ADSORPTION OF CATIONIC SURFACTANTS ON SILICA SURFACES WITH DIFFERENT SURFACE-CHARGE

In this work the adsorption of the cationic surfactants hexadecylpyridinium chloride and dodecylammonium chloride was investigated on silica gel and precipitated silica at varying pH and electrolyte concentration, The enthalpy changes cuased by the displacement process were determined by means of flow microcalorimetry. The surface thermodynamic potential functions were calculated from the adsorption and calorimetric data to characterize the adsorption exchange processes and the structure of the adsorption layers. The magnitude and sign of the surface charge determine the adsorption mechanism (ionic or molecular) of these ionic surfactants on silica surfaces, The adsorption exchange processes and the adsorption layer structure can be influenced by changing the pH and the electrolyte concentration of the bulk liquid phase. Large exotermic adsorption heats were determined in the case of adsorption of surfactant molecules and micelles explained by the breaking of ,,iceberg water'' structure at the surface.