Interfacial rate constant molecule

The complexation proceeded almost completely at the interface. The values of the interfacial complexation rate constants are listed in Table 10,3. The rate constant, fc = 5.3 X 10 M s , was determined in the aqueous solution using stopped-flow spectrometry in the region where the formation rate was independent of pH. The conditional interfacial rate constants represented by k = /ki/k2[HL] /(A2 + A-ilH ]) were larger at the heptane/water interface than at the toluene/water interface for both metal ions. The MD simulations of the adsorptivities of 5-Br-PADAP at the heptane/water and toluene/water interfaces suggested that 5-Br-PADAP could be adsorbed at the interfacial region more closely to the aqueous phase, but 5-Br-PADAP at the toluene/water interface was still surrounded by toluene molecules, which lowered the probability of reaction with aqueous Ni(II) ions [6]. 

FIG. 29 Oxygen-transfer rate constants derived from Fig. 28 as a function of the reciprocal of the interfacial area per molecule. (Reprinted from Ref 19. Copyright 1998 American Chemical Society.)... 

Subscript (ads) denotes adsorption via a thiolate linkage, while (ps) stands for a physisorbed and/or adsorbed state via different interactions. However, large dimensions of the studied molecules and their amphiphilic nature make the surface reaction mechanism more complex than in case of cystine/cysteine. Interfacial microstructure plays an important role in the determination of the surface behavior of the adsorbed molecules. From the study on the charge-transfer kinetics, the transfer coefficient a was calculated as slightly less than 0.50, while the rate constant (based on Laviron s derivations [105]) was of the order of 10 s k The same authors [106] have shown earlier that the adsorption rate constant of porcine pancreatic phospholipase A2 at mercury via one of its disulfide groups is of the order of 10 s h... 

Broadly speaking, dynamic electron transfer involves two steps. The first step is the diffusion controlled formation of an encounter complex between the electron/hole acceptor molecule and the particle. The second step is the electrochemical interfacial charge-transfer, which may be characterized by a rate constant kct. Albery et al. [143] and Gratzel et al. [129] have independently arrived at the same result, relating the observed effective bimolecular rate constant for hole/electron acceptor oxidation/re-duction, /cobs (m3mor s I) to reactant diffusion and A ct. 

The dynamic behaviour of the 2-hydroxy oxime and its adsorptivity at the interface were well depicted by the molecular dynamics (MD) simulations [34]. It was revealed that the polar groups of —OH and =N—OH of the adsorbed 2-hydroxy oxime molecule were accommodated in the aqueous phase side so as to react with the Ni(II) ion in the aqueous phase [35]. This was thought to explain that the magnitude of the reaction rate constants of Ni(II) at the heptane/water interface and that in the aqueous phase were similar to each other. The diffusive and adsorptive behaviour of LIX65N around the interface was also simulated for 1 ns. The molecule was active around the interfacial region, moving... 

In practice, the amount of solid molecules on the surface being exposed to the solution is difficult or even impossible to quantify. Instead, the solid surface area to solution volume ratio is often used to quantify the amount of solid reactant. Therefore, experimentally determined second-order rate constants for interfacial reactions have the unit m s h As the true surface area of the solid is very difficult to determine, the BET (Brunauer-Emmett-Teller) surface area is fte-quentiy used. The maximum diffusion-controlled rate constant for a particle suspension containing pm-sized particles is ca 10 m s and for mm-sized particle suspensions the corresponding value is I0 m s h Unfortunately, the discrepancy between the true surface area and the BET surface area and the non-spherical geometry of the solid particles makes it impossible to exactly determine the theoretical diffusion-controlled rate constant. 

Based upon a detailed analysis of reaction transients, a mechanism was proposed for chlorophyll a-photosensitized transmembrane oxidation-reduction of aqueous phase donors and acceptors that included electron transfer between juxtaposed Chi a+ r-cations and Chi a molecules as the transmembrane charge-transfer step [112]. The maximum apparent first-order rate constant for this step was 10 s , which seems large for thermal electron transfer between chlorophyll molecules located at the opposite membrane interfaces, even considering that nuclear activation barriers may be relatively small for this reaction. Transverse flip-flop diffusion of Chi b across the membrane is 10 -fold slower than transmembrane redox under these conditions, so this alternative mechanism is almost certainly unimportant. Kinetic mapping studies have shown that some of the Chi a becomes localized within the membrane at sites that are inaccessible to aqueous phase electron acceptors, presumably within the membrane interior [114]. This suggests the possibility of a transverse hopping mechanism involving electron transfer over relatively short distances from buried Chi a to interfacial Chi a+, followed by electron transfer from Chi a at the opposite interface to the buried Chi a" ". 

These forms are equivalent to the interfacial electrochemical ET rate constants (Eqs. (2.2) and (2.3)). Thus is the rate constant for ET from the reduced molecule to the substrate and k"/" the rate constant for ET from the tip to the oxidized molecule. The parameter represents the potential drop between the electrode and the solution. In the tunneling gap with approximately the same spatial extension as the electrochemical double layer(s), and /cannot, however, be regarded as independent but are correlated as [48-50, 60]... 

The highest interfacial electron transfer rate constant yet reported (about 14,000 s ) is for a c-type cytochrome from Aquifex aolicus This protein has a 62-amino acid linker domain by which it is usually anchored to the periplasmic side of the inner membrane this linker has a cysteine as the terminal residue before the signal region, and the sulfur atom provides an anchor point. The cytochrome adsorbs strongly onto a Au electrode that is already modified with a hexane-thiol SAM (note this requires that the molecules in the SAM move or vacate to allow this). The results are striking. 

These rate constant forms are completely equivalent to the interfacial electrochemical rate constant forms in eqns. (8-2) and (8-3). F is thus the rate constant for electron transfer from the reduced form of the molecule to the substrate and the rate constant for electron transfer from the tip to the oxidized form of the molecule. 

A neutral molecule solubilized in the micelle can be located in several positions or microenvironments. As early as the 1930s it was suggested by Lawrence that the site of a solubilized molecule would be dependent on the hydrophobic/hydrophilic composition of the solubilizate. Two extremes are easily identified the core of the micelle providing a hydrocarbon-like microenvironment, and the palisade layer providing an aqueous or water-rich interfacial environment. It seems logical to assume, then, that nonpolar solutes like alkanes would prefer the micellar core and that polar molecules would be anchored at the surface. However, this is an oversimplification available data tend to contradict it. First, the solubility of alkanes in micelles is significantly lower than expected if compared to solubility in hydrocarbon solvents. Second, the size of a micelle is normally such that part of the solute would be close to the surface at any time. Sepulveda et al. state that for SDS micelles at least half of the solute will be within 4 to 5 A of the surface. We should also consider the timescale of the experiments, as the timescale for intramicellar migration is short. The rate constants of entry and exit of molecules to and from micelles is of the order 1(F and... 

Simulation studies offer the following picture of the dynamics of water in the hydration layer of micelles. Within the interfacial region, there is a constant exchange of water molecules between the three states, IBW2, IBWl, and IFW. The microscopic reactions between IBW2 and IBWl on the one hand, and IBWl and IFW on the other, are reversible, and are described by four distinct rate constants, as described below [8]. 

Table 4.17 Expressions for gas-particle mass transfer n , g, Mq and Haq molecule density of the same substance far from the particle, close to the particle, at particle surface and within the particle (droplet) p — gas partial pressure far from the droplet, c g - aqueous-phase concentration, k - mass transfer coefficient (recalculable into spjecific rate constant) g - gas-phase, aq - aqueous-phase, het - interfadal layer (chemistry), in - interfacial layer (transport), coll - collision, ads — adsorption (surface striking), sol - dissolution, diff -diffusion in gas-phase, des - desorption.

More complex situations such as solubilization of oils also exhibit nonequilibrium behavior near interfaces, but only at times. Carroll (1981) found that the rate of solubilization of a very insoluble oil drop by a micellar solution of nonionic surfactant (in moles per unit time per unit interfadal area) was iudepmdent of time and drop volume in a batch experiment. This behavior indicates that interfacial resistance, not mass transfer in the micellar solution, controls the rate of solubilization. His view was that first micelles completely demicellized to individual molecules in the immediate vicinity of the oil-water interface with the rate constant discussed in Chapter 4. These surfactant molecules were then... 

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