Electrokinetic studies of monodisperse hematite particles: effects of inorganic electrolytes and amino acids

Abstract

In order to examine the possibilities of using monodisperse (hydrous) oxide particles as model colloids, this paper studies the effect of simple inorganic electrolytes (1-1, 2-1 and 3-1) and of five different amino acids (alanine, glycine, lysine, threonine and glutamic acid) on the electrophoretic mobility μ e of monodisperse hematite ( α- Fe 2 O 3) particles. Maxima were found in the mobility-electrolyte concentration curves, unexpected for presumably indifferent electrolytes. Since no anion or cation adsorption seems to be present, the possible occurrence of high surface conductances is assumed to explain the results. Concerning the interactions of the amino acids with hematite particles, our data indicate that pH appears to be a determinant factor in the way such interactions occur. Measurements of the adsorption of amino acids glycine, lysine and glutamic acid onto hematite demonstrate that glycine adsorbs to a much lower extent than lysine or glutamic acid. The effect of pH on adsorption is very significant, a maximum in adsorption being found at a pH close to the isoelectric point of each amino acid. These results show that electrostatic interactions alone cannot explain completely the adsorption process: other effects, probably involving acid-base interactions must also be considered. In any case, our data show a reasonable agreement between electrophoretic mobility and adsorption experiments.