Chemico-osmosis in geologic membranes: Role of membrane potential gradient

Chemico-osmosis is conventionally regarded to occur in the positive direction, i.e., from low to high concentration reservoirs. However, experiments have shown that chemico-osmosis in clay membranes can occur in the opposite direction, i.e., from high to low concentration reservoirs. Conceptual interpretations of this negative osmosis suggest that the diffused electrolytes exert “drag” on water molecules, thus driving the entire solution toward the low concentration reservoir. This paper proposes a quantitative interpretation of this phenomenon considering the role of the induced membrane potential gradient. To this end, a model, which involves the expansion of pore-scale electrokinetics to continuum-scale chemico-osmosis behaviors, is developed for quantification of this membrane potential gradient. The numerical investigation based on the model shows that the membrane potential gradient (1) is caused by the requirement of electroneutrality in the solutions on either side of the membrane; (2) is directly proportional to the difference in effective diffusivity for cations and anions; and (3) contributes to retarding cation migration, enhancing anion migration, and driving the solution flux from low to high concentration reservoirs. These three observations thus suggest that a likely cause of negative osmosis is a decreasing tendency for the solution flux from low to high concentration reservoirs caused by a decreasing membrane potential gradient. Using these findings, previous experiments are discussed and interpreted with success from the electrodynamic perspective of the membrane potential gradient.