Fluid Phase Equilibria 240 (2006) 161–166 Excess properties of aqueous mixtures of methanol: Simulation versus experiment Diego Gonz´ alez-Salgado a, 1 , Ivo Nezbeda a,b, ∗ a E. H ´ ala Laboratory of Thermodynamics, ICPF, Academy of Sciences, 165 02 Prague 6, Suchdol, Czech Republic b Department of Chemistry, J.E. Purkinje University, 400 96 ´ Ust´ ı n. Lab., Czech Republic Received 9 October 2005; received in revised form 6 December 2005; accepted 6 December 2005 Abstract We report molecular simulation results for both the excess mixing and partial molar properties of water–methanol mixtures over the entire concentration range with the particular emphasis on the low concentration ends. It is shown that the mixing properties are very sensitive to potential models and that the used realistic potentials (TIP4P for water and OPLS for methanol) give a reasonably good agreement with experiment only for volumetric properties although the qualitative trend of the partial molar volume at low concentrations is not reproduced. As regards excess enthalpy, the results are rather bad and only its sign is predicted correctly. © 2006 Elsevier B.V. All rights reserved. Keywords: Aqueous mixtures; Water–methanol mixtures; Solutions of alcohol; Excess mixing properties; Partial molar properties 1. Introduction Aqueous solutions of alcohols have attracted a good deal of attention of both scientific and engineering community for decades for a number of reasons. From the technological point of view, such mixtures have served as useful industrial solvent media for a variety of separation processes; moreover, recently it has become popular to use them also in solar thermal systems [1]. Physical chemists have been attracted by their eccentric, unusual non-ideal behavior, especially in the low concentration range [2]. Aqueous solutions of alcohols are also ones of the sim- plest aqueous solutions with the solute having both a hydrophilic head and hydrophobic tail. They therefore attract nowadays also attention of biochemists because it is believed that the full under- standing of their behavior may provide insight into the behavior of certain biological systems and may thus serve as a spring- board for studying and modeling aqueous solutions of complex amphiphiles that are difficult to simulate. ∗ Corresponding author. Tel.: +420 2 20390296; fax: +420 2 209 20661. E-mail address: IvoNez@icpf.cas.cz (I. Nezbeda). 1 On leave of absence from: Department of Applied Physics, Faculty of Science, University of Vigo, 32004 Ourense, Spain. It is well known that the thermodynamic properties of water– alcohol mixtures are significantly smaller than the values that might be expected from an ideal mixture of the pure fluids. Structural properties underlying this behavior were reviewed by Frank and Ives in 1966 [3] and again in 1985 [4]. However, the wide-spread and commonly accepted explanation of these effects in terms of an enhanced structuring of water does not seem to be supported by modern diffraction experiments [5,6]. Neutron scattering experiments supported by molecular simula- tions based on an empirical potential obtained directly from the diffraction data show highly heterogeneous mixing across the entire concentration range despite apparent miscibility of both components [6]. In the light of the yet unsettled controversy as for the origin of the observed anomalies it is understandable that overwhelming majority of molecular simulation studies have focussed on the structure with very little attention paid to the thermodynamic properties [7–11]. However, it has been well established that not all interactions acting between the molecules contribute equally to establishing the structure [12]. Thus, although the observed macroscopic properties are superimposed on the underlying structure, they may result from a more complex interplay between various intermolecular interactions. Detailed simulation studies of the thermodynamic behavior revealing such relations seem therefore equally important. 0378-3812/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2005.12.007