  Methanol is an amphiphilic solute whose aqueous solutions exhibit distinctive
physical properties. The volume change upon mixing, for example, is negative
across the entire composition range, indicating strong association. We explore
the corresponding behavior of a Jagla solvent, which has been previously shown
to exhibit many of the anomalous properties of water. We consider two models of
an amphiphilic solute: (i) a "dimer" model, which consists of one hydrophobic
hard sphere linked to a Jagla particle with a permanent bond, and (ii) a
"monomer" model, which is a limiting case of the dimer, formed by
concentrically overlapping a hard sphere and a Jagla particle. Using discrete
molecular dynamics, we calculate the thermodynamic properties of the resulting
solutions. We systematically vary the set of parameters of the dimer and
monomer models and find that one can readily reproduce the experimental
behavior of the excess volume of the methanolwater system as a function of
methanol volume fraction. We compare the pressure and temperature dependence of
the excess volume and the excess enthalpy of both models with experimental data
on methanol-water solutions and find qualitative agreement in most cases. We
also investigate the solute effect on the temperature of maximum density and
find that the effect of concentration isorders of magnitude stronger than
measured experimentally.

Soft Condensed Matter