Effect of head group size, temperature and counterion specificity on cationic micelles

Specific conductivity vs mole fraction of CTACl at different temperature. [Display omitted] ► In this study, we have reported experimental results concerning the temperature variations of the cmc of four cationic micelles differing in head group size and counterion type. ► The conductance plots for cmc determinations were used to estimate the extent of counterion binding to the micelle. ► The micelle ionization degree increases with the size of the ionic head group and with the size of the counterion. ► Thermodynamic analysis of micelle formation has often been made only from cmc change with temperature and using a polynomial-function description of this dependence. The critical micelle concentration (cmc) and ionisation degree ( α), of micelles of cetyltrimethylammonium bromide (CTABr), cetyltrimethylammonium chloride (CTACl), cetyltripropylammonium bromide (CTPABr) and cetyltripropylammonium chloride (CTPACl) have been measured over a narrow temperature range at 2 degree intervals using electrical conductivity. CTPACl and CTPABr are very soluble in water and were measured in the temperature range 275.15–323.15 K. The Krafft temperatures for CTABr and for CTACl are 293.15 K and 284.15 K, respectively and established a lower temperature limit for our studies on these two surfactants. The cmc vs temperature curves have a smooth minimum near room temperature and α linearly increases with temperature. The changes of cmc and α with temperature are smaller than those associated with the modification of head group size or counterion nature. Using these results, basic thermodynamic quantities associated with the phenomena of micellization have been evaluated. Thermodynamic properties of the surfactant solutions were discussed in terms of temperature dependence of the free energy, enthalpy and entropy of micellization. A close similarity between the effects of change in temperature on protein folding and micellization process appears from the data.