Adsorption characteristic time

A possible approach to interpretation of a low-frequency region of the G ( ) dependence of filled polymers is to compare it with a specific relaxation mechanism, which appears due to the presence of a filler in the melt. We have already spoken about two possible mechanisms — the first, associated with adsorption phenomena on a filler s surface and the second, determined by the possibility of rotational diffusion of anisodiametrical particles with characteristic time D 1. But even if these effects are not taken into account, the presence of a filler can be related with the appearance of a new characteristic time, Xf, common for any systems. It is expressed in the following way... 

In case when the concentration of adsorption particles increases t > -j4co ) formulas (1.108) and (1.109) become valid only for times t < tee, where tee is the characteristic time determined from equation > which is the condition of leveling-off of criti-... 

Observe that p is a Damkdhler number since it can be interpreted as the ratio of a characteristic contact time for flow in a thin film (RT/i/aQ), to a characteristic time for adsorption, (rQ/[k... 

To establish the well drainage boundaries and fluid flow patterns within the TFSA-waterflood pilot, an interwell chemical tracer study was conducted. Sodium thiocyanate was selected as the tracer on the basis of its low adsorption characteristics on reservoir rocks (36-38), its low and constant background concentration (0.9 mg/kg) in produced fluids and its ease and accuracy of analysis(39). On July 8, 1986, 500 lb (227 kg) of sodium thiocyanate dissolved in 500 gal (1.89 m3> of injection brine (76700 mg/kg of thiocyanate ion) were injected into Well TU-120. For the next five months, samples of produced fluids were obtained three times per week from each production well. The thiocyanate concentration in the produced brine samples were analyzed in duplicate by the standard ferric nitrate method(39) and in all cases, the precision of the thiocyanate determinations were within 0.3 mg/kg. The concentration of the ion in the produced brine returned to background levels when the sampling and analysis was concluded. 

Smooth and uniform polymer surface after vacuum plays a key role to ensure good OFRR sensing performance. We have observed in experiments that toluene after vacuum is prone to leave a number of cavities of a few micrometers in diameter on the surface. These cavities will induce additional scattering loss for the WGMs in the OFRR, which greatly degrade the g-factor, and hence the detection limit of the OFRR vapor sensor. Moreover, these small cavities have different adsorption characteristics compared to smooth polymer surface. Vapor molecules may be retained for a longer time at the cavity, which increases the response time and recovery time. Acetone and methanol are found to be better candidates for solvents because they usually leave uniform and smooth surface after vacuum. 

Another adsorption characteristic which can be deduced from the primary experiments described above is the evaporation rate E. This i.s the number of molecules of nitrogen which leave the surface per square centimeter per second and which previously were chemisorbed as atoms. That nitrogen leaves the surface as molecules rather than as atoms follows from the experimental fact that the flash-off experiments yield the same values as the pressure-time curves. If the nitrogen came off as atoms, it would stick to glass walls and would not reach the ion gauge bulb to be recorded as a sudden increase in pressure. Only at very high flash-off temperatures is the evaporation rate of atoms comparable with the evaporation rate of molecules. 

Phenol and dodecyl benzene sulfonate are two solutes that have markedly different adsorption characteristics. The surface diffusion coefficient of phenol is about fourteen times greater than that for dodecyl benzene sulfonate. The equilibrium adsorption constants indicate that dodecyl benzene sulfonate has a much higher energy of adsorption than phenol (20,22). The adsorption rates from a mixture of these solutes can be predicted accurately, if (1) an adequate representation is obtained for the mixture equilibria, and (2) the diffusion rates in the solid and fluid phases are not affected by solute-solute interactions. 

Another tool used to study friction on the molecular scale is the quartz crystal microbalance (QCM) introduced in Section 9.4.1. The QCM has been used to monitor the adsorption of thin films on surfaces via the induced frequency shift [385], In the years since 1986, Krim and coworkers could show that the slippage of adsorbed layers on the QCM leads to a damping of the oscillator [472], This damping is reflected as a decrease in the quality factor Q of the oscillator. From the change in Q, a characteristic time constant rs, the so-called slip-time, can be derived. This corresponds to the time for the moving object s speed to fall to 1 /e, i.e. 

The molecular sieve behaviour of zeolites can be controlled by a hydrolytic process at elevated temperatures. Water vapour in contact with zeolite crystals at elevated temperatures results in a variation of the zeolitic adsorption characteristics. The amount of water vapour, the pretreatment temperature and the pretreatment time, can control the effect pore size of zeolites. It appears, that a steam treatment causes a cation migration and a cation hydrolysis of the exchangeable cations. However, the effect of steam on the adsorption behaviour of zeolites is influenced by the nature of the initial exchangeable cations. 

A large size of the surfactant polymer molecules (e.g., the PAA molecule consists of ca. 105 monomers units) and interaction between the segments of these molecules decrease considerably the conformational mobility of both the whole molecule and single segment of its chain. This may increase the characteristic time of sulfur atoms and other particles adsorption-desorption to 1 hour and more, i.e., to the time typical for the observed dark relaxation process. 

The aforesaid may be schematically presented as a time scale hierarchy, where the characteristic times of overall xreac and establishment of the adsorption-desorption equilibrium Tad.des are indicated. Let us estimate these times for the reaction system under study. 

As noted above, the characteristic time of establishing the PAA adsorption-... 

Thus, the adsorption layer at the colloidal particle surface is represented by the segments (areas) of at least two types, which differ significantly in the time of establishing the adsorption-desorption equilibrium with respect to the characteristic time of photocatalytic overall reaction. A part of the surface of particle is not coated with the PAA molecules, and for this part... 

An interesting avenue for investigation is to examine the adsorption characteristics on single crystals concurrently with electrical measurements. Thus, any relationship which possibly exists between the slow states and the chemisorption might be positively revealed. Examination of the adsorption characteristics of reduced germanium crystals and the effect of the fast states would also be of interest. These studies have been initiated. It remains clear at this time, however, that the semiconductor properties of the germanium influence the surface properties of the thin oxide films supported thereon. The influence is clear in the case of propanol adsorption and the differences are even more dramatic in the case of water adsorption. 

The above formulas may become inapplicable for systems with adsorption processes or/and coupled chemical steps in solution whose characteristic times are comparable with the inverse frequency within the impedance measurement interval. In this case the charge-transfer resistance, Rct, must be replaced by a complex charge-transfer impedance, Zct. Another restriction of this treatment is its assumption of the uniform polarization of the m s interface which requires to ensure a highly symmetrical configuration of the system. Refs. [i] Sluyters-Rehbach M, Sluyters JH (1970) Sine wave methods in the study of electrode processes. In Bard A/ (ed) Electroanalytical chemistry, vol. 4. Marcel Dekker, New York, p 1 [ii] Bard A], Faulkner LR (2001) Electrochemical methods, 2nd edn. Wiley, New York [iii] Retter U, Lohse H (2005) Electrochemical impedance spectroscopy. In Scholz F (ed) Electroanalytical methods. Springer, Berlin, pp 149-166 [iv] Bar-soukov E, Macdonald JR (ed) (2005) Impedance spectroscopy. Wiley, Hoboken... 

Heterogeneously catalyzed reactions are usually studied under steady-state conditions. There are some disadvantages to this method. Kinetic equations found in steady-state experiments may be inappropriate for a quantitative description of the dynamic reactor behavior with a characteristic time of the order of or lower than the chemical response time (l/kA for a first-order reaction). For rapid transient processes the relationship between the concentrations in the fluid and solid phases is different from those in the steady-state, due to the finite rate of the adsorption-desorption processes. A second disadvantage is that these experiments do not provide information on adsorption-desorption processes and on the formation of intermediates on the surface, which is needed for the validation of kinetic models. For complex reaction systems, where a large number of rival reaction models and potential model candidates exist, this give rise to difficulties in model discrimination. 

---