The Influence of Tetraalkylammonium Counterions on the Drainage and Stability of Thin Films and Foams Stabilized by Dilute Aqueous Solutions of Sodium Dodecyl Sulfate
It was demonstrated that the presence of tetraalkylammonium counterions in aqueous foams and thin film lamella stabilized by sodium dodecyl sulfate (at concentrations below the CMC) can act either to promote or to prevent foam stability. The increase in stability was explained by the congenial incorporation of the smaller and more hydrophilic counterions (tetraethylammonium cations and, to some extent, tetrabutylammonium cations) into the interfacial anionic layers. It was suggested that this increased the interfacial cohesive interaction and resiliency within the thin film lamella. This causes regular thin film drainage rates (within reasonable agreement with Reynold's theory) and was shown to promote black films at low surfactant concentrations in model horizontal thin film lamella. Also, high bulk foam lifetimes were observed, as determined by handshaking. The decrease in stability was associated with rapid drainage rates and was explained by the penetration of larger more hydrophobic tetrapentylammonium cations in the layers. This could disrupt and weaken the molecular interactions (less cohesion). This result is quite different from previously reported studies describing the action of tetraalkylammonium counterions on sodium dodecyl sulfate (at concentrations above the CMC). At these higher surfactant concentrations, a defoaming action was generally observed and could be explained in terms of a reduction in the dynamic stability (Gibbs surface elasticity).
Manev
E
Institute for Surface Chemistry, Stockholm, S-11486, Sweden
Pugh
RJ