Non-steady-state streaming potential in multilayer partially-hydrophobic diaphragms and membranes

The problem of streaming potential in diaphragms with distributed hydraulic capacity is considered theoretically. It is shown that such a capacity may occur in partially-hydrophobic diaphragms due to the compressibility of eventually trapped air. Its presence causes non-stationarities in the hydrostatic pressure profiles inside the diaphragm. However, if the diaphragm is macroscopically homogeneous in terms of electrokinetic properties, that does not give rise to non-stationarities in the streaming potential. Only if the diaphragm is inhomogeneous, in particular multilayer, non-stationarities in the streaming potential do occur. The example of bi-layer composite membranes consisting of hydrophilic active layers and partially hydrophobic supports is considered in some detail. The pattern of time transients of streaming potential depends on whether the active layer or support makes a decisive contribution into the streaming potential. When it is controlled by the active layer, the potential decreases monotonely upon the pressure switch-on and changes sign after the pressure switch-off. When the streaming potential is controlled by the support, there is a gradual increase upon the switch-on and a gradual decrease upon the switch-off. The latter pattern is qualitatively similar to that caused by concentrational polarisation of active layers.