Cluster composition distribution at the liquid surface of alcohol–water mixtures and evaporation processes studied by liquid ionization mass spectrometry

Abstract

Liquid ionization mass spectrometry (LPI-MS) gives information about hydrogen-bonded clusters at a liquid surface. By improving the way in which samples are introduced, mass spectra showing clusters at the liquid surface or in a gas phase were obtained for 20% (v/v) ethanol and methanol aqueous solutions. Observed cluster ions were expressed as (ROH) m (H 2O) n H +, R = CH 3 or C 2H 5, and mass spectra gave the molar ratios of methanol to water ( M/ W) or ethanol to water ( E/ W) close to the ratio corresponding to the concentration of the respective solution. Binary cluster ions containing two molecules of alcohol ( m = 2) were abundant for both solutions. The molar ratios calculated from the cluster compositions, m − n, of the most abundant cluster ions were also close to the ratio corresponding to the concentration of the solution. The results suggest that the composition distribution of cluster ions observed as LPI mass spectrum may be similar to the composition distribution of clusters existing at the liquid surface. The cluster compositions at the liquid surface vary very quickly due to evaporation of the liquid. The clusters in the vapor were also measured using another device for sample introduction. The evaporation processes occurring in the nano-space at and above the liquid surface were mainly the loss of water molecules from larger clusters. The following collisions between smaller clusters leaded to the association of alcohol molecules accompanied with further loss of water molecules, resulting in the increase of the number of alcohol molecules in each cluster. Even ethanol clusters, (C 2H 5OH) m H +, were formed from the aqueous ethanol solutions. Reasonable differences between ethanol–water and methanol–water were observed for the mass spectra measured in the gas phase. Liquid ionization mass spectrometry gives nano-scale information concerning the cluster compositions at the liquid surface and the evaporation processes.