Effect of Headgroup Size on the Thermodynamic Properties of Micellization of Dodecyltrialkylammonium Bromides

The micellization of dodecyltrimethyl/ethyl/propyl/butylammonium bromide (C12NM, C12NE, C12NP, and C12NB) was investigated by electrical conductivity measurements at different temperatures T. In the investigated temperature range, (15 to 45) degrees C, both the critical micelle concentration (cmc) and the degree of counterion association (beta) decreased with the increase of headgroup size. The cmc as a function of T showed a typical U-shaped relationship. The temperature of the minimum of U-shaped curve (T-min) increased with the incremental chain length in the headgroups, which was the reverse to the previous results for quaternary ammonium surfactants that T-min decreased with the increment in the hydrophobic tails. The beta approximately exhibited a linear decrease with raising T. Nonlinearity was observed both in In(cmc) versus N-c and in Delta(mic)G degrees versus T, where N-c was the carbon number in the headgroups and Delta(mic)G degrees was the standard Gibbs energy of micellization. C12NM and C12NE resembled each other in the thermodynamics of micellization, but they behaved considerably differently from either C12NP or C12NB in the dependence of In(cmc) on N-c, in the decreasing rate of Delta(mic)G degrees versus T, and in the variation of thermodynamic properties with T. All of the surfactants exhibited the enthalpy-entropy compensation phenomenon. The large value of standard entropy of micellization (Delta S-mic degrees) of C12NB compared with the others could be attributed to the strong hydrophobicity of the tributyl headgroup.