (ESR) are reported. For all of alkanols, the main characteristic ESR temperatures connected with the slow to fast motion regime transition, namely, T_50′s and T_X1^fast’s are situated above the corresponding glass temperatures, T_g, and for the first five shorter members T_50G‘s lie in the vicinity of melting point, T_m, while for the longer ones the T_50G < T_m relationship indicates that the TEMPO molecules are in the local disordered regions of the crystalline media. The T_X1^fast’s are compared with the dynamic crossover temperatures, parametrized as T_X^VISC = 8.72M^0.66, as obtained by fitting the viscosity data in the liquid n-alkanols with the empirical power law. In particular, for N_C = 6-10 T_X1^fast’s lie rather relatively close to T_X^VISC as for apolar n-alkanes, while for N_C = 1-5 they are situated above the respective T_X^VISC in the vicinity of T_m. The absence of such a coincidence for lower 1-alkanols indicates that the slow to fast motion transition is significantly influenced by the mutual interaction between the polar TEMPO and the protic polar medium due to the increased polarity and proticity which are destroyed at higher temperatures region connected to the larger-scale solid-to-liquid transition.