Temperature dependence of the structure of the liquid-vapour interface of aqueous methanol solutions

The temperature dependence of the structure of the liquid-vapour interface of aqueous methanol solution containing 25 mol% methanol has been studied on the basis of Monte Carlo computer simulations. The performed simulations cover a 50 K wide temperature interval, ranging from 248 K to 298 K. It is found that the interface is characterized by a double layer structure; at its vapour side the methanol concentration is considerably higher, whereas at its liquid side somewhat lower than in the bulk liquid phase. Both layers become more pronounced at lower temperatures. On the other hand, no noticeable effect of the temperature is observed on the orientational ordering of the molecules. Correlation of the molecular orientation with the interface is only observed among the molecules of the methanol adsorption layer. Here the water molecules are found to prefer an alignment tilted by about 20° relative to the plane of the interface, without any twist about their dipolar axis, whereas methanol molecules prefer to align perpendicular to the interface pointing by their methyl group straight toward the vapour phase. These preferred orientations correspond to the alignment of a hydrogen bonded water-methanol pair, in which the H-accepting water molecule is located closer to the liquid, whereas the H-donating methanol closer to the vapour phase.