Thermodynamic and NMR study of aggregation of dodecyltrimethylammonium chloride in aqueous sodium salicylate solution

The complex aggregation processes of dodecyltrimethylammonium chloride (DTAC) have been studied in dilute solutions of sodium salicylate (NaSal) by isothermal titration calorimetry and electrical conductivity at temperatures between 278.15 K and 318.15 K. A structural transformation that was dependent on the concentrations of DTAC and NaSal was observed. The micellization process in dilute solutions of DTAC has been subjected to a detailed thermodynamic analysis and shown to occur at considerably lower critical micelle concentrations than reported for DTAC in water and NaCl solutions. Gibbs free energy, Δ _mic G ^o, and entropy, Δ _mic S ^o, were deduced by taking into account the degree of micelle ionization, β, estimated from conductivity measurements. From the temperature dependence of the enthalpy of micellization, Δ _mic H ^o, the heat capacities of micellization, D_mic c_p^o {\Delta_{{{\rm mic} }}}c_p^o were determined and discussed in terms of the removal of large areas of non-polar surface from contact with water upon micellization. The process is exothermic at all temperatures, indicating, in addition to the hydrophobic effect, the presence of strong interactions between surfactant and salicylate ions. These were confirmed by ¹H NMR spectroscopy and diffusion NMR experiments. Salicylate ions not only interact with the headgroups but also insert further into the micelle core. At c _NaSal/c _DTAC > 2.5, the structural rearrangements occur even at relatively low concentrations of NaSal.