Probing the specificity of polyurethane foam as a ‘solid-phase extractant’: Extractability-governing molecular attributes of lipophilic phenolic compounds

Abstract

The long-proposed use of open-cell polyurethane foam (PUF) as a convenient ‘solid-phase extractant’ for aqueous organic compounds has been hindered by a critical lack of understanding on the underlying specificity of its extraction mechanism. In this work, we tasked ourselves to understand the hierarchy of molecular structure, properties, and partitioning characteristics of compounds in PUF and aqueous phase by targeting lipophilic phenolic compounds (LPCs) as a group of primary targets for PUF extraction. Using six structurally related bisphenol analogs as comparative probes, we identified molecular lipophilicity and H-bond acidity as key molecular attributes that governed their extractability by PUF. Molecular modeling study on H-bonding interactions between PUF surrogates, bisphenols, and water molecules elucidated the governing effect of H-bond acidity in the binding affinity of guest molecules onto PUF lone-pair donors. A holistic view must be adopted when assessing the extractability of LPCs with reactive lone-pair donors e.g. bisphenol S which forms multidentate H-bond adducts with water molecules. We validated our theory on two model groups of monofunctional LPCs, alkylphenols and chlorophenols, with the observation that the presence of a second proton-donating moiety dramatically enhanced the extractability of bisphenol molecules. The specificity of PUF rendered it selective towards compounds with correlating molecular attributes against other structural analogs and co-existing matrix organics. For LPCs, the PUF macromolecular structure can be conceptualized as a flexible 'molecular zipper network' that is most affinitive towards nonionic, permeable, and lipophilic guest molecules with multiple reactive proton donors.