SEDIMENTATION FIELD-FLOW FRACTIONATION IN THE DETERMINATION OF SURFACE CONCENTRATION OF ADSORBED MATERIALS

Due to their large surface-to-volume ratios, colloidal materials are often used as substrates in adsorption studies, where evaluation of the surface concentration of an adsorbed component requires exact knowledge of both the surface area of the substrate and the mass of the adsorbed species. Sedimentation field-flow fractionation (FFF) is frequently used to determine the size of colloidal materials of known density from their degree of retention in the fractionator. However, if the colloid is allowed to interact with an adsorbing species, the density of the adduct becomes a function of the level of adsorption and is, as such, not directly applicable to evaluations of adduct size. Here, we show that it is possible to extract information from sedimentation FFF retention which permits characterization of the adlayer in terms of the amount adsorbed per particle or related quantities such as surface concentration and the area occupied per adsorbed molecule. The method is applied to evaluate the adsorption of a PEO-PPO-PEO triblock polymeric surfactant to standard PS latex particles of different sizes, and the results are shown to compare well with quantification based on isotope labeled surfactant.