Molecular motion, adsorbates

Many of the fiindamental physical and chemical processes at surfaces and interfaces occur on extremely fast time scales. For example, atomic and molecular motions take place on time scales as short as 100 fs, while surface electronic states may have lifetimes as short as 10 fs. With the dramatic recent advances in laser tecluiology, however, such time scales have become increasingly accessible. Surface nonlinear optics provides an attractive approach to capture such events directly in the time domain. Some examples of application of the method include probing the dynamics of melting on the time scale of phonon vibrations [82], photoisomerization of molecules [88], molecular dynamics of adsorbates [89, 90], interfacial solvent dynamics [91], transient band-flattening in semiconductors [92] and laser-induced desorption [93]. A review article discussing such time-resolved studies in metals can be found in... 

Deuterium NMR has recently been used to study molecular motion of organic adsorbates on alumina (1.) and in framework aluminosilicates (2). The advantage of NMR is that the quadrupole interaction dominates the spectrum. This intramolecular interaction depends on the average ordering and dynamics of the individual molecules. In the present work we describe NMR measurements of deuterated benzene in (Na)X and (Cs,Na)X zeolite. 

NMR signals are highly sensitive to the unusual behavior of pore fluids because of the characteristic effect of pore confinement on surface adsorption and molecular motion. Increased surface adsorption leads to modifications of the spin-lattice (T,) and spin-spin (T2) relaxation times, enhances NMR signal intensities and produces distinct chemical shifts for gaseous versus adsorbed phases [17-22]. Changes in molecular motions due to molecular collision frequencies and altered adsorbate residence times again modify the relaxation times [26], and also result in a time-dependence of the NMR measured molecular diffusion coefficient [26-27]. 

The physical properties of probe molecules adsorbed in the confined space of porous materials are known to vary in dependence of structural constraints on molecular motion. Detailed investigations of adsorption geometries are possible, when well-defined sites and loadings exist. This was the case for the adsorption of strongly interacting probe molecules, such as pyridine, on SiOH groups in the... 

The motional dynamics of O J adsorbed on Ti supported surfaces has been analyzed over the temperature range 4.2-400 K in a recent paper by Shiotani et al. (66). Of the several types of 02, a species noted as 02 (III), and characterized by gxx = 2.0025, gyy = 2.0092, g12 = 2.0271 at 4.2 K, exhibited highly anisotropic motion. While gxx and gzz varied with increasing temperature and were accompanied by drastic line shape changes, gyy was found to remain constant. This observation indicates that the molecular motion of this 02 can be described by rotation about the y axis perpendicular to the internuclear axis of 02 and perpendicular to the surface with the notation given in Fig. 4. The EPR line shapes were simulated for different possible models and it was found that a weak jump rotational diffusion gave a best fit of the observed spectra below 57.4 K, whereas some of the models could fit the data above this temperature. The rotational correlation time was found to range from 10 5 sec (below 14.5 K) to 10 9 sec (263 K), while the... 

Quadrupolar interactions can offer direct information on the dynamics of organics within zeolite crystals. Eckman and Vega (304) studied the 2H quadrupolar echo decay in perdeuterated p-xylene adsorbed on zeolite ZSM-5. The deuterium quadrupolar interaction usually dominates the spin Hamiltonian, so that the powder pattern can be used as a test for models of molecular motion. At -75°C and 25°C typical rigid-lattice spectra were obtained. At 100°C however, the resonance arising from the aromatic deuterons was motionally narrowed, while the methyl resonance was not, The authors conclude that p-xylene molecules reorient about an axis which passes through the C3 axes of the methyl groups. 

C relaxation studies on AW-dimethylaniline adsorbed on Si02 gel and octadecylsilanized Si02 gel provided information on mechanisms of molecular motion of these adsorbed species.130... 

It is well known that both nanometre and nanosecond-picosecond resolutions at an interface can be achieved by total internal reflection (TIR) fluorescence spectroscopy. Unlike steady-state fluorescence spectroscopy, fluorescence dynamics is highly sensitive to microscopic environments, so that time-resolved TIR fluorometry at water/oil interfaces is worth exploring to obtain a clearer picture of the interfacial phenomena [1]. One of the interesting targets to be studied is the characteristics of dynamic motions of a molecule adsorbed on a water/oil interface. Dynamic molecular motions at a liquid/liquid interface are considered to be influenced by subtle changes in the chemical/physical properties of the interface, particularly in a nanosecond-picosecond time regime. Therefore, time-resolved spectroscopy is expected to be useful to study the nature of a water/oil interface. 

Summary Solid state NMR studies of molecular motions and network structure in poly(dimethylsiloxane) (PDMS) filled with hydrophilic and hydrophobic Aerosil are reviewed and compared with the results provided by other methods. It is shown that two microphases with significantly different local chain mobility are observed in filled PDMS above the glass transition, namely immobilized chain units adsorbed at the filler surface and mobile chain units outside this adsorption layer. The thickness of the adsorption layer is in the range of one to two diameters of the monomer unit ( 1 nm). Chain units in the adsorption layer are not rigidly linked to the surface of Aerosil. The chain motion in the adsorption layer depends significantly on temperature and on type of the filler surface. With increasing temperature, both the fiaction of less mobile adsorbed chain units and the lifetime of the chain units in the adsorbed state decrease. The lifetime of chain units in the adsorbed state approaches zero at approximately 200 K and 500 K for PDMS chains at the surface of hydrophobic and hydrophilic Aerosil, respectively. 

According to this model, the temperature dependence of molecular motions for adsorbed and non-adsorbed chain units in filled PDMS containing hydrophilic Aerosil is shown in Fig. 9 [9]. The lowest temperature motion is a C3 rotation of the CH3 groups around the Si-C bond (line 1 in Fig. 9). The rate of the a-relaxation (points 2 in Fig. 9) in filled PDMS is close to that for unfilled sample (line 2 in Fig. 9). It has been proposed that independence of the mean average frequency of a-relaxation process on the filler content in filled PDMS is due to defects in the chain packing in the proximity of primarily filler particles [7]. Furthermore, the chain adsorption does not restrict significantly the local chain motion, which is due to high flexibility of the siloxane main chain as well as due to fast adsorption-desorption processes at temperatures well above Tg. 

The broad-band dielectric study of highly filled PDMS is complementary to the NMR study of molecular motions in filled PDMS. The dielectric experiments were performed in the frequency range of 10" -10 Hz [27], A combined analysis of the dielectric spectra both for filled PDMS and the pure components of the mixtures was used to assign the dielectric losses to motions of adsorbed and non-adsorbed PDMS chain units. As discussed above, the interpretation of the results is based on a two-phase model assiuning the exchange of chain units at the surface of Aerosil between adsorbed and non-adsorbed states. 

In the present study, the surface activity of Aerosil is characterized by molecular motions of adsorbed chain units. Highly fille PDMS has been studied by H Ti NMR relaxation experiments and H NMR spectra [8, 10, 21]. The H NMR spectra are compared in Fig. 12 for highly filled samples containing hydrophilic and hydrophobic Aerosil [21]. 

Paramagnetic ions are now being used quite extensively to study adsorption phenomena. Mn ions have been used as probes for studying molecular motion in synthetic zeolites, (350) Co and Ni ions have been used for studying the complexation of molecular hydrogen on the surface of zeolites, (351) and these same ions have been used in a variety of studies of adsorption on Aerosil surfaces. (352-358) Adsorbed molecules studied include olefins, saturated hydrocarbons, alcohols, and benzene. From the measured line-shifts the number of active surface sites can be deduced in favourable cases. (357, 358)... 

Freezing of a dipolar liquid is accompanied by a rapid decrease in its electric permittivity [8-10]. Following solidification, dipole rotation ceases and the electric permittivity is almost equal to n, where n is refractive index, as it arises from deformation polarisation only. Investigation of the dynamics of a confined liquid is possible from the frequency dependences of dielectric properties, which allows both the determination of the phase transition temperature of the adsorbed substance and characteristic relaxation frequencies related to molecular motion in particular phases. 

Special techniques in surface science to study molecular motion in adsorbed layers are a problem that has found little attention in surface science, even though it is of utmost importance. Electron spin resonance is one such method. We discuss diffusion of NO2, rotational motion of ((CH3)3C)2NO and melting of self-assembled layers of spin-labeled fatty acids adsorbed on Al203(0001). 

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