Self-association processes of short-chain anionic surfactant in silica dispersions from 13C NMR

Structural properties of the ionic surfactant potassium nonanoate in aqueous solutions with pH values of 2 and 6 containing a silica dispersion (SiO2) were investigated by means of 13C MAS NMR techniques. It is shown that the chemical shifts δ and the spin-lattice relaxation times T1 of separate segments of the hydrophobic chain reflect rather sensitively the process of surfactant adsorption on a solid surface as well as the self-association of the surfactants in the bulk solution. The values and the character of the changes of Δδ and T1 are mainly determined by the surface charge, which is generally assumed to change with the initial pH value of the silica aqueous dispersion. Most sensitive to the surface charge are the 13C NMR parameters of the first and second carbon atoms in the hydrophobic chain of the surfactant molecules. In a system with pH = 2, the greatest dependence of these parameters on the surfactant concentration was observed. Comparing the NMR parameters for the surfactant–water–silica systems with similar parameters found for aqueous surfactant solutions, it is shown that at low surfactant concentrations in a solution (1.5–3 wt.%) with pH = 6, the surfactant molecules are oriented parallel to the surface. They are mainly adsorbed due to a hydrophobic interaction with the surface. In the same range of concentration, for systems with pH = 2 the adsorption occurs primarily on hydroxyl groups by hydrogen bond between these groups and polar head of the surfactant molecules. The adsorbed molecules then are oriented normal to the surface. At concentrations between 10 and 20 wt.%, which considerably exceed the CMC value, the chemical shifts and relaxation times in both ternary systems are approximately equal to the values measured in solution because of the molecular exchange process between adsorbed state and bulk solution.