Electrolyte effect on mixed micelle and interfacial properties of binary mixtures of cationic and nonionic surfactants

In the present work, the adsorption behavior at the liquid–air interface and micellization characteristics of mixtures of cetyltrimethylammonium bromide (CTAB) and p-(1,1,3,3-tetramethylbutyl) polyoxyethylene (TritonX-100) in aqueous media containing different concentrations of NaBr were investigated by surface tension and potentiometry measurements. From plots of surface tension (γ  ) as a function of solution composition and total surfactant concentration, we determined the critical micelle concentration (CMC), minimum surface tension at the CMC (γCMCγCMC), surface excess (ΓmaxΓmax), and mean molecular surface area (AminAmin). On the basis of regular solution theory, the compositions of the adsorbed film (Z  ) and micelles (XMXM) were estimated, and then the interaction parameters in the micelles (βMβM) and in the adsorbed film phase (βσβσ) were calculated. For all mole fraction ratios, the results showed synergistically enhanced ability to form mixed micelles as well as surface tension reduction. Furthermore β was calculated by considering nonrandom mixing and head group size effects. It was observed that, for both the planar air/aqueous interface and micellar systems, the nonideality decreased as the amount of electrolyte in the aqueous medium was increased. This was attributed to a decrease of the surface charge density caused by increasing the concentration of bromide ions.

Excess Gibbs free energy as a function of mole fraction of CTAB in the mixed micelle (X1∗) for CTAB/TritonX-100 mixtures at α=0.5α=0.5 at different salt concentrations.Figure optionsDownload as PowerPoint slide