Sorption/desorption measurement

Direct measurement of solute sorption-desorption kinetics in chromatographic systems provides some useful insights into the mechanism... 

An additional difficulty in studies of slow chemisorption is evidenced by the Taylor-Liang technique of measuring adsorption isobars at successively higher temperatures without pumping between runs, a technique that has been reviewed in an earlier volume of this series (8). The ad-sorption-desorption phenomena observed between room temperature and about 150°C. clearly show in this temperature range the existence on the surface of zinc oxide of two types of hydrogen chemisorption (9). 

The integral permeability coefficient P may be determined directly from permeation steady-state flux measurements or indirectly from sorption kinetic measurements 27 521 activity is usually replaced by gas concentration or pressure (unless the gas deviates substantially from ideal behaviour and it is desired to allow for this) and a<>, ax (p0, Pi) are the boundary high and low activities (pressures) respectively in a permeation experiment, or the final (initial) and initial (final) activities (pressures) respectively in an absorption (desorption) experiment. 

Basically, perecnl relative humidiiy is an indicator ol the water vapor saturation deficit of ihe gas mixture, rather ihun an indicator of sorption, desorption, comfort, or evaporation. A measurement of RH without a corresponding measurement of dry bulb temperature is not of particular value, since the water vapor content cannot be determined from % RH alone... 

The data shown here for diffusion refer to the system polyethyl acrylate-benzene, and have been determined by Kishimoto and Enda (unpublished) by using the sorption technique. Values of D calculated from both absorption and desorption measurements agreed with each other over the range of benzene concentration studied at each temperature of the experiment, and the D0 values were determined by appropriate graphical extrapolation of those D values. In Fig. 15 are shown... 

The automated sorption apparatus developped can be useful for easy and rapid adsorption and desorption measurements of solutes in supercritical fluids. 

Earlier studies of intracrystalline diffusion in zeolites were carried out almost exclusively by direct measurement of sorption rates but the limitations imposed by the intrusion of heat transfer and extra-crystalline mass transfer resistances were not always fully recognized. As a result the reported diffu-sivities showed many obvious inconsistencies such as differences in diffusivity between adsorption and desorption measurements(l-3), diffusivities which vary with fractional uptake (4) and large discrepancies between the values measured in different laboratories for apparently similar systems. More recently other experimental techniques have been applied, including chromatography and NMR methods. The latter have proved especially useful and have allowed the microdynamic behaviour of a number of important systems to be elucidated in considerable detail. In this paper the advantages and limitations of some of the common experimental techniques are considered and the results of studies of diffusion in A, X and Y zeolites, which have been the subject of several detailed investigations, are briefly reviewed. 

Water sorption-desorption isotherms can be carried out by using thermobalances. Now specific instruments allow to measure water sorption-desorption isotherms at different constant temperatures (e.g., dynamic vapor sorption instrument (DVS), Surface Measurement Systems Ltd., Monarch Beach, US). 

Nitrogen sorption/desorption isotherms of membrane samples (carefully dried at room temperature) were obtained by Micrometries Instrument Corp., Norcross, Georgia. About 1.5g of each membrane samples was used for measurements of BET surface areas, 21 point sorption and 22 point desorption isotherms. The Instrument used was DigiSorb 2500 with fully automated control. The most significant experimental error introduced was from saturation pressure variation of about + 1.5 mm Hg (that is, about 0.2% error in relative pressure). 

The majority of sorption kinetic stndies have ntilized either batch or flow-through methods coupled with aqueous measurements for determination of the concentrations of species of interest. More recent work has focused on molecular-scale approaches, including spectroscopic and microscopic techniques that allow for observations at increased spatial and temporal resolution to be made, often in situ and in real time. Complementary to both macroscopic and molecular-scale observations has been the utilization of theoretical techniques, such as molecular mechanics and quantum mechanics, to model surface complexes computationally. It has been through the integration of macroscopic, molecular-scale, and theoretical approaches that some of the most profound observations of sorption-desorption phenomena over the past decades have been made. 

---