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.

Due to the growing world population, there is an ever-increasing need to develop better receptors to recover and recycle phosphate for use in agricultural processes. This need is driven by agricultural demand and environmental concerns because phosphate eutrophication has a damaging effect on fresh water supplies by fueling algal blooms. The air/water interface provides a unique region with a dielectric constant (ε) that diminishes from high in bulk water (ε = 80) to significantly lower (e.g., ε < 40) near the monolayer surface to potentially enhance affinities during molecular recognition. The work presented here uses a model system of phosphate binding to an amino acid, arginine, and utilizes the interfacial properties of the phospholipid monolayer, 1,2dipalmitoyl-sn-glycero-3-phosphatidic acid, as the phosphate source to quantify binding. Employing arginine as a probe molecule allows for the evaluation of its guanidinium moiety for phosphate chelation. Surface pressure−area isotherms from Langmuir monolayer studies, and the corresponding infrared reflection absorption spectroscopy, were used along with Brewster angle microscopy for in situ determination of molecular binding interactions and the surface binding constants of the phosphate−guanidinium complex, which are shown here to be greater than 10 3 M −1 . The binding constant in bulk solution, determined by NMR titrations of phosphate and arginine, is determined to be on the order of 0.1 M −1 . The greater than 10 000-fold increase from the bulk aqueous solution to the air/water interface reveals that the interface provides a region of enhanced binding affinity.

The selectivities and driving forces governing phosphate recognition by charged receptors at prevalent aqueous interfaces is unexplored relative to the many studies in homogeneous solutions. Here we report on electrostatic binding versus hydrogen-bond-assisted electrostatic binding of phosphate (H2PO4 –) for two important receptor classes in the unique microenvironment of the air–water interface. We find that the methylated ammonium receptor (U-Ammo + ) is dominated by electrostatic binding to phosphate anions and fails to be selective for phosphate binding over chloride, whereas the highly phosphate-selective guanidinium receptor (U-Guan + ) provides synergistic hydrogen-bonding and electrostatic interactions. Apparent binding constants were evaluated in situ for U-Ammo + and U-Guan + using temperature-controlled infrared reflection–absorption spectroscopy with Langmuir-type fitting. Thermodynamic quantities showed enthalpically driven binding affinities of U-Guan + and U-Ammo + receptors (ΔH°b = −71 (±9) kJ/mol and ΔH°b = −155 (±13) kJ/mol, respectively). U-Guan + revealed a nearly fourfold smaller entropic barrier to binding (ΔS°b = −132 (±34) J/mol K) than the U-Ammo + receptor (ΔS°b = −440 (±45) J/mol K), attributed to hydration differences. The larger entropic penalty for the U-Ammo + receptor is correlated with a molecular expansion shown in surface pressure–area isotherms, whereas the smaller entropic penalty of the U-Guan + receptor conversely correlated with no expansion. The U-Guan + receptor also revealed anti-Hofmeister selectivity for phosphate over chloride, while the non-hydrogen-bonding U-Ammo + receptor followed Hofmeister selectivity. Our results indicate that hydrogen bonding is an integral chemical design element for achieving anti-Hofmeister selectivity for phosphate.

There is a critical need for receptors that are designed to enhance anion binding selectivity at aqueous interfaces in light of the growing importance of separation technologies for environmental sustainability. Here, we conducted the first study of anion binding selectivity across a series of prevalent inorganic oxoanions and halides that bind to a positively charged guanidinium receptor anchored to an aqueous interface. Vibrational sum frequency generation spectroscopy and infrared reflection absorption spectroscopy studies at the water–air interface reveal that the guanidinium receptor binds to an oxoanion series in the order SO4 2– > H2PO4 – > NO3 – > NO2 – while harboring very weak interactions with the halides in the order I– > Cl– ≈ Br–. In spite of large dehydration penalties for sulfate and phosphate, the more weakly hydrated guanidinium receptor was selective for these oxoanions in contradiction to predictions made from ion partitioning alone, like the Hofmeister series and Collins’s rules. Instead, sulfate binding is likely favored by the suppression of dielectric screening at the interface that consequently boosts Coulombic attractions, and thus helps offset the costs of anion dehydration. Geometric factors also favor the oxoanions. Furthermore, the unique placement of iodide in our halide series ahead of the stronger hydrogen-bond acceptors (Cl–, Br–) suggests that the binding interaction also depends upon single-ion surface partitioning from bulk water to the interface. Knowledge of the anion binding preferences displayed by a guanidinium receptor sheds light on the receptor architectures needed within designer interfaces to control selectivity.

The composition and lifetime of sea spray aerosols are driven by the molecular and biological complexity of the air−seawater interface. We explore in situ the surface properties of marine algal bloom diatom monocultures by utilizing surface techniques of Brewster angle microscopy (BAM) imaging, vibrational sum-frequency generation (SFG) spectroscopy, and infrared reflection−absorption spectroscopy (IRRAS). Over the course of the bloom, the marine algae produce surface-active biogenic molecules that temporally partition to the topmost interfacial layers and are selectively probed through surface imaging and spectroscopic measurements. BAM images show morphological structural changes and heterogeneity in the interfacial films with increasing density of surface-active biogenic molecules. Film thickness calculations quantified the average surface thickness of a productive bloom over time. The image results reveal an ∼5 nm thick surface region in the late stages of the bloom, which correlates with typical sea surface nanolayer thicknesses. Our surface-specific SFG spectroscopy results show significant diminishing in the intensity of the dangling OH bond of surface water molecules consistent with organic molecules partitioning and replacing water at the air−seawater interface as the algal bloom progresses. Interestingly, we observe a new broad band appear between 3500 and 3600 cm −1 in the late stages of the bloom that is attributed to weak hydrogen bonding interactions of water to the surface-active biogenic matter. IRRAS confirms the presence of organic molecules at the surface as we observe an increasing intensity of vibrational alkyl modes and the appearance of a proteinaceous amide band over time. Our work shows the often overlooked but vast potential of tracking changes in the interfacial regime of small-scale laboratory marine algal blooms. By coupling surface imaging and vibrational spectroscopies to complex, time-evolving, marine-relevant systems, we provide additional insight into unraveling the temporal complexity of sea spray aerosol compositions.