Microcalorimetric and Conductivity Studies with Micelles Prepared from Multi-Headed Pyridinium Surfactants

A set of novel single-chain surfactants bearing one (P1), two (P2), and three (P3) pyridinium headgroups have been synthesized in an attempt to achieve control over the aggregate properties. The critical micellar concentrations (cmc's) and the degrees of counterion dissociation (\alpha) of micelles of these surfactants were determined by conductometry. The cmc and the values increased with increase in the number of headgroups of the surfactant. The thermodynamics of micellization of these surfactants were investigated by microcalorimetry, and the results were compared with that of well-known single-chain/single-headgroup surfactant, cetylpyridinium bromide (CPB). The relationship between the cmc of surfactant in solution and its free energy of micellization $(\Delta G^\circ_m )$ was derived for each surfactant. Exothermic enthalpies of micellization $(\Delta H^\circ_m )$ and positive entropies of micellization $(\Delta S^\circ_m )$ were observed for all the surfactants. $ \Delta H^\circ_m $ values were found to be more negative for CPB than P1, and it increased with a negative sign from P1 to P2 and decreased for P3. In contrast the $ \Delta S^\circ_m $ values decreased with increase in the number of headgroups. The $ \Delta G^\circ_m $ values progressively became less negative with increase in the number of headgroups. This implies that micelle formation becomes less favorable as more headgroups are incorporated in the surfactant