Linear free energy relationship for osmotic water flow through a membrane.

A linear free-energy relationship has been found for the osmotic water flux through membranes in a broad variety of systems including electrolytes, organic compounds, intact biological cells and industrial scale filtration. In all cases, broad concentration ranges were found in which the equation 1n v = alpha 1n m + beta (v, flux (in kg cm-2 min-1; m, molarity] was valid. The parameters alpha and beta were interpreted in terms of molecular weight, mean ionic radius, enthalpy of solvation, electronic structure and H-bonding propensity. The equation is independent of the membrane material and of the presence of other solutes and precipitates, as long as the latter are incompressible. Its parameters are only slightly dependent upon the temperature. The contributions from different solutes to the osmotic flux are at appreciable concentrations even additive. The relationship permits the prediction of the osmotic water flux and of the rate of filtration of systems of known composition. For simple systems it permits determination of the molecular weight, mean ionic radius, degree of hydration and enthalpy of solvation. It is suggested that osmosis is primarily due to the shift of hydration equilibria and that guanidine hydrochloride, in a realistic concentration range, forms practically infinite clathrates with water. The properties of the urea and Gdn X HCl systems indicate that these solutes either reversibly change the membrane structure and/or display intrinsic hysteresis.