SURFACTANT ADSORPTION ON A CLAY MINERAL - APPLICATION OF RADIATION SCATTERING

Aqueous mixtures of the synthetic clay mineral Laponite and cetyltrime thylammonium bromide (CTAB) in dilute suspension have been investigate d by small-angle neutron scattering (SANS) and dynamic light scatterin g (DLS). The SANS intensity pattern from the suspension of the pure cl ay is consistent with that from disks or platelets with diameters of a bout 30 nm. After addition of CTAB to the clay suspensions, CTAB adsor bs on the face of the clay platelets by cationic exchange and the resu lting complex contains excess CTAB in an amount corresponding to about four multiples of the clay's cation exchange capacity (CEC). Contrast matching by isotopic substitution in the solvent indicates that the C TAB/clay complex retains a disk morphology, even if the CTAB in the aq ueous medium is in considerable excess. Analyses of the DLS data are i nterpreted to indicate two possible stable clay platelet configuration s in the suspensions-free clay platelets or clay platelets with CTAB a dsorbed in an amount above the CEC and presumably enough to form a sta ble surfactant bilayer. The effect of CTAB concentration in solution o n the aggregation of the clay and its subsequent redispersion is repor ted, and the influence of clay on CTAB micelle formation is also discu ssed briefly.