Effects of Ethanol and Dimethyl Sulfoxide on the Molecular Organization of H2O as Probed by 1-Propanol

We characterized the effects of ethanol (ET) and dimethyl sulfoxide (DMSO) on H2O within a limited H2O-rich region by the 1-propanol (1P)-probing methodology developed by us earlier. The results are displayed on a two-dimensional map with twin coordinates: one pertaining to hydrophobicity and the other to hydrophilicity. The locus of ET on this map was at a point in between methanol (ME) and 2-propanol (2P) as expected from our earlier findings by thermodynamic studies. That for DMSO, however, was surprisingly more hydrophilic than ME. Similar to N-methyl groups discussed recently (J. Phys. Chem. B2011, 115, 2995), it was argued that the methyl groups attached to the S atom are made susceptible for direct hydrogen bonding to the surrounding H2O molecules due possibly to the electronegativity of the S atom. In view of these findings, we suggest caution to be exercised for the conventional general trend of taking any methyl groups to be hydrophobic.