Head Group hydrophobicity determines properties of Zwitterionic Surfactants

Surfactants that form micelles usually comprise a hydrophobic alkyl carbon chain and a hydrophilic head group, which can be cationic, anionic, zwitterionic, or uncharged. Here, we studied the properties of three different surfactants, N-Tetradecyl-N,N-dialkylammonium propane sulfonate, where the alkyl groups can be two methyls (TMPS), two ethyls (TEPS), and two propyls (TPPS) groups, to analyze whether the headgroup hydrophobicity affects micellar properties. Measure micellization enthalpy and critical micellar concentration (cmc) of each surfactant at 25, 35, 45, and 55 °C to understand the effect of surface hydrophobicity on micellar properties. N-Tetradecyl-N,N-dimethylammonium propane sulfonate was SIGMA analytical grade. Gianfranco Saveli, from Universitá degli Studi di Perugia, Italy, synthesized N-tetradecyl-N, N-diethylammonium propane sulfonate, and N-tetradecyl-N,N-dipropylammonium propane sulfonate, and the purity of the samples were verified by mass spectrometry. By increasing temperature from 25 to 55°C the micellization enthalpy of the three surfactants was increasingly negative, showing that the process was progressively exothermic. Upon increasing the hydrophobicity of the polar headgroup of the surfactants, the enthalpy increased, showing that the more hydrophobic the polar head, the micellization was less exothermic in the process analyzed, even reaching and turning to an endothermic phenomenon. The cmc of TMPS was higher than its analogs. TMPS micellization enthalpies was more exothermic than that of TEPS and TPPS, showing a direct influence of the hydrophobicity of the polar head on the micellization process. These results can be rationalized assuming that the enthalpy contribution to the cmc is higher than the entropy, highlighting the role of enthalpy over entropy at higher temperatures.