Interfacial chemistry can be found in myriad environments of scientific significance including biological membranes, ocean and atmospheric chemistry, and electrochemistry. The interface represents a unique coordination environment with properties distinct from those of the bulk. In the image charge picture, derived from early surface tension measurements, ions have traditionally been considered to be depleted from the liquid/vapor interface. However, recent works have contradicted this simple picture, finding some ions exhibit enhanced surface concentration, with their respective surface affinities often following the well-known Hofmeister series.1 Of particular importance is that of the air-water interface as it is involved in many atmospheric reactions. However, it is difficult to probe the interface because the interfacial signal tends to be overwhelmed by the bulk signal, and thus not readily discernible. Fortunately, with the advent of ultrafast laser spectroscopy, we are now able to employ second order nonlinear optical techniques, Sum Frequency Generation (SFG) and Second Harmonic Generation (SHG), to probe ions and molecules at aqueous interfaces. Under the electric dipole approximation, due to necessary symmetry constraints, these techniques are interface specific and thus give no signal from bulk. Of particular importance is that of the air-water interface as it is involved in many atmospheric reactions. However, it is difficult to probe the interface because the interfacial signal tends to be overwhelmed by the bulk signal, and thus not readily discernible. Fortunately, with the advent of ultrafast laser spectroscopy, we are now able to employ second order nonlinear optical techniques, Sum Frequency Generation (SFG) and Second Harmonic Generation (SHG), to probe ions and molecules at aqueous interfaces. Under the electric dipole approximation, due to necessary symmetry constraints, these techniques are interface specific and thus give no signal from bulk. 

To achieve a better understanding of selective ion adsorption, we have developed deep-UV resonant electronic second harmonic generation, which exploits the resonant charge-transfer-to-solvent transitions to directly probe ions at the interface.2 Using this technique, we have measured the thermodynamics of ion adsorption at aqueous interfaces and found that, in agreement with predictions, certain ions such as iodide and thiocyanate are indeed enhanced at the air/water interface.3
Collaborating with theorists, it was determined that the driving force for this surface activity is the enthalpic gain of moving weakly-coordinated water molecules from the interface and hydration shell into the bulk.4 Recently, we have also measured ion adsorption thermodynamics for the water/graphene interface.5 Current work is focused on probing the adsorption thermodynamics at the metal/water interface, with the end goal of re-writing the general description of how ions behave at interfaces.

Extending the multiplex homodyne broadband electronic sum frequency generation (ESFG) technique in the visible wavelengths developed by Tahara et al.6 into the deep ultraviolet (below ~250 nm), we recently developed broadband Deep-UV ESFG spectroscopy.7 By using a white light continuum as one of the input pulses, we can obtain a broadband interfacial electronic spectrum in a single laser shot, allowing us to analyze number, shape, and position of electronic resonances.
Recently, using our broadband DUV-ESFG setup, the CTTS spectrum of iodide7,8 and thiocyanate,9 and a molecular transition of nitrite10 at the air-water interface were measured. Our current work focuses on measuring the spectra of other biologically and/or atmospherically relevant anions and molecules, expanding the detection range, and improving the S/N of the system.

Characterization of buried interfaces, such as those involved in water purification membranes and catalytic nanoparticles is essential to understanding the behavior of these complex systems.11 Second Harmonic Scattering (SHS) enables study of these interfaces in colloidal solutions. For example, SHS can be used to determine the adsorption thermodynamics of micropollutants at the surface of polystyrene beads (PSBs) in aqueous solutions. Information about the orientation of the adsorbing molecule can be obtained by recording the scattering pattern of the solution.12 This information can help elucidate the differences in behavior of the micropollutants at a membrane surface relative to the bulk.
By employing a competitive adsorption methodology, we can extend our study of micropollutants to non-resonant molecules.13 Recently in our group, we determined the adsorption thermodynamics of several non-resonant molecules by fitting the signal decay to a competitive Langmuir isotherm model.14 This proof-of-concept experiment can be extended to more practically relevant systems by changing the surface of the colloidal media. Specifically, functionalizing silica beads can allow for the characterization of water purification membranes as well as catalytic nanoparticle systems.