A molecular dynamics study of the equation of state and the structure of supercooled aqueous solutions of methanol

We perform molecular dynamics computer simulations in order to study the equation of state and the structure of supercooled aqueous solutions of methanol at methanol mole fractions xm = 0.05 and xm = 0.10. We model the solvent using the TIP4P/2005 potential and the methanol using the OPLS-AA force field. We find that for xm = 0.05 the behavior of the equation of state, studied in the P − T and P − ρ planes, is consistent with the presence of a liquid-liquid phase transition, reminiscent of that previously found for xm = 0. We estimate the position of the liquid-liquid critical point to be at T = 193 K, P = 96 MPa, and ρ = 1.003 g/cm3. When the methanol mole fraction is doubled to xm = 0.10 no liquid-liquid transition is observed, indicating its possible disappearance at this concentration. We also study the water–water and water–methanol structure in the two solutions. We find that down to low temperature methanol can be incorporated into the water structure for both xm = 0.05 and xm = 0.10.