Micellar Growth in the Presence of Salts and Aromatic Hydrocarbons:  Influence of the Nature of the Salt

Viscosity measurements under Newtonian flow conditions have been performed at 40 °C to study the effect of aromatic hydrocarbons (benzene, toluene, or o -xylene) on aqueous micellar solutions of 0.1 M cetylpyridinium bromide (CPB) containing different salts. Two series of salts, viz. (i) inorganic (KX; X = Cl, Br, or NO 3 ) and (ii) symmetrical quaternary ammonium (R 4 NBr; R = H, CH 3, C 2 H 5, n -C 3 H 7, or n -C 4 H 9 ), were used to explore the effect of their nature and concentration. The hydrocarbons had marginal effect on viscosity when added to CPB solutions having no salt. However, in the presence of salts, the viscosity behavior was quite different (synergistic effect). Relative viscosity (η r ) versus concentration of hydrocarbon plots were constructed for various fixed salt concentrations. Most of the time, after reaching a maximum value, η r decreased on further addition of hydrocarbons, showing a peaked behavior. The peak position (maximum) as well as the viscosity at the maximum, η r max, was found to be dependent on the nature/concentration of salts, hydrocarbons, and counterions. However, the viscosity behavior was different with the R 4 N salts having a longer alkyl (R) part (the synergism progressively diminished). The effect of concentration of salt was reversed and peaked behavior was also lost. This reversal and change in behavior have been explained in terms of the salting-in nature of these salts as compared to the salting-out nature of the salts of series i.