Working Paper Article Version 1 This version is not peer-reviewed

The Hydrophilic and Hydrophobic Effects on the Structure and Thermodynamic Properties of Confined Water: Water in Solutions

Version 1 : Received: 5 March 2021 / Approved: 5 March 2021 / Online: 5 March 2021 (17:01:05 CET)

A peer-reviewed article of this Preprint also exists.

Mallamace, F.; Mallamace, D.; Chen, S.-H.; Lanzafame, P.; Papanikolaou, G. Hydrophilic and Hydrophobic Effects on the Structure and Themodynamic Properties of Confined Water: Water in Solutions. Int. J. Mol. Sci. 2021, 22, 7547. Mallamace, F.; Mallamace, D.; Chen, S.-H.; Lanzafame, P.; Papanikolaou, G. Hydrophilic and Hydrophobic Effects on the Structure and Themodynamic Properties of Confined Water: Water in Solutions. Int. J. Mol. Sci. 2021, 22, 7547.

Journal reference: Int. J. Mol. Sci. 2021, 22, 7547
DOI: 10.3390/ijms22147547

Abstract

NMR Spectroscopy is used, in the temperature range 180-350K, to study local order and transport in liquid water (pure and confined) and its solutions with glicerol and methanol at different molar fractions. Being the liquid water thermodynamic dominated by polymorphism (two coexisting liquid phases: high- and low-density HDL and LDL) - with the LDL due to the hydrophilic HB interactions, originating in the supercooled regime the tetrahedral networking and the liquid-liquid transition – we focused our interest to hydrophobic effects (HE) on these. Nowadays, if compared to the hydrophilicity, little is known about hydrophobicity so that the main purpose of this study is to gain new information on it. We measured the relaxation times (T1 and T2) and the self-diffusion (DS). From the times we took advantage of the NMR property to follow the behaviors of each molecular component (the hydrophilic and hydrophobic groups) separately; they are studied directly and DS in terms of the Adam-Gibbs model: obtaining the configurational entropy (Sconf ) and the specific heat contributions (CP,conf ). Due to the HE all the studied quantities, behave differently. For water-glycerol the HB interaction is dominant for all the conditions, whereas for water-methanol are observable two different T-regions above and below 265 K, dominated respectively by the hydrophilicity and hydrophobicity. A situation linked to the water polymorphism. Below this temperature, where the LDL phase and the HB networking develops and grows, the times and CP,conf change behaviors leading to maxima and minima. Above it, where the HB becomes weak and less stable and the HDL dominates, the hydrophobicity determines the solution properties.

Keywords

water; local order; relaxation times; self-diffusion; hydrophobic effect

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