Vibrational spectroscopy liquid-solid surfaces

Because of the generality of the symmetry principle that underlies the nonlinear optical spectroscopy of surfaces and interfaces, the approach has found application to a remarkably wide range of material systems. These include not only the conventional case of solid surfaces in ultrahigh vacuum, but also gas/solid, liquid/solid, gas/liquid and liquid/liquid interfaces. The infonnation attainable from the measurements ranges from adsorbate coverage and orientation to interface vibrational and electronic spectroscopy to surface dynamics on the femtosecond time scale. 

Novotny et al. [41] used p-polarized reflection and modulated polarization infrared spectroscopy to examine the conformation of 1 -1,000 nm thick liquid polyperfluoropropy-lene oxide (PPFPO) on various solid surfaces, such as gold, silver, and silica surfaces. They found that the peak frequencies and relative intensities in the vibration spectra from thin polymer films were different from those from the bulk, suggesting that the molecular arrangement in the polymer hlms deviated from the bulk conformation. A two-layer model has been proposed where the hlms are composed of interfacial and bulk layers. The interfacial layer, with a thickness of 1-2 monolayers, has the molecular chains preferentially extended along the surface while the second layer above exhibits a normal bulk polymer conformation. 

Wall-coated flow tube reactors have been used to study the uptake coefficients onto liquid and solid surfaces. This method is sensitive over a wide range of y (10" to 10 1). For liquids this method has the advantage that the liquid surface is constantly renewed, however if the uptake rate is fast, the liquid phase becomes saturated with the species and the process is limited by diffusion within the liquid, so that corrections must be applied [70,72,74]. Many experiments were designed to investigate the interaction of atmospheric species on solid surfaces. In this case the walls of the flow tube were cooled and thin films of substrate material were frozen on the wall. Most of the reaction probabilities were obtained from studies on flow tubes coated with water-ice, NAT or frozen sulfate. Droplet train flow tube reactors have used where liquid droplets are generated by means of a vibrating orifice [75]. The uptake of gaseous species in contact with these droplets has been measured by tunable diode laser spectroscopy [41]. 

Inelastic neutron scattering (INS) is a spectroscopic technique in which neutrons are used to probe the dynamics of atoms and molecules in solids and liquids. This book is the first, since the late 1960s. to cover the principles and applications of INS as a vibrational-.spectroscopic technique. It provides a hands-on account of the use of INS. concentrating on how neutron vibrational spectroscopy can be employed to obtain chemical information on a range of materials that are of interest to chemists, biologists, materials scientists, surface scientists and catalyst researchers. This is an accessible and comprehensive single-volume in imary text and reference source. 

Yeganeh MS, Dougal SM, Pink HS (1999) Vibrational spectroscopy of water at liquid/solid interfaces Crossing the isoelectric point of a sohd surface. Phys Rev Lett 83 1179-1182 Zabinsky SI, Rehr JJ, Ankudinov A, Albers RC, Eller MJ (1995) Multiple-scattering calculations of X-ray-absorption spectra. Phys Rev 52 2995-3009... 

In-situ vibrational spectroscopy has long been used to study the electrified solid/ liquid interface. By using the information given by peak position, width, and lifetime, vibrational spectroscopy can provide the chemical identity of the adsorbate, an estimation of surface coverage, and the orientation and even dynamics of molecules at the electrode. Three different types of vibrational spectroscopy are relevant to the solid/liquid interface. The first two of these, Raman and infrared spectroscopy, are thoroughly discussed in this book. A third technique successfully used to probe the Uquid/soUd electrochemical interface is vibrational sum frequency generation (SFG). SFG was developed as a surface probe some 20 years ago [1], and its use was extended to the electrochemical interface by Tadjeddine over a decade ago [2]. Several reviews examining the use of SFG in non-electrochemical environments exist [3-11]. Tadjeddine wrote two reviews on the application of SFG to electrochemical problems [12, 13). This chapter updates the Tadjeddine work and focuses on the promise and problems of state-of-the-art electrochemical SFG. 

Summary Ab initio calculations predict the existence of anti and gauche rotamers for the disilanes /BuXaSiSiXatBu (X = Br and I). For the chloro compound a third backbone conformer with a CSiSiC dihedral angle of 95° ortho) was located on the potential energy surface. Due to the fact that the vibrational spectra of these disilanes are hardly sensitive to the conformation around the Si-Si bond we have not been able to determine energy differences between conformers from variable-temperature Raman spectra. Infrared and Raman spectroscopy suggest that anti and twisted conformers are present in liquid and solid tBuCbSiSiCh/Bu. For X = Br and I the anti conformation is adopted in the solid state, as proven by X-ray diffraction and vibrational spectroscopy. 

Modern methods of vibrational analysis have shown themselves to be unexpectedly powerful tools to study two-dimensional monomolecular films at gas/liquid interfaces. In particular, current work with external reflection-absorbance infrared spectroscopy has been able to derive detailed conformational and orientational information concerning the nature of the monolayer film. The LE-LC first order phase transition as seen by IR involves a conformational gauche-trans isomerization of the hydrocarbon chains a second transition in the acyl chains is seen at low molecular areas that may be related to a solid-solid type hydrocarbon phase change. Orientations and tilt angles of the hydrocarbon chains are able to be calculated from the polarized external reflectance spectra. These calculations find that the lipid acyl chains are relatively unoriented (or possibly randomly oriented) at low-to-intermediate surface pressures, while the orientation at high surface pressures is similar to that of the solid (gel phase) bulk lipid. 

The colloidal properties are controlled to a great extent by the species present at the solid-liquid interface. The vibrational spectra of the adsorbed or chemically attached species can be followed by infrared(21,22) or Raman(23) spectroscopy, usually on the centrifugate. Fluorescence of the high surface area substrates often prevents the use of Raman spectroscopy. 

Ionic liquids at the gas-liquid and solid-liquid interface have been extensively studied by a variety of surface analytical techniques. The most prominent technique for surface orientational analysis proves to be SFG. Other vibrational spectroscopic and surface-sensitive techniques such as surface-enhanced Raman spectroscopy (SERS) and total internal reflection Raman spectroscopy (TIR Raman) have been employed for studying surface processes these techniques, however, have not been applied yet specifically for the study of ionic hquids. 

---