Sorption of ethylene glycol monoethyl ether (EGME) on homoionic montmorillonites

A B S T R A C T: Sorption isotherms of ethylene glycol monoethyl ether (EGME) on homoionic Na-, K-, Mg- and Ca-montmorillonites at 25~ were determined. The extent of sorption was found to depend on the exchangeable cations, being larger for the bivalent ionic forms. Uptake of EGME was compared with the retention value obtained by evacuation of the sample wetted with EGME and also interpreted from XRD data. The results indicate that the montmorillonite-EGME complex formed by sorption at the saturation vapour pressure is stable and less ambiguous than after wetting by EGME. Montmorillonites are not suitable reference materials for surface area measurements. Dyal and Hendricks (1950) introduced a method for estimating surface areas of soil species based on wetting the sample with ethylene glycol, equilibrating, and then removing the excess liquid by evacuation. The area was computed from the mass of retained liquid on the assumption that a monolayer was formed. Determinations with ethylene glycol proved very time consuming and Carter et al. (1965) suggested the use of a more volatile liquid, ethylene glycol monoethyl ether (EGME), also called 2-ethoxyethanol. They calibrated the method by experiments with montmorillonite, but subsequent calibration measurements with montmorillonite-EGME complexes made by other workers (Novak & Cicel, 1970; Eltantawy & Arnold, 1973) were contradictory. This has led us to study these complexes by sorption experiments, an approach which does not seem to have been applied previously. Retention experiments have also been made for comparison. MATERIALS The montmorillonite samples were prepared from a Wyoming bentonite supplied by BDH (prod. 26022). The crude material was purified by soaking in mild HCI and H202 for two hours, centrifuged and then washed with distilled water. A < 2 t.tm suspension was produced by sedimentation/decantation and this divided into four aliquots, which were converted to different cationic forms by repeated treatment with the appropriate metal chloride solutions. The final products were washed ~20 times with distilled water until the electrical conductivity of the wash liquid indicated removal of chloride. The CEC of the Camontmorillonite at pH 7.0 was 0.76 mEq/g. The cation-exchange samples were preserved in dried form and ground in an agate mortar before use. The EGME used was Merck analytical grade and was further purified by distillation using a long column.