Role of ammonium ion on the aggregation and adsorption properties of sodium dodecylsulfate

ABSTRACT

The role of mixed counterion toward the aggregation behavior of sodium dodecylsulfate (SDS) is quantified by determining the critical micelle concentration (cmc) of SDS micelle in the presence of added NH₄Cl salt. The dependence of cmc of SDS on added salt is explained in the light of the modified Corrin–Harkins equation and the nonlinearity of this plot is shown due to the exchange of Na⁺ ion by added ion. The aggregation number (N) of SDS micelle is also calculated using the steady-state fluorescence method, where sudden change in N value was observed in two different concentration regions of the modified CH plot. Furthermore, the hydrodynamic diameter (d_H) of dodecyl sulfate micelles was determined using the dynamic light scattering method. In scattering studies, a sharp change in the value of d_H occurred at around 50 mM of added salt, which might hint toward the shape change of dodecyl sulfate micelle. The surface excess values of SDS in the presence of mixed inorganic counterion are also calculated in the light of the Gibbs adsorption isotherm.

GRAPHICAL ABSTRACT

KEYWORDS:

• Cmc
• counterion-binding
• surface-excess

Acknowledgments

The author would also like to thank Prof. K. Ismail for his assistance.