Calorimetric evidence of the formation of half-cylindrical aggregates of a cationic surfactant at the graphite/water interface

The adsorption of a cationic surfactant, dodecyltrimethylammonium bromide (C(12)TAB), from aqueous solutions on graphitized carbon black has been investigated from 288.15 to 318.15 K using a versatile, automated measuring system, which has been designed for the simultaneous measurement of the adsorption isotherm and the calorimetric enthalpies of displacement at the solid/liquid interface. At low concentrations, the surfactant molecules form a flat monolayer on the graphite surface. The enthalpy of monolayer formation is apparently not (or only slightly) dependent on the temperature and is independent of the surface coverage (-61 kJ.mol(-1)). The adsorption proceeds further as the concentration in the bulk solution is increased to the cmc. In this region, the enthalpy of displacement is again nearly independent of the surface coverage but depends strongly on the temperature: -10.7, -17.5, and -29 kJ.mol(-1) at 288.15, 298.15, and 318.15 K, respectively. The mechanism of the adsorption and the morphology of the high-density adsorbate structure have been analyzed in terms of the classical reorientation model (reorientation of the adsorbed surfactant molecules from a horizontal to a vertical orientation, accompanied by further adsorption from the bulk solution) and in terms of a recent concept of interfacial aggregation (formation of half-cylindrical surface micelles templated by an epitaxially bound surfactant monolayer). The results of the thermodynamic analysis strongly support the formation of C(12)TAB half-cylinders at the graphite/water interface.