Thermodynamics of Micellization of Sulfobetaine Surfactants in Aqueous Solution

The thermodynamics of micellization of the sulfobetaine (SB) amphoteric surfactants, that is N-alkyl-N,N-dimethyl-3-ammonio-1-propanesulfonate and N-alkyl-N,N-dimethyl-3-ammonio-1-butanesulfonate (the carbon atom number of the alkyl chain is 12, 14 and 16 respectively) in aqueous solution, have been studied by surface tension measurements with the temperature range from 298.15 to 318.15 K. The critical micelle concentrations (CMC) of SB n-3 and SB n-4 surfactants were determined from the drop-volume methods at different temperatures. The obtained results indicated that the values of critical micelle concentration strongly depended on the surfactants species and temperatures. Thermodynamic parameters (\( \Updelta G_{\text{mic}}^{ \circ } \), \( \Updelta H_{\text{mic}}^{ \circ } \) and \( \Updelta S_{\text{mic}}^{ \circ } \)) of the micelle formation were determined. The micellization was found to be enthalpy-driven at lower temperatures, while this process was entropy-driven at higher temperatures. The enthalpy–entropy compensation were also investigated. The compensation temperature T c and \( \Updelta H_{\text{mic}}^{*} \) decreased, while \( \Updelta S_{\text{mic}}^{*} \) increased with the increase in the hydrophobic chain length.