Thermodynamics of micellization of multiheaded single-chain cationic surfactants

Abstract

The energetics of micelle formation of three single-chain cationic surfactants bearing single (h = 1), double (h = 2), and triple (h = 3) trimethylammonium [⁺N(CH₃)₃] headgroups have been investigated by microcalorimetry. The results were compared with the microcalorimetric data obtained from well-known cationic surfactant, cetyl trimethylammonium bromide (CTAB), bearing a single chain and single headgroup. The critical micellar concentrations (cmc's) and the degrees of counterion dissociation (α) of micelles of these surfactants were also determined by conductometry. The cmc and the α values increased with the increase in the number of headgroups of the surfactant. The relationship between the cmc of the surfactant in solution and its free energy of micellization (ΔG_m) was derived for each surfactant. Exothermic enthalpies of micellization (ΔH_m) and positive entropies of micellization (ΔS_m) were observed for all the surfactants. Negative ΔH_m values increased from CTAB to h = 1 to h = 2 and decreased for h = 3 whereas ΔS_m values decreased with increase in the number of headgroups. The ΔG_m values progressively became less negative with the increase in the number of headgroups. This implies that micelle formation becomes progressively less favorable as more headgroups are incorporated in the surfactant. From the steady-state fluorescence measurements using pyrene as a probe, the micropolarities sensed by the probe inside various micelles were determined. These studies suggest that the micelles are more hydrated with multiheaded surfactants and the micropolarity of micelles increases with the increase in the number of headgroups.