Calorimetric study of the adsorption of short-chain nonionic surfactants on silica glass and graphite:: Dimethyldecylamine oxide and octyl monoglucoside

The material and enthalpy balances of adsorption of the nonionic surfactants N,N-dimethyldecylamine-N-oxide (C(10)DAO) and n-octyl beta -D-monoglucoside (C(8)G(1)) from dilute aqueous solutions onto hydrophilic silica glass and hydrophobic graphite (graphitized carbon black) were determined at 298.15 K up to the critical micelle concentration. An automated flow sorption/microcalorimeter system was used for simultaneous measurements of the adsorption isotherm and the enthalpy isotherm of displacement. The formation of the adsorption layer is discussed in terms of the differential molar enthalpy data of adsorption as a function of surface coverage, and the results are related to the aggregated structure of nonionic surfactants at silica/solution and graphite/solution interfaces studied by atomic force microscopy. On silica, a low-density adsorption region (exothermic) is followed by a high-density adsorption region to produce globular surface aggregates of both C(8)G(1) and C(10)DAO. On graphite, the formation of a flat, ordered monolayer (exothermic) is followed by the formation of C(8)G(1) surface hemicylinders or, probably, a flat, less ordered bilayer of C(10)DAO. In either case, the adsorption in the high-density adsorbate region is endothermic, like micelle formation in aqueous bulk solution, as is to be expected on the basis of current models of surface aggregation of nonionic surfactants on hydrophilic and hydrophobic surfaces.