Ion-exchange equilibria of aliphatic amino acid cations on a liquid sulphonic ion exchanger: Comparison with ion-exchange resins

Equilibria of ion exchange of neutral aliphatic amino acid cations (A(+) = Gly(+), Ala(+), Val(+), Leu(+)) and H+ have been studied on a liquid sulfon ic type ion exchanger - dinonylnaphthalene sulphonic acid dissolved in hept ane. It has been established that the difference in selectivity of extracti on of these cations is much greater than on sulphostyrene resins, which is a favourable precondition for their selective extraction. The selectivity c oefficients of the H+-A(+) exchange have a region of a low dependence on th e degree of loading of liquid ion exchanger ((X) over bar = 0-0.5) and a re gion of a sharp decrease ((X) over bar > 0.5). This phenomenon is explained as a result of formation of very stable complexes of the amino acid cation s and the anion of dinonylnaphthalene sulphonic acid containing one unsubst ituted proton per 3-6 A(+) cations. The LR spectra confirm this assumption. The dependence of the selectivity coefficient on the degree of H+-A(+) exc hange has been described using the previously developed mathematical model of ion-exchange processes. The model is consistent with the experimental da ta in an assumption that four nearest neighbours of an exchange centre in t he associates of the liquid ion exchanger influence the energy of ion excha nge on a given exchange site. (C) 1998 Elsevier Science B.V. All rights res erved.