Adsorption studies of a polymerizable surfactant by optical reflectivity and quartz crystal microbalance

A quartz crystal microbalance (QCM) and an optical reflectometer (OR) have been used to investigate the adsorption behavior of two different variants of the surfactant-hydrotropic counterion system, alkane trimethylammonium vinylbenzoate (C_nTVB), onto silica surfaces. The C₁₈TVB variant, with a longer hydrocarbon tail, produced a threestage adsorption isotherm in the OR, whereas the C₁₆TVB surfactant showed a two-stage adsorption isotherm. This was explained in terms of the greater degree of hydrophobicity of the C₁₈ carbon chain requiring a significantly higher concentration of surfactant to be present on the surface before any further adsorption can occur. A concentration dependent adsorption rate was observed for both surfactants, with the faster adsorption rate being detected for C₁₈TVB. The OR data showed that each surfactant could be completely rinsed off with the flow of water into the OR cell. This was not observed with the QCM data, where only a partial rinse off was seen. The difference between the two techniques was hypothesized to be due to the ability of the QCM to detect both interfacial and bulk behavior thus complicating the interpretation of the adsorption data.