WATER-INDUCED STRUCTURAL-CHANGES WITHIN THE L(2) PHASE OF DIDODECYLDIMETHYLAMMONIUM BROMIDE-CYCLOHEXANE-WATER SYSTEMS

The structural changes which occur as a function of water concentration w (=[H2O]/[DDAB]) in the dilute L(2) phase (0.008 < phi < 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 Angstrom and length in the range of 250-90 Angstrom 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 Angstrom and the overall radius including the DDAB monolayer of ca. 30 Angstrom were obtained by neutron contrast variation. The SANS results are interpreted in terms of the ratio rho/rho(0) in the film bending energy model of microemulsions, where rho(0) is the spontaneous radius and rho is the equivalent sphere radius for a given composition. The cylinder-to-sphere transition observed at w x 10.0 and the solubilisation phase boundary at w approximate to 12.0 are in good agreement with the predictions.