Aggregation properties of novel cationic surfactants with multiple pyridinium headgroups. Small-angle neutron scattering and conductivity studies

Abstract

Novel single-chain cationic surfactants bearing one, two, and three pyridinium headgroups have been synthesized. These surfactants upon solubilization in water produced micelles or large lamellar structures depending on the number of headgroups they possess. The critical micellar concentrations (cmc) of these surfactants were determined by measuring the specific conductance of the aqueous solution of the surfactants. The cmc values were found to increase with every increase in the number of headgroups in the surfactant. Detailed small-angle neutron scattering (SANS) studies were carried out with their aqueous solutions (D₂O) to study their aggregation properties. The data have been analyzed using the Hayter and Penfold model for macro-ion solution to compute the interparticle structure factor S(Q), taking into account the screened Coulomb interactions between the micelles. SANS analysis clearly indicated that the nature of the aggregate structure of these cationic micelles depends on the number of the headgroups present in the surfactants. It was found that a single-headed new pyridinium amphiphile formed the lamellar structure, whereas surfactants with double and triple headgroups formed micelles in water. The aggregates shrank in size with an increase in the number of headgroups in the surfactants. The aggregation number (N) continually decreased and the fractional charge (α) increased with a greater number of headgroups on the surfactants. The semimajor axis (a) and semiminor axis (b = c) of the micelle also decreased with the increase in the number of headgroups in the surfactants. It indicates that hydrocarbon chains in such micelles prepared from multiheaded surfactants adopt a bent conformation and no longer stay in extended conformation. The effects of variation of the concentration and temperature on aggregation properties for all the surfactants have also been investigated. In the case of h = 1, the lamellar structure remains approximately the same with the variation of concentration, but with an increase in temperature there was transition from lamellar to rodlike micelles. For other surfactants, with an increase in concentration the aggregation number and the size of the micelles increased. The micelles became progressively smaller in size, the aggregation number decreased, and the fractional charge increased with an increase in temperature.