Spectroscopy at Liquid Interfaces

Spectroscopic studies of liquid interfaces provide important information about the composition and structure of the interfacial region. Early work was mainly carried out at the solid liquid interface and involved techniques such as neutron and X-ray diffraction, and reflection FTIR spectroscopy. More recently, powerful techniques have been developed to study the liquid liquid and liquid gas interfaces. These studies are especially important because of their relevance to biological systems such as cell membranes. The techniques described here are second-harmonic generation (SHG) and vibrational sum frequency spectroscopy (VSFS). They are both second-order non-linear optical techniques which are specific to the interfacial region. Since the second-order effects involve signals of low intensity, they rely on high-power lasers. 

When electromagnetic radiation interacts with a medium it induces a polarization P which is proportional to the electrical field E due to the radiation  

The proportionality constant is called the linear electrical susceptibility. It is related in a simple way to the permittivity of the medium (see equation (4.5.3)). Thus, 

When a molecule in the gas phase experiences a very large electromagnetic field, the resulting polarization in no longer a linear function of the field. In this case, the polarization is given by 

The coefficient Pp is called the second-order non-linear polarizability or the hyperpolarizability. The corresponding equation in a condensed medium is 

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Benjamin, L, Static and dynamic electronic spectroscopy at liquid interfaces, Chem Rev,No. 106, (2006) p. 1212. 

I. Benjamin, Chem. Rev., 106, 1212 (2006). Static and Dynamic Electronic Spectroscopy at Liquid Interfaces. 

The use of neutron reflectivity at liquid interfaces, which is a method sensitive to both surface roughness and surfactant layer thickness, was reviewed with the examples of polydimethylsiloxane-surfactant layers.633 Sum-frequency generation (SFG) vibrational spectroscopy was applied to study surface restructuring behavior of PDMS in water in an attempt to understand antifouling properties of silicones.6 ... 

Nonlinear optical spectroscopies such as second harmonic generation (SHG) and sum frequency generation (SFG) are finding increasing use in probing species at interfaces (e.g., Eisenthal, 1996). For example, SHG was used by Donaldson et al. (1995) to detect a surface-bound S02 species, and SFG has been applied to elucidate the structure of dimethyl sulfoxide at liquid interfaces (Allen et al., 1999). These techiques are... 

One of the fundamental problems in chemistry is understanding at the molecular level the effect of the medium on the rate and the equilibrium of chemical reactions which occur in bulk liquids and at surfaces. Recent advances in experimental techniques[l], such as frequency and time-resolved spectroscopy, and in theoretical methods[2,3], such as statistical mechanics of the liquid state and computer simulations, have contributed significantly to our understanding of chemical reactivity in bulk liquids[4] and at solid interfaces. These techniques are also beginning to be applied to the study of equilibrium and dynamics at liquid interfaces[5]. The purpose of this chapter is to review the progress in the application of molecular dynamics computer simulations to understanding chemical reactions at the interface between two immiscible liquids and at the liquid/vapor interface. 

Potential-modulated fluorescence spectroscopy at liquid/liquid interfaces between immiscible liquids has been reported and a cell design has been provided [78], The dependence of the adsorption of the free, bare or the water-soluble porphyrins at the polarized water/l,2-dichloroethane interface has been studied [79], Observed spectral differences suggest a solvation structure at the interface that is different from that inside the bulk of the respective solution phases. For further studies with related porphyrins at the same interface, see [79]. Details of the transfer mechanism of the rose bengal dianion across the water/l,2-dichloroethane interface have been elucidated [80],... 

The Structure of the Metal-Vacuum Interface The Study of Simple Consecutive Processes in Electrochemical Reactions Surface Analysis by Electron Spectroscopy Surface-Enhanced Raman Scattering (SERS Surface Potential at Liquid Interfaces Surface States on Semiconductors... 

Richmond G L 1997 Vibrational spectroscopy of molecules at liquid/liquid interfaces Ana/. Chem. 69 A536-43... 

Surface forces measurement is a unique tool for surface characterization. It can directly monitor the distance (D) dependence of surface properties, which is difficult to obtain by other techniques. One of the simplest examples is the case of the electric double-layer force. The repulsion observed between charged surfaces describes the counterion distribution in the vicinity of surfaces and is known as the electric double-layer force (repulsion). In a similar manner, we should be able to study various, more complex surface phenomena and obtain new insight into them. Indeed, based on observation by surface forces measurement and Fourier transform infrared (FTIR) spectroscopy, we have found the formation of a novel molecular architecture, an alcohol macrocluster, at the solid-liquid interface. 

Real Time Monitoring of Molecular Structure at Solid/Liquid Interfaces by Non-Linear Spectroscopy... 

SFG spectroscopy is an ideal technique to investigate the mechanism of interfacial processes at solid/liquid interfaces [5, 6, 10-16]. 

Here, we describe the basic principles and detailed experimental arrangement of SFG spectroscopy and present several examples of SFG study at solid/liquid interfaces. HRS is also described briefly. 

Fujiyoshi, S., Ishibashi, T. and Onishi, H. (2006) Molecular vibrations at a liquid-liquid interface observed by fourth-order Raman spectroscopy. J. Phys. Chem. B, 110, 9571-9578. 

While characterization of the electrode prior to use is a prerequisite for a reliable correlation between electrochemical behaviour and material properties, the understanding of electrochemical reaction mechanisms requires the analysis of the electrode surface during or after a controlled electrochemical experiment. Due to the ex situ character of photoelectron spectroscopy, this technique can only be applied to the emersed electrode, after the electrochemical experiment. The fact that ex situ measurements after emersion of the electrode are meaningful and still reflect the situation at the solid liquid interface has been discussed in Section 2.7. 

Titration calorimetry and cylindrical internal reflection-Fourier transform infrared (CIR-FTIR) spectroscopy are two techniques which have seldom been applied to study reactions at the solid-liquid interface. In this paper, we describe these two techniques and their application to the investigation of salicylate ion adsorption in aqueous goethite (a-FeOOH) suspensions from pH 4 to 7. Evidence suggests that salicylate adsorbs on goethite by forming a chelate structure in which each salicylate ion replaces two hydroxyls attached to a single iron atom at the surface. 

Inspired by these Surface Science studies at the gas-solid interface, the field of electrochemical Surface Science ( Surface Electrochemistry ) has developed similar conceptual and experimental approaches to characterize electrochemical surface processes on the molecular level. Single-crystal electrode surfaces inside liquid electrolytes provide electrochemical interfaces of well-controlled structure and composition [2-9]. In addition, novel in situ surface characterization techniques, such as optical spectroscopies, X-ray scattering, and local probe imaging techniques, have become available and helped to understand electrochemical interfaces at the atomic or molecular level [10-18]. Today, Surface electrochemistry represents an important field of research that has recognized the study of chemical bonding at electrochemical interfaces as the basis for an understanding of structure-reactivity relationships and mechanistic reaction pathways. 

Many important heterogeneous catalytic reactions occur at the interface between a solid catalyst and liquid or liquid-gas reactants. Notwithstanding the importance of solid-catalyzed reactions in the presence of liquid reactants, relatively little attention has been paid to spectroscopic methods that allow researchers to follow the processes occurring at the solid-liquid interface during reaction. This lack can be explained in part by the fact that there are only a few techniques that give access to information about solid-liquid interfaces, the most prominent of them being attenuated total reflection infrared spectroscopy (ATR-IR) and X-ray absorption fine structure (XAFS) spectroscopy. 

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