SDS, Brij58 and CTAB at the dodecane-water interface

The dynamics of interfacial tension of the n-dodecane-water interface in the presence of three different types of surfactants viz. SDS, Brij58, and CTAB, was investigated using the classical dynamic version of the drop-volume method for drop times ranging from a few seconds to several hundred seconds. Experiments were conducted with aqueous sub-micellar solutions of each surfactant at three concentrations. The effects of addition of 0.1 M KCl into each of these systems was also studied. The results are presented as "long time approximation (LTA)" plots of interfacial tension versus 1/root t, where t is the adsorption time. In every LTA plot, two distinct regions are seen: region I, early time; and region II, long time. In most cases, there was a steep fall in the interfacial tensions in region II. For each region, the variation of interfacial tension with 1/root t follows a straight line relationship indicating a diffusion-controlled adsorption process. Using the appropriate surface equation of state for each surfactant system, adsorption in both regions were estimated and diffusion coefficients were calculated from the slopes in the LTA plot. Diffusion coefficients in region I correlate remarkably well with published literature values. The behavior of the two ionic surfactants (SDS and CTAB) was similar. For both, in the absence of KCI, the diffusivities in region LI was at least an order of magnitude lower than in region I, suggesting that diffusional transport is "slowed down" in this region. This situation was reversed upon the addition of KCI whereby the diffusivities in region II become higher than in region I. As expected, for Brij58, although a small increase was seen in the effective diffusivities in the presence of KCl, the presence or absence of the neutral salt did not have any major effect on the interfacial tension dynamics. Results are rationalized based on previous interpretations of tension relaxation at interfaces that deal with effects due to electrical retardation or intermolecular cohesion. (C) 1998 Elsevier Science B.V.