Impact of surfactant chain length on dynamic surface properties of alkyltrimethylammonium bromide/polyacrylic acid solutions

The dynamic surface properties of mixed solutions of polyacrylic acid (PAA) and alkyltrimethylammonium bromides (CnTAB) were measured by the oscillating ring method as a function of surface age and surfactant concentration. The concentration dependencies of the surface tension and dilational dynamic surface elasticity were similar to the corresponding results for other polyelectrolyte/surfactant solutions, which have been investigated earlier. The kinetic dependencies of the surface elasticity proved to be non-monotonic for all the investigated surfactants at n = 8, 10, 12, 14, 16 and displayed two local maxima at surfactant concentrations close to the critical micellar concentration (CMC). The first maximum was caused by an aggregate formation in the surface layer while the second maximum occurred simultaneously with a multilayer formation at the gas-liquid interface. The surface tension corresponding to the beginning of the aggregate formation decreased and the surface elasticity in the region of the first local maximum increased with the hydrophobicity of CnTAB/PAA complexes caused by the increasing hydrocarbon chain length of the surfactant. The hydrophobic interactions influenced also the height and position of the second local elasticity maximum. These effects were connected with changes in the rate of adsorption layer and aggregate structure formation. Comparable values of the real and imaginary parts of the dynamic surface elasticity at concentrations for aggregate formation in the surface layer indicated that the relaxation time of the mass exchange between the aggregates and the surrounding adsorption layer is comparable with the period of oscillations, i.e. ≤10 s.