Effect of adsolubilized solutes on 2-D structure of cationic admicelles

Adsorption isotherms of aqueous C12E6 and cetyltrimethylammonium bromide surfactants on solid surfaces in the presence of low molecular weight coadsorbents

In this work, we evaluate the effects of the low molecular weight compounds toluene, phenol, and 1-hexanol on the adsorption of two surfactants on one solid surface. The surfactants are cetyltrimethylammonium bromide (CTAB, cationic) and hexaethylene glycol monododecyl ether (C12E6, nonionic). The surface is gold, although X-ray photoelectron spectroscopic analysis reveals the presence of a large number of oxygenated sites that render the surface hydrophilic (contact angle 10 degrees). Adsorption isotherms are measured using a quartz crystal microbalance with dissipation monitoring (QCM-D). Although our measurements do not allow the determination of the morphology of the aggregates directly, we rationalize our results by referring to AFM images from the literature. On the basis of primarily the dissipative signal and on AFM studies done by others, our results are consistent with CTAB forming a patchy cylindrical structure and C12E6 likely yielding a monolayer structure. The presence of cosolutes almost doubles the mass of surface aggregates and increases the rigidness of the aggregates for CTAB, consistent with a morphological change from cylinders to flat bilayers. Part of the increase in adsorbed mass is likely due to increased surface area covered by admicelles. For C12E6, cosolutes cause small changes in the mass adsorption and essentially no change in the flexibility of surface aggregates.