Thermodynamics of multilayer adsorption of aqueous butanol solution onto Printex and graphitised Printex carbon blacks

Abstract

The surface excess isotherms of the adsorption of n-butanol from water onto Printex and graphitised Printex carbon blacks are sigmoid in shape, indicating that multilayer formation takes place near the solubility limit. Flow microcalorimetric studies showed that the displacement of water by butanol on these non-porous carbon surfaces is an exothermic process up to monolayer formation, after which it becomes more and more endothermic, although the enthalpy of adsorption still remains negative. The free enthalpy decreases during the displacement process, while the entropy of displacement is predominantly an increasing function of the butanol concentration. This peculiar behaviour is connected with the hydrophobic dehydration of the alcohol molecules as they become adsorbed. The mechanism of mono- and multi-layer formation and the effect ot the surface heterogeneity on the surface layer composition are discussed.