A connection between interfacial self-assembly and the inhibition of hexadecyltrimethylammonium bromide adsorption on silica by poly-L-lysine

Coadsorbing poly-L-lysine hydrobromide inhibits hexadecyltrimethylammonium bromide adsorption to silica surfaces. Using optical reflectometry to measu re surface excess concentrations, we fmd that this inhibition depends on th e concentration of added I:I electrolyte, both quantitatively and qualitati vely. In the absence of added salt, the polyelectrolyte causes a rather uni form decrease in the extent of surfactant adsorption, and the shape of the surfactant coadsorption isotherm is qualitatively similar to the isotherm f or adsorption in the absence of polyelectrolyte. In particular, the surfact ant concentration marking the onset of cooperative adsorption, where admice lles are formed at bulk concentrations below the critical micelle concentra tion, is unchanged by the presence of the polyelectrolyte. In contrast, whe n the surfactant coadsorbs in the presence of 10 mill KBr, the polyelectrol yte eliminates this adsorption regime altogether. Admicelles do not form un til the bulk surfactant concentration exceeds the critical micelle concentr ation. Upon addition of the 1:1 electrolyte, the inhibition mechanism chang es from a simple competition for available surface area to a more profound disruption of surfactant interfacial self-assembly. By comparing a low mole cular; weight oligolysine with three higher molecular weight polylysine sam ples, we find that this change in inhibition mechanism can be traced to the effect of salt on the relative adsorption energies of the surfactant and t he polyelectrolyte, but kinetically trapped or frustrated states exert a la rge influence on the composition of the mixed adsorbed layer in the case of higher molecular weight polyelectrolytes.