Novel Anion Exchangers with Spatially Distant Trimethylammonium Groups in Linear and Branched Hydrophilic Functional Layers

Abstract

Novel polystyrene–divinylbenzene (PS-DVB) based anion exchangers having branched structure of ion exchange functional groups are prepared and characterised. Two proposed synthetic methods include acylation of PS-DVB microspherical particles followed by reductive amination with dimethylamine or methylamine, which results in obtaining tertiary or secondary amino groups on the surface, respectively. Further alkylation of amino groups was provided by reaction with either (3-chloro-2-hydroxypropyl)trimethylammonium chloride or glycidyltrimethylammonium chloride, which allowed the simultaneous insertion of trimethylammonium functional group and hydrophilic spacer into the structure. The other considered option is reaction with diglycidyl ethers, which requires further amination of terminal epoxide rings with trimethylamine. The proposed approaches resulted in preparation of the ion exchangers with extended linear or branched anion exchange structure with trimethylammonium groups located at a distance from the surface and connected via hydrophilic linkers. The ion exchange selectivity and separation efficiency of obtained anion exchangers are studied for the model mixture of inorganic anions (F⁻, HCOO⁻, Cl⁻, NO₂ ⁻, Br⁻, NO₃ ⁻, H${\text{PO}}_{ 4}^{2 - }$ and ${\text{SO}}_{4}^{ 2- }$) using hydroxide eluents. The adsorbents with branched ion exchange layer having three positively charged sites in the structure of attached groups demonstrate superior column efficiencies and better peak symmetry as compared with anion exchanger having linear structure with two positively charged sites in the chain. In case of branched anion exchangers the calculated values of column efficiencies for polarisable NO₃ ⁻ and Br⁻ are 44,000 and 50,000 N/m, respectively, and significantly higher than 7,000 and 8,000 N/m obtained for anion exchanger with linear structure of the ion exchange layer.