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 different growths of micelles for different electrolytes. In this connection, small-angle neutron scattering (SANS) experiments on aqueous micellar solutions of ionic surfactants cetyltrimethylammonium chloride (CTAC) and sodium dodecyl sulphate (SDS) in presence of various alkali-metal halide electrolytes are reported. The measurements have been carried out from 0.1 M CTAC for varying concentrations of $\mathrm{KBr}(=0.0,0.03,0.05,0.07,$ and 0.1 M) and $\mathrm{KCl}(=0.0,0.05,0.1,0.3,0.5,0.7,$ and 1.0 M). It is found that micelles grow on addition of KBr, but their size is constant over a wide range of KCl concentration. When compared with similar data on CTAB micellar solutions, these data suggest that the differences in the micellar growths in the above systems are not connected with the common ion effect. To examine the effect of size and hydration behavior of ions of the electrolytes on the micellar growth, SANS measurements from SDS micellar solutions have been carried out in presence of alkali halide electrolytes $(AX,A=\mathrm{Na},\mathrm{K},\mathrm{Cs}$ and $X=\mathrm{Cl},\mathrm{Br},\mathrm{I})$ for the fixed concentrations of surfactant (= 0.3 M) and the electrolyte (= 0.1 M). It is found that micellar growth strongly depends on the counterions and there is negligible effect of the coions. The growth is more when the counterion hydration is smaller.