Sorption of neutral components in ion-exchange resins. 1. Effect of cross-link density and counterion on selective sorption of water-ethanol mixturesin sulfonated PS-DVB resins

Experimental data for the distribution of water-ethanol mixtures between a solid phase and a liquid phase at 298 K are reported. The solid phases stud ied were gel-type sulfonated poly(styrene-co-divinylbenzene) resins of diff erent degrees of cross-linking (4-8% DVB) and carrying different, counterio ns (Na+, Ca2+, and La3+). The shear moduli of the resin beads were also mea sured to characterize their elastic properties. All resins absorb water sel ectively, and the selectivity increases with increasing cross-link density. The low selectivity of the less densely cross-linked resins is shown to be mainly due to the mutual interaction of the solvents in the resin phase re sulting in a pronounced maximum in the ethanol sorption isotherms. The infl uence of the counterion on the selectivity is more complex. At high water c ontents, the water selectivity of the Na+ resin is higher than that of the Ca2+ and La3+ resins, whereas the selectivities are approximately equal at low water contents. The elastic properties of the resin beads remain unchan ged from pure water to water mole fractions of around 0.4, where a sharp ri se in the shear modulus occurs. The data are analyzed by means of a model b ased on the UNIQUAC equation and the affine network theory of elasticity. T he effect of cross-link density an the selectivity and solvent content of t he resina can be explained satisfactorily with the model. However, the calc ulated and experimental sorption isotherms for the La3+ resins deviate appr eciably at low external water contents. The discrepancies are discussed on the basis of the elastic properties of the resins and the specific solvatio n interactions.