Aqueous surfaces and interfaces are important in many physical, chemical, and biological processes in our world. The adsorption, dissolution, and reaction of atmospheric gases at the surfaces of atmospheric aerosols and oceanic waters play a key role in the composition of our atmosphere and the sustainability of plant and animal species in land waters. The transport and exchange of ions and solutes across the interface between an aqueous phase and hydrophobic biomolecular assemblies underlies some of the most important processes in living plants and animals. Membrane formation, protein folding, and micelle formation all involve, and are often controlled by, bonding interactions with water molecules at their surfaces. The unique physical, chemical, and biological properties of aqueous surfaces arise from the strong hydrogen bonding that occurs between water molecules and the asymmetry in this otherwise tetrahedral bonding coordination that results from the termination of the bulk water phase. Although there has been increased experimental and theoretical effort in recent years focused on developing a molecular picture of the structure and bonding of water layers to other solid, liquid, and gaseous media, consensus on the details of interfacial bonding has been slow or nonexistent in many areas. The adsorption of ions, surfactants, and solute molecules at these interfaces adds a level of complexity to the Geraldine Richmond holds the Richard M. and Patricia H. Noyes Professor of Chemistry position at the University of Oregon. She received her Ph.D. degree under the mentorship of George Pimentel at the University of California, Berkeley, in 1980. From 1980 to 1985 she was on the faculty at Bryn Mawr College and moved to the University of Oregon in 1985 as an associate professor. Richmond is recognized for her fundamental studies of the structure, dynamics, and bonding characteristics of surfaces and interfaces. Her research group uses a combination of linear and nonlinear optical methods, thermodynamic measurements, and theory to characterize interactions at aqueous surfaces, metal and semiconductor surfaces in contact with liquids and adsorbates, and liquid/liquid interfaces. Richmond has received several recent honors for these studies including the 2002 ACS Spectrochemical Analysis Award, the 2001 Oregon Scientist of the Year, and the 1996 Olin-Garvan Medal of the ACS and has been a Fellow of the American Physical Society since 1993. 2693 Chem. Rev. 2002, 102, 2693−2724

Molecular bonding and interactions at aqueous surfaces as probed by vibrational sum frequency spectroscopy.

Sum frequency generation vibrational spectroscopy (SFG-VS) has been proven to be a uniquely effective spectroscopic technique in the investigation of molecular structure and conformations, as well as the dynamics of molecular interfaces. However, the ability to apply SFG-VS to complex molecular interfaces has been limited by the ability to abstract quantitative information from SFG-VS experiments. In this review, we try to make assessments of the limitations, issues and techniques as well as methodologies in quantitative orientational and spectral analysis with SFG-VS. Based on these assessments, we also try to summarize recent developments in methodologies on quantitative orientational and spectral analysis in SFG-VS, and their applications to detailed analysis of SFG-VS data of various vapour/neat liquid interfaces. A rigorous formulation of the polarization null angle (PNA) method is given for accurate determination of the orientational parameter D = ⟨cos θ ⟩/⟨cos3 θ⟩, and comparison between the PNA me...

Quantitative spectral and orientational analysis in surface sum frequency generation vibrational spectroscopy (SFG-VS)

Recently, sum frequency generation (SFG) vibrational spectroscopy has been developed into a powerful technique to study surfaces of polymer materials. This review summarizes the significant achievements in understanding surface molecular chemical structures of polymer materials obtained by SFG. It reviews in situ detection at the molecular level of surface structures of some common polymers in air, surface segregation of small end groups, polymer surface restructuring in water, and step-wise changed polymer blend surfaces. Studies of surface glass transition and surface structures modified by rubbing, plasma deposition, UV light irradiation, oxygen ion and radical irradiation, and wet etching are also discussed. SFG probing of polymer surfaces provides valuable insights into the relations between polymer surface structures and surface properties, which will assist in the design of polymer materials with desired surface properties.

Studies of polymer surfaces by sum frequency generation vibrational spectroscopy

Quantitative spectral and orientational analysis in surface sum frequency generation vibrational spectroscopy (SFG-VS). Int Rev Phys Chem

A qualitatively new understanding of the nature of ions at the liquid water surface is emerging Traditionally, the characterization of liquid surfaces has been limited to macroscopic experimental techniques such as surface tension and electrostatic potential measurements, wherein the microscopic picture then has to be inferred by applying theoretical models Because the surface tension of electrolyte solutions generally increases with ion concentration, all inorganic ions have been thought to be repelled from the air-water interface, leaving the outermost surface layer essentially devoid of ions This oversimplified picture has recently been challenged: first by chemical kinetics measurements, then by theoretical molecular dynamics simulations using polarizable models, and most recently by new surface sensitive experimental observations Here we present an overview of the nature of the interfacial structure of electrolyte solutions and give a detailed description of the new picture that is emerging

On the nature of ions at the liquid water surface

An experiment was designed to measure sum-frequency generation (SFG) spectra in both reflection and transmission. The result from an octadecyltrichlorosilane monolayer on a substrate reveals that while the resonant spectra in the two geometries both originate from the surface monolayer, the bulk substrate has a significant contribution to the nonresonant background in the transmitted spectra. Study of a polyethylene film also shows that the bulk contribution to the SFG spectra is only significant for the transmission geometry, exhibiting a resonant mode not observable in the reflected spectra.

Evaluation of Surface vs Bulk Contributions in Sum-Frequency Vibrational Spectroscopy Using Reflection and Transmission Geometries†

The structure of the liquid/vapor interface of acetone is investigated theoretically by molecular dynamics simulation and experimentally by infrared-visible sum frequency vibrational spectroscopy. The simulation yields the molecular orientational distribution at the liquid surface. In conjunction with the ab initio calculation, the sum-frequency vibrational spectra for the interface is then computed, allowing a direct comparison to the measured spectra. As the calculation agrees well with the experiment, we can conclude that the molecules at the liquid surface are polar ordered, with one of the methyl groups pointing away from the bulk and the molecular plane perpendicular to the surface. This orientation is similar to that of molecules in the layer planes of crystalline acetone. Together with the appreciable surface tension of acetone, it suggests that the acetone liquid surface has a more ordered structure than the bulk and is more or less crystal-like.

Hermann Held

Papers

Evaluation of Surface vs Bulk Contributions in Sum-Frequency Vibrational Spectroscopy Using Reflection and Transmission Geometries†

Structure of the acetone liquid/vapor interface