ADSORPTION FROM AQUEOUS-SOLUTIONS BASED ON A COMBINATION OF HYDROGEN-BONDING AND HYDROPHOBIC INTERACTIONS

By limiting the number of adsorptive mechanisms, we believe it is possible to develop highly selective sorbents. In this work, our goal was to develop a sorbent that was capable of selectively adsorbing solutes from water through the formation of a hydrogen bond. When water was the solvent, a polycarboxylic ester resin was the sorbent, and a homologous series of phenylalkanols were used as solutes, it appeared that hydrophobic interactions were a predominant mechanism for adsorption. This conclusion is based on the observation that the affinity for adsorption increased with the number of methylene groups of the solute and that the free energy change for adsorption per methylene group was -0.83RT. This value is similar to free energy changes per methylene group observed for various phenomena that result from hydrophobic interactions. In contrast, when hexane was used as the solvent, we observed that the affinity for adsorption of this homologous series of phenylalkanols was independent of the the number of methylene groups. Thus it appears that the mechanism of adsorption onto this polycarboxylic ester sorbent varies depending on the solvent. With hexane as the solvent, we believe adsorption results from the formation of a hydrogen bond between the hydroxyl group of the solute and the carbonyl group of the sorbent. To exploit the specificity of the hydrogen bond for the adsorption of solutes from aqueous solutions, we developed a modified sorbent in which the hydrogen bonding site of the sorbent is retained in a nonpolar environment. For this modified sorbent, we filled the pores of the sorbent with hexane. Solutes that adsorb onto this modified sorbent must first partition from the aqueous phase into the hexane pore-phase, and then the solute must adsorb from the hexane pore-phase onto the hydrogen bonding site of the sorbent. Our results demonstrate that adsorption onto this modified sorbent can be quantitatively described by these individual steps. Thus this modified sorbent is capable of adsorbing solutes from aqueous solution through a combination of hydrophobic interactions (i.e. the water-hexane partitioning step) and hydrogen bonding (i.e. solute adsorption from the hexane pore-phase).