Specific counterion effects on the competitive co-adsorption of polyelectrolytes and ionic surfactants.

Counterions affect not only the bulk and interfacial self-assembly of ionic surfactants but also their competitive adsorption with similarly charged polyelectrolytes. Here, we explore the specific effects of bromide, chloride, and the bulky, somewhat hydrophobic tosylate counterion on the adsorption of hexadecyltrimethylammonium surfactants (CTA(+)), the adsorption of polylysine (PL), and the co-adsorption of CTA(+) and PL on negatively charged silica surfaces. Similar to bulk self-assembly, increasing the micellar binding affinity of the counterion from chloride to bromide to tosylate promoted interfacial self-assembly in the absence of polylysine. During co-adsorption, the presence of the polylysine decreased the adsorbed amount of CTA(+) in all cases. Polylysine was more effective at hindering CTA(+) adsorption when the surfactant concentration was below the critical micelle concentration. Although these systems were strongly influenced by persistent nonequilibrium states, it was possible to demonstrate that polylysine was able to prevent CTA(+) admicelle formation below the cmc only when the thermodynamic driving forces for adsorption of the polymer and the surfactant were comparable. Solution compositions where that condition was met depended on the identity of the counterion. Below the bulk cmc, CTAT adsorption displayed the greatest degree of cooperativity, and it also was the most susceptible to hindered adsorption by polylysine.