Influence of a base electrolyte on the adsorption behavior of an anionic surfactant as observed by second harmonic generation

The adsorption process of sodium-[4-(4'-trifluormethylphenylazo)phenyl-1-ox yethylsulfonate] (NaTAPOS) at the water/1,2-dichlorethane interface was inv estigated by second harmonic generation and ellipsometry. The combination o f both techniques yields the surface excess and the orientation of the amph iphiles within the adsorption layer. These data were used for an investigat ion of equilibrium properties of the system. The average tilt angle of the chromophore within the adsorption layer remains constant in the whole conce ntration range. The mean tilt angle was found to be 39 +/- 3 degrees and th e area per molecule at maximum surface coverage is 45.2 +/- 6 Angstrom(2). The present study focuses on the impact of a base electrolyte on the adsorp tion process. Neither the limiting area per molecule nor the orientational order was changed by base electrolyte dissolved in the aqueous phase. The m aximum surface coverage with and without base electrolyte is achieved at th e same bulk concentration and is within the error limit identical; however, at lower concentrations a significantly higher surface excess is observed in the absence of base electrolyte. This is counterintuitive since one migh t expect an electrostatic shielding of the prevailing interaction. The adso rption isotherms with and in absence of base electrolyte can be described b y Frumkin's equation of state. This model accounts for regular surface beha vior and takes into account the lateral interaction of the absorbed species . The model fit yields two quantities: the free enthalpy of adsorption and the interaction parameter. Only slight changes of the free enthalpy of adso rption were observed by the presence of base electrolyte; however, a signif icant increase of the corresponding interaction parameter was observed. The se findings are in contradiction to the Debye-Huckel theory and are explain ed by a model in analogy to the polyelectrolyte layer-by-layer adsorption.