Micelles of asymmetric chain catanionic surfactants

The properties of solutions of asymmetric chain catanionic surfactants are reported. The materials are n-propyl-, n-butyl-, n-hexyl- and n-heptylammonium dodecylsulphates (abbreviated as C3-C12, C4-C12, C6-C12 and C7-C12 respectively). At 25 degrees C the critical micelle concentrations (CMCs) were determined by air-water surface tension, gamma, and electrical conductivity, kappa, measurements. It was found that log (CMC) was proportional to the chain length n(c). The mean area per molecule, a(s), and per hydrocarbon chain, a(c), were estimated by applying the Gibbs isotherm to pre-CMC gamma versus In(c) data. For C3-C1.2, a(c) approximate to 29 Angstrom(2), increasing to 34 Angstrom(2) for the C7-C12 material. These values reflect efficient packing in the monolayer as compared with single chain ionic surfactants like sodium dodecyl sulphate, where a(s) approximate to 45 Angstrom(2). The mesophase sequences were determined at 25 degrees C using polarising microscopy. As n(c) increases from four to six the hexagonal phase, H-1 (fans), is eliminated, and only L(a) phases (oily streaks and myelin) form. Small-angle neutron scattering (SANS) experiments and data modelling suggest that the micelles of C4-C12, C6-C12 and C7-C12 are essentially uncharged, and that a cylinder-to-disc transition occurs between n(c) = 4 and 6. Mean values for the micellar dimensions, number density, n(p), aggregation number, n(agg), and area per molecule, a(s), were obtained from the SANS data. n(agg) increases from around 300 for the cylindrical to about 6000 for the disc micelles. The value of a(s) decreases from around 64 Angstrom(2) (cylinders) to about 56 Angstrom(2) (discs), and this is used to calculate the surfactant packing parameter, P, as a function of n(c). The micellar shapes are consistent with the calculated values of P.