Micellar association in simultaneous presence of organic salts/additives

Abstract

Viscosity measurements under Newtonian flow conditions had been performed on cetyltrimethylammonium bromide (CTAB) aqueous solutions in the combined presence of sodium salts of aromatic acids (sodium salicylate, NaSal; sodium benzoate, NaBen; sodium anthranilate, NaAn) and organic additives (1-hexanol, C 6 OH; n-hexylamine, C 6 NH 2 ) at 30°C. On addition of C 6 OH or C 6 NH 2 , the viscosity of 25 mM CTAB solution remained nearly constant without salt as well as with a lower salt concentration. This is due to low CTAB concentration which is not sufficient to produce structural changes in this concentration range of salts. However, as the salt concentration was increased further, the effect of C 6 OH/C 6 NH 2 addition was different with different salts: The viscosity first increased; then a decrease was observed with the former while with C 6 NH 2 a decrease followed by constancy appeared in plots of relative viscosities (η r ) vs. organic additive concentrations. At further higher salt concentration, the magnitude of η r was much higher. The viscosity increase is explained in terms of micellar growth and the decrease in terms of swollen micelle formation (due to interior solubilization of organic additive) or micellar disintegration (due to formation of water + additive pseudophase).Paper no. S1267 in JSD 5, 55-59 (January 2002).Numerous studies have been carried out on the effect of salts (1-3) and organic additives (4-6) on the micellar structural transitions in aqueous medium. Recently, we reported a kind of "synergism" (e.g., significant increase in viscosity) in such systems when salts and organic additives are present concurrently in the micellar solutions (7-11). In these studies the nature of salts and additives was found to play a crucial role toward such synergisms.Micelles of quaternary ammonium halides [e.g., cetyltrimethylammonium bromide (CTAB)] grow from spherical to rod-shaped on the addition of different counterions (12). Halide anions associate only moderately with surfactant headgroups, and micellar growth is gradual. However, with anions that associate strongly, such as aromatic salt anions (e.g., salicylate, Sal − ), rod-shaped micelles grow rapidly even at low surfactant and salt concentrations (13), and the solutions exhibit a remarkable viscosity increase (14,15).Aqueous micelles are capable of solubilizing organic molecules with quite distinct polarities and degrees of hydrophobicity (16). The addition of different types of molecules leads to large deviations of packing parameters in micellar assembly (17). Many counterions and organic additives are strongly adsorbed at the micellar interface, and depending on the degree of penetration, this may change the mean distance between polar headgroups or may increase the volume of the micellar core. The aromatic acid counterions are most efficient with cationic micelles. 1 H nuclear magnetic resonance (NMR) studies have shown that the Sal − anion orients in such a way that the negatively charged site (COO − group) stands perpendicular to the CT...

Microstructural transition of aqueous CTAB micelles in the presence of long chain alcohols

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Mixing Behavior of Cationic Hydrotropes with Anionic Surfactant Sodium Dodecyl Sulfate

“…[7,8] Many investigations have been carried out in the literature on the formation of viscoelastic wormlike micelles in a variety of surfactant systems with the addition of the above type of organic salts. [9][10][11][12][13][14][15][16][17][18] The organic salt screens the electrostatic repulsion between the charged head groups, and induces one dimensional micellar growth by decreasing the effective cross-sectional area per head group of the surfactant molecule. [19] Anionic surfactants are widely produced and consumed in vast quantities than any other surfactants because of their high detergency and low cost of manufacture.…”

A Rheological Approach to Viscoelastic Worm‐Like Micelles of Tunable Properties

“…The rod-like micelles are formed at 0.1 mol L -1 KBr and the worm-like micelles are formed at above 0.2 mol L -1 KBr. Kumar et al [13] and Desai et al [14] studied the micellar growth in presence of alcohol and amines and they found that octanol is more effective in promoting the sphere-to-rod transitions in surfactant systems; the penetration of the surfactant rich film by octanol helps to overcome the head group repulsion of the surfactant molecules [15,16].…”