Thermodynamic Properties of Hydration Water around Solutes: Effect of Solute Size and Water-Solute Interaction A. Oleinikova* and I. Brovchenko Physical Chemistry, Dortmund University of Technology, Otto-Hahn-Str. 6, Dortmund, D-44227, Germany ABSTRACT: Density, thermal expansion, and heat capacity of hydration water around various model solutes have been studied as a function of temperature and pressure. The radius of spherical structureless solute was varied from 3 to 10 Å, and the water-solute interaction was varied from strongly hydrophobic to strongly hydrophilic. Thermodynamic proper- ties of hydration water around solutes were compared with those near the inner surface of large cylindrical pores with a radius of 25 Å. For all systems studied, the energy of water- water interactions per molecule in the hydration shell is found less negative than in the bulk. This is the result of the missing neighbor effect, which leads to the liquid density depletion even near strongly hydrophilic surfaces. This effect enhances near concave surfaces and diminishes near convex surfaces, which causes an essential increase of hydration water density around small solutes. Liquid density depletion near surfaces is accompanied by an essential increase of the thermal expansion coefficient of hydration water: at low temperatures, it exceeds the bulk value even near strongly hydrophilic surfaces. The constant volume heat capacity of hydration water is close to the bulk value; it is practically not sensitive to water-surface interaction and slightly increases upon decreasing solute size. The constant pressure heat capacity of hydration water increases upon weakening water-surface interaction and is practically not sensitive to solute size. Increase of the constant pressure heat capacity of water near hydrophobic surfaces is found to be the result of the increasing thermal expansion coefficient. I. INTRODUCTION Thermodynamic properties of fluids (density, heat capacity, etc.) are local in the presence of a surface. Far from the liquid- vapor critical point, the intrusion of the surface perturbation into the bulk fluid is noticeable in close proximity of a surface only. In particular, properties of liquid water differ strongly from the bulk ones in the first surface layer (hydration water), the difference is still noticeable in the second layer, and it is practically absent further from the surface. 1 Hydration water makes an important contribution to the properties of various aqueous systems (confined water, aqueous solutions, etc.), and therefore, it is important to know the thermodynamic properties of hydration water and their dependence on the surface characteristics. In low-hydrated systems, where most of the water molecules are presumably adsorbed at the surface, the properties of hydration water can be estimated experimentally from the difference in the properties of hydrated and dry systems. 2,3 It is much more difficult to measure experimentally properties of hydration water in systems with high water content (liquid water near extended surfaces, aqueous solutions, completely filled mesopores, etc.). Some nondirect information about the thermodynamic properties of hydration water can be obtained from the measurements of the average properties of aqueous systems. For example, the average density and the average thermal expansion coefficient of confined water differ from the respective bulk values 4-8 and this difference gains more importance in smaller pores, where the fraction of hydration water is larger. 6,5 The experiments on dissolution of organic molecules in liquid water evidence that the hydration contribution to the constant pressure heat capacity is positive for apolar groups and negative for polar and charged groups. 9,10 More direct information about the profiles of hydration water can be obtained using reflectivity and diffraction techniques. For example, the reflectivity measurements evidence the depletion of liquid water density near extended hydrophobic surfaces and allow estimation of the total water density deficit near a surface. 11-13 The density profile of liquid water near a hydrophilic surface has been obtained from the reflectivity data assuming an oscillating shape of the profile. 14 Use of the simulation technique (“empirical potential structure refine- ment”) for the analysis of the experimental diffraction data allows estimation of the density profiles of water in narrow silica pores. 15,16 Such experimental studies of liquid water density near surfaces are rather rare, and we are not aware of the direct experimental studies of liquid water thermal expansivity or heat capacity near surfaces. The local properties of fluids near surfaces can be studied in detail by simulations. To simulate a liquid near a surface, it should be confined in pore geometry with periodic boundary conditions in up to two dimensions and the pore should be large enough to minimize the shift of the liquid-vapor phase Received: July 9, 2012 Revised: October 19, 2012 Published: November 21, 2012 Article pubs.acs.org/JPCB © 2012 American Chemical Society 14650 dx.doi.org/10.1021/jp306781y | J. Phys. Chem. B 2012, 116, 14650-14659