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

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.

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.

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.