Osmosis in Negatively Charged Nanocapillaries and Its Enhancement by an Anionic Surfactant

Abstract

Osmotic flows through track-etched membranes with pore radii of 10–50 nm have been measured in a water/membrane/salt solution system at a salt concentration on the order of several millimoles per liter. It has been found that water is intensely transported through the pores only when a solute dissociates into ions (KCl, K2SO4, sodium dodecyl sulfate, etc.). Molecules of low-molecular-mass nonelectrolytes induce actually no osmotic flows at these capillary radii, thereby indicating the key role of an electrical double layer in the mechanism of the osmotic transport. It has been shown that the transport through the pores under the action of the osmotic pressure is of a convective nature; i.e., the Poiseuille law for a viscous flow through a cylindrical capillary is fulfilled. The values of the osmotic pressure have been determined at different salt concentrations, and reflection coefficients have been calculated. The reflection coefficients correlate with the values of apparent diffusion coefficients of salts in nanopores, and these diffusion coefficients are markedly smaller than bulk values. The sorption of an anionic surfactant increases the membrane surface charge and enhances the osmotic effect.