Viscosity of aqueous solutions of monohydric alcohols in the normal and supercooled states

Abstract

Viscosities of solutions of Met−, Et−, and n-PropOH were measured by quasielastic light scattering of polystyrene lattice spheres of 800 Å diameter, in the low-concentration, low-temperature ranges where effects of alcohols on the structure and properties of liquid water are most pronounced. Raw data already indicate that alcohols promote the formation of clathrate-like structures of H bonds which add to those occurring naturally in the pure solvent. Evaluation of the number of water molecules taking part in longer-lived structures further indicates that this promotion is most effective in the infinite-dilution limit. Available thermodynamic data agree with this evaluation. Mismatches among solute-promoted clathrate-like cages are evidenced. The known disruptive effects of alcohols on the anomalous properties of cold and supercooled water are thus understood in terms of limitations to the correlation length of density fluctuations, set by mismatches. A contribution of OH groups to cage promotion is also evidenced at least at low temperature. Constraints or hindrances to the motion of water molecules, irrespective of their nature, appear to be effective in favoring structures of stabler, longer-lived H bonds, corresponding to water molecules of lower mobility. This offers a unified view of hydrophobic and hydrophilic interactions which agrees with earlier work and with more recent computer experiments. The possibility here evidenced for structures of H bonds promoted in solvent water of being conflictual or synergistic, adds to the microscopic understanding of solvent-mediated interactions, e.g., of biosolutes.