Stability of micelles in aqueous salt solutions of dodecylpyridinium halides

Stability of spherical and rod-like micelles of cationic surfactants in aqueous salt solutions have been treated on the basis of a statistical thermodynamic theory, corrected by incorporation of the micellar surface energy term. The modified equations for the double logarithmic relationship between micellar aggregation number and ionic strength of the solution have been derived. The newly derived equations have been applied to aqueous sodium halides solutions of dodecylpyridinium halides including those of chloride, bromide and iodide. Effect of salt concentration on the degree of ionization and the energy of hydration of both spherical and rod-like micelles have been demonstrated. Degree of ionization of the micellar associates decreases with increasing salt concentration for all surfactants studied. Energy of hydration of both spherical and rod-like micelles is negative and it increases with increasing salt concentration.