Analysis of ionic strength effects on the adsorption of simple amino acids

Abstract

Hypothesis: Amino acid adsorption by metals and/or oxide surfaces is important in many biomedical and industrial processes, however limited information exists discussing ionic strength influences on the mechanism of adsorption. A comparison of pure water solution and added 1:1 electrolyte should highlight the effects of electrolyte on amount adsorbed. ATR spectroscopy of the adsorbed phase should demonstrate the effects on the mechanism of adsorption.

Experiments: Low solution concentration adsorption isotherms for glycine, lysine and glutamic acid on Aerosil 200 silica were generated in pure water and 10 and 100 mmol/L sodium chloride solutions. A systematic study of the adsorption isotherms as well as adsorbent and adsorptive solution properties was performed. ATR-IR spectroscopy was used to analyse the adsorbed phase in solution.

Findings: Glycine adsorbs primarily through electrostatic interactions; lysine also adsorbs through electrostatic interactions in a parallel conformation with the surface. Glutamic acid adsorbs via hydrogen bonding and forms intermolecular clusters around an adsorbed nucleus. ATR-IR spectrum deconvolution shows a peak shift for glycine and lysine associated with the δ(ad)NH3(+) vibration, indicating interaction through the amino moieties. Amount adsorbed was decreased significantly by the addition of 10 mmol/L sodium chloride and completely inhibited by the addition of 100 mmol/L sodium chloride.

Keywords: ATR-IR spectroscopy; Adsorption; Amino acids; Deconvolution; Glutamic acid; Glycine; Ionic strength; Lysine; Silica.