Membrane surface characterisation by contact angle measurements using the immersed method

A new concept of contact angle measurements by the immersed method is applied to membrane surface characterisation. The general system of two immiscible liquids is used wherein a droplet of CCl4 is deposited on the membrane surface immersed in an aqueous solution. When the solution is pure water, membrane hydrophilicity is evaluated. With aqueous solutions of a cationic surfactant (below its critical micelle concentration) at different pH values, membrane titration curves are obtained. From these curves we obtain the acid-base behaviour of each membrane and the pH value at which half of the membrane surface groups are ionised, pK(1/2). This method is tested with five nanofiltration membranes, a series of three cellulose acetate membranes, CA-316, with increasing hydraulic permeabilities and two commercial thin film composite membranes, CD-NF-50 of poly(trans-2,5-dimethyl)piperazin thiofurazanamide/polyethersulfone and HR-98-PP of polyamide/polysulfone. The results show that the method (i) is easy to perform and avoids dynamic measurements (requires 5 min of drop deposition); (ii) is reproducible (maximal deviations of 7 degrees); (iii) simulates multiple membrane technical environments (e.g. pure water, aqueous solutions); (iv) is not affected by the presence of pores in the nanofiltration range of operation; and (v) is sensitive to membrane hydrophilicity and membrane acidity/basicity (titration curves and pK(1/2) values). Furthermore, the results show the direct effect of the annealing treatment on the acidity of the CA-316 membranes and they evidence the importance of the membrane chemical properties (hydrophilicity, acidity/basicity) on the membrane permeation performance.