Enthalpic Effects in Competitive Polymer Adsorption: Adsorption Isotope Effect and Chain End Effect

Competitive adsorption was studied of deuteriopolystyrene (PS), protio PS, and carboxylic acid terminated PS, from cyclohexane solutions (0.05 or 0.1 mg mL^-1) onto a single surface of silicon oxide at 30.0°C. Surface excess mass of the polymer was measured by infrared spectroscopy in the mode of attenuated total reflection. In competitive adsorption using a family of polymer fractions, preferential adsorption was observed of the deuterio (versus protio) isotopes and of the carboxylic acid terminated (versus hydrogen-terminated) chains. From considerations of relative enthalpic and entropic tendencies toward adsorption, it was concluded that (a) the magnitude of the differential sticking enthalpy (deuterio versus protio isotope) was ≈80 J mol^-1 (0.03 kT) per polystyrene repeat unit and (b) the magnitude of the differential sticking enthalpy (carboxylic acid terminated versus hydrogen-terminated chain) was ≈1.6 × 10⁴ J mol^-1 (6.4 kT) per carboxylic acid end group. The implications of these findings for competitive adsorption when samples contain traces of polar impurities (for example, of oxidation impurities) are discussed quantitatively.