Interfacial Supramolecular Structures of Amphiphilic Receptors Drive Aqueous Phosphate Recognition

Phosphate remediation is important for preventing eutrophication in fresh waters and maintaining water quality. One approach for phosphate removal involves the utilization of molecular receptors. However, our understanding of anion recognition in aqueous solution and at aqueous interfaces is underdeveloped, and the rational design of surface-immobilized receptors is still largely unexplored. Herein, we evaluated the driving forces controlling phosphate binding to elementary amphiphilic receptors anchored at air-water interfaces. We designed biologically inspired receptors with neutral thiourea, positively charged guanidinium, and thiouronium units that all formed Langmuir monolayers. Phosphate binding was quantitatively examined using surface pressure-area isotherms and infrared reflection-absorption spectroscopy (IRRAS). The receptors within this homologous series differ in functional group, charge, and number of alkyl chains to help distinguish the fundamental components influencing anion recognition at aqueous interfaces. The two charged receptors bearing two alkyl chains each displayed strong phosphate affinities and 10³- and 10¹-fold anti-Hofmeister selectivity over chloride, respectively. Neutral thiourea and the single-chain guanidinium receptor did not bind phosphate, revealing the importance of electrostatic interactions and supramolecular organization. Consistently, charge screening at high ionic strength weakens binding. Spectroscopic results confirmed phosphate binding to the double alkyl chain guanidinium receptor, whereas surface pressure isotherm results alone showed a minimal change, thus emphasizing the importance of interfacial spectroscopy. We found that the binding site identity, charged interface created by the electrical double layer, and supramolecular superstructure all affect interfacial binding. These detailed insights into phosphate recognition at aqueous interfaces provide a foundation to develop efficient receptors for phosphate capture.