Effects of nonpolar solutes on the thermodynamic response functions of aqueous mixtures

We investigate the effect of adding nonpolar solutes at atmospheric pressure on water's temperature of maximum density, isothermal compressibility, and isobaric heat capacity, using a statistical mechanical model of water solutions [H. S. Ashbaugh, T. M. Truskett, and P. G. Debenedetti, J. Chem. Phys. 116, 2907 (2002)]. We find that the temperature of maximum density increases with solute hydrophobicity, as characterized by its size, and decreases with its van der Waals attractive parameter a, in agreement with experiment. We predict similar trends for the addition of solutes on the isothermal compressibility and isobaric heat capacity: solute hydrophobicity causes an upward shift in water's anomalies, whereas dispersive interactions as measured by the solute's van der Waals attractive parameter shift the anomalies to lower temperatures. The locus along which the competing contributions of solute size σ and interaction strength a to the shift in water's response functions balance each other obeys the scaling relationship σ6 ∼a.