Water-induced structural changes within the L2 phase of didodecyldimethylammonium bromide–cyclohexane–water systems

Abstract

The structural changes which occur as a function of water concentration w(=[H₂O]/[DDAB]) in the dilute L₂ phase (0.008 < ϕ < 0.16) of the didodecyldimethylammonium bromide (DDAB)–water–cyclohexane system have been studied using viscometry and small-angle neutron scattering (SANS). For 2.0 < w < 8.0 cylindrical reverse micelles are present. The SANS data are consistent with a constant cylinder of cross-section radius ca. 15 Å and length in the range of 250–90 Å depending on w. At w= 10.0 there is an abrupt structural change and for w > 10.0 spherical aggregates are present. The solubilisation limit is reached at w= 12.0 where spherical water-in-oil microemulsion droplets co-exist with an excess water phase (Winsor II system). For the spherical droplets values for the mean water core radius of ca. 19 Å and the overall radius including the DDAB monolayer of ca. 30 Å were obtained by neutron contrast variation. The SANS results are interpreted in terms of the ratio ρ/ρ_o in the film bending energy model of microemulsions, where ρ_o is the spontaneous radius and ρ is the equivalent sphere radius for a given composition. The cylinder-to-sphere transition observed at w≈ 10.0 and the solubilisation phase boundary at w≈ 12.0 are in good agreement with the predictions.