Effect of Ionic Strength and Competitive Adsorption of Na+ on the Flocculation of Lauryl Sulfate Micelles with Al3+

The binding of Al 3+ to SDS micelles is best described by means of Guoy−Chapman−Stern isotherms. A method is described to estimate parameters for a Guoy−Chapman−Stern competitive adsorption isotherm for the system Na + /Al 3+ /SDS from data of the composition of micellar flocculates obtained in conditions of optimum flocculation. The dissociation coefficient K Al 3+ is found to be equal to 17.67 if the bound fraction is defined as the ratio of bound Al 3+ to SDS ions in the micelle. Experiments performed in the presence of a range of NaCl concentrations between 0 and 0.5 M show that the absence of flocculates in micellar solutions of lauryl sulfate in the presence of high [Al 3+ ] cannot be explained by a drop in the chemical activity of Al 3+ at high ionic strengths, although the chemical activity of Al 3+ strongly influences the binding of Al 3+ to SDS micelles. One effect of increasing [Na + ] is to increase the critical micelle concentration of the surfactant in solution, from a value close to that of Al(DS) 3 to another close to that of SDS. This reduces the proportion of surfactant in micellar form and susceptible therefore of being flocculated.