Quantitative correlation between counterion-affinity to cationic micelles and counterion-induced micellar growth

Abstract

The fascinating and serendipitous discovery, in 1976, of the characteristic viscoelastic behavior of wormlike micelles of cetyltrimethylammonium salicylate (CTASa) surfactant solution at ~2×10−4M CTASa became a catalyst for an increasing interest in both industrial application and mechanism of the origin of micellar growth of this and related wormlike micellar systems. It has been perceived for more than three decades, based upon qualitative evidence, that the extent of the strength of the counterion (X) binding to ionic micelles determines the counterion-induced micellar structural transition from spherical-to-small rodlike-to-linear long stiff/flexible rodlike/wormlike-to-entangled wormlike micelles. This perception predicts the presence of a possible quantitative correlation of counterionic micellar binding constants (KX) with counterion-induced micellar growth. The quantitative estimation of counterion binding affinity to cationic micelles, in terms of the values of the degree of counterion binding (βX), is concluded to be either inefficient or unreliable for moderately hydrophobic counterions (such as substituted benzoate ions). The values of KX, measured in terms of conventional ion exchange constants (KXY), can provide a quantitative correlation between KX or KXY (with a reference counterion Y=Cl− or Br−) and counterion-induced ionic micellar growth. A recent new semi-empirical kinetic (SEK) method provides the estimation of KXY for Y=Br as well as ratio of counterionic micellar binding constants KX/KBr (= RXBr) where the values of KBr and KX have been derived from the kinetic parameters in the presence of cationic spherical and nonspherical micelles, respectively. The SEK method has been used to determine the values of KXBr or RXBr for X=2-, 3- and 4-ClC6H4CO2−. Rheometric measurements on aqueous CTABr/MX (MX=2-,3- and 4-ClBzNa) solutions containing 0.015M CTABr and varying values of [MX] reveal the presence of spherical micelles for MX=2-ClBzNa and long linear as well as entangled wormlike micelles for MX=3- and 4-ClBzNa. The respective values of KXBr or RXBr of 5.7, 50 and 48 for X=2-, 3- and 4-ClBz− give a quantitative correlation with the rheometric measurements of the structural features of micelles of 0.015M CTABr solutions containing 2-, 3- and 4 ClBzNa.