Orientation of the 3-methylpyridine molecules at the liquid-vapour interface of their aqueous solution

The orientation of the 3-methylpyridine (3MPy) molecules at the liquid-vapour interface of their 3 mol% solution is analysed in detail at 298 K on the basis of a Monte Carlo simulation. The 3MPy molecules are found to have a strong tendency for being adsorbed at the interface, exhibiting a dual orientational preference. At the vapour side of the interface they are preferentially aligned perpendicular to the interface, in such a way that the para carbon atom of the pyridine ring points straight to the vapour phase, whereas the C-Me bond declines by about 60° from the interface normal. On the other hand, at the liquid side of the interface the preferred orientation of the 3MPy molecules is close to the parallel alignment with the plane of the interface.