Influence of CaCl2 on microheterogeneity of methanol-water mixtures

Molecular dynamics simulations were performed for methanol-water mixtures and CaCl2 solutions in this binary solvent. Radial distribution functions and running integration numbers were calculated for binary and ternary systems and for the bulk solvent in CaCl2 solutions. To illustrate the influence of ions on the correlation between the solvent molecules differences of the radial distribution functions in ternary and binary systems and in the bulk solvent and binary mixtures were computed. The presence of ions hinders the correlation between unlike molecules, but promotes the correlation between like molecules. The influence of ions extends beyond their coordination shells. At xm = 0.3 strong correlation between methanol molecules in the bulk solvent was observed. This excess of interaction between methanol molecules evidences their aggregation. Similar excess of the correlation between the water molecules was found in the bulk solvent of the water rich solutions. Such behaviour suggests an enhancement of a micro-segregation of methanol and water. In water clusters the tetrahedral structure still exists, but the H-bonded network is partly distorted; distances between water's oxygens and hydrogens are more accidental and the water molecule has more than four nearest neighbours.