The Adsorption Time

A useful parameter to characterize adsorption is the adsorption time. Let us first assume that no forces act between the surface and a gas molecule. Then, if a molecule hits the surface, it is reflected elastically with the same energy. An energy transfer between the surface and gas 

An attractive force between the gas molecule and the surface increases the average residence time of the molecule at the surface to 

Another useful parameter is the accommodation coefficient a. The accommodation coefficient is defined by the temperature of the molecules before the impact T), the surface temperature 7 2, and the temperature of the reflected molecules T3 [364]  

For an elastic reflection, the mean velocity of the molecules before and after hitting the surface are identical and so are the temperatures T = 7 s. Then a = 0. If the molecules reside a long time on the surface they have the same temperature, after desorption, as the surface T2 = 7 s and a = 1. Thus, the accommodation coefficient is a measure of how much energy is exchanged before a molecule leaves the adsorbent again. 

A useful approach to the phenomenon of adsorption is from the point of view of the adsorption time, as discussed by de Boer [4]. Consider a molecule in the 

In the case of polyatomic molecules, one may consider separately the accommodation coefficients for translational and for vibrational energy. Values of the latter, civ, are discussed by Nilsson and Rabinovitch [7]. 

If attractive forces are present, then according to an equation by Frenkel (see Ref. 2), the average time of stay t of the molecule on the surface will be 

In addition to Q and r, a quantity of interest is the surface concentration F, where 

if Z is expressed in moles of collisions per square centimeter per second, r is in moles per square centimeter. We assume the condensation coefficient to be unity, that is, that all molecules that hit the surface stick to it. At very low Q values, F as given by Eq. XVII-3 is of the order expected just on the basis that the gas phase continues uniformly up to the surface so that the net surface concentration (e.g., F2 in Eq. XI-24) is essentially zero. This is the situation 

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The rate constants k and ki may be related to the concepts of the preceding section as follows. First, k is simply the reciprocal of the adsorption time, that is. 

Fig. 5.10. Series of sequential in-situ AFM images of the growth of ODS on silicon. The numbers indicate the adsorption time in minutes.

It is reasonable to assume, at least for oppositely charged polymers and particles, that aA > ag, which means that the adsorption time is always expected to be shorter than the coagulation time under perikinetic conditions. Consequently, perikinetic flocculation rates are very likely not to be adsorption rate limited. The ratio of orthokinetic adsorption time to orthokinetic coagulation time is... 

What is the significance of the constants kad and kde is the inverse of the adsorption time ... 

Figure 30 shows the CTL glow curves for the catalyst pre-adsorbed ethanol or acetone vapor. The catalyst is heated at a rate of 0.5 °C/s in synthetic air. The CTL intensity increases at high temperatures, and the total amount of CTL intensity L depends on the adsorption time At and the gas concentration C during adsorption. We can measure the gas at a very low concentration by measuring the value of L because L is proportional to the product of At and C in a region of low AtC where a Henry-type adsorption isotherm holds. 

The absence of CSDAPM adsorbed layer aging can be the sequence of this PE nature this polyelectrolyte is weak and has low molecular weight in comparison with PDADMAC. Hence, the reconstruction inside the adsorbed layer may occur during less time, in our case during the adsorption time scale. 

Our study revealed the time effect on the structure of adsorbed layer. It was noted that the deformation ability of PDADMAC layer decreased in one day after its formation. This means that the conformation rearrangement time is more than the adsorption time. 

Ethyltriethoxysilane (ETES). The critical surface tensions of the ETES films obtained on silica by retraction from 1% solutions in a-chloro-naphthalene are plotted in Figure 2A. The adsorption time—i.e., the abscissa—is the time the silica substrate was allowed to remain in contact with the adsorbate solution. Adsorption times longer than 20 hours did not produce any further decrease in yc and solutions containing 0.1% and 5% ETES gave values within 1 dyne/cm. of those in Figure 2A. Attempts to obtain ETES films from solution in isopropylbicyclohexyl were unsuccessful—the solutions did not retract from the test surfaces even after 20 hours adsorption time. Isopropylbicyclohexyl has a surface tension of 34.4 dynes/cm. so if the ETES adsorbed to form films having yc values of 33-35 dynes/cm., as it had from -chloronaphthalene, then the bicyclohexyl solution would not be expected to retract. 

The 0H2o values for the VTES films were anomalously low on air-dried silica, as had been found for the ETES films. The anomaly persisted even when the adsorption time was extended to many days. On flamed silica, however, the anomaly was small or did not appear at all for films obtained from the 1% and 5% solutions after 2 hours or more adsorption time. 

Acid-Base Catalyzed Adsorption. It was found that the addition of low molecular weight organic acids and bases to the adsorbate solutions produced ETES and VTES films that were much more hydrophobic and resistant to the water desorption test than the films formed by the tri-ethoxysilanes alone. Tables I and II list the values of yc and 0H2o for the films produced in the presence of various acids and bases. Also included are the data for the films formed by ETES and VTES alone. In all cases the adsorption time was 20-24 hours, the solvent was a-chloro-naphthalene and the triethoxysilane concentration was 1% by weight. 

Potentiodynamic gas sensors have a schematic structure that is practically equal to that of amperometric gas sensors. They are -> electrochemical cells that measure a -> current signal directly related to the concentration of the analyte, but are not necessary operated in a region where -> mass transport is limiting. They are typically employed to detect less reactive species such as benzene and halogenated hydrocarbons that require a previous accumulation step at a suitable -> adsorption potential to be then reduced or oxidized according to a given potential scan [iii]. The adsorption time can be automat-... 

At 175°C, t > 0.2 s for E, > 100 kJ.mok. For the value of 50 kJ.mol given by Welipitiya et al., the adsorption time is only 7 x 10 s. The extreme sensitivity on does not allow a precise estimation of the adsorption time in the present case. However, the value of 50 kJ.mof proposed for E is certainly underestimated, because it concerns an atomically smooth surface. On a surface under growth conditions that are far from the Frank-van der... 

Depletion factor (raw gas concentration/clean gas concentration), working capacity (mass-related loading as a percentage for each test taking into account the residual loading present after the previous test) and the adsorption time before breakthrough (set at 10% of the raw gas concentration value) were considered as important parameters for evaluating the measurement... 

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