The nature of surface complexation: a continuum approach

An alternative continuum approach for studying the nature of surface complexation reactions is presented. This new technique may enhance our understanding of surface complexation reactions in heterogeneous media that are often encountered in natural and engineered systems, Interpretation of particle hydrophilicity, as determined by contact angle, provides insights into the nature of the surface complex. This new continuum approach is demonstrated by evaluating two phosphate-oxide surface complexation reactions of a hydrous ferric oxide matrix (HFM). An increase in HFM polarity as a function of bound phosphate was observed. A net gain of two hydrogen-bonding sites per bound phosphate molecule was determined. The surface free energy gained per hydrogen bond added was ca. -0.431 kT/H bond. The increase in hydrophilicity, observed near neutral pH values, did not appear to derive from changes in surface charge. Consequently, phosphate was hypothesized to form a mononuclear complex with the HFM surface. These findings suggest that changes in interfacial polarity may be a useful heuristic supplement to spectroscopic measurements when evaluating the nature of a surface complex in heterogeneous systems.