Surface Enrichment in Alcohol−Water Mixtures

Molecular beams generated from the vapors above the surfaces of alcohol−water mixtures have been examined by mass spectrometry. The alcohols examined are methanol, ethanol, n-propanol, and n-butanol. The variation of the vapor-phase mole fraction of the alcohol, estimated from the cluster populations in the molecular beam, with the liquid mole fraction is found to be identical to that of the surface concentration of the alcohol in the liquid obtained 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 the case of n-butanol and n-propanol compared to that of ethanol.