Temperature and pressure dependences of the solvation properties of alcohols in water Aoi Taira 1 , Ryuichi Okamoto 2 , Tomonari Sumi 1 , Kenichiro Koga 1 1 Okayama University, Japan, 2 University of Hyogo, Japan The thermodynamic characterization of the solvation of amphiphilic molecules, i.e., molecular species that possess both hydrophobic and hydrophilic moieties, is important for understanding microscopic and macroscopic phase separation of aqueous solutions, micelle formation, and the related biological phenomena. However, in contrast to hydrophobic hydration, solvation of amphiphiles is less studied and the effect of hydrophilic groups on the solvation properties is not fully examined. We report here the solvation free energy calculations of two kinds of alcohols, methanol and 1,2-hexanediol, in water and discuss the temperature and pressure dependences of the solvation free energy μ*. The solvation process of an amphiphilic solute in water can be divided into three basic steps, with each of which the contribution to μ* is associated. The first is the work of cavity formation, the second is the one due to the addition of weak, attractive interactions (the van der Waals forces) to purely repulsive interactions of a solute molecule with surrounding solvent molecules, and the third one is the free energy arising from the electrostatic interactions between the solute and water. (The sum of the first two contributions corresponds to the solvation free energy of a hypothetical non-polar molecule with the same structure as the amphiphilic molecule of interest.) Some insights into the solvation characteristic of amphiphiles are obtained from numerical evaluation of each contribution. The hydration free energy of a non-polar solute as a function of temperature has a maximum in the temperature range below the boiling point at ambient pressure 1 ; however, the solvation free energy of amphiphiles increases monotonically. It is found that the difference results from the fact that the electrostatic contribution is nearly linear in temperature and the slope is sufficiently large. Another finding is that the ratio of temperature derivatives of the electrostatic contribution to the solvation free energy of methanol and 1,2-hexanediol is close to 1:2. The implication of this result is discussed in terms of the solvation enthalpy of each hydrophilic group. The solvation volume and the contributions to it from the three steps are also evaluated and discussed. We also evaluate relative accuracies of the free energy calculation methods including a mean-field-approximation (MFA) method, a perturbation combining (PC) method, and the Bennet Acceptance Ratio method. It is found that the MFA method, whose validity has been confirmed for the solvation of a small non-polar solute in bulk water and at liquid-vapor interfaces [2,3] , works reasonably well for the non-polar contribution to the solvation free energy of polyatomic amphiphiles and that the PC method is effective for evaluating the electrostatic contribution. References 1. e.g., A. Ben-Naim, Solvation Thermodynamics , Plenum Press, New York, 1987; C. Tanford, The hydrophobic Effect : Formation of Micelles and Biological Membranes, 2nd ed ., Wiley, New York, 1980.
2. K. Koga, and N. Yamamoto, J. Phys. Chem. B ., 2018, 122 , 3655-3665. 3. K. Abe, T. Sumi, and K. Koga., J. Phys. Chem. B ., 2016, 120 , 2012-2019.
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