Van der Waals adsorption


Emmett P H and Brunauer S 1937 The use of low temperature van der Waals adsorption isotherms in determining the surface area of iron synthetic ammonia catalysts J. Am. Chem. See. 59 1553-64  [c.1896]

Brunauer S, Deming L S, Deming W S and Teller E A 1940 Theory of the van der Waals adsorption of gases J. Am. Chem. See. 62 1723-32  [c.1896]

In such an experiment the material actually adsorbed by the solid (the adsorbent) is termed the adsorbate, in contradistinction to the adsorptive which is the the general term for the material in the gas phase which is capable of being adsorbed. The adsorption is brought about by the forces acting between the solid and the molecules of the gas. These forces are of two main kinds—physical and chemical—and they give rise to physical (or van der Waals ) adsorption, and chemisorption respectively. The nature of the physical forces will be dealt with in the next section meanwhile it is convenient to note that they are the same in nature as the van der Waals forces which bring about the condensation of a vapour to the liquid state.  [c.2]

In such an experiment the material actually adsorbed by the solid (the adsorbent] is termed the adsorbate, in contradistinction to the adsorptive which is the the general term for the material in the gas phase which is capable of being adsorbed. The adsorption is brought about by the forces acting between the solid and the molecules of the gas. These forces are of two main kinds—physical and chemical—and they give rise to physical (or van der Waals ) adsorption, and chemisorption respectively. The nature of the physical forces will be dealt with in the next section meanwhile it is convenient to note that they are the same in nature as the van der Waals forces which bring about the condensation of a vapour to the liquid state.  [c.2]

There is always some degree of adsorption of a gas or vapor at the solid-gas interface for vapors at pressures approaching the saturation pressure, the amount of adsorption can be quite large and may approach or exceed the point of monolayer formation. This type of adsorption, that of vapors near their saturation pressure, is called physical adsorption-, the forces responsible for it are similar in nature to those acting in condensation processes in general and may be somewhat loosely termed van der Waals forces, discussed in Chapter VII. The very large volume of literature associated with this subject is covered in some detail in Chapter XVII.  [c.350]

The thermodynamics of Ae wetting transition as the critical point is approached has been treated for simple fluids [10-12] and polymer solutions [13], and the influence of vapor adsorption on wetting has been addressed [14]. Widom has modeled the prewetting transition with a van der Waals-like theory and studied the boundary tension between two coexisting surface phases [15]. The line tension at the three phase contact line may diverge as the wetting transition is approached [16]. The importance of the precursor film (see Section X-7A) on wetting behavior was first realized by Marmur and Lelah in 1980 in their discovery of the dependence of spreading rates on the size of the solid surface [17]. The existence of this film has since received considerable attention [1, 18, 19].  [c.466]

All gases below their critical temperature tend to adsorb as a result of general van der Waals interactions with the solid surface. In this case of physical adsorption, as it is called, interest centers on the size and nature of adsorbent-adsorbate interactions and on those between adsorbate molecules. There is concern about the degree of heterogeneity of the surface and with the extent to which adsorbed molecules possess translational and internal degrees of freedom.  [c.571]

As discussed in connection with Table XVI-1, one can expect important surface structural changes to accompany adsorption whenever the adsorbent-adsorbate bond is comparable in energy to the heat of sublimation of the adsorbent, that is, to the adsorbent-adsorbent bond energy. In the case of refractory solids, such as metals and ionic crystals and oxides, sublimation energies are in the range of 20-100 kcal/mol, which is also that for chemisorption. One thus expects surface reconstruction to be likely under chemisorption conditions. The energy of physical adsorption, which is due to van der Waals forces, is typically around 5-10 kcal/mol, about the same as the cohesion energy of molecular solids and of chain-chain interactions in polymers. One concludes that physical adsorption should not perturb the structure of a refractory adsorbent but should be able to do so in the case of a molecular solid. As an example, surface restmcturing is indicated when n-hexane is adsorbed on ice above -35°C [121,122]. The surface structure of polymers has been estimated from LEIS (see p. 296) measurements here, one can expect the surface packing of polymer chains to be affected when physical adsorption of a vapor occurs.  [c.591]

The quantity zoi will depend very much on whether adsorption sites are close enough for neighboring adsorbate molecules to develop their normal van der Waals attraction if, for example, zu is taken to be about one-fourth of the energy of vaporization [16], would be 2.5 for a liquid obeying Trouton s rule and at its normal boiling point. The critical pressure P, that is, the pressure corresponding to 0 = 0.5 with 0 = 4, will depend on both Q and T. A way of expressing this follows, with the use of the definitions of Eqs. XVII-42 and XVII-43 [17]  [c.614]

It must be remembered that, in general, the constants a and b of the van der Waals equation depend on volume and on temperature. Thus a number of variants are possible, and some of these and the corresponding adsorption isotherms are given in Table XVII-2. All of them lead to rather complex adsorption equations, but the general appearance of the family of isotherms from any one of them is as illustrated in Fig. XVII-11. The dotted line in the figure represents the presumed actual course of that particular isotherm and corresponds to a two-dimensional condensation from gas to liquid. Notice the general similarity to the plots of the Langmuir plus the lateral interaction equation shown in Fig. XVII-4.  [c.624]

Physical adsorption may now, in fact, be seen as a preamble to chemisorption in heterogeneous catalysis, that is, as a precursor state (see Section XVIII-4). Physical adsorption is not considered to involve chemical bond formation, however, and the current theoretical approaches deal mainly with the relatively long range electrostatic and van der Waals types of forces. The quantum mechanics of chemical bonding is thus largely missing although aspects of it appear in the treatment of physisorption on metal surfaces. Steele [8] provides an extensive review of molecular interactions in physical adsorption generally, and for the case of molecules adsorbed on the graphite basal plane in particular [95].  [c.634]

One may choose 6(Q,P,T) such that the integral equation can be inverted to give f Q) from the observed isotherm. Hobson [150] chose a local isotherm function that was essentially a stylized van der Waals form with a linear low-pressure region followed by a vertical step tod = 1. Sips [151] showed that Eq. XVII-127 could be converted to a standard transform if the Langmuir adsorption model was used. One writes  [c.656]

The second general cause of a variable heat of adsorption is that of adsorbate-adsorbate interaction. In physical adsorption, the effect usually appears as a lateral attraction, ascribable to van der Waals forces acting between adsorbate molecules. A simple treatment led to Eq. XVII-53.  [c.700]

The saturation coverage during chemisorption on a clean transition-metal surface is controlled by the fonnation of a chemical bond at a specific site [5] and not necessarily by the area of the molecule. In addition, in this case, the heat of chemisorption of the first monolayer is substantially higher than for the second and subsequent layers where adsorption is via weaker van der Waals interactions. Chemisorption is often usefLil for measuring the area of a specific component of a multi-component surface, for example, the area of small metal particles adsorbed onto a high-surface-area support [6], but not for measuring the total area of the sample. Surface areas measured using this method are specific to the molecule that chemisorbs on the surface. Carbon monoxide titration is therefore often used to define the number of sites available on a supported metal catalyst. In order to measure the total surface area, adsorbates must be selected that interact relatively weakly with the substrate so that the area occupied by each adsorbent is dominated by intennolecular interactions and the area occupied by each molecule is approximately defined by van der Waals radii. This  [c.1869]

Fig. 3.24 Test of the tensile strength hysteresis of hysteresis (Everett and Burgess ). TjT, is plotted against — Tq/Po where is the critical temperature and p.. the critical pressure, of the bulk adsorptive Tq is the tensile strength calculated from the lower closure point of the hysteresis loop. C), benzene O. xenon , 2-2 dimethyl benzene . nitrogen , 2,2,4-trimethylpentane , carbon dioxide 4 n-hexane. The lowest line was calculated from the van der Waals equation, the middle line from the van der Waals equation as modified by Guggenheim, and the upper line from the Berthelot equation. (Courtesy Everett.) Fig. 3.24 Test of the tensile strength hysteresis of hysteresis (Everett and Burgess ). TjT, is plotted against — Tq/Po where is the critical temperature and p.. the critical pressure, of the bulk adsorptive Tq is the tensile strength calculated from the lower closure point of the hysteresis loop. C), benzene O. xenon , 2-2 dimethyl benzene . nitrogen , 2,2,4-trimethylpentane , carbon dioxide 4 n-hexane. The lowest line was calculated from the van der Waals equation, the middle line from the van der Waals equation as modified by Guggenheim, and the upper line from the Berthelot equation. (Courtesy Everett.)
Forces of Adsorption. Adsorption may be classified as chemisorption or physical adsorption, depending on the nature of the surface forces. In physical adsorption the forces are relatively weak, involving mainly van der Waals (induced dipole—induced dipole) interactions, supplemented in many cases by electrostatic contributions from field gradient—dipole or —quadmpole interactions. By contrast, in chemisorption there is significant electron transfer, equivalent to the formation of a chemical bond between the sorbate and the soHd surface. Such interactions are both stronger and more specific than the forces of physical adsorption and are obviously limited to monolayer coverage. The differences in the general features of physical and chemisorption systems (Table 1) can be understood on the basis of this difference in the nature of the surface forces.  [c.251]

Adsorption on a nonpolar surface such as pure siUca or an unoxidized carbon is dominated by van der Waals forces. The affinity sequence on such a surface generally follows the sequence of molecular weights since the polarizabiUty, which is the main factor governing the magnitude of the van der Waals interaction energy, is itself roughly proportional to the molecular weight.  [c.252]

Although very few systems conform accurately to the Langmuir model, this model provides a simple quaUtative representation of the behavior of many systems and it is therefore widely used, particularly for adsorption from the vapor phase. According to the Langmuir model the heat of adsorption should be independent of loading, but this requirement is seldom fulfilled in practice. Both increasing and decreasing trends are commonly observed (Eig. 5). Eor a polar sorbate on a polar adsorbent (ie, a system in which electrostatic forces are dominant) a decreasing trend is normally observed, since the relative importance of the electrostatic contribution declines at high loadings as a result of preferential occupation of the most favorable sites and consequent screening of cations. In contrast, where van der Waals forces are dominant (nonpolar sorbates), a rising trend of heat of adsorption with loading is generally observed. This is commonly attributed to the effect of intermolecular attractive forces, but other explanations are also possible (11). In homologous series such as the linear paraffins the heat of adsorption increases linearly with carbon number (12).  [c.255]

Adsorption at Solid—Liquid Interface. Practical appHcations of surfactants usually iavolve some manner of surfactant adsorption on a sohd surface. This adsorption is always associated with a decrease ia free-surface eaergy, the magnitude of which must be determined indirecdy. The force with which the adsorbate is held on the adsorbent may be roughly classified as physical, ionic, or chemical. Physical adsorption is a weak attraction caused primarily by van der Waals forces. Ionic adsorption occurs between charged sites on the substrate and oppositely charged surfactant ions, and is usually a strong attractive force. The term chemisorption is appHed when the adsorbate is joined to the adsorbent by covalent bonds or forces of comparable strength. The surface condition of a sohd adsorbent markedly affects its adsorption characteristics. Important considerations include smoothness, cleanliness, particle size, packing of powders, and presence of capillary systems. Typical substrates on which adsorption effects are important include metals, glass, plastics, textile fibers, sand, cmshed minerals, plant foHage, paper, and the mixed sohd dirt that coUects on clothing, linen, walls, and floors. Analysis of the shape of adsorption isotherms has been the technique most widely used in investigations of adsorption mechanisms, but their interpretation has not been estabhshed for the numerous combinations of adsorbent—adsorbate and interactions in multicomponent surfactant systems (see Adsorption, LIQUID separation).  [c.236]

The size of particles removed by such filters is less than the size of the passages. The mechanism of removal includes adsorption (qv) of the impurities at the interface between the media and the water either by specific chemical or van der Waals attractions or by electrostatic interaction when the medium particles have surface charges opposite to those on the impurities to be removed.  [c.276]

These traits have led to a wide diversity of theoretical and analytical advances needed to interpret coUoidal behavior properly. Various traits that relate to the properties and behavior of coUoids have developed since the middle of the twentieth century (21,25,26). Notable examples include experimental use of the Mie and inelastic theories of light scattering, photon correlation spectroscopy, neutron and x-ray scattering, electrical double-layer models, quantitative formulation of van der Waals interactions, gas adsorption theory, electron microscopy, ultracentrifugation, various spectroscopic techniques based on electron, ion, and photon probes, pulsed nuclear magnetic resonance methods, and direct measurement of interparticle forces. Many of these can be used, either directly or indirectly, to study the changes that a coUoidal system undergoes, with the appearance or disappearance of the coUoidal state being the most critical event and a change in the tendency to agglomerate (soUds) or to coalesce (Uquids, gases) being the most common industrial problems.  [c.394]

Adsorption (qv) is a phenomenon in which molecules in a fluid phase spontaneously concentrate on a sohd surface without any chemical change. The adsorbed molecules are bound to the surface by weak interactions between the sohd and gas, similar to condensation (van der Waals) forces. Because adsorption is a surface phenomenon, ah practical adsorbents possess large surface areas relative to their mass.  [c.506]

Adhesion is a phenomenon by which two materials form a contact region that is able to sustain or transmit stress. There are a variety of mechanisms or factors that contribute to the adhesion between two materials. These include interfacial van der Waals forces that lead to adsorption interdiffusion of molecules across the interface interfacial chemical bonding and/or hydrogen bonding mechanical interlocking electrostatic interactions and so on. It should be emphasized that in a given engineering or real situation, one or more of the above-mentioned mechanisms may be operative. It may also be noted that the van der Waals molecular level interactions are universally present in all situations, and that these interactions are usually attractive in nature. The details of the origin and nature of interfacial forces are discussed in several text books and review articles. (See references [7-11], for example.)  [c.76]

Degree of agglomeration. Some fillers such as clay, carbon blacks and fumed silicas have a natural tendency to agglomerate. Van der Waals forces are primarily responsible for agglomeration of fillers during production and storage. However, agglomeration of filler particles is complex as it can be also influenced by the particle size, chemical groups on the filler surface, moisture level, and the method of filler production. For instance, water adsorption is mainly responsible for the agglomeration of titanium dioxide, while interactions between silanol groups on the filler surface are responsible for the agglomeration of fumed silicas.  [c.630]

Physical adsorption theory proposes, that if there is intimate contact between the substrate and the adhesive, interatomic and intermolecular forces will operate across this interface, resulting in primary bonding. These interatomic and intermolecular forces include the low energy London dispersion and van der Waals forces, which encompass interactions between both permanent and induced dipoles. Higher energy intermolecular forces such as hydrogen bonding and acid-base interactions are also included in this mechanism. Donor-acceptor interactions are sometimes included in this mechanism, although their bond strength is intermediate between the physical forces and chemical bonds.  [c.695]

Adsorption is the selective collection and concentration onto solid surfaces of particular types of molecules contained in a liquid or a gas. By this unit operation gases or liquids of mixed systems, even at extremely small concentrations, can be selectively captured and removed from gaseous or liquid streams using a wide variety of specific materials known as adsorbents. The material which is adsorbed onto the adsorbent is called the adsorbate. The two mechanisms involved, chemical adsorption and physical adsorption, focus specifically on carbon adsorption. When gaseous or liquid molecules reach the surface of an adsorbent and remain without any chemical reaction, the phenomenon is called physical adsorption or physisorption. The mechanism of physisorption may be intermolecular, electrostatic or van der Waals forces, or may depend on the physical configuration of the adsorbent such as the pore structure of the adsorbent. Physical absorbents typically have large surface areas. The properties of the material being adsorbed (molecular size, boiling point, molecular weight, and polarity) and the properties of the surface of the adsorbent (polarity, pore size, and spacing) together serve to determine the quality of adsorption. There are also the following parameters which can be used to improve physical adsorption increase the adsorbate concentration increase the adsorbate area select the best absorbent for the specific gas system remove contaminants before adsorption reduce the adsorption temperature increase the adsorption contact time frequently replace or regenerate the adsorbent.  [c.276]

The adsorption of ions and molecules at an electrified interface is of fundamental importance in all aspects of aqueous corrosion, and it is therefore relevant to consider it briefly in the context of this discussion on the nature of the electrified interfaceGases may be adsorbed on solid surfaces in various ways, and when the adsorbed layer is held by the residual forces around the gas molecule, which are responsible for the liquefaction of gases and the cohesion in liquids, it is referred to as physical or van der Waals adsorption. Physical adsorption is reversible and the adsorbed gas can be removed by evacuation or by heating to moderate temperatures the enthalpy of physical adsorption is small ([c.1175]

The classic explanation for the presence of an activation energy in the case where dissociation occurs on chemisorption is that of Lennard-Jones [113] and is illustrated in Fig. XVIII-12 for the case of O2 interacting with an Ag(llO) surface. The curve labeled O2 represents the variation of potential energy as the molecule approaches the surface there is a shallow minimum corresponding to the energy of physical adsorption and located at the sum of the van der Waals radii for the surface atom of Ag and the O2 molecule. The curve labeled O + O, on the other hand, shows the potential energy variation for two atoms of oxygen. At the right, it is separated from the first curve by the O2 dissociation energy of some 120 kcal/mol. As the atoms approach the surface, chemical bond formation develops, leading to the deep minimum located at the sum of the covalent radii for Ag and O. The two curves cross, which means that O2 can first become physically adsorbed and then undergo a concerted dissociation and chemisorption process, leading to chemisorbed O atoms (see Ref. 113a for a more general diagram). In this type of sequence, the molecularly adsorbed species is known as a precursor state (see Refs. 115 and 116).  [c.703]

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.  [c.294]

If we knew the variation m A as a fiinction of coverage 0, this would be the equation for the isothenn. Typically the energy for physical adsorption in the first layer, -A E, when adsorption is predominantly tlnongh van der Waals interactions, is of the order of lO/rJ where T is the temperature and /rthe Boltzmann constant, so that, according to equation (B1.26.6), the first layer condenses at a pressure given by PIPq. 10  [c.1871]

Finally, ethane merits special mention on account of its early appearance in the field of surface area evaluation from adsorption isotherms usually at 78 and 90 K. Its saturation vapour pressure at these temperatures is so low (0 0017 and 0-0083 Torr respectively) that the dead space correction for unadsorbed gas is nearly or quite negligible. In 1947, Brown and Uhlig, in their estimates of the surface roughness of chrome-plated nickels from ethane adsorption at 90 K, used the value a (C2H6) = 20-5 A, calculated from the lattice spacing of sotid ethane (m.p. of ethane = 90-4 K) this figure has been adopted by a number of workers. Later, Kiselev and his collaborators, working with graphitic carbon of known area, obtained 22-7 A from the BET monolayer capacity (at 173 K), as compared with 20-4 A from a molecular model and 22-2 A from the liquid density (taking the van der Waals thickness of an ethane monolayer as 4 A). The use of ethane for surface area determination seems to have fallen olT in recent years, however.  [c.80]

Second-harmonic generation, and xps measurements (246,247), as well as near edge X-ray absorption fine stmcture spectroscopy (nexafs) studies confirm the two-step mechanism (248). Studies also showed pronounced differences between the short (n < 9) and long (n > 9) alkanethiolates, probably owing to the decreased rate of the second step which results from the diminution of the interchain VDW attraction energy. In the case of simple alkyl chains, the masking of adsorption sites by disordered chains is not a serious problem. However, if the chain contains a bulky group, the two steps are coupled, and the chemisorption kinetics is greatly impeded by the chain disorder (249). A direct competition between / fZ-butyknercaptan and -octadecylmercaptan reveals that the latter adsorbed onto gold at greater efficiency than the former by a factor of 290—710 from ethanol (250). The additive effects of the stabilizing van der Waals interactions in the alkyl mercaptan monolayer and the stericaHy hindered / fZ-butylmercaptan explain the clear preference of the linear molecules.  [c.540]

However, polymer stabilization is sensitive to the properties of the environment. This has been described in an eady calculation of polymer conformation at an interface (21). The fraction of adsorbed polymer was plotted as a function of its total adsorption energy (= number of adsorbing groups times their individual adsorption energy) at constant molecular weight, showing the range for usehd polymer stabilization to be extremely narrow. Adsorption energies lower than the optimal range (A, Fig. 10) give no adsorption of the polymer and no stabilization. At a higher adsorption energy (B, Fig. 10), the polymer adsorbs dat at the interface. Such an adsorption is also without stabilization effect because at short distances the van der Waals potential has already reached such large negative values that the potential well is too deep for the droplets to be deaggregated. Only in a limited range of adsorption energies, in which loops and tails are formed, does the polymer serve. In addition, the same phenomenon means that a minimum molecular weight is necessary to obtain stabiUty.  [c.200]

Adsorption involves, in general, the accumulation (or depletion) of solute molecules at an interface (including gas-liquid interfaces, as in foam fractionation, and hquid-liquid interfaces, as in detergency). Here we consider only gas-solia and liquid-solid interfaces, with solute distributed selectively between the fluid and solid phases. The accumulation per unit surface area is small thus, highly porous solids with veiy large internal area per unit volume are preferred. Adsorbent surfaces are often physically and/or chemically heterogeneous, and bonding energies may vary widely from one site to another. We seek to promote physical adsorption or physlsoiption, which involves van der Waals forces (as in vapor condensation), and retard chemical adsorption or chemisorption, which involves chemical bonding (and often dissociation, as in catalysis). The former is well suited for a regenera-  [c.1496]

When a gas comes in contact with a solid surface, under suitable conditions of temperature and pressure, the concentration of the gas (the adsorbate) is always found to be greater near the surface (the adsorbent) than in the bulk of the gas phase. This process is known as adsorption. In all solids, the surface atoms are influenced by unbalanced attractive forces normal to the surface plane adsorption of gas molecules at the interface partially restores the balance of forces. Adsorption is spontaneous and is accompanied by a decrease in the free energy of the system. In the gas phase the adsorbate has three degrees of freedom in the adsorbed phase it has only two. This decrease in entropy means that the adsorption process is always exothermic. Adsorption may be either physical or chemical in nature. In the former, the process is dominated by molecular interaction forces, e.g., van der Waals and dispersion forces. The formation of the physically adsorbed layer is analogous to the condensation of a vapor into a liquid in fret, the heat of adsorption for this process is similar to that of liquefoction.  [c.736]

Adsorption on solids is a process in which molecules in a fluid phase are concentrated by molecular attraction at the interface with a solid. The attraction arises from van der Waals forces, which are physical interactions between the electronic fields of molecules, and which also lead to such behavior as condensation. Attraction to the surface is etihanced because the foreign molecules tend to satisfy an imbalance of forces on the atoms in the surface of a solid compared to atoms within the solid where they are surrounded by atoms of the  [c.246]


See pages that mention the term Van der Waals adsorption : [c.417]    [c.16]    [c.637]    [c.901]    [c.1870]    [c.1871]    [c.1871]    [c.1874]    [c.81]    [c.159]    [c.254]    [c.12]   
Corrosion, Volume 2 (2000) -- [ c.13 , c.20 ]