Inhomogeneity of viscous aqueous solutions

Brownian principles for homogeneous media fail to explain the effect of viscous cosolvents on reaction rates. We examined the possibility that solvent viscosity in the microenvironment of a probe ${\mathrm{\eta }}^{\mathrm{*}}$ (microviscosity) differs from the bulk viscosity η (macroviscosity). ${\mathrm{\eta }}^{\mathrm{*}}$ was derived from the diffusion constant D of water tracers, measured in the presence of viscous cosolvents. We found that (a) ${\mathrm{\eta }}^{\mathrm{*}}$∝${\mathrm{\eta }}^{\mathrm{\alpha }}$, where 0≤α≤1, and (b) the temperature dependence of D is insensitive to the presence of these cosolvents. We propose that viscous aqueous solutions have an inhomogeneous microstructure with waterlike domains.