Interfacial kinetics measurement examples

The interfacial area can be measured in specific systems using chemical reactions in which the absorption rate kinetics are a known function of the gas-liquid interfacial area. For example, Vasquez et al. (2000) compared three different chemical methods (i) Danckwerts method using the absorption of CO2 in sodium or potassium carbonate buffer solutions, (ii) the sodium sulfite method involving the oxidation of sulfite ions, and (iii) the sodium dithionite method involving the oxidation of dithionite ions. All three methods were shown to produce similar interfacial area measurements. 

A second approach is to use the SECM tip as a passive sensor of the concentration boundary layer of a freely reactive crystal. For example, a shear-force constant-distance scanning potentiomet-ric system with a Ca + ion-selective probe was used to map Ca + from a calcium carbonate surface under slightly acidic conditions [47]. These studies provide insights into interfacial concentrations, but do not exploit the high mass transport rates that can be generated in SECM to allow quantitative high-resolution kinetic measurements. 

The combination of photocurrent measurements with photoinduced microwave conductivity measurements yields, as we have seen [Eqs. (11), (12), and (13)], the interfacial rate constants for minority carrier reactions (kn sr) as well as the surface concentration of photoinduced minority carriers (Aps) (and a series of solid-state parameters of the electrode material). Since light intensity modulation spectroscopy measurements give information on kinetic constants of electrode processes, a combination of this technique with light intensity-modulated microwave measurements should lead to information on kinetic mechanisms, especially very fast ones, which would not be accessible with conventional electrochemical techniques owing to RC restraints. Also, more specific kinetic information may become accessible for example, a distinction between different recombination processes. Potential-modulation MC techniques may, in parallel with potential-modulation electrochemical impedance measurements, provide more detailed information relevant for the interpretation and measurement of interfacial capacitance (see later discus-... 

There is currently little understanding of the influence of interfacial composition and (nano)structure on the kinetics of enzymatic hydrolysis of biopolymers and lipids. However, a few preliminary studies are beginning to emerge (McClements et al., 2008 Dickinson, 2008). Thus, for example, Jourdain et al. (2009) have shown recently that, in a mixed5 sodium caseinate + dextran sulfate system, the measured interfacial viscosity increased from qs = 220 mN s m 1 without enzyme to qs = 950 mN s m 1 with trypsin present. At the same time, the interfacial elasticity was initially slightly reduced from (7S = 1.6 mN m 1 to (h = 0.7 mN m, although it later returned to close to its original value. Conversely, in the... 

While the quasistatic method is quite accurate, it requires a long time to determine a complete adsorption kinetics curve. This is because a new drop has to be formed at the tip of the capillary to determine one single measurement point. For example, if ten dynamic interfacial tension values are to be determined over a period of 30 min, -180 min will be required to conduct the entire measurement. On the other hand, the constant drop formation method is often limited because a large number of droplets have to be formed without interruption, which may rapidly empty the syringe. Furthermore, the critical volume required to cause a detachment of droplets depends on the density difference between the phases. If the density difference decreases, the critical volume will subsequently increase, which may exacerbate the problem of not having enough sample liquid for a complete run. 

A rigorous kinetic description of interfacial catalysis has been hampered by the ill-defined physical chemistry of the lipid—water interface (Martinek et ai, 1989). Traditional kinetic assumptions are undermined by the anisotropy and inhomogeneity of the substrate aggregate. For example, the differential partitioning of reactants (enzyme, calcium ion, substrate) and products (lysolecithins, fatty acids) between the two bulk phases prevents direct measurement of enzyme and substrate concentrations. This complicates dissection of the multiple equilibria that contribute to the observed rate constants. Only recently has it become possible to describe clearly the activity of SPLA2S in terms of traditional Michaelis— Menten kinetics. Such a description required the development of methods to reduce experimentally the number of equilibrium states available to the enzyme (Berg etai, 1991). 

However, in kinetic regime D, the variations in the average rate of absorption with the concentration of each component should conform to Eq. (99), and the experimental values measured in equipment where the interfacial area A is known should lead to the values of the partial orders. Examples of such variation in the average volumetric rate of absorption of oxygen

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The natural first question to ask is whether the crystal-liquid surface free energy can be measured experimentally by some method that is independent of nucleation kinetics. In gas-liquid nucleation studies, for example, it is routine to measure the surface tension of the liquid and to use its equality with the gas-liquid surface free energy to make predictions of nucleation rates and compare them with experiment. For the liquid-solid transition, the situation is quite different, however. This is true first because the surface tension and the surface free energy are no longer strictly equal due to the possible existence of strains in the crystal. The second reason is that measurements of liquid-solid free energies or interfacial tensions are by no means simple to devise or carry out, and so are available only in certain special cases. These limited experimental data are summarized in this section. 

With both staged equipment and differential contactors, availability oradequate phase-equilibrium models and rate expressions would allow application of existing correlations and simulation algorithm). For example. knowledge of metal-extraction kinetics in terms of interfacial species concentrations conld be combined with correlations of film mass transfer coefficients in a particular type of equipment to obtain the inlerfacial flux as a fuuction of bulk concentrations. Correlations or separate measurements of inierfacial area and an estimate of dispersion characteristics would allow calculation of extraction performance as a... 

To summarize, although rate constants (or, perhaps, apparent rate constants) for IT across the ITIES have been reported for more than 20 years, there is still controversy about the interpretation of this phenomenon, not least because the reported rate constants have increased over the years as experimental measurements became more and more sophisticated [127]. As noted above, a general problem has been that the characteristic timescales of the (apparent) kinetic process are often not markedly lower than the timescales of the experimental technique, a fact that has been remarked upon in the literature [128]. For example, in some of the recent data [94, 96] the time constants of the technique are frequently of the same order of magnitude as the timescales of the process they are purporting to measure. The question therefore becomes whether the rate constants reported for IT using nanopipettes [104, 107, 108] will increase in the future or whether these represent true values. Clearly at this point it is reasonable to ask whether theory predicts that a barrier to interfacial IT should exist and, if so, what the physical origin of such a barrier might be. 

Adsorption kinetics, mainly studied by dynamic surface tension measurements, shows many features very much different from that of typical surfactants (Miller et al. 2000). The interfacial tension isotherms for standard proteins such as BSA, HSA, (3-casein and (3-lactoglobulin were measured at the solution/air interface by many authors using various techniques. The state of the art of the thermodynamics of adsorption was discussed in Chapter 2 while isotherm data for selected proteins were given in the preceding Chapter 3. Here we want to give few examples of the dynamic surface pressure characteristics of protein adsorption layers. 

The kinetics of dissolution of ionic solids has traditionally been studied via the measurement of bulk concentration changes in, for example, stirred suspensions. Using the equilibrium perturbation mode of the SECM, the experimenter can control directly the interfacial undersaturation and make measurements at single crystal surfaces or with micrometer spatial resolution [24]. 

Rate measurements with well-stined vessels may be continuous or batch. An example of the latter is simply a stirred flask. The former include the AKUFVE apparatus and bench-scale models of mixer-settler contactors. Because the interfacial area and mass transfer conditions in these systems are not known, th generally do not yield intrinsic heterogeneous rate data for metal-extraction systems. Nevertheless, th are useful for screening relative rates and identifying cases where slow kinetics may be a problem. 

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