Mobility of water near charged interfaces

Abstract

The location of the hydrodynamic shear surface is discussed for micelles of Na dodecyl sulfate and for clay particles (platelets of montmorillonite and of vermiculite). Micelles are characterized by a combination of experiments: light scattering, micellar self-diffusion, intrinsic viscosity, electrophoresis and electric conductance. The concerted interpretation of these experiments shows that the shear surface of micelles coincides within 0.1 nm with the surface enveloping the heads of the micellized ions. Claims of structured water in clays and an abnormally high viscosity of clay-held water have been based on the low self-diffusion of water in swollen clays, and on the temperature dependence of the hydraulic resistance of clay plugs (anomalous activation energy). It is shown that the self-diffusion of water between the platelets requires corrections for a wall effect and for the hydration of the exchangeable, slow moving cations. After application of such corrections, the viscosity of water in clay is found to be about the same as of bulk water, with the shear surface located at 0.1 ± 0.1 nm from the clay/water interface. The small anomaly in the activation energy of water in clay plugs is reasonably explained by a slight change with temperature of the pore size distribution in the plugs. Approximate calculations of the dielectric constant of water in electric double layers suggest some restriction in the orientation of water molecules in the first layer next to highly charged interfaces such as vermiculite/water. The various results all indicate that changes in the water mobility induced by a charged interface are small and do not reach beyond the first layer of water molecules.