Adsorption of 1-alkyl-4-methylpyridinium salts at solid-liquid and water-air interfaces

Determinations of the adsorption at both solid-liquid and water-air interfaces of 1-alkyl-4-methylpyridinium salts have been performed. At the water-air interface, measurements of the surface tension as a function of the bulk concentration of 1-dodecyl-4-methylpyridinium chloride were performed. Mathematical treatment of the resulting isotherm allowed the size of the adsorbed molecule and the CMC to be obtained. The solid adsorbent was a titanium dioxide sample prepared in the laboratory. Adsorption isotherms of 1-dodecyl-4-methylpyridinium chloride and 1-butyl-4-methylpyridinium chloride were obtained at the same pH and ionic strength. For the shorter-chain surfactant, a second isotherm at a higher base electrolyte concentration was obtained. The experimental isotherms are discussed in the light of literature results and analyzed on the basis of the Frumkin-Fowler-Guggenheim adsorption equation. X-ray photoelectron spectroscopic analyses were performed both on the bare adsorbent and in the presence of the adsorbates. The binding energy of the surfactant elements (both nitrogen and carbon) was observed to undergo significant modifications when the compound was in the adsorbed state. The features of the adsorption of the two organic ions are discussed in light of the different experimental results.