Measurement of Adsorbed Amounts

In the manometry technique coupled with CPG or MS, the determination of the amount of each component i adsorbed (n ) consists to perform a mass balance on the gas phase from the total pressure and the mole fraction of component i in the gas phase, measured before and after adsorption [9]. As the system is closed, we consider that the amount of matter leaving the gas phase is equal to that one adsorbing on the solid. 

let us consider the adsorption of a single component in a closed system (Fig. 7.13). The volume of the system containing the adsorbent previously evacuated 

By introducing Eq. 7.1 in the right hand side and as Qie volume of the adsorbed phase 

In the manometric technique, the measure of the concentration (pressure) of the gas before and after adsorption does not lead to the adsorbed amount but to the surface excess. However, if the volume of the adsorbed phase is small compared to that one of the system (V V ) and if the concentration of the gas is not too much high, Eq. 7.24 can be reduced to  

the surface excess can be assimilated to the adsorbed amount if the volume of the reactor is enough large and the pressure enough low. 

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The combined measurements of adsorbed amount and electrical effects in the present work indicate that monolayer models are not adequate to explain our observations. In analogy with the behaviour of the lipid type of amphiphiles, which adsorb as bilayers on metal oxides [3], the bilayer structure is a natural model to consider also in the case of amphiphilic proteins. 

If the van t Hoff and isosteric methods are simple ways for estimating the adsorption enthalpy of single component from isothermal adsorption data, they have the disadvantage to not take into account the temperature dependence on the enthalpy and entropy and to be not enough accurate. Moreover they are not adapted to the adsorption of gas mixtures. The best mean to determine the adsorption and coadsorption enthalpy is to measure them by using a differential calorimetry technique coupled with others techniques allowing the measure of adsorbed amount and composition as for example the manometry and the chromatography. 

Over the past fifty years, the gas adsorption has become the main technique for the characterization of porous or finely divided solids. This rise is due to the success of the BET and B3H theories (and related procedures), as well as to the growing needs of characterization data for the inorganic, organic and pharmaceutical industries or research centers. Other needs concern crude measurements of adsorbed amounts (chemisorption, gas separation data). Consequently, many laboratories now buy commercially available devices, or build them, to perform these measurements. The development of the computer science has also provided facilities for the automation of the entire experimental run and data reduction. 

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