Physical properties of sodium carboxymethyl cellulose molecules adsorbed on a polyacrylonitrile ultrafiltration membrane

Ultrafiltration experiments showed that the graphical relationship between flux and pressure was a straight line through the origin, provided that the wall shear rate of the bulk fluid was higher than a certain critical value or the pressure was below a critical value. A higher critical shear rate corresponded to a higher critical pressure. For these conditions the total hydrodynamic resistance was only slightly greater than the resistance of a clean membrane for pure water. This additional resistance is attributed to a (mono-) molecular layer of macromolecules which is adsorbed on the membrane in the absence of both a concentration polarization layer and a conventional gel layer.At steady state ultrafiltration conditions, an increase of the flux was obtained after replacing the bulk solution by distilled water at constant experimental conditions, which is attributed to the removal of the concentration polarization layer whereas a mono-molecular layer of macromolecules remained adsorbed on the membrane. For these conditions the flux vs. pressure relationship showed a qualitatively similar behaviour as for ultrafiltration conditions.At a constant shear rate the flux vs. pressure relationship was a straight line through the origin for pressures below the critical pressure, the value of which increased with the shear rate. This linear relationship was reversible, showing no hysteresis. However, if the pressure was higher than its critical value, the flux vs. pressure relationship was no longer a straight line as a consequence of the occurrence of an additional hydrodynamic resistance which did not disappear entirely upon lowering the pressure below its critical value. For the explanation of these phenomena it is assumed that freely moveable parts of the adsorbed macromolecules can block the entrance region of the pores in the membrane if the pressure is beyond its critical value.On the other hand, for pressures below the critical pressure or shear rates beyond the critical shear rate, the pores of the membrane are deblocked. This blocking and deblocking of pores by parts of adsorbed macromolecules apparently takes place in a partly reversible way.