Influence of the alkyl chain length, temperature, and added salt on the thermodynamics of micellization: Alkyltrimethylammonium chlorides in NaCl aqueous solutions

The thermodynamics of micelle formation of the cationic surfactants decyltrimethylammonium chloride (DeTAC) and tetradecyltrimethylammonium chloride (TTAC) in water and aqueous NaCl solutions have been investigated. Isothermal titration calorimetry (ITC) was used to study the effect of added salt on the critical micelle concentration, cmc, and enthalpy of micellization, ΔmicH∘ΔmicH∘, between the temperatures (278.15 and 328.15) K. From the temperature dependence of ΔmicH∘ΔmicH∘, the heat capacities of micellization, Δmiccp∘, were determined. The results obtained are compared with reported data for dodecyltrimethylammonium chloride (DTAC).The values of Δmiccp∘ are negative for all surfactants in all solvents relating to the removal of water accessible non-polar surfaces also in the presence of excess counterions. The temperature dependence of Δmiccp∘ is evident at DeTAC, leading to the assumption that, especially at high temperatures, the non-polar parts of surfactant molecules are still in perceivable contact with water.

► Heat upon micellization was measured for DeTAC and TTAC in aqueous NaCl solutions. ► Three procedures for the determination of cmc and ΔmicH∘ΔmicH∘ were applied and compared. ► Δmiccp∘ were determined from the temperature dependence of ΔmicH∘ΔmicH∘. ► For DeTAC Δmiccp∘ is strongly temperature dependent. ► Relation between Δmiccp∘ and the removal of water ASAnp of surfactant is discussed.