Measurement of adsorption isotherms

Several experimental methods are applied to measure adsorption isotherms. The main problem is to determine the amount adsorbed. One method is a gravimetric measurement. In a gravimetric measurement the weight increase as a function of the pressure is determined. The adsorbent, usually in the form of a powder, is placed into a bulb and kept at the desired temperature. The bulb is mounted on a sensitive balance. Before the experiment, the bulb is 

In volumetric measurements the volume of an adsorbed gas at constant pressure and temperature is determined. Therefore, we first determine the dead space or volume of the bulb by admitting some nonadsorbing (or weakly adsorbing) gas such as Helium. Then, after evacuating the bulb, the gas of interest is admitted into the bulb. This is done at constant pressure and temperature. The volume admitted into the bulb minus the dead space is the amount adsorbed. 

More recently methods have also been developed to measure the adsorbed amount on single surfaces and not onto powders. Adsorption to isolated surfaces can, for instance, be measured with a quartz crystal microbalance (QCM) [383]. The quartz crystal microbalance consists of a thin quartz crystal that is plated with electrodes on the top and bottom (Fig. 9.11). Since quartz is a piezoelectric material, the crystal can be deformed by an external voltage. By applying an AC voltage across the electrodes, the crystal can be excited to oscillate in a transverse shear mode at its resonance frequency. This resonance frequency is highly sensitive to the total oscillating mass. For an adsorption measurement, the surface is mounted on such a quartz crystal microbalance. Upon adsorption, the mass increases, which lowers the resonance frequency. This reduction of the resonance frequency is measured and the mass increase is calculated [384-387], 

---

Only measurements of adsorption isotherms and determination of the conformation of the adsorbed chains could help the interpretation. One could indeed expect large loops in the first hypothesis and more flatted conformation if hydrogen bonds are predominant. 

Using this methodology via measurement of adsorption isotherms, Guiochon and coworkers investigated site-selectively the thermodynamics of TFAE [51] and 3CPP [54] on a tBuCQD-CSP under NP conditions using the pulse method [51], the inverse method with the equilibrium-dispersive model [51, 54], and frontal analysis [54]. 

One approach commonly used to study how strongly an aqueous ion binds to a sohd surface is measurement of adsorption isotherms as a function of pH, ionic strength, and total metal-ion concentration, in the presence or absence of other ions or organic coadsorbents, ft is often assumed that significant inhibition of adsorption with increasing ionic strength at a given pH indicates that the sorption complexes are dominantly of the weakly bound, outer-sphere type [123]. In contrast, when there is... 

Figure 1. Determination of the isosteric heat of adsorpton from the measurement of adsorption isotherms for the system Xe/ Ni(100) [13].

The following check list is recommended to assist authors in the measurement of adsorption isotherms and the presentation of the data in the primary literature. The reporting of results along generally accepted lines would considerably facilitate the compilation of data in the secondary literature and would thus promote interdisciplinary scientific cooperation [6]. 

John Sinfelt feels "that the most generally valuable tool for a long time has been the measurement of adsorption isotherms, including the BET method for determination of total surface area and various selective chemisorption methods for determining the amount of surface associated with particular component. The BET method has been widely used by catalytic chemists for almost half a century for the characterization of catalytic materials. It is among the foremost developments in surface science during the twentieth century". 

The determination of the adsorption of gases on solids can be a time consuming matter. Already in 1969 Jantti suggested to measure three points of the initial course of the kinetic adsorption curve and to extrapolate the equilibrium value (3PM). When the specific molecular model of the adsorption of a gas on a solid surface is known and when it can be expected that only one kind of adsorption is at stake, this method delivers good results and allows a very fast stepwise measurement of adsorption isotherms-. ... 

The objective of this chapter is to present the fundamental theories of adsorption followed by the description and discussion of experimental techniques for the measurements of adsorption isotherms and for the determination of surface area and pore size distribution. The adsorption of gases on microporous membranes and the inter-relation between adsorption and permeation are then discussed. The adsorption in liquid phase is briefly presented. The chapter concludes with a brief summary. 

Also, heats of immersion are frequently measured as a function of the preadsorption pressure. At the same time, these popular experiments are rarely accompanied by the measurements of adsorption isotherm. 

Note that the measurement of adsorption isotherms is not necessary for repulsive interactions, though it would be convenient to get an alternative value of l g to check the accuracy of the result. 

Measurement of adsorption isotherm Activated alumina is activated in a stream of N2 gas at 200°C for 4 hrs. About 1.0 g of sample is exactly weighed and taken in a 50 ml conical flask with a airtight stopper. Teflon tape is wound round the stopper to avoid loss of material due to evaporation. To this is added styrene of known TBC concentration and weighed. The conical flask is kept in a thermostated waterbath (Julabo SW 1) with constant stirring. The temperature is maintained at 15°C with an accuracy of + 0.1 C. After an equilibration period of 24 hrs. TBC in the solution phase is measured and the amount of TBC adsorbed is calculated by following relation... 

What has not been dealt with at nearly the same level of intensity is the relationship between adsorption and catalysis, and in particular between the energy terms in the parameters of the rate expression and catalyst performance. There are good reasons for this neglect, not the least being the lack of a clear and measurable relationship between adsorption and the rate of reaction - the activity - on a given catalyst. This is the problem that mechanistic rate expressions are well suited to illuminate by simulation, although the direct measurement of adsorption isotherms pertinent to the active species on the catalyst surface is not yet possible. At some future time the use of a TS-SSR (see Chapter 5) may give us a better appreciation of the adsorption properties of catalysts. 

X-ray powder diffraction (XRPD), thermo gravimetric (TGA) analysis, solid-state nuclear magnetic resonance (NMR), and measurements of adsorption isotherms are key methods for characterizing zeolite-like behavior. However, a simple proof for observing structural changes during the sorption processes is XRPD. 

Measurements of adsorption isotherms of organic vapours on clean glass surfaces have been performed by Bottomley and co-workers. They find that, in general, monolayer coverage (based on liquid densities) is reached at approximately 75 per cent of the saturation pressure. For typical pK-apparatus they calculate that adsorption can produce errors in B as large as 30 to 100 cm mol when the adsorption occurs on a clean surface. Minute traces of grease... 

A large number of methods for measurements of adsorption isotherms and kinetic data (mostly diffusion coefficients) have been developed and published. A number of good reviews in this area can be found in classical adsorption books. 

Flow-type measurement of adsorption isotherms—10, 20, and 30 cm long pieces of square Berea core (2.5 x 2.5 cm) were cut, dried, and cast in epoxy. Several pore volumes of NaCl brine were injected into the cores and the pore volumes were determined from differences in core weights. Surfactant solutions were then injected into the cores at linear velocities not exceeding 1.25 cm/ hour (1 ft/day) and samples were collected in enclosed vials since the early experiments showed that a substantial error occurs due to the evaporation of the solution if the outlet of the core is left open to the air. Volumes of solutions injected into the cores were measured and the concentration of surfactant in each sample was determined. 

Figure 2.10. Sensor gas calorimeter (SGC) for simultaneous measurements of adsorption isotherms and enthalpies. IFT, University of Siegen, 2003.

---