Physisorption

If the coupling between a foreign atom or a molecule and a surface does not involve the formation of a chemical bond, the adsorption is referred to as physisorption. The underlying mechanism is known in molecular physics as a van der Wools interaction and is due to the interaction between instantaneous dipoles which are generated in interacting molecules because of the quantum-mechanical fluctuations of their electron clouds. The van der Waals potential is attractive and decreases with distance r between the molecules as 

Let us have a look at the image charges induced by an atomic dipole in the polarizable substrate. According to classical electrodynamics (Stratton 1941), the electrostatic field of a charge q located at a distance z from a flat surface of a medium with real dielectric function e can be described by introducing a charge 

Note that the components of the dipole moments parallel to the surface ( ) are antiparallel to each other, whereas those perpendicular to the surface (+) are of the same direction (see Fig. 2.12). The interaction energy of the real dipole with its image is then determined as (Landau and Lifshitz 1980) 

In contrast to the van der Waals potential between two molecules, the atom-surface potential decreases as 1 /z with distance from the surface. 

Equation (2.98) describes the frequency shift of a classical harmonic oscillator, 5E , located above a surface at distances much less than the wavelength of its oscillations (Chance et al. 1975b). A rigorous quantum-electrodynamical treatment of the problem (Wylie and Sipe 1984, 1985) predicts, besides this classical term, a correction term arising from the van der Waals interaction between atom and surface. The total shift of the electronic energy level a can be written as 

When an atom or molecule approaches the surface it feels the potential energy set up by the metal atoms in the solid. The interaction is usually divided into two regimes, namely physisorption and chemisorption, which we discuss separately. 

Physisorption is a weak interaction characterized by the lack of a true chemical bond between adsorbate and surface, i.e. no electrons are shared. The physisorption interaction is conveniently divided into to parts A strongly repulsive part at close distances and Van der Waals interactions at medium distances of a few A. 

Concepts of Modem Catalysis and Kinetics. I. Chorkendorff, J. W. Niemantsverdriet Copyright 2003 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-30574-2 

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The physisorption bond, being relatively weak, is even more difficult to characterize than the chemisorption one. Some aspects of this are covered in Section XVII-10. 

Infrared Spectroscopy. The infrared spectroscopy of adsorbates has been studied for many years, especially for chemisorbed species (see Section XVIII-2C). In the case of physisorption, where the molecule remains intact, one is interested in how the molecular symmetry is altered on adsorption. Perhaps the conceptually simplest case is that of H2 on NaCl(lOO). Being homo-polar, Ha by itself has no allowed vibrational absorption (except for some weak collision-induced transitions) but when adsorbed, the reduced symmetry allows a vibrational spectrum to be observed. Fig. XVII-16 shows the infrared spectrum at 30 K for various degrees of monolayer coverage [96] (the adsorption is Langmuirian with half-coverage at about 10 atm). The bands labeled sf are for transitions of H2 on a smooth face and are from the 7 = 0 and J = 1 rotational states Q /fR) is assigned as a combination band. The bands labeled... 

Adsorbates can physisorb onto a surface into a shallow potential well, typically 0.25 eV or less [25]. In physisorption, or physical adsorption, the electronic structure of the system is barely perturbed by the interaction, and the physisorbed species are held onto a surface by weak van der Waals forces. This attractive force is due to charge fiuctuations in the surface and adsorbed molecules, such as mutually induced dipole moments. Because of the weak nature of this interaction, the equilibrium distance at which physisorbed molecules reside above a surface is relatively large, of the order of 3 A or so. Physisorbed species can be induced to remain adsorbed for a long period of time if the sample temperature is held sufficiently low. Thus, most studies of physisorption are carried out with the sample cooled by liquid nitrogen or helium. 

Note that the van der Waals forces tliat hold a physisorbed molecule to a surface exist for all atoms and molecules interacting with a surface. The physisorption energy is usually insignificant if the particle is attached to the surface by a much stronger chemisorption bond, as discussed below. Often, however, just before a molecule fonus a strong chemical bond to a surface, it exists in a physisorbed precursor state for a short period of time, as discussed below in section AL7.3.3. 

Note that chemisorption often begins with physisorption into a weakly bound precursor state. While in this... 

The energies of the selective adsorption resonances are very sensitive to the details of the physisorption potential. Accurate measurement allied to computation of bound state energies can be used to obtain a very accurate quantitative fonn for the physisorption potential, as has been demonstrated for helium atom scattering. For molecules, we have... 

The chemisorption of a molecule is often a precursor [31] to fiirther reactions such as dissociation (see section A3,9.5.2). that is, the molecule must reside in the precursor state exploring many configurations until finding that leading to a reaction. Where there is more than one distinct chemisorption state, one can act as a precursor to the other [32], The physisorption state can also act as a precursor to chemisorption, as is observed for the 02/Ag(l 10) system [33],... 

Sing K S W, Everett D H, Haul RAW, Mosoul L, Pierotti R A, Rouguerol J and Siemieniewska T 1985 Reporting physisorption data to the determination of surface area and porosity Pure Appl. Chem. 57 603-19... 

The hydrophobic character exhibited by dehydroxylated silica is not shared by the metal oxides on which detailed adsorption studies have been made, in particular the oxides of Al, Cr, Fe, Mg, Ti and Zn. With these oxides, the progressive removal of chemisorbed water leads to an increase, rather than a decrease, in the affinity for water. In recent years much attention has been devoted, notably by use of spectroscopic and adsorption techniques, to the elucidation of the mechanism of the physisorption and chemisorption of water by those oxides the following brief account brings out some of the salient features. 

Other workers have followed a similar approach prolonged evacuation at a low temperature, usually 25°C, is assumed to remove physisorbed, but not chemisorbed water, so that the subsequent isotherm can be ascribed to physisorption. 

Physisorption and chemisorption of water on alumina, titania and ferric oxide selection of results (Morimoto ef a/. )... 

The earlier interpretation of point X in terms of a close-packed monolayer of water would thus seem untenable. As has been clearly demonstrated, the total uptake at X, 327pmolg" , contains a contribution of ISOpmolg" from chemisorption thus physisorption accounts for only 177pmolg, which corresponds to 21 h per molecule of water. The fact that the total uptake at X corresponds to 11-2A, and is therefore close to the figure 10-5 for a close-packed monolayer, must be regarded as fortuitous. 

The first stage in the interpretation of a physisorption isotherm is to identify the isotherm type and hence the nature of the adsorption process(es) monolayer-multilayer adsorption, capillary condensation or micropore filling. If the isotherm exhibits low-pressure hysteresis (i.e. at p/p° < 0 4, with nitrogen at 77 K) the technique should be checked to establish the degree of accuracy and reproducibility of the measurements. In certain cases it is possible to relate the hysteresis loop to the morphology of the adsorbent (e.g. a Type B loop can be associated with slit-shaped pores or platey particles). 

A manual entitled Reporting Physisorption Data for Gas/Solid Systems with Special Reference to the Determination of Surface Area and Porosity has been prepared as a provisional publication by Commission 1.6 of the International Union of Pure and Applied Chemistry (lUPAC). The purpose of the manual is to draw attention to problems involved in reporting physisorption data and to provide guidance on the evaluation and interpretation of isotherm data. The general conclusions and recommendations are very similar to those contained in Chapter 6. 

A second class of monolayers based on van der Waal s interactions within the monolayer and chemisorption (in contrast with physisorption in the case of LB films) on a soHd substrate are self-assembled monolayers (SAMs). SAMs are well-ordered layers, one molecule thick, that form spontaneously by the reaction of molecules, typically substituted-alkyl chains, with the surface of soHd materials (193—195). A wide variety of SAM-based supramolecular stmctures have been generated and used as functional components of materials systems in a wide range of technological appHcations ranging from nanoHthography (196,197) to chemical sensing (198—201). 

Molecules arrive at the surfaces of traps and baffles by volume flow and surface creep. Molecules are trapped in vacuum systems by binding with energies much greater than kT of the surface, where k is Boltzmann s constant and Tthe absolute temperature, or by lowering the temperature of the surface in such a way that kT is less than the heat of physisorption of a molecular species on a surface. 

Chemical Potential. Equilibrium calculations are based on the equaHty of individual chemical potentials (and fiigacities) between phases in contact (10). In gas—soHd adsorption, the equiHbrium state can be defined in terms of an adsorption potential, which is an extension of the chemical potential concept to pore-filling (physisorption) onto microporous soHds (11—16). 

Sing, K. S. W., Everett, D. H., Haul, R. A. W., Moscou, L., Pierotti, R. A., Rouqudrol, J. and Siemieniewska, T., Reporting physisorption data for gas solid systems with special reference to the determination of surface area and porosity. Pure Appl. Cliem., 1985, 57(4), 603 619. 

Coupling to a mineral surface requires the presence of active hydroxyls on the substrate. The coupling reaction is a multi-step process that proceeds from a state of physisorption through hydrogen bond formation to actual covalent bond formation through condensation of surface hydroxyls with silanols ... 

PDMS based siloxane polymers wet and spread easily on most surfaces as their surface tensions are less than the critical surface tensions of most substrates. This thermodynamically driven property ensures that surface irregularities and pores are filled with adhesive, giving an interfacial phase that is continuous and without voids. The gas permeability of the silicone will allow any gases trapped at the interface to be displaced. Thus, maximum van der Waals and London dispersion intermolecular interactions are obtained at the silicone-substrate interface. It must be noted that suitable liquids reaching the adhesive-substrate interface would immediately interfere with these intermolecular interactions and displace the adhesive from the surface. For example, a study that involved curing a one-part alkoxy terminated silicone adhesive against a wafer of alumina, has shown that water will theoretically displace the cured silicone from the surface of the wafer if physisorption was the sole interaction between the surfaces [38]. Moreover, all these low energy bonds would be thermally sensitive and reversible. 

Theoretically, these intermolecular interactions could provide adhesion energy in the order of mJ/m. This should be sufficient to provide adhesion between the adhesive and the substrate. However, the energy of adhesion required in many applications is in the order of kJ/m. Therefore, the intermolecular forces across the interface are not enough to sustain a high stress under severe environmental conditions. It is generally accepted that chemisorption plays a significant role and thus, physisorption and chemisorption mechanisms of adhesion both account for bond strength. 

When gaseous or liquid molecules adhere to thesurface of the adsorbent by means of a chemical reaction and the formation of chemical bonds, the phenomenon is called chemical adsorption or chemisorption. Heat releases of 10 to 100 kcal/g-mol are typical for chemisorption, which are much higher than the heat release for physisorption. With chemical adsorption, regeneration is often either difficult or impossible. Chemisorption usually occurs only at temperatures greater than 200 C when the activation energy is available to make or break chemical bonds. 

Physisorption occurs when, as a result of energy differences and/or electrical attractive forces (weak van der Waals forces), the adsorbate molecules become physically fastened to the adsorbent molecules. This type of adsorption is multilayered that is, each molecular layer forms on top of the previous layer with the number of layers being proportional to the contaminant concentration. More molecular layers form with higher concentrations of contaminant in solution. When a chemical compound is produced by the reaction between the adsorbed molecule and the adsorbent, chemisorption occurs. Unlike physisorption, this process is one molecule thick and irreversible... 

J. Mai, W. von Niessen. The influence of physisorption and the Eley-Rideal mechanism on the surface reaction C0-(-02. Chem Phys 156 63-69, 1991. 

H. J. Kreuzer, Z. W. Gortel. Physisorption Kinetics. Berlin Springer-Verlag, 1986. 

The strength of the adsorptive interaction as expressed in values of AG jj depends on the mode of interaction between the adsorbate molecule and the electrode surface. Weak adsorption (physisorption) is based mostly on van-der-Waals-interactions. It shows t5q)ically values of AG jj > - 20 kJ mol Stronger... 

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