Self-association of potassium nonanoate molecules in aqueous aerosil dispersions:: 13C NMR data

The association of potassium nonanoate molecules in aqueous Aerosil dispersions at high and low surface charges of its particles are studied by the C-13 NMR technique at frequencies of 75 and 125 MHz with the rotation of a sample at a magic angle. It is shown that. using the chemical shifts and spin-lattice relaxation times of C-13 nuclei of single segments of the hydrophobic chain of a surfactant molecule. one can determine whether the segments are located in the adsorption layer on a solid surface or in the associates formed in the bulk solution. The values of the chemical shift and spin-lattice relaxation Lime depend. above all. on the value of the surface charge. The NMR parameters of two carbon atoms in the hydrophobic chain nearest to the polar group of the surfactant molecule appeared to be the most sensitive to the value of the adsorbent surface charge. It is discovered that, upon adsorption of potassium nonanoate from solutions with concentrations of 1.5-3 wt % and pH 6, the surfactant molecules are located in the adsorption layer parallel to the surface. At a higher surface charge, adsorption in the same concentration range occurs mainly on the hydroxyl groups; in this case. adsorbed molecules are arranged mostly along the normal to the surface. At surfactant concentrations much higher than the CMC (10-20%), the values of the chemical shift and relaxation time become closer to those measured in surfactant solutions in the absence of Aerosil.