Effect of surfactant and solvent properties on the stacking behavior of non-aqueous suspensions of organically modified clays

Montmorillonite clay was treated with quaternary ammonium surfactants with 1-3 long chains of 10-18 carbons to form organoclays which can be suspended in non-aqueous solvents. The effects of surfactant chain length, number of long chains, and the properties of the solvent on the colloidal behavior of the surfactant coated clay plates were studied using small-angle X-ray scattering. The scattering data were modeled using a one-dimensional aggregation theory to describe the stacking of the clay plates. The plates self-organize into stacks with a reproducible basal spacing in the range of 30-50 angstrom, and for each surfactant, the basal spacing falls into one of two preferred distances. We interpret this by considering that the surfactant layer on the clay has two strata, one being the polar near-clay headgroup region and the other the nonpolar alkane chain region. Polar solvents will swell the polar stratum preferentially while nonpolar solvents will swell the nonpolar stratum of the surfactant. As the nonpolar stratum is larger than the polar one, the nonpolar solvents increase the basal spacing between the clay plates more than the polar solvents. The number of long chains on the surfactant does not have an effect on the basal spacing, as the density of surfactant molecules on the surface is low enough to allow the unimpeded swelling of the chains. The one-dimensional aggregation theory can be used to determine the number of plates in a stack, but the effect of changing clay particle size or concentration is not as great as would be expected from this theory. This may be due to the formation of large-scale structures in the suspensions which prevent a true equilibrium stack size being attained.