Gas Adsorption and Surface Area Measurement

Ok, I believe the best way to judge if you understand certain circumstances is to write it down. How organized and prepared the sentences reflect your understanding level. So I decided to give shoot on what I am doing right now on research. Of course this is just preliminary.

When a gas or vapor is brought into contact with a solid, part of it is taken up by the solid. The molecules that dissappear from the gas either enter the inside of the solid, or remain on the outside attached to the surface. The former phenomenon is termed absorption (or dissolution). When the phenomena occur simultaneously, the process is termed sorption.

The solid that takes up the gas is called the adsorbent, and the gas or vapor taken up on the surface is called the adsorbate. It is not always easy to tell whether the gas is inside the solid or merely at the surface because most practical' absorbents are very porous bodies with large internal' surfaces. It is not possible to determine the surface areas of such materials by optical or electron microscopy because of the size and complexity of the pores and channels of the material. The gas adsorption itself, however, can be used to determine the accesible surface area of most absorbents.

Theory

Molecules and atoms can attach themselves onto surfaces in two ways. In physisorption (physical adsorption), there is a weak van der Waals attraction of the adsorbate to the surface. The attraction to the surface is weak but long ranged and the energy released upon accommodation to the surface is of the same order of magnitude as an enthalpy of condensation (on the order of 20 kJ/mol). During the process of physisorption, the chemical identity of the adsorbate remains intact, i.e. no breakage of the covalent structure of the adsorbate takes place.

In chemisorption (chemical adsorption), the adsorbate sticks to the solid by the formation of a chemical bond with the surface. This interaction is much stronger than physisorption, and, in general, chemisorption has more stringent requirements for the compatibility of adsorbate and surface site than physisorption. The chemisorption may be stronger than the bonds internal to the free adsorbate which can result in the dissociation of the adsorbate upon adsorption (dissociative adsorption).

The energetics of adsorption depend on the extent to which the available surface is covered with adsorbate molecules. This is because the adsorbates can interact with each other when they lie upon the surface (in general they would be expected to repel each other).

At any temperature, the adsorbate and the surface come to a dynamic equilibrium, that is, the chemical potentials of the free adsorbate and the surface bound adsorbate are equal. The chemical potential of the free adsorbate depends on the pressure of the gas, and the chemical potential of the bound adsorbate depends on the adsorption coverage. Thus the coverage at a given temperature is a function of the applied adsorbate pressure. The variation of adsorption coverage with p at a given T is called an adsorption isotherm.

Several adsorption isotherms have proven useful in understanding the process of adsorption. The simplest isotherm is attributed to a pioneer in the study of surface processes, Langmuir, and is called the Langmuir isotherm. If one assumes:

1. Adsorption cannot proceed beyond the point at which the adsorbates are one layer thick' on the surface (monolayer)
2. All adsorption sites are equivalent.
3. The adsorption and desorption rate is independent of the population of neighboring sites.

Instrument

The ASAP 2020 analyzer is equipped with two independent vacuum systems — one for sample preparation and one for sample analysis. Having two separate systems, as well as separate preparation ports, allows sample preparation and sample analysis to occur concurrently without interruption. Inline cold traps are located between the vacuum pump and the manifold in both the analysis and the degas systems.The sample saturation pressure (Psat) tube is located next to the sample analysis port. Gas inlet ports and cable connections are located conveniently on the side panel of the analyzer for easy access.

Compressed gases are required for analyses performed by the ASAP 2020 analyzer. Gas bottles or an outlet from a central source should be located near the analyzer. Appropriate two-stage regulators which have been leak-checked and specially cleaned are required. Pressure relief valves should be set to no more than 30 psig (200 kPag).