Counterions in the growth of ionic micelles in aqueous electrolyte solutions: A small-angle neutron scattering study

Ionic micelles in aqueous solution usually grow in size on addition of the electrolytes. This paper deals with the understanding of the reasons for di fferent growths of micelles for different electrolytes. In this connection, small-angle neutron scattering (SANS) experiments on aqueous micellar solu tions of ionic surfactants cetyltrimethylammonium chloride (CTAC) and sodiu m dodecyl sulphate (SDS) in presence of various alkali-metal halide electro lytes are reported. The measurements have been carried out from 0.1 M CTAC for varying concentrations of KBr(= 0.0,0.03,0.05,0.07, and 0.1 M) and KCl( = 0.0,0.05,0.1,0.3,0.5,0.7, and 1.0 M). It is found that micelles grow on a ddition of KBr, but their size is constant over a wide range of KCI concent ration. When compared with similar data on CTAB micellar solutions, these d ata suggest that the differences in the micellar growths in the above syste ms are not connected with the common ion effect. To examine the effect of s ize and hydration behavior of ions of the electrolytes on the micellar grow th, SANS measurements from SDS micellar solutions have been carried out in presence of alkali halide electrolytes (AX,A=Na,K,Cs and X = Cl,Br,I) for t he fixed concentrations of surfactant (= 0.3 M) and the electrolyte (= 0.1 M). It is found that micellar growth strongly depends on the counterions an d there is negligible effect of the colons. The growth is more when the cou nterion hydration is smaller.