A molecular view of glycerol-water hydrogen bonding patterns*

Photoionization dynamics of micro-hydrated molecules in the gas phase when combined with theory can reveal information on the solution phase. Synchrotron based molecular beam mass spectrometry of water-glycerol clusters in the gas phase and terahertz time domain spectroscopy in solution coupled to calculations reveal the nature of hydrogen bonding in glycerol water solutions and provides a general picture of the interaction of polyols with water. A novel hydrogen bonding pattern is theoretically observed at specific glycerol to water concentrations. A significant shortening (and strengthening) of hydrogen bond lengths (~1.6 Å) are observed between H₂O molecules where one of the H₂O molecules is held in a “tweezed” fashion by the two hydroxyl groups of glycerol. This hydrogen bond shortening is also observed to reach beyond the first hydration shell. Experimental results show a clear preference for certain clusters to retain two water molecules due to the enhanced hydrogen bonding strengths. This shortening of hydrogen bonds is also proposed to explain the experimentally observed decrease of excess molar volumes with concentrations for various polyols.