ANION EFFECTS ON THE MICELLIZATION OF A ZWITTERIONIC SURFACTANT

Surfactants form micelles when the concentration is above the critical micellar concentration (CMC). This process can be affected by the interaction of different salts, as well as the temperature of the system. In this study, we analyzed the micellization process of tetradecyldimethylammonium propanesulfonate (TMPS), varying the temperature and adding different hydrotropic salts (which have a hydrophobic and a hydrophilic portion on their structure, but do not form aggregates), like benzoate, benzenesulfonate, triflate, and sodium acetate. Our data and published results show specific ion effects for TMPS. We measured the micellization enthalpy and CMC of TMPS at 25, 35, 45, 55, and 65°C, varying the salt concentration, and compared the results with those obtained with analog surfactants. TMPS was SIGMA analytical grade. The sodium salts benzoate (NaBzo) (Sigma-Aldrich >99%), trifluoromethanesulfonate (NaTf), and benzenesulfonate (NaBzs) were synthesized by the neutralization with NaOH of respective acids (trifluoroacetic acid Sigma-Aldrich ≥98%; trifluoromethanesulfonic acid Sigma-Aldrich ≥98%; benzenesulfonic acid Sigma-Aldrich ≥98%). The CMC and the micellization enthalpy of TMPS follow the order Cl⁻ < Br⁻ < I⁻ < ClO4⁻, with both parameters decreasing from Cl⁻ to ClO4⁻. Hydrotropic anions such as benzoate (Bzo⁻) and benzenesulfonate (Bzs⁻) reduced the micellization enthalpy more than Br⁻ and Cl⁻ at the same salt concentration (10 mM). Consequently, the decreasing trend in cmc and micellization enthalpy was: Cl⁻ < Br⁻ < Bzo⁻ < I⁻ ≈ Bzs⁻ < Tf⁻ < ClO4⁻. The specific anion effect on the cmc and micellization enthalpy of TMPS confirmed the selectivity of anions in the process of micellization for this surfactant. Hydrotropic anions and perchlorate had a significant effect on the CMC and enthalpy of the zwitterionic detergent TMPS.