Orientation of adsorbed molecules

however, worthwhile to mention that the orientation of a molecule in the adsorbed phase cannot be determined directly. It can only be assumed that the orientation adopted by a given adsorbate molecule is the same as in the case of vapor phase adsorption on the concerned solid although this may not be completely trae in all cases of adsorption from binary solutions. For example, stearic acid is oriented with the major axis perpendicular to the surface as close packed films on 

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IRE Infrared emission [110] Infrared emission from a metal surface is affected in angular distribution by adsorbed species Orientation of adsorbed molecules... 

Surface SHG [4.307] produces frequency-doubled radiation from a single pulsed laser beam. Intensity, polarization dependence, and rotational anisotropy of the SHG provide information about the surface concentration and orientation of adsorbed molecules and on the symmetry of surface structures. SHG has been successfully used for analysis of adsorption kinetics and ordering effects at surfaces and interfaces, reconstruction of solid surfaces and other surface phase transitions, and potential-induced phenomena at electrode surfaces. For example, orientation measurements were used to probe the intermolecular structure at air-methanol, air-water, and alkane-water interfaces and within mono- and multilayer molecular films. Time-resolved investigations have revealed the orientational dynamics at liquid-liquid, liquid-solid, liquid-air, and air-solid interfaces [4.307]. 

In EMIRS and SNIFTIRS measurements the "inactive" s-polarlsed radiation is prevented from reaching the detector and the relative intensities of the vibrational bands observed in the spectra from the remaining p-polarised radiation are used to deduce the orientation of adsorbed molecules. It should be pointed out, however, that vibrational coupling to adsorbate/adsorbent charge transfer (11) and also w electrochemically activated Stark effect (7,12,13) can lead to apparent violations of the surface selection rule which can invalidate simple deductions of orientation. 

Introducing two-dimensional unit vectors for azimuthal orientations of adsorbed molecules... 

In passing from the first to the second problem, a feature of importance should be borne in mind. The periods of the orientational structure (3.1.9) can exceed those of the basic Bravais sublattice, Ai, A2. If this is the case, the unit cell A, A2 should be enlarged so that conditions (3.1.10) can be met and translations onto the new vectors R can reproduce the orientations of adsorbed molecules. Then the excitation Hamiltonian (3.1.3) can be represented in the Fourier form with respect to the wave-vector K as... 

The adsorption of amino acids on rutile and hydroxyapatite exhibits some characteristics of specific adsorption. The results can be interpreted in terms of electrostatic models of adsorption, however, if reorientation of adsorbed molecules is taken into consideration. The electrokinetic behavior of hydroxyapatite in glutamic acid is complicated because of a chemical reaction, possibly involving calcium ions. The study shows that it is necessary to take into consideration the orientation of adsorbed molecules, particularly for zwitterionic surfactants. 

III. Determination of the Orientation of Adsorbed Molecules by Elastic Neutron Diffraction... 

A promising aspect of RS for probing surface chemistry involves its ability to evaluate the molecular orientation of monolayer coverages via polarization measurements (146). The orientation of a surface active dye, Suminol Milling Brilliant Red BS, has been studied at a water-carbon tetrachloride interface (147). As the surface area per molecule was reduced the spectra showed a transition which was interpreted as a change from a mixture of orientations to one predominantly perpendicular with respect to the surface. A thorough theoretical analysis of the use of depolarization ratios for the prediction of primary surface orientations of adsorbed molecules has also been reported (148). Similar developments are occurring in IR spectrscopy and a determination of the molecular orientations in a series of polymers has been reported (149). 

In the second chapter, Anil Agiral and Han J.G.E. Gardeniers take us to a fascinating world wherein "chemistry and electricity meet in narrow alleys." They claim that microreactor systems with integrated electrodes provide excellent platforms to investigate and exploit electrical principles as a means to control, activate, or modify chemical reactions, or even preparative separations. Their example of microplasmas shows that the chemistry can take place at moderate temperatures where the reacting species still have a high reactivity. Several electrical concepts are presented and novel principles to control adsorption and desorption, as well as the activity and orientation of adsorbed molecules are described. The relevance of these principles for the development of new reactor concepts and new chemistry is discussed. 

Soriaga, M. P. and Hubbard, A. T. (1982) Determination of the orientation of adsorbed molecules at solid-liquid interfaces by thin-layer electrochemistry Aromatic compounds at platinum electrodes. J. Am. Chem. Soc. 104, 2735-2742. 

Observation of the ion angular distribution after electron stimulated desorption of chemisorbed species (ESDIAD) can provide direct quantitative information on the orientation of adsorbed molecules on surfaces. Electrons incident on the surface can excite chemical bonds into non-bonding states, causing molecular decomposition. The excess energy can be converted into kinetic energy, which accelerates an ionic fragment of the molecule along the axis... 

Comparing the intensity of corresponding peaks in ppp and ssp SFG spectra allows one to deduce the molecular orientations of adsorbed molecules. However, the analysis is quite involved and not always unambiguous (see discussion in Reference ISO)). Details of the theory and analysis of such spectra can be found elsewhere 125,149,150,277,431). 

The amplitude of the the electric field vectors in the different directions can be used to study the orientations of adsorbed molecules at the interface of the internal reflection element [33]. 

If one is interested in the orientation of adsorbed molecules one can, as well as using TIRF with polarized light and NMR, also have recourse to non-linear optical techniques, including the analysis of so-called second harmonics. The second harmonic is a wave with twice the frequency of the incident one it is... 

Adsorption isotherms are commonly used to describe adsorption processes and these represent a functional relationship between the amount adsorbed and the activity of the adsorbate at a constant temperature. The shape of the adsorption isotherm gives useful information regarding the mechanisms of the adsorption process. A classification of adsorption phenomena based on the shape of the isotherms is given by Giles et al. (1960) as shown in Fig. 4.1. Mainly four major classes of isotherms have been identified based on the initial part of the isotherms (a) S-type isotherm with a convex shaped initial portion where adsorption rate increases with adsorption density and is indicative of vertical orientation of adsorbed molecules at the surface (b) L-type (Langmuir type) isotherm, characterized by a concave initial region, represents systems in which the solvent is relatively inert and adsorption rate decreases with adsorption density. This is usually indicative of molecules adsorbed flat on the surface or ions vertically adsorbed with strong intermolecular attraction. 

The thickness of the adsorbed phase depends upon the number of adsorbed molecular layers and upon the orientation of adsorbed molecules. A useful approximation for most liquid-solid chromatographic systems is the assumption of monolayer adsorption. This limits the possible volume of the adsorbed phase within narrow limits and leads to a mathematical basis for correlating relative adsorption with adsorbent surface area (see Section 6-2A). 

This chapter will also present a selected number of experiments representative for the various models discussed. This refers especially to surfactant systems where recently new phenomena have been observed and explained, i.e. the possibility of changes in the orientation of adsorbed molecules are alternatively the formation of aggregates at the interface. Adsorption kinetics as well as interfacial relaxation experiments will be reported and the results discussed in terms of the specific parameters of these new theories. This will include also some data on proteins as particular type of surface active molecules able to change their conformation at an interface and hence changing the molar area at the interface. 

Two other important characters of SEIRA should be addressed. First, the enhanced surface EM field around the metal particles is essentially polarized along the surface normal at any point on the particles, as shown by the small arrows in Fig. 8.3 [25]. Consequently, only the vibrations that give transition dipole components perpendicular to the local surface can be excited. The surface selection rule can be explained also by the interaction of the adsorbate dipole with its image induced in the metal the adsorbate dipole that is perpendicular to the surface constructively interacts with its image dipole to enhance the absorption, while the adsorbate dipole that is parallel to the surface destmctively interacts with its image dipole to reduce the absorption [26]. The surface selection rule is identical to that in IRAS [27] and the orientations of adsorbed molecules can be elucidated by using this rule, as will be described in more detail later. 

Quantitative SNIFTIRS was introduced in 2002 and so far has been applied to the investigation of the orientation of adsorbed molecules in three systems pyridine at Au(llO) [40], citrate at Au(lll) [53], and 2-mercaptobenzimidazole at Au(lll) [54, 55]. Pyridine adsorption at gold single-crystal surfaces has long been used as a model system to study the coordination of organic molecules to metal electrode surfaces. The thermodynamics of pyridine adsorption has been thoroughly investigated with the help of the chronocoulometric technique [56-64]. The availability of the thermodynamic data made this an ideal system to test the performance of the quantitative SNIFTIRS. 

In any anisotropic system, the depolarization factor may be used as a valuable source of information about the orientation of molecules, e. g. about the orientation of polymer chains in fibers or the orientation of adsorbed molecules on surfaces. 

The fact that many surfactant systems cannot be adequately described by the Frumkin mode was the reason that other models have been derived. A comprehensive overview of such models was given recently elsewhere (Fainerman et al. 1998). We want to discuss two of the most recent models considering changes in orientation of adsorbed molecules and formation of two-dimensional aggregates (Fainerman et al. 2002). These new models are suitable to describe quite a number of surfactant adsorption layers much better than classical models do. 

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