Surfactant adsorption on a clay mineral: Application of radiation scattering

Aqueous mixtures of the synthetic clay mineral Laponite and cetyltrimethylammonium bromide (CTAB) in dilute suspension have been investigated by small-angle neutron scattering (SANS) and dynamic light scattering (DLS). The SANS intensity pattern from the suspension of the pure clay is consistent with that from disks or platelets with diameters of about 30 nm. After addition of CTAB to the clay suspensions, CTAB adsorbs on the face of the clay platelets by cationic exchange and the resulting 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 CTAB/clay complex retains a disk morphology, even if the CTAB in the aqueous medium is in considerable excess. Analyses of the DLS data are interpreted to indicate two possible stable clay platelet configurations in the suspensions-free clay platelets or clay platelets with CTAB adsorbed in an amount above the CEC and presumably enough to form a stable surfactant bilayer. The effect of CTAB concentration in solution on the aggregation of the clay and its subsequent redispersion is reported, and the influence of clay on CTAB micelle formation is also discussed briefly.