Dynamic surface potential and adsorption kinetics of nonionic surfactants at the air-water interface

The surface potential kinetics of the nonionic surfactants tridecyldimethyl phosphine oxide and dodecanoic acid at the air/water interface have been in vestigated by the vibrating plate method, An influence of the thickness of the solution layer on the surface potential and adsorption kinetics was obs erved. A mathematical solution for the case of diffusion controlled adsorpt ion kinetics is presented. The experimental data were fitted for different models, As a result, the depth of the solution layer has to be taken into a ccount in dynamic processes for small concentrations of these amphiphiles. This is important for strong surfactants. The fit parameters have reasonabl e values for the whole set of data by use of the derived equations. In the case of very low concentrations, a peculiar behavior in the surface potenti al kinetics during the initial adsorption time (t < 1 min) was found. We su ppose this to be due to reorientation of the adsorbed surfactant molecules when the number of the molecules at the interface increases. This was previ ously observed by one of us on the basis of equilibrium surfactant adsorpti on isotherm analysis.