Molecular Motion of Adsorbates

Whereas adsorption isotherms and microcalorimetry give information on the thermodynamics of adsorption (including the entropy of adsorption, which is related to the motion of adsorbed species), other techniques are required to measure molecular motion within the pores directly. This motion can take the form of either re-orientation (discussed below) or diifusional transport through the pores, which is addressed in Section 7.4. 

Molecules adsorbed within micropores have motional modes and frequencies of re-orientation that depend on the geometric constraints of the pores and the strength of the adsorption interaction. Deuterium ( H) wideline spectroscopy is one of the most powerful experimental methods to study this quasi-elastic neutron scattering is another. Molecular dynamics simulations are also of great value, as described in Section 4.5.3, although the timescales involved only extend to nanoseconds, rather than the timescale of microseconds or greater that is accessible to NMR measurements. 

Simulated NMR spectral lineshapes at the fast limit of motion for QD benzene (centre) undergoing motion around the molecule s Ce axis and (right) for H3C-C5D4-CH3 p-xylene undergoing n flips around the molecule s para axis. These are compared with (left) the spectrum of static 

The static (or slow MAS) spectra can be simulated assuming modes and frequencies for independent mechanisms of re-orientation, and simulations proceed iteratively to match the spectra. Several computer codes are available to perform the simulation, while some groups calculate spectra directly from analytical functions. wideline NMR studies have been applied to many adsorbate-microporous solid systems, including both physisorbed and chemisorbed species. The lineshape-matching process can sometimes be ambiguous, so that additional constraints on the possible mechanisms of motion, such as those provided by molecular dynamics or (time-averaged) by crystallography, are very helpful. 

In a recent example in our own laboratory that demonstrates the effect of small structural changes in pore structure on the constrained motion of adsorbates, we studied the motion of dg-benzene in the two aluminium 

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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]. 

The dynamic characteristics of adsorbed molecules can be determined in terms of temperature dependences of relaxation times [14-16] and by measurements of self-diffusion coefficients applying the pulsed-gradient spin-echo method [ 17-20]. Both methods enable one to estimate the mobility of molecules in adsorbent pores and the rotational mobility of separate molecular groups. The methods are based on the fact that the nuclear spin relaxation time of a molecule depends on the feasibility for adsorbed molecules to move in adsorbent pores. The lower the molecule s mobility, the more effective is the interaction between nuclear magnetic dipoles of adsorbed molecules and the shorter is the nuclear spin relaxation time. The results of measuring relaxation times at various temperatures may form the basis for calculations of activation characteristics of molecular motions of adsorbed molecules in an adsorption layer. These characteristics are of utmost importance for application of adsorbents as catalyst carriers. They determine the diffusion of reagent molecules towards the active sites of a catalyst and the rate of removal of reaction products. Sometimes the data on the temperature dependence of a diffusion coefficient allow one to ascertain subtle mechanisms of filling of micropores in activated carbons [17]. 

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. 

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... 

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... 

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. 

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). 

More than one decade ago we have reported an ESR study on the molecular motion of NO2 adsorbed on porous Vycor glass [8,9] and it was found that the NO2 adsorbed on the Vycor glass and Cu-metal supported on Vycor glass gave strongly temperature-dependence ESR spectra. The present study aimed to further apply our dynamic ESR technique to elucidate motional dynamics of NO2 adsorbed on various zeolite with different structures. The results can provide fundamental and useful information which are required to develop new active catalyst and to clarify the reaction mechanism. 

PST/MAS experiments the intensity of the peak is governed by the NOE enhancement. In the CP/MAS spectrum, the intensity of peak 1 is more intense than that of peak 2, but in the PST/MAS spectrum (Fig. 9.15(b)) the intensity of peak 2 is relatively increased compared with the CP/MAS spectrum. This means that the molecular motion of the methylene carbons for peak 2 is faster than that for peaks 1 and 3. The CP/MAS spectral pattern of adsorbed polyethylene is very different from that of bulk polyethylene. The fractions of the mobile components for these polyethylene samples are different from each other. The fraction of the mobile component in adsorbed polyethylene is larger than that of bulk polyethylene. The chemical shift value for peak 1 is close to that for tran -zigzag methylene carbons and the difference in C chemical shift between peaks 1 and 3 is 4.4 ppm which corresponds to the 1 y-gauche effect value. From these results, it can be said that peaks 1 and 3 come from the trans and gauche parts in which the molecular motion is frozen on the NMR timescale. This is caused by adsorption on the surface of silica gel (this part is designated as the region A). 

Nmr relaxation measurements allow the molecular motions of the segments of a polymer molecule to be investigated. The spectral line width, which is proportional to I/T2 (i.e. the reciprocal of the spin-spin relaxation time of proton nmr) is a direct measure of segment mobility. It would be anticipated that the anchor segments in the trains attached to the surface of the adsorbent particle would display low mobility. On the other hand, those segments in loops and tails that project into the continuous phase should possess much higher mobility. Two distinct linewidths would therefore be expected for the two different types of segments. 

Investigation of the motion of adsorbed molecules, which give mechanisms and rates of re-orientation and diffusion, require alternative approaches. For systems that contain highly mobile species. Molecular Dynamics (MD) techniques are widely used. However, for many adsorbates the timescales of motion are much longer than can feasibly be simulated, so that MD is only relevant either for small molecules or at high temperatures. In order to simulate slower diffusion, the process must be considered in terms of rare events with significant activations. The activated processes are then usefully treated by transition state theory, and the associated processes treated over extended timescales and volumes by, for example, Kinetic Monte Carlo (KMQ techniques. 

Surface diffusion implies a thermal motion of adsorbed molecules. It should be distinguished from interstitial diffusion or intracrystalline diffusion, which is more similar to solid solution than adsorption. This intracrystalline diffusion is strongly affected by the molecular size and it decreases with an increase in the molecular size. The decrease is much faster than 1 / Vm, observed for the case of Knudsen diffusion. 

FIGURE 1.51 Activation energy of molecular motion of unfrozen water at T<213 K and different hydration of A-300 and 200DF with coadsorbed methane. (Adapted from Appl. Surf Sci., 258, Gun ko, V.M., Turov, V.V., Bogatyrev, V.M. et al.. The influence of pre-adsorbed water on adsorption of methane on fumed and nanoporous silicas, 1306-1316, 2011c. Copyright 2011, with permission from Elsevier.)... 

In addition to dynamics associated with atomic and molecular motions of the adsorbate molecule and zeolite framework that induce chemical changes and affect diffusion, temperature can also affect the desorption of chemisorbed and physisorbed species and even collapse of the framework structure of zeolite ( above 800°K the structure of zeolite Y, with Si/Al=2.5, collapses to an amorphous residue ). Effects associated with the thermal redistribution of the... 

A considerable element of the model is the assumption connected with the possibility of the kinetic motion of adsorbed molecules. When the motion of molecules in the z direction is restricted but molecules are able to move freely in the (x,y) plane, the process is classified as mobile adsorption. However, if the lateral translation is also hindered, the process is classified as localized adsorption. The motion of admolecules is controlled by the energetic topography of the surface, molecular interactions, and thermal energies. The adsorbed molecule is considered as localized on a surface when it is held at the bottom of a potential well with a depth that is much greater than its thermal energy. Except for extreme cases, adsorption is neither frilly localized nor frilly mobile and can be termed partially mobile [8]. Because temperature strongly affects the behavior of the system, adsorption may be localized at low temperatures and become mobile at high temperatures. 

When a macromolecule adsorbs at a solid-liquid interface, the molecular motion of the polymer s backbone becomes slower, and the longer correlation time of the motion is reflected in the relaxation times of protons ( H NMR) or free electrons (electron paramagnetic resonance, EPR) that are attached closely to the backbone. Provided there is slow exchange between segments associated with the surface (trains) and those in loops or tails, the spectrum of the whole molecule will be resolvable into the fraction of the chain in each state. As discussed in more detail later, these fractions obtained by EPR and NMR may well be quite different from, but complementary to, those obtained by IR measurements. 

Nuclear magnetic resonance (NMR) provides a powerful method for the study of molecular motion. The techniques can distinguish molecular reorientation and translation and have proved particularly valuable for the study of self-diffusion in bulk liquids. The molecular motion of liquids in the confined geometry provided by their containment in porous materials has been of considerable interest for many years. It is of importance both as a fundamental scientific problem and because of its technological importance in such diverse systems as oil recovery from rocks and catalytic agents. The purpose of this paper is to question the reliability of many previous investigations and the validity of their interpretation. Potential sources of error are demonstrated by measurements on mobile liquids adsorbed into porous silicas with different geometrical characteristics. The principles illustrated are equally valid for other porous systems. Preliminary measurements of the diffusion coefficient of n-butane in silica as a fimction of temperature and the effect of pore dimensions are presented. 

Zimdars D, Dadap J I, Eisenthal K B and Heinz T F 1999 Anisotropic orientational motion of molecular adsorbates at the air-water interface J. Chem. Phys. 103 3425-33... 

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]. 

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