Protonation at the Aqueous Interface of Polymer Nanoparticles with Second Harmonic Generation

The protonation reaction of the carboxyl (−COOH) functional group fixed at the surface of polystyrene carboxylate (PSC) microspheres, with a radius of 457 ± 14 nm, dispersed in aqueous medium has been studied using the nonlinear optical method of second harmonic generation (SHG). We have demonstrated that the “χ (3) method,” along with the use of the Gouy−Chapman model describing the surface potential, can be used to determine the p K a of carboxyl groups at the particle−liquid interface. A surface p K a of 5.4 ± 0.3 has been determined for the PSC colloidal particles, which corresponds to a decrease in the acidity; that is, K a is a factor of 4 smaller at the interface than that of long chain carboxyl acids in bulk aqueous solution.