Thermodynamics of adsorption of O-n-dodecylpentaethylene glycol and O-n-dodecyloctaethylene glycol from aqueous solutions on to graphitised carbon

Abstract

Adsorption isotherms and enthalpies of adsorption are reported for two dodecylpolyethylene glycol non-ionic surfactants (C₁₂E₅ and C₁₂E₈) from aqueous solution on to graphitised carbon. Adsorption at low coverages involved both the hydrophobic and hydrophilic parts of the surfactant molecules lying flat on the carbon surface. Subsequent displacement of adsorbed oxyethylene segments by incoming dodecyl groups was accompanied by dehydration of the hydrophilic groups. Segments of adsorbed alkyl groups were lifted from the surface as the adsorbed layer approached the maximum attainable surface excess concentration. At this stage the differential enthalpy of adsorption of C₁₂E₈ on carbon was similar to a literature value for the standard enthalpy of adsorption of C₁₂E₈ at the water/air interface.