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 surfactant s N,N-dimethyldecylamine-N-oxide (C(10)DAO) and n-octyl beta -D-monoglucosi de (C(8)G(1)) from dilute aqueous solutions onto hydrophilic silica glass a nd hydrophobic graphite (graphitized carbon black) were determined at 298.1 5 K up to the critical micelle concentration. An automated flow sorption/mi crocalorimeter system was used for simultaneous measurements of the adsorpt ion isotherm and the enthalpy isotherm of displacement. The formation of th e 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/solut ion 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 monolay er (exothermic) is followed by the formation of C(8)G(1) surface hemicylind ers 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 m icelle formation in aqueous bulk solution, as is to be expected on the basi s of current models of surface aggregation of nonionic surfactants on hydro philic and hydrophobic surfaces.