Partially mobile adsorbates

Figure 1.18. Patchwise partially mobile adsorbates. and are the fraction of the surface on which the adsorption Is localized and the fraction of the adsorbate that Is localized, respectively. Figure (a) localized fraction, figure (b) Isotherms. Parameters in the moleculeir partition functions correspond to nitrogen atoms, ignoring rotation.

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. 

In theoretical studies, two different concepts of the adsorption system are considered. The first assumes that the adsorbed film forms an individual thermodynamic phase, being in thermal equilibrium with the bulk uniform part of the system. This model has been very effectively used to describe various adsorption systems [5,6]. It allows one to derive the relatively simple equations for adsorption equilibrium by utilizing the quality of the chemical potentials of a given component in both phases. Numerous models of the surface (adsorbed) phase are considered it may be assumed to be a monolayer or multilayer and either localized, mobile, or partially mobile, molecular interaction can be taken into account or neglected, and so on. However, the thermodynamical correctness of the concept of surface phase is coniroversial. 

Experimental results have elearly shown that during the formation of the monolayer, a change from nonlocalized to locahzed adsorption occurs. Several theoretical studies have been made of the so-called partially mobile or partially localized adsorption models [11,60,161,164,228,230]. These theories must explain the phase transitions in the adsorbed monolayers and may also be useful in describing surface transport phenomena [11]. 

Grahame introdnced the idea that electrostatic and chemical adsorption of ions are different in character. In the former, the adsorption forces are weak, and the ions are not deformed dnring adsorption and continne to participate in thermal motion. Their distance of closest approach to the electrode surface is called the outer Helmholtz plane (coordinate x, potential /2, charge of the diffuse EDL part When the more intense (and localized) chemical forces are operative, the ions are deformed, undergo partial dehydration, and lose mobility. The centers of the specifically adsorbed ions constituting the charge are at the inner Helmholtz plane with the potential /i and coordinate JCj < Xj. 

The most important feature of monolithic media is that the mobile phase flows exclusively through the separation unit. In contrast, there is no flow inside the conventional porous chromatographic particles and only a partial flow through the perfusion beads. Just as with the membrane adsorbers, monolith stationary phases may be operated with a minimum in mass transfer resistance with the concomitant advantages in terms of speed and throughput. 

For Example 9 the order is 0.7, suggesting a model for which the logL value is between that for Step 2 (0.5 order, log L = 15) and Step 1 (1.0 order, log L = 22). Thus, the rate-determining step may be a reaction on a partially filled surface. Since the log L value calculated in this way for 0.7 order is rather large, some surface mobility and/or rotation is indicated. The zero-order reactions of Examples 11 and 19 are clearly surface reactions for which expected site densities are obtained. For Example 11 Tottrup (25) suggested that the rate-determining step is C-O scission in adsorbed CO. 

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). 

Standard state free energies (AG°,js) and entropies (ASacjs) may also be determined from GSC retention data if ideal conditions are assumed. For the adsorbate behaving as an ideal gas in the mobile phase, the standard state is defined as a partial pressure of 1 atm. The adsorbed standard state is defined as a two-dimensional perfect gas at 1 atm where the mean distance between adsorbed molecules is the same as in the three dimensional gas phase standard state. Thus, the sorbate equilibrium surface concentration Cg becomes 4.07 x 10 9/T (moles/cm2) and the gas phase sorbate concentration becomes 4.07 x 10- /TK ,. 

This description shows first, that due to the independent character of elementary diffusion acts between atoms of various substances, their diffusion mobility is controlled by the different partial diffusion coefficients second, that the diffusion of atoms and molecules adsorbed on the surface takes place due to their "overjumps to neighbouring unoccupied sites (vacancies). 

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