VARIOUS SHAPES OF THE ENTHALPY CURVE OBSERVED IN ADSORPTION OF A CATIONIC SURFACTANT FROM AQUEOUS-SOLUTIONS ON CRYSTALLINE AND AMORPHOUS SILICA SURFACES

Adsorption of a cationic surfactant, benzyldimethyldodecylammonium bro mide (BDDAB), from aqueous solutions on negatively charged surfaces of precipitated silica S91-16 and crystalline quartz at 298 K and free p H has been studied using adsorption microcalorimetry. Differential mol ar enthalpy of displacement was correlated with the experimental adsor ption isotherm and plotted as a function of the amount adsorbed. The g eneral (qualitative) shape of the enthalpy curve appears to be depende nt upon the crystalline structure of silica (amorphous-crystalline). T he experimental calorimetric and adsorption data for BDDAB were compar ed with those for benzyltrimethylammonium bromide (BTMAB), a molecule containing no alkyl chain, in order to demonstrate an ion-exchange mec hanism of individual surfactant adsorption onto silica S91-16 at low s urface coverages.