Adsorption and Ordering of Surface Active Molecules and Particles at Solid Interfaces and in the Bulk

Abstract: Factors that influence the process of adsorption and order of dilute and concentrated systems of molecules and particles are explored in this thesis.  The results are based largely on neutron scattering techniques.  Study of the adsorption from dilute solutions of a common surfactant, AOT (sodium bis (2-ethylhexyl) sulfosuccinate), at a solid/liquid interface showed that AOT molecules adsorbed to the interface in a bilayer structure, with the hydrocarbon tails at maximum packing density even at very low concentrations.  At higher AOT concentrations, a stack of fluctuating layers each separated by large amounts of water next to the dense bilayer was seen.  The driving force for adsorption is dominated by self-assembly of AOT.  It was found that an oriented lamellar phase wets the interface below the bulk concentration for formation of this phase.Proteins can be viewed as polymeric surfactants.  The adsorption of proteins from seeds of the Moringa oleifera tree to a silicon oxide surface was studied to elucidate the mechanism of the protein as a flocculent in water treatment processes.  The protein was found to adsorb at the interface as dense layers with a thickness suggestive of co-adsorption rather than single isolated molecules.  The strong adsorption and tendency to associate in solution suggest mechanisms for flocculating particulate impurities in water.As with surfactants, dispersions of colloidal particles can assemble in regular structures by self-assembly.  Polystyrene latex particles were studied and could form large three-dimensional crystals of about 1×1 cm2 in a 2 mm path cell.  The diffraction pattern indicated a close packed structure with the 110 axis perpendicular to the container wall.  The crystal was well-aligned and oriented by the direction of flow.  At the solid interface large two-dimensional domains of about 20 cm2 of highly oriented particles were formed.  The particle-particle separation at the surface and in the bulk was determined by the charge repulsion of the particles.