Adsorbate, adsorbent

The adsorption of nonionic surfactants, such as the alcohol ethoxylates, are in many cases Langmuirian, like those of most other highly surface active solutes adsorbing from dilute solutions, and the adsorption is generally reversible. However, several other adsorption types are produced [27] giving several steps that may be explained in terms of the various adsorbate-adsorbate, adsorbate-adsorbent and adsorbate-solvent interactions forming bilayers, hemi-miceUes, and micelles on the particle surface. 

The H-K method is based upon the model suggested by Everett and Powl100 which describes the interaction potential of a single adsorbate molecule between two parallel planes of the atoms of graphitized carbon. In the H-K expansion of the Everett and Powl s work, the space between the parallel carbon planes, i.e., the pore is assumed to be filled with adsorbed gas molecules. Thus, the contribution of adsorbate-adsorbate-adsorbent interaction to the total interaction potential is considered along with that of adsorbate-adsorbent interaction. 

In the case of the pore filled with adsorbate molecules one must account for adsorbate-adsorbent, and adsorbate-adsorbate-adsorbent interactions. Walker57 expressed the potential energy minimum, e, corresponding to these two respective interactions in terms of two dispersion constants, Ae.a, and Aa.a, as well as the number density of adsorbate and adsorbent atoms per unit area. This potential minimum takes the form ... 

Table I shoves that, for the small molecules, substances 1-4, the maximum number of adsorbed molecules considerably exceeds the number of adsorption centers of the zeolite. For the larger molecules, substances 5-7, the maximum number of adsorbed molecules is close to the number of adsorption centers. Therefore, 2 limiting cases are typical for adsorption on zeolites. The first case corresponds to the adsorption of relatively larger molecules (as compared with the void sizes for the zeolite in hand), which is determined to a great extent by the interaction of the molecules adsorbed with the adsorption centers of the zeolite even for the maximal filling of the zeolite voids. In the second case, after the filling of the adsorption centers, there may remain a free space in the zeolite voids for adsorption as a result of the manifestation of both dispersion forces (adsorbent-adsorbate interaction) and the forces of interaction between the molecules adsorbed (adsorbate-adsorbate interaction).

The process of polymer adsorption is quite different in many aspects from that of small molecules the latter has been studied extensively in the past. These differences in their adsorption characteristics arise in turn from the obvious flexibility of the larger polymer molecules, so that in addition to the nsnal adsorption factors considered, such as the adsorbate-adsorbent, adsorbate-solvent, and adsorbent-solvent interactions, a major aspect to be understood is the conformation of polymer molecnles at the interface and its role in dispersion. Polymers have a large number of functional groups, each of which can potentially adsorb at the surface, whereas smaller molecules are mostly monofunctional. 

The simplest representation of adsorbate - adsorbate- adsorbent interactions is the Fowler -Guggenheim isotherm which is formally similar to the Bragg-Williams approximation. This isotherm differs from the Langmuir one in the exponential term, accounts for the interactions. 

However, no clear justification could be found in the literature for incorporating the adsorbate-adsorbate-adsorbent interaction in this manner. Further, e(z) in Eq. 4.9 may be split as follows ... 

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. 

Type of Adsorbate-Adsorbent Interaction Type of Adsorbent ... 

The immediate site of the adsorbent-adsorbate interaction is presumably that between adjacent atoms of the respective species. This is certainly true in chemisorption, where actual chemical bond formation is the rule, and is largely true in the case of physical adsorption, with the possible exception of multilayer formation, which can be viewed as a consequence of weak, long-range force helds. Another possible exception would be the case of molecules where some electron delocalization is present, as with aromatic ring systems. 

SERS. A phenomenon that certainly involves the adsorbent-adsorbate interaction is that of surface-enhanced resonance Raman spectroscopy, or SERS. The basic observation is that for pyridine adsorbed on surface-roughened silver, there is an amazing enhancement of the resonance Raman intensity (see Refs. 124—128). More recent work has involved other adsorbates and colloidal... 

Fig. XVn-8. T vo additional types of adsorption isotherms expected for nonwetting adsorbate-adsorbent systems. (From Ref. 37.)...

Two other examples will sufhce. Methane physisorbs on NaCl(lOO) and an early study showed that the symmetrical, IR-inactive v mode could now be observed [97]. In more recent work, polarized FTIR rehection spectroscopy was used to determine that on being adsorbed, the three-fold degeneracies of the vs and v modes were partially removed [98]. This hnding allowed consideration of possible adsorbate-adsorbent geometries one was that of a tripod with three of the methane hydrogens on the surface. The systems were at between 4 and 40 K so that the equilibrium pressure was very low, about 10 atm. 

We conclude with the matter of adsorbate-adsorbate interactions these give rise to deviations from Henry s law behavior. These may be expressed in the form of a virial equation, much as is done for imperfect gases. Following Steele [8], one can write... 

I. Adsorption Heats and Entropies. It is not necessary, phenomenologically, to state whether the process is adsorption, absorption, or solution, and for the adsorbent-adsorbate complex formal equations can be written, such as... 

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. 

Since in chemisorption systems it is reasonable to suppose that the strong adsorbent-adsorbate interaction is associated with specific adsorption sites, a situation that may arise is that the adsorbate molecule occupies or blocks the occupancy of a second adjacent site. This means that each molecule effectively requires two adjacent sites. An analysis [106] suggests that in terms of the kinetic derivation of the Langmuir equation, the rate of adsorption should now be... 

---