The influence of hydration and structure-promoting effects on the viscosity of binary solvent mixtures

Abstract

The viscosity of binary mixtures of water and aprotic or protic polar solvents has been examined and compared with the viscosity of binary mixtures of aprotic polar solvents. In the latter case, ideal behaviour is seen in the sense that the viscosity of the mixture can be expressed as a sum of viscosities. On the contrary, mixtures of water plus a polar solvent (either protic or aprotic) show a maximum in viscosity (expressed as Δη max) corresponding to a well-defined molar ratio of components (expressed as a hydration number, HN max) and resulting from specific water-solvent interactions. Relationships between the two viscosity parameters and various physicochemical properties of the solvents have been investigated, showing that HN max increases or decreases with the capacity of solvents to donate or accept H-bonds, respectively. In contrast, Δη max results from structure-promoting effects associated with the capacity of the solvent to accept H-bonds from water and to interact hydrophobically