Abstract
Studies of clusters are important for understanding the chemical reactions in solutions. Liquid ionization (LPI) mass spectrometry (MS) gives information about hydrogen-bonded clusters at liquid surface. By improving techniques for introducing liquid smples, the LPI mass spectra of a 40% (v/v) ethanol aqueous solution showed that mixed cluster ions, (C2H5OH)m(H2O)nH+, containing 2∼6 molecules of ethanol were abundant and the composition (m-n) of the base peaks were 3-18 and 4-17. The molar ratios of ethanol to water (E/W) for 3-18 and 4-17 are 0.17 and 0.24, respectively, and the E/W ratio calculated from the whole mass spectrum was 0.20. These molar ratios agreed with the ratio (0.21) calculated from the ethanol concentration of the solution, suggesting that the liquid may be an aggregation of these observed clusters. Although the LPI mass spectra of a 20% (v/v) ethanol solution have not yet agreed with the ethanol concentration, these spectra gave nano-scale information about the evaporation processes. In the early stage of evaporation (close to the bulk), the number of ethanol molecules in clusters did not change, but the number of water molecules in clusters decreased rather quickly, resulting in the increase of E/W. Later, the number of ethanol molecules increased during evaporation. It has been well known that the ethanol concentrations in the vapor phase are much higher than those (0∼50%) in the liquid phase. LPI-MS gives useful nano-scale information about liquids and evaporation phenomena.
