Synergistic Effect of Salts and Organic Additives on the Micellar Association of Cetylpyridinium Chloride

Abstract

Viscosity measurements under Newtonian flow conditions have been made to study the effect of organic additives on the micellar association of 0.2 M cetylpyridinium chloride (CPC) in aqueous medium at 30 °C containing different fixed concentrations of potassium salts (mainly KCl). 1-Hexanol (C6OH) and n-heptylamine (C7NH2) were used as the organic additives, and relative viscosity (ηr) vs [additive] plots were constructed for various KCl concentrations (0−2 M). In each case, after reaching a maximum, the |ηr| decreased on further addition of the additive (i.e., a peaked behavior). An interesting phenomenon of progressive shifting of the peak position toward lower [additive] was observed with increase in KCl concentration. This peak shift was found to be dependent on nature of the counterion but almost independent on the co-ion nature. The start of viscosity decrease at lower [additive] may be due to the fact that in the presence of KCl (because of its salting-out nature), the palisade layer gets saturated earlier and the excess content begins to solubilize inside the micellar corethe evidence to this effect is provided by performing identical experiments with n-hexane and n-octane. Availability of the additive in the micellar interior could provided more flexibility as well as swelling (of higher degree of sphericity), which forms the basis of viscosity decrease. For a constant CPC content, a well-defined linear relationship was found to exist between [KCl] and [additive] at the viscosity maximum (ηrmax); these results could be helpful to tune viscosities of surfactant formulations.