Second harmonic generation as a probe of multisite adsorption at solid-liquid interfaces of aqueous colloid suspensions

Chemistry at the surface of solid particles in colloidal dispersions is important in the description of such diverse topics as nutrient and contaminant flow in the environment and the creation of better paints and cosmetics. Particles in these systems are often characterized by molecular-level, surface heterogeneity. This molecular heterogeneity influences many aspects of the chemical processes of interest, among the simplest of which is physisorption. In this study we construct adsorption isotherms for the triphenylmethane dye Malachite Green on various latex particles using the flow through Second Harmonic Generation (ftSHG) technique. This technique makes it possible to construct significantly higher resolution isotherms than conventional separation methods. Here we chose both particles that we expect to be chemically homogeneous (plain polystyrene) and particles whose surface we expect to have molecular level heterogeneity (carboxylated and hydroxylated polystyrene). We find that, using the ftSHG isotherms, we are able to quantify the adsorption energies and site densities of multiple adsorption sites on the carboxylated and hydroxylated polystyrene particles but find only a single type of adsorption site on the plain polystyrene. We expect the ability to measure adsorption site energy and density on chemically heterogeneous latex colloids will be useful in the creation of better biosensors and that the application of ftSHG to other chemically heterogeneous colloids should provide insight into a variety of natural and engineered processes.