Physical properties of homoionic montmorillonite and kaolinite complexed with amino acids and peptides

Abstract

X-ray diffractometry was used to investigate whether various amino acids and peptides expanded montmorillonite (M) homoionic to different cations, and the electrophoretic mobility (EPM) was measured to determine whether the binding of these amino acids and peptides influenced the net surface charge of M and kaolinite (K). Neither aspartic acid, adsorbed at equilibrium at concentrations of 20–250 nM mg −1 M homoionic to Ca or Zn, nor cysteine, at 50–830 nM mg −1 M homoionic to Zn or Al, caused expansion, whereas proline, at 90–870 nM mg −1 clay, caused expansion of M homoionic to Ca or Zn, and both proline and arginine, at 60–1150 nM mg −1 clay, caused expansion of M homoionic to H or Al. The extent of expansion caused by proline and arginine was related to the concentration of the amino acids and the exchangeable cation on the homoionic M. Only M-A1 complexed with arginine remained expanded after extensive washing of the clay-amino acid complexes. The expansion of M was apparently dependent on both the type of exchangeable cation on the clay and the characteristics of the amino acid (i.e. the functional groups, dielectric constant increment, dipole moment and concentration). M homoionic to H, Na, Zn, Al or La was not expanded by a series of di- to tetrapeptides bound on these clays and none of the homoionic K-amino acid or peptide complexes was expanded. The EPM of the homoionic M and K and of the corresponding clay-amino acid or peptide complexes was identical, indicating that the concentration of the amino acids or peptides bound was not sufficient to cover enough of the surface of the clays to alter significantly their net charge. The lack of differences in the bulk pH of the clays after binding with amino acids or peptides (with the exception of complexes with arginine) also indicated limited coverage of the clays by these organics.