Absolute mass adsorbed

Both concepts of masses of an adsorbate discussed so far - the Gibbs surface excess (m g) based on proposition (PI) and calculated by Eq. (1.27) and the absolute mass adsorbed (m ) based on proposition (P2) and calculated by either Eq. (1.34) or (1.35) - do have their physical limitations. Hence it is desirable to mention other possibilities to define and to measure masses of adsorbates in gas adsorption systems. 

Data present absolute masses adsorbed (m ) calculated from Eq. (1.34), i. e. assuming V = and V = m /p with pj = 0.600 g/cm having been determined via tbe optimization procedure (1.33) using the Langmuirian adsorption isotherm (AI)... 

In principle it also should be possible to determine absolute masses adsorbed by analogously combined magneto-calorimetric or by magnetodielectric measurements, the later being probably the most favorable in view of the highly sensitive experimental equipment available today. It also would be desirable to compare results of measurements of absolute masses adsorbed with those of numerical simulation models to get new insight for both the analytical model used and the experimental technique proposed here [1.54]. 

Similarly one may proceed in order to get the absolute masses adsorbed (mi,m2), in this case using for (V ) instead of the helium approximation (VhJ (2.7) the model equation (2.9). 

Absolute mass adsorbed of both components (nco2 t>di4)- ( ) ... 

Table 4.2. Gibbs excess masses and absolute masses of a CO2 / CH4 co-adsorbed phase on AC D47/3 at T = 293 K for pressures up to 1.4 MPa, [4.17]. For calculation of absolute masses adsorbed cp. also (2.7), (2.9), (2.31).

Figure 5.8. Reduced masses resulting from osciUometric and gravimetric adsorption measurements (n sc, figrav) of CO2 on activated carbon (Norit R1 Extra) at 293 K. Gibbs excess masses () and absolute masses adsorbed ( m ...

It follows that a standard crystal with a natural frequency of 9 MHz and a surface area of about one square centimeter will manifest a change in frequency of about 200 Hz for each microgram of adsorbed solute. Now frequency changes can be measured to within 0.1 Hz with normal equipment consequently a change in mass adsorbed of 0.2 ng (lO g) should be detectable. It follows that this type of device should be very sensitive but it appears that, so far, it has not been made available commercially, at least, not as a GC detector. One attractive feature of this detecting system is that it basically measures mass and therefore could be considered to be an absolute detector. 

Figure 1.20. Plane surface sorption system (PSSS) in a box of total volume (V ) including a certain mass of sorptive gas (m ) part of it being adsorbed on the surface of the sorbent, the sorbate having the absolute mass (m ) (layer model) and the Gibbs surface excess mass (mog). ...

These data also present absolute masses of N2 adsorbed on the AC Norit R1 which have been calculated by proposition (P4) (1.43), i. e. taking the (changing) volume of the adsorbate phase (V = m /pj ) into account. The volume (V ) of the AC impenetrable to the N2 molecules has been calculated by the optimization procedure (1.44) using the adsorption isotherm (1.49). Data can easily be fitted by the Langmuir isotherm (1.49). Also the thermodynamic stability condition (1.47) holds. 

Outline of Calorimetric-Dielectric Measurements of Absolute Masses of Adsorbates... 

Absolute masses of adsorbates defined by Eq. (1.14) in principle can experimentally be determined by combined dielectric and calorimetric measurements. In this section we only will present the basic thermodynamic idea of this method and give an example. Details can be found in the literature [1.54]. 

Figure 1.24. Absolute masses (m ) measured by combined dielectric-calorimetric experiments and Gibbs surface excess masses (mj,g) measured manometiically and also gravimetrically of carbon dioxide CO2 (4.5) adsorbed on Wessalite (Degussa DAY) atT= 298 K, [1.54].

Figure 4.15. Kinetics of the adsorption process of a gasmixture (CO2, CH4) with equilibrium sorptive gas concentrations (yco2 20 %mol, = 80 %mol). The upper curve (Am) indicates the total absolute mass, cp. (2.31), of CH4 and CO2 adsorbed as function of time. Data have been taken from the microbalance recordings. The lower curves show the time variation of the pressure and the temperature of the sorptive gas mixture inside the adsorption vessel. As can be seen adsorption equilibrium is approached asymptotically and realized - in technical terms - after approximately 4 hours.

Because of its sensitivity to the top layer of the surface, ISS has proven very useful in providing surface compositional analysis of materials such as catalysts and alloys. ISS can also provide structural information on metal, semiconductor, metal o.vidc, and adsorbate surfaces. The major limitations of ISS are associated with difficulties in providing quantitative results, because of neutralization reactions and other interactions. Collisional processes and inelastic energy losses also make absolute mass determinations difficult. In compositional analysis, ISS is complementary to AES although not as generally applicable. 

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