Surface enrichment in alcohol-water mixtures

Molecular beams generated from the vapors above the surfaces of alcohol-wat er mixtures have been examined by mass spectrometry. The alcohols examined are methanol, ethanol, n-propanol, and n-butanol. The variation of the vapo r-phase mole fraction of the alcohol, estimated from the cluster population s in the molecular beam, with the liquid mole fraction is found to be ident ical to that of the surface concentration of the alcohol in the liquid obta ined from surface-tension measurements. The populations of the neat alcohol clusters, as distinct from those of alcohol-water clusters, also exhibit a comparable trend. Surface enrichment is considerably more pronounced in th e. case of n-butanol and n-propanol compared to that of ethanol.