Absolute masses of adsorbates

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]. 

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. 

These data present absolute amounts of adsorbed masses calculated from 2-databy using proposition (P3), i. e. optimization procedure (1.38) with the Langmuirian isotherm (1.49). The data fit is reasonably well, cp. Table 1.7. Also data increase monotonously with increasing gas density, i. e. the stability condition (1.47)... 

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. 

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).

The quantities of interest are (i) n, moles of adsorbate (ii) m, mass of adsorbent (iii) V, volume (iv) p, pressure (v) T, absolute temperature (vi) R, molar ideal gas constant (vii) A, surface area of the adsorbent (viii) Q heat (ix) U, internal energy (x) H, enthalpy (xi) 5, entropy and (xii) G, Gibbs free energy. Superscripts refer to differential quantities (d) experimentally measured quantities (exp) integral quantities (int) gas phase (g), adsorbed phase (s) and solid adsorbent (sol) quantities standard state quantities (°). Subscript (a) refers to adsorption phenomena (e.g. AaH) [13, 91]. 

Since the latex is slightly polydisperse the specific surface area of the latex cannot be calculated with sufficient accuracy. We will therefore present the adsorption results per unit mass of the latex. Note, that we are in this work only concerned with the surfactant composition on the surface and not the absolute value of the amount of adsorbed surfactant per unit area. 

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. 

Recently, Keller published a method allowing for the absolute determination of the adsorbed mass m of porous materials [5]. It consists of a combination of calorimetric and impedance spectroscopic measurements. Unfortunately, that method is experimentally difficult and needs improvements for practical applications [6]. 

Maximum stability is attained when saturation of the adsorbed surfactant layer is reached. In the case of high-volume fraction at a specific surfactant concentration, if the adsorbed layer is unsaturated, the emulsion stability decreases. In a low-volume fraction case, if the specific surfactant concentration is too high, emulsion stability is also decreased. Therefore, while an increase in surfactant concentration may increase the stability of the internal phase, the absolute stability of the ELM may be decreased [88]. Carrier Concentration Mass transfer rates can be increased by increasing the carrier concentration [41], however, increasing the carrier concentration usually increases swelling and lowers the emulsion stability [33,108]. Other studies have found limits to the carrier concentration where further increases do not lead to an increase in extraction rates as the mobility of the carrier is stifled due to an increase in viscosity [34,41]. 

When normalized to unit surface area, the adsorption density of the anionic surfactant is higher on quartz than on Berea sandstone because quartz carries a more positive surface charge than the clays (The clays provide most of the surface area for adsorption in Berea sandstone). If it is assumed that the betaine adsorbs on sandstone at least in part by its cationic group, then the lower adsorption density of the betaine on quartz than on Berea sandstone can also be attributed to electrostatic interactions. Matrix grains of the size encountered in typical reservoir rocks have low specific surface areas. Accordingly, the absolute amount of surfactant adsorbed or the amount adsorbed per unit mass of rock is lower for a clean sand than for a sand containing clays (12, 34, 82). Therefore, the... 

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