Studies of cetylpyridinium chloride and cetylpyridinium salicylate in solution and adsorbed on silica surfaces using X- and W-band electron paramagnetic resonance spectroscopy

The electron paramagnetic resonance (EPR) spin probe 4-[N,N-dimethyl-N-(n-h exadecyl)ammonium]-2,2,6,6-tetramethylpiperindinyl-N'-oxyl (HTAB*) has been used to study the adsorption and aggregation of cetylpyridinium chloride ( CPC) and cetylpyridinium salicylate (CPSa) on silica particles. In CPC mice lles, the local viscosity, as determined from the rotational correlation ti mes of HTAB*, was found to be approximately constant, as was the local pola rity determined from A(iso), g(iso), and HTAB* equilibrium constants. In CP Sa micelles at concentrations above the sphere-to-rod transition, significa nt decreases in polarity and local viscosity were observed. The affinity of CPC for the strong binding site on the silica surface was found to be high er than the affinities of CPSa and HTAB. The polarities and local viscositi es of the surfactant aggregates and the corresponding micelles were found t o be similar. From analysis of the line widths of the EPR spectra, it was c oncluded that CPSa on silica surfaces forms two coexisting aggregate phases , one which excludes HTAB* and one in which HTAB* is concentrated. The HTAB * is believed to be excluded from the former phase because of ordering of t he salicylate counterion and pyridinium headgroups as proposed by Favoriti and Treiner.(1).