Interfacial reagent concentration

Abstract Vesicles prepared with synthetic amphiphiles constitute useful microreactors, where reaction rates can be delicately controlled. Here we review our work on quantitative analysis of reaction rates in vesicles and show that reaction at several vesicular sites can be probed and controlled. Vesicles prepared with dialkyldimethylammonium halides, (DODA)X, can accelerate bimolecular reactions by more than a million fold. Quantitative analysis of the vesicular effect on ester thiolysis, using a pseudophase ion exchange formaUsm, suggests that the rate increase is primarily due to reagent concentration in the bilayer and interfacial effects on ion distribution, as well as contributions from enhanced nucleophile reactivity. Vesicle-containing solutions exhibit a variety of potential reaction sites the inner and outer surfaces, bilayer and internal aqueous compartment. 

Essentially, the reason for the appearance of varied interfacial tension is the variable reagent concentration of the interface. When using a surfactant in the droplet generation process, the low adsorption rate of the surfactant is caused by its low mass transfer rate, even for small-molecule surfactants... 

Phosphoric acid ester was used as a model for the estimation of concentration of a reagent in an adsorbed layer by optical measurements of the intensity of a beam reflecting externally from the liquid-liquid interface. The refractive index of an adsorbed layer between water and organic solution phases was measured through an external reflection method with a polarized incident laser beam to estimate the concentration of a surfactant at the interface. Variation of the interfacial concentration with the bulk concentration estimated on phosphoric acid ester in heptane and water system from the optical method agreed with the results determined from the interfacial tension measurements... 

The methodology of surface electrochemistry is at present sufficiently broad to perform molecular-level research as required by the standards of modern surface science (1). While ultra-high vacuum electron, atom, and ion spectroscopies connect electrochemistry and the state-of-the-art gas-phase surface science most directly (1-11), their application is appropriate for systems which can be transferred from solution to the vacuum environment without desorption or rearrangement. That this usually occurs has been verified by several groups (see ref. 11 for the recent discussion of this issue). However, for the characterization of weakly interacting interfacial species, the vacuum methods may not be able to provide information directly relevant to the surface composition of electrodes in contact with the electrolyte phase. In such a case, in situ methods are preferred. Such techniques are also unique for the nonelectro-chemical characterization of interfacial kinetics and for the measurements of surface concentrations of reagents involved in... 

Interfacial tension studies are particularly important because they can provide useful information on the interfacial concentration of the extractant. The simultaneous hydrophobic-hydrophilic nature of extracting reagents has the resulting effect of maximizing the reagent affinity for the interfacial zone, at which both the hydrophobic and hydrophilic parts of the molecules can minimize their free energy of solution. Moreover, as previously mentioned, a preferential orientation of the extractant groups takes place at the interface. Conse-... 

It must be observed that when this mechanism holds, the rate of the extraction reaction is independent of the interfacial area, Q, and the volume of the phases, V. The expected logarithmic dependency of the forward rate of extraction on the specific interfacial area S = Q/V), the organic concentration of the extracting reagent and the aqneons acidity, is shown in Fig. 5.7, case 1. 

Fig. 5.8 Concentration profile for an interfacial reaction with reagent depletion in the diffusion film.

Mass transport processes - diffusion, migration, and - convection are the three possible mass transport processes accompanying an - electrode reaction. Diffusion should always be considered because, as the reagent is consumed or the product is formed at the electrode, concentration gradients between the vicinity of the electrode and the bulk solution arise, which will induce diffusion processes. Reactant species move in the direction of the electrode surface and product molecules leave the interfacial region (- interface, -> interphase) [i-v]. The - Nernst-Planck equation provides a general description of the mass transport processes. Mass transport is frequently called mass transfer however, it is better to reserve that term for the case that mass is transferred from one phase to another phase. 

Reaction of dissolved gases in clouds occurs by the sequence gas-phase diffusion, interfacial mass transport, and concurrent aqueous-phase diffusion and reaction. Information required for evaluation of rates of such reactions includes fundamental data such as equilibrium constants, gas solubilities, kinetic rate laws, including dependence on pH and catalysts or inhibitors, diffusion coefficients, and mass-accommodation coefficients, and situational data such as pH and concentrations of reagents and other species influencing reaction rates, liquid-water content, drop size distribution, insolation, temperature, etc. Rate evaluations indicate that aqueous-phase oxidation of S(IV) by H2O2 and O3 can be important for representative conditions. No important aqueous-phase reactions of nitrogen species have been identified. Examination of microscale mass-transport rates indicates that mass transport only rarely limits the rate of in-cloud reaction for representative conditions. Field measurements and studies of reaction kinetics in authentic precipitation samples are consistent with rate evaluations. 

Solution of the coupled mass-transport and reaction problem for arbitrary chemical kinetic rate laws is possible only by numerical methods. The problem is greatly simplified by decoupling the time dependence of mass-transport from that of chemical kinetics the mass-transport solutions rapidly relax to a pseudo steady state in view of the small dimensions of the system (19). The gas-phase diffusion problem may be solved parametrically in terms of the net flux into the drop. In the case of first-order or pseudo-first-order chemical kinetics an analytical solution to the problem of coupled aqueous-phase diffusion and reaction is available (19). These solutions, together with the interfacial boundary condition, specify the concentration profile of the reagent gas. In turn the extent of departure of the reaction rate from that corresponding to saturation may be determined. Finally criteria have been developed (17,19) by which it may be ascertained whether or not there is appreciable (e.g., 10%) limitation to the rate of reaction as a consequence of the finite rate of mass transport. These criteria are listed in Table 1. 

Examination of mass-transport limitation to the H202-S(IV) reaction is given in Figure 10 for assumed concentration of each reagent of 1 ppb. This examination indicates no gas-phase limitation, and aqueous-phase limitation only at quite low pH (<2.5). Interfacial limitation would be appreciable only for values of a 10-3. 

Equation (5) indicates that by monitoring the gas concentration change as a function of time, the accommodation coefficient may be deduced using a computer program. However, precautions must be undertaken to satisfy the boundary condition that the surface concentration of the dissolved gas must be negligible at all times. This can be accomplished in principle by agitation and by addition of proper chemical reagents in the aqueous phase to remove the dissolved gas as quickly as it is absorbed. In addition, the system must be operated at sufficiently low pressure so that the gas-phase resistance is much smaller than interfacial resistance. 

It is well known that interfacial reaction rates increase as stirring speed is increased until reaching a plateau around 600-1700 rpm (revolutions per minute) [25]. On the other hand, PTC reaction rates are independent of the stirring speed above 200-350 rpm, necessary to level concentration gradients at the interphase. When neutral reagents are involved, it is thus possible to exclude the contribution of interfacial phenomena to the PTC processes. 

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