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 counterion s 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 count erions (tetraethylammonium cations and, to some extent, tetrabutylammo nium cations) into the interfacial anionic layers. It was suggested th at this increased the interfacial cohesive interaction and resiliency within the thin film lamella This causes regular thin film drainage ra tes (within reasonable agreement with Reynold's theory) and was shown to promote black films at low surfactant concentrations in model horiz ontal thin film lamella. Also, high bulk foam lifetimes were observed, as determined by handshaking. The decrease in stability was associate d with rapid drainage rates and was explained by the penetration of la rger more hydrophobic tetrapentylammonium cations in the layers, This could disrupt and weaken the molecular interactions (less cohesion), T his result is quite different from previously reported studies describ ing the action of tetraalkylammonium counterions on sodium dodecyl sul fate (at concentrations above the CMC), At these higher surfactant con centrations, a defoaming action was generally observed and could be ex plained in terms of a reduction in the dynamic stability (Gibbs surfac e elasticity). (C) 1997 Academic Press.