Adsorption of Cetylpyridinium Chloride at Silica Nanoparticle/WaterInterfaces (II): Dependence of Surface Aggregation on Particle Size

Herein, we report a thermodynamic model that relates the adsorption(aggregation) parameters of surfactants at solid/liquid interfaces to particle radius (r). Theadsorption (aggregation) parameters include adsorption amounts, equilibrium constants(or the standard Gibbs free energy changes), the critical surface micelle concentration(csmc), and the average aggregation number of surface micelles (n). The model predictsthe size dependence of the surface aggregation of surfactants, which is determined by thechanges in the interfacial tension and the molar volume of surface components caused byadsorption. In addition, the adsorption of cetylpyridinium chloride (CPyCl), a cationicsurfactant, on silica nanoparticles with differentrvalues (ca. 6-61 nm) was determined at298 K and pH 4, showing an obvious size dependence, consistent with the prediction ofthe model. With an increase inr, the adsorption isotherm changes from the double-plateautype to the Langmuir type, accompanied by obvious changes in the adsorption parameters. The size-dependent adsorption data canbe well described using the model equations, indicating that the model presented here is acceptable. In addition, the model canextract information on the interfacial tensions from adsorption data. We think that the model deepens the understanding of theaggregation phenomena of surfactants at solid/liquid interfaces.