Viscometric studies on aqueous gemini micelles in the presence of additives

Surfactant molecules produce different types of morphologies in water above their critical micelle concentrations. This article presents the viscosity results obtained on gemini surfactants alkanediyl-α,ω-bis(cetyldimethylammonium bromide) (C16H33 Me2N+ –(CH2)s– N+Me2C16H33, 2Br−; 16-s-16, 2Br−, where s = 4, 5, 6) in pure aqueous solutions and also in the presence of different additives (KBr, n-hexanol, n-pentanol, n-butanol and n-hexylamine). The data are compared with a conventional surfactant cetyltrimethylammonium bromide (n-C16H33N+Me3, Br−; CTAB). In pure aqueous solutions, the viscosity was found to increase rapidly with geminis in comparison to CTAB on increasing surfactant concentration. It was also observed that among geminis, the viscosity increase was sharper lower the spacer chain length. This is due to the fact that with a lower spacer chain length, the headgroup area is smaller that leads to more orderly packing in the micelle (i.e., micellar growth) resulting in sharp viscosity changes. Addition of KBr to 30 mM surfactant solutions again shows that viscosities are much higher with geminis (of lower spacer length) in comparison to CTAB. Addition of n-alcohols and n-hexylamine also causes increase in viscosity of 30 mM 16-4-16 solutions. However, the increase was more with n-hexanol which can be understood in the light of hydrophobic ranking of the two types of additives. A combined presence of KBr and either of the alcohols or n-hexylamine shows a synergistic effect on the viscosity increase of 16-4-16 solution. This increased effectiveness of the organic additives in presence of added KBr has been discussed in terms of electrostatic and hydrophobic forces operating in the solution. The data allow to conclude that in micellar growth the presence of a salt and organic additive produces favorable conditions which are absent when the additive is present singly.