Micellization process - Temperature influence on the counterion effect

The micellization process of dodecyltrimethylammonium chloride (DTAC) and b romide (DTAB) was studied at 313 K. Nuclear magnetic resonance and calorime tric methods were used. The calorimetric titration curves permitted determi nation of the critical micelle concentration (CMC) and enthalpy of the mice llization process (DeltaH(m)) of the compounds studied. The results obtaine d were compared to those obtained at 298 K. It was found that calorimetric curves obtained at 313 K for both compounds were similar to each other in c ontrast to 298 K. Especially a great difference in the shape of curves was observed for DTAC. NMR (H-1 NMR and C-13 NMR) spectra were taken below and above the CMC value s and chemical shifts (delta) analysed as a function of concentration of th e compounds. Comparison of chemical shift-concentration plots with those ob tained from measurements performed at lower temperature showed that chemica l shifts are of very similar character in both cases for analyzed groups. H owever, there are some quantitative differences that indicate at smaller di fference in hydration of DTAB and DTAC micelles at elevated temperature. Th is may be the reason of decrease of differences between micellization proce sses of DTAC and DTAB compounds. The smaller hydration may be, in turn, the result of diminishing differences in physicochemical properties of bromide and chloride ions with temperature.