Electrokinetic characterization of hollow fibers by streaming current, streaming potential and electric conductance

The electrokinetic properties of hollow fiber polymer membranes were investigated from tangential streaming current/streaming potential and electric conductance measurements. The experiments were conducted with a number of fibers n between 1 and 10 and for three fiber lengths l. The quite good linearity of (i) streaming current/potential data versus pressure difference and (ii) streaming current coefficient and “SP × G” (SP: streaming potential coefficient; G: cell electric conductance) data versus n/l shows that expressions of the streaming current and streaming potential derived in laminar flow are also valid for turbulent flux conditions (provided the electrical double layer lies within the laminar sublayer near the surface). The high experimental conductance, the nonlinear dependence of electric conductance on the number of fibers and the variation of streaming potential coefficient with n and l suggest that the solution in which fibers are immersed makes contribution to the cell electric conductance. A non negligible part of the total streaming current is likely to flow through the macroporous body of fibers. Unlike flat membranes, the contributions of the skin surface and the porous body of the fibers to the streaming current cannot be separated for this type of material due to the impossibility of varying channel cross section. The conversion of tangential electrokinetic measurements into zeta-potential of lumen surface is then no more possible. In such cases, it is advisable to carry out streaming current measurements (or to combine streaming potential measurements with electric conductance measurements) because the streaming current (or the product SP × G) is not affected by the cell electric conductance and can then be considered a property of membrane surface.