MEMBRANE-SURFACE CHARACTERIZATION BY CONTACT-ANGLE MEASUREMENTS USINGTHE IMMERSED METHOD

A new concept of contact angle measurements by the immersed method is applied to membrane surface characterisation. The general system of tw o immiscible liquids is used wherein a droplet of CCl4 is deposited on the membrane surface immersed in an aqueous solution. When the soluti on is pure water, membrane hydrophilicity is evaluated. With aqueous s olutions of a cationic surfactant (below its critical micelle concentr ation) 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 ioni sed, pK(1/2). This method is tested with five nanofiltration membranes , a series of three cellulose acetate membranes, CA-316, with increasi ng hydraulic permeabilities and two commercial thin film composite mem branes, CD-NF-50 of poly(trans-2,5-dimethyl)piperazin thiofurazanamide /polyethersulfone and HR-98-PP of polyamide/polysulfone. The results s how that the method (i) is easy to perform and avoids dynamic measurem ents (requires 5 min of drop deposition); (ii) is reproducible (maxima l deviations of 7 degrees); (iii) simulates multiple membrane technica l environments (e.g. pure water, aqueous solutions); (iv) is not affec ted 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 resul ts 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 membran e permeation performance.