Role of incorporation of multiple headgroups in cationic surfactants in determining micellar properties. Small-angle-neutron-scattering and fluorescence studies

Abstract

Three new single-chain surfactants bearing one, two, and three headgroups have been synthesized. Each of these surfactants formed micelles upon solubilization in water. The critical micellar concentrations (cmc) of these surfactants were determined using a micelle-solubilized, extrinsic fluorescent probe, pyrene. The cmc values were found to increase with every increase in the number of headgroups of the surfactant. Detailed small-angle-neutron-scattering (SANS) studies were performed with the micellar solutions in D₂O to study their aggregate properties. The data were analyzed using the Hayter and Penfold model for macroion solution to compute the interparticle structure factor, S(Q), taking into account the screened coulomb interactions between the micelles. SANS analysis has shown that the extent of aggregate growth of these cationic micelles depends on the number of the headgroups present in the surfactants. It has been observed that the micelles become progressively smaller in size with every increase in the number of headgroups of the surfactants. The aggregation number (N) is continually decreased and the fractional charge (a) is gradually increased with the increase in the number of headgroups. The semiminor axis and semimajor axis of the micelle decrease strongly with the increase in the number of headgroups. To mitigate charge repulsion, the hydrocarbon chains in the multiheaded surfactants appear to take up bent conformations. The effects of concentration and temperature on micellar growth have also been examined. While the N and a for the micelles of single-headed surfactant increased and decreased, respectively, with concentration, no significant alteration in these parameters was observed with micelles of surfactants with double and triple headgroups upon increase in concentration. Increase in temperature brought about transformation of micelles of surfactant with one headgroup more toward spherical morphology. However, such temperature-induced changes were less-pronounced with micelles of double- and triple-headed surfactants.