Preparation and characterisation of anion exchangers with dihydroxy-containing alkyl substitutes in the quaternary ammonium functional groups

Novel poly(styrene-divinylbenzene) (PS-DVB) based anion exchangers having one and two (2,3-dihydroxypropyl) substitutes in the quaternary ammonium functional groups are prepared and characterised by ion chromatography (IC). The introduction of bulky and hydrophilic substitutes to the anion-exchange groups allows the elimination of non-ionic interactions between the polarisable anions and the aromatic rings from the matrix and the improvement of separation selectivity as compared with the traditional trimethylammonium (TMA) functionalised ion exchangers. The synthesis of the ion exchangers includes acylation of PS-DVB particles with acetic anhydride followed by reductive amination either with methylamine hydrochloride or dimethylamine and further alkylation with oxiranes under varied conditions. The ion exchange selectivity and separation efficiency of nine adsorbents having different structure of bonded groups, ion exchange capacity (9–98 μequiv. g⁻¹) or particle size is studied for model mixture of inorganic anions (F⁻, Cl⁻, NO₂⁻, Br⁻, NO₃⁻, HPO₄²⁻ and SO₄²⁻) using carbonate and hydroxide eluents. The adsorbents with more hydrophilic substitutes provided superior columns efficiencies and better peak symmetry as compared with analogues having hydrophobic functional groups. The calculated values of column efficiencies for polarisable NO₃⁻ and HPO₄²⁻ are 18,500 and 29,000 N/m, respectively, for anion-exchanger, having N-methyl-N′,N″-di-(2,3-dihydroxypropyl)ammonium groups which is significantly higher than 1800 and 12,000 N/m obtained for these anions with anion exchanger bearing TMA functional groups.