Adsorbed film depth

Using the assumption that the adsorbed film depth in a pore is the same as that on a plane surface for any value of relative pressure, one can write... 

In the second decrement the liquid volume desorbed iVuq)2 i st be corrected for the decrease in the adsorbed film depth remaining on the walls of previously emptied pores. By assuming the pores are cylindrical, the core volume V )2 can be calculated from the decrease in statistical thickness t, as... 

If the depth of the adsorbed film when condensation or evaporation occurs is t, then the actual pore radius r is given by... 

In Chapter 4, it was emphasized that the surface of an adsorbent is never covered with an adsorbed film of uniform depth but rather with stacks of... 

Shull assumed that the adsorbate molecules were packed one on top of the other in the film and deduced the monolayer depth to be 4.3 A for nitrogen. A more realistic assumption is that the film structure is close-packed hexagonal, leading to a monolayer depth of 3.54 A as shown by equation (8.23). Therefore, as stated previously, the statistical depth t of the adsorbed film is... 

Adsorbed films between two immiscible liquids. The question of the meaning of the term pn in the surface layer has been raised by Crax-ford, Gatty, and Teorell,2 without, however, coming to any very clear decision. Danielli s estimate was a very rough one, based on the application of the Donnan equilibrium between the surface layer and the interior, and suffers from the difficulties always attending an attempt to consider concentrations in the surface layer in a similar way to concentrations in a bulk phase the surface layer is not homogeneous. pH is closely related to, and is determined by, the electrochemical potential (see Chap. VIII, pp. 304 ff.), and this depends on the electrostatic potential, which varies rapidly at different levels near to the surface it appears possible that the only satisfactory definition of pa in the surface may be one which varies rapidly at different depths. The question appears one which would repay... 

The first two types of interaction are certainly physical adsorption the fourth is certainly chemisorption the third, which is a postulated mechanism whose actual occurrence has not yet been adequately demonstrated, exists in a twilight zone that defies classification as either physical or chemical adsorption. Previous papers of this series have not been concerned with the source of the adsorptive potential, but have postulated only that the adsorbed film be mobile their considerations could apply, therefore, to all adsorption situations other than No. 4. The present paper is devoted to a consideration of the adsorptive potential arising from 1 and 2—that is, physical adsorption properly so called. This general topic has been discussed by de Boer (5) for a number of specific cases the theory and techniques that we have developed enable us to derive from experimental data a quantity, Pads, the depth of the adsorptive potential well, which can also be calculated a priori from physical properties of the system. We are therefore able to compare theory and observation and so have an independent test of conclusions reached in previous papers. 

In the preceding paper (9) we quoted the observed values of the isosteric heats of adsorption of neon, argon, krypton, and xenon on P33 (2700°) graphite, as well as corresponding values of UJ and v obtained from a kinetic-molecular model of the adsorbed film. From these latter quantities we can derive the depth of the adsorption potential well, given by... 

The interactions between water and ceramic particles are complex and important for processes ranging from the rheology of slurries to the drying of particulate solids. An in-depth discussion of water-particle interactions is beyond the scope of this chapter. For the discussions that follow, it is sufficient to understand the forms that water takes within a particulate ceramic [27], At the lowest contents, water is present as partial, complete, or multiple layers adsorbed (physical) on the surface of the particles. After the surfaces are covered with a continuous adsorbed film, liquid water can condense in the pores between particles. Finally, at the highest water... 

In recent years, the measurement of thick adlayers has received an increasing interest in the field of so-called polyelectrolyte multilayer films. These films sometimes have a thickness of several micrometers. Such large thicknesses are obviously not easily monitored using conventional waveguide sensors because of the limited penetration depth into the adlayer. Simply, waveguide sensors loses their sensitivity when the adsorbed layer thickness exceeds 2 3 times the penetration depth of the evanescent field and cannot be used to monitor films thicker than 350 nm12. 

With the foregoing ideas in mind, one characteristic of the adsorbed monolayer becomes apparent molecular orientation at surfaces. For a film of RX on water, the picture that emerges is one in which the polar groups are incorporated into the aqueous phase with the hydrocarbon part of the molecule oriented away from the water. Such details as the depth of immersion of the tail and the configuration of the alkyl group are best approached by considering how the properties of the monolayer depend on the physical variables. 

Finely divided metal samples can also be prepared in the form of evaporated films in high vacuum, usually deposited on IR-transparent alkali halide plates (76-78). Such spectra are of interest in themselves, but tend to be much weaker than those obtained from the metal-particles-in-depth, oxide-supported catalysts. The rough surfaces of films of Cu, Ag, and Au, prepared by deposition on cold surfaces, can lead to very high-quality surface-enhanced Raman spectra (27, 28, 79, 80). The results from such experiments will be discussed in the later sections devoted to particular adsorbed hydrocarbons and metals, alongside the majority of spectra that are obtained on oxide-supported samples. 

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