Comparative study of amino acid adsorption on bare and octadecyl silica from water using high-performance liquid chromatography

Equilibrium constants (K) for amino acid adsorption on bare and octadecyl silica from water and the corresponding free energy (Delta G degrees), enthalpy (Delta H degrees) and entropy (Delta S degrees) changes were calculated from the retention values measured by means of high-performance liquid chromatography in the temperature range 20-60 degrees C. On bare silica gel, for all amino acids Delta G degrees values were positive and K was < I, thus providing evidence of very weak adsorption. Regression analysis of the Van't Hoff plots was used to derive Delta H degrees and Delta S degrees values for the adsorption process. For the case of octadecyl silica, some amino acids (Val, Nva, Leu, Ile, Nle, Tyr, Phe, Trp and Met) had negative Delta G degrees values and K was > 1. The K values decreased and Delta G degrees values correspondingly increased with temperature. Regression analysis of the Van't Hoff plots was used to derive Delta H degrees and Delta S degrees values for the adsorption process. All these values, for both silicas, were negative which proves that the adsorption process is enthalpy-driven and generally unfavorable. Plotting Delta H degrees vs. Delta S degrees and Delta G degrees gave nearly linear general trends indicative of compensation behavior, when all the amino acids are adsorbed according to essentially the same mechanism. Based on analysis of the graphical inter-relations In K vs. isoelectric points and In K vs. hydrophobicity coefficients, it was concluded that adsorption on bare silica is governed by ionic interactions, whereas both hydrophobic interactions with octadecyl groups and ionic interactions with residual silanol groups contribute in the case of octadecyl silica.