Adsorption interaction between molecules

The Langmuir isotherm assumes an homogeneous surface and negligible lateral interactions. However, adsorption interaction between molecules and surface heterogeneity plays an important role in many cases. 

Replacement of gas by the nonpolar, e.g., hydrocarbon phase (or oil phase) is used to modify the interactions between molecules in a spread film of investigated long-chain substances [6,15,17,18]. The nonpolar solvent-water interface possesses the advantage over that between gas and water, that the cohesion (i.e., interactions between adsorbed molecules due to dipole and van der Waals forces) is negligible. Thus, at the oil-water interfaces behavior of adsorbates is much closer to ideal, but quantitative interpretation may be uncertain, in particular for the higher chains which are predominantly dissolved in the oil phase to an unknown activity. Adsorption of dipolar substances at the w/a and w/o interfaces changes surface tension and modifies the surface potential of water [15] ... 

The basic assumption of the Langmuir adsorption isotherm is that the adsorbed molecules do not interact. This condition is not always fulfilled for adsorption, particularly on electrodes. The Frumkin adsorption isotherm includes interaction between molecules in the adsorption film,... 

Adsorption. This step depends on the possible interaction between molecules and the catalyst surface. When the reactants reach the active sites, they chemisorb on adjacent active sites. The chemisorption may be dissociative and the adjacent active sites may be of the same or different origin. The chemisorbed species react and the kinetics generally follow an exponential dependence on temperature, exp( EfRT), where E3 is the activation energy of chemisorption. 

How exactly the molecules are oriented inside the channels depends on their specific shape and on the adsorption interaction between the dyes and the channel walls or charge compensating cations. Because of the dye s oblongness, a double-cone-like distribution in the channels is a reasonable model. This distribution is illustrated in Fig. 19a. The arrows represent the transition moments of the dyes and a describes the half-opening angle of the double cone. The hexagonal structure of the zeolite L crystal hence allows six equivalent positions of the transition moments on this double cone with respect to the channel axis. 

Temkin (1941) approached the development of an adsorption isotherm by considering a heterogeneous surface (Section 6.8.3) where no molecular interactions exist. He divided the surface into different patches, and since there are no interactions between molecules, in each patch the Langmuir isotherm can be applied [see Eq. (6.196)] (Fig. 6.98). Thus, for the ith patch,... 

Silica gel is a polar material. The presence of silanol groups is responsible for the acidic catalytic effect of this material (the pK of Si OH is comparable to that of phenol). The mode of action of silica gel is based on adsorption (Fig. 3.9), a phenomenon that leads to the accumulation of a compound at the interface between the stationary and mobile phases. In the simplest case, a monolayer is formed (known as a Langmuir isotherm) but there is also some attraction and interaction between molecules that are already adsorbed and those still in solution. This contributes to the asymmetry of the elution profile. Although it demonstrates good resolution and a high adsorption capacity, bare silica gel is seldom used for analytical purposes. For most applications, it must be deactivated by partial rehydration (in 3-8% water). 

Another major advantage of molecular graphics is that it allows real time manipulation of several interacting molecules while quantitatively monitoring the stereochemistry and even the interaction energies. These methods are only beginning to be used for studies of interactions between molecules and only in the most preliminary way for the simulation of protein adsorption 29). 

Chemisorption [9] is an adsorptive interaction between a molecule and a surface in which electron density is shared by the adsorbed molecule and the surface. Electrochemical investigations of molecules that are chemisorbed to electrode surfaces have been conducted for at least three decades. Why is it, then, that the papers that are credited with starting the chemically modified electrode field (in 1973) describe chemisorption of olefinic substances on platinum electrodes [10,11] What is it about these papers that is different from the earlier work The answer to this question lies in the quote by Lane and Hubbard at the start of this chapter. Lane and Hubbard raised the possibility of using carefully designed adsorbate molecules to probe the fundamentals of electron-transfer reactions at electrode surfaces. It is this concept of specifically tailoring an electrode surface to achieve a particularly desired goal that distinguishes this work from the prior literature on chemisorption, and it is this concept that launched the chemically modified electrode field. 

Despite claims by Spry and Sawyer (1975) of analytical measurements verifying asphaltene molecular sizes in the 100 A range at ambient conditions, it is unlikely that molecules this bulky exist at reaction conditions. The good predictive capability of the model may therefore result from a compensation effect. Electrostatic and adsorption interactions between solute molecules and the pore walls not explicitly accounted for with the purely geometric partition coefficient may result in the diffusing molecules appearing larger than they are at reaction conditions. 

The method as a rule used for the determination of the specific surface of a material is the Brunauer-Emmet-Teller (BET) method [2,4,5], The BET theory of multilayer adsorption for the calculation of specific surface area, S, was originally developed by Brunauer, Emmett, and Teller [2,4,5], The adsorption process, within the frame of the BET theory, is considered as a layer-by-layer process. In addition, an energetically homogeneous surface is assumed so that the adsorption field is the same in any site within the surface. Additionally, the adsorption process is considered to be immobile, that is, each molecule is adsorbed in a concrete adsorption site in the surface. Subsequently, the first layer of adsorbed molecules has an energy of interaction with the adsorption field, and a vertical interaction between molecules after the first layer,, is explicitly analogous to the liquefaction heat of the adsorbate. Besides, adsorbed molecules do not interact laterally. 

If both the reactant and product are adsorbed on the electrode surface, the pre-wave appears if the adsorption of product is stronger than the adsorption of reactant, while the post-wave appears in the opposite case. If the adsorptions of both the reactant and product are equally strong, only a main wave can appear, but this is a simplification which neglects possible interactions between molecules in the first and second adsorbed layers. 

Note that in this specific model, desorption is neglected, and sites get regenerated upon adsorption, so the classic Langmuir blocking of sites is uncommon for MBE modeling. Furthermore, the diffusion-adsorption model for the terrace is only approximate since interactions between molecules are not accounted for. As a result, this hybrid model cannot handle nucleation between terraces, and applies only to small supersaturations or high temperatures [note that for high temperatures, one needs to include desorption in Eq. (2)] where the adatom concentration on terraces is relatively low. 

Heterogeneous eatalysis results from interactions between molecules in a gas or liquid phase and aetive sites at the surface of a solid. Mossbauer spectroseopy, whieh is an elemental probe teehnique, may be used to investigate adsorption sites eontaining appropriate isotopes and thereby gain information about adsorption and reaetion proeesses. 

This isotherm model was designed by Fowler and Guggenheim [12] to correct for the first-order deviations from the Langmuir isotherm. It assumes ideal adsorption on a set of localized sites on a homogeneous surface, with weak interactions between molecules adsorbed on neighboring sites. It assumes also that the interaction energy between two sorbate molecules is small enough that the random... 

The set of parameters collected from the linear regressions for the homogeneous surface model and from the linear regression and best fit for the molar differential heats of adsorption for heterogeneous surface model, taking into account the interactions between molecules... 

The L-type, would follow the Langmuir model, which is site adsorption without any lateral interaction between the adsorbate molecules. The concavity of the curve, in normal scale, is always directed toward the concentration axis. The S-type would follow a more complex model in which lateral interactions between molecules are to be taken into account, using, e.g., the Bragg-Williams approximation [15]. A concavity of the adsorption isotherm directed toward the y-axis is a very strong indication of lateral interactions between molecules. If one looks at the lUPAC classification of gas adsorption isotherms [1], the same remark holds this type of concavity is related with phenomena involving interactions between adsorbate molecules capillary condensation, multilayer formation, 2-D phase changes, etc. 

One of the prominent ways to reduce the barriers is to adsorb the substrate on the catalyst surface. The adsorptive interaction between catalyst surface and CO2 molecule change the molecular structure from linear to bent. The bending of the carbon dioxide depends on the nature of catalyst surface. Carbon dioxide adsorbs on a metal surface by nine different ways [22b]. The interaction of CO2 with the metal surface is beyond the scope of this document. CO2 interacts with semiconductor surface in different ways as shown in Fig. (3). 

There are B equivalent sites for localized adsorption in the first layer, interactions between molecules in the first layer being neglected. This is just Fowler s model for Langmuir adsorption on a uniform surface without interactions. 

Solution adsorption studies also deserve to be set apart. A start was made in 1956 by following 1-butanol adsorption out of water onto Graphon by heat of immersion measurements [31]. A model of preferential adsorption of the butanol, plus the measured adsorption isotherm and measured heat effects due to wetting of the adsorbed film at various pertinent concentrations, allowed the heats of immersion to be calculated. Interaction between molecules in the adsorbed film were taken to be the same as in the bulk solution. The calculated values were in excellent agreement with the experimental heats of immersion, as illustrated in Figure 7. No further studies of this kind have been performed on other fimctional groups, different chain lengths, or other molecular structures. 

Adhesion interactions at the solids/polymers interfaces are first and foremost adsorption interactions between the sofid surface and polymer molecules [1—11]. After polymerization there is a low molecular-weight fraction of coupling agents, which can decrease the cohesion and adhesion of the polymer film. If the molecules from this fraction interact with the filler particles preferentially (which can be reached due to the filler surface modification) instead of with the material surface covered, then the boundary layer of the film can be free from this fraction and adhesion increases as strengthening the boundary layer of the coating leads to stronger adhesion of the coating to the covered surfaces [46]. 

Its characteristic is low interaction between molecules and solid surfaces. The resulting forces are of the same order of the van der Waals forces, and the enthalpy of adsorption is in the same range of the condensation enthalpy or evaporation gases... 

The extension of the potential theory was studied by Bering et al (1963), Doong and Yang (1988) and Mehta and Dannes (1985) to multicomponent systems. We shall present below a brief account of a potential theory put forward by Doong and Yang (1988). The approach is simple in concept, and it results in analytical solution for the multicomponent adsorption isotherm. The basic assumption of their model is that there is no lateral interaction between molecules of different types and pure component isotherm data are described by the DA equation. With this assumption, the parameters of the DA equation (Wq, Eq, n) of each species are not affected by the presence of the other species, but the volume available for each species is reduced. This means that the volume available for the species i is ... 

Langmuir localised adsorption and no interaction between molecules... 

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