Very slow relaxation observed in the surface tension of surfactant solutions

A light scattering study was made on ripplons generated over the surface of C(n)TAB (n-cetyltrimethyl ammonium bromide) solutions (in the present case n=14 and 16). After preparing the sample solution we measured its surface tension for 3 h and confirmed that the tension did decrease with time exponentially but very slowly. The relaxation time depended on the surfactant concentration of solutions and increased sensitively with the progress of dilution, e.g., 3 min at 10(-2) mol/l to 80 min at 10(-4) mol/l for C(16)TAB. The results obtained were correlated with surface adsorption densities estimated from the observed surface pressure vs concentration curves using Langmuir's adsorption equation. Two mechanisms were proposed to interpret such a slow relaxation process: One is a model in which reorientation or redistribution of the surfactant molecules adsorbed on the solution is considered, while the other is a two-state model that assumes the existence of a high energy barrier for surfactant molecules between those on the surface and those in the subsurface. A question still remains as to which of the two is more probable.