Micelle formation by a long-chain cation surfactant in aqueous solutions of the lower quaternary ammonium bromides

The critical micelle concentrations (CMC) of long-chain cationic and anionic surfactants in water are lowered by the addition of ordinary salts like NaCl. This is usually ascribed to electrostatic effects. It has been found that the CMC of hexadecyltrimethylammonium bromide in water at 30°C. is at first decreased by the addition of the lower quaternary bromides and then increases markedly at higher salt concentration. The initial CMC decrease is in the order K⁺ < (CH₃)₄N⁺ < (C₂H₅)₄N⁺ < (n − C₃H₇)₄N < (n − C₄H₉)₄N⁺. There is a continually decreasing value of the CMC in increasingly concentrated KBr solutions. However, there is a minimum CMC value for each added quaternary salt. Those for (CH₃)₄N⁺ and (C₂H₅)₄N⁺ are found in approximately 0.09 M and 0.01 M salt, respectively, that for the n-propyl compound is at 10⁻³ M solution. In more concentrated salt solutions the CMC is three to five times greater than in water, with the greatest increase found in (n − C₄H₉)₄ NBr solution, and the smallest increase in (CH₃)₄NBr at equal concentrations. The results are interpreted in terms of the effects of added electrolytes on the hydrogen-bonded structure of water. It is concluded that micelle formation is rendered more difficult by electrolytes which organize the solvent structure, and occurs at lower soap concentrations in the presence of structure-disrupting electrolytes like the alkali halides.