(144e) Characterizing Membrane Surface Charge by Contact Angle Titration

Reverse osmosis (RO) and nanofiltration (NF) membrane processes are on the cutting edge of water treatment today, and thus, their development and application require more direct and reliable characterization techniques. Specifically, accurate characterization of membrane surface charge is vital for understanding RO/NF membrane separation performance as well as surface fouling phenomena. Surface fouling is caused by the adsorption, deposition, or crystallization of rejected matter on the membrane surface and/or within membrane pores. The principal consequence of membrane fouling is an increased hydraulic resistance, and thus, greater energy required to operate the filtration process. Fouling is one of the biggest hurdles associated with membrane desalination, and thus, it is crucial to be able to understand and reduce the fouling propensity of RO/NF membranes. Electrokinetic phenomena, particularly streaming potential, are often used to characterize a membrane surface (zeta) potential, which is related to the charge functionality at the membrane-solution interface as well as the solution pH and electrolyte composition. While zeta potential is an important parameter for understanding colloidal and interfacial phenomena, it is an indirect, relative description of membrane surface charge. In particular, streaming potential depends on electrolyte composition and may be highly misleading due to specific ion adsorption and surface conductance related phenomena. Our objective is to develop a rapid technique to characterize directly the charge functionality on RO and NF membrane surfaces. Contact angle titration involves measuring sessile drop equilibrium contact angles on a membrane surface using both buffered and unbuffered aqueous solutions adjusted to a range of pH values. The breakpoint of unbuffered titration curves gives the net concentration of surface ionizable groups (proportional to acid or base concentration in solution). Using buffered drop solutions eliminates the influence of surface functional groups on the pH of the drop, which effectively act to buffer the drop of unbuffered solutions. Hence, the inflection point of buffered contact angle titration curves provides the apparent dissociation constant (pKa) for the predominant type of surface ionizable groups, while the local contact angles of the buffered solution provide the extent of surface ionization with respect to pH. The ionization curve gives the relationship between the fraction of ionized functional groups and the pH of the drop solution. Contact angle titration differs from traditional titrimetric methods because it deals only with the surface functionality and does not require further analysis of the titrant solution. We will present data from contact angle titrations of commercial and synthesized polyamide RO/NF composite membranes. While our primary motivation in developing the contact angle titration method was to provide a more direct, rapid measure of membrane surface charge, we will also present a method of predicting the zeta potential based on the Grahame and Gouy-Chapman models of electrical double layers formed about charged interfaces. These model predictions will be compared to zeta potentials estimated from precisely measured streaming potential analyses. Finally, we will discuss the advantages, disadvantages, and complimentary nature of surface charge characterization via contact angle titration and classical electrokinetic characterization techniques.