Stoichiometry of surface complex

The stoichiometry of surface complexes as well as the total surface coverage is important in determining electron transfer rate. 

Because of our ignorance about the stoichiometries of surface complexes, comparisons between dissolved complexes and surfaces work only at the scale of reactivity trends and then only in tightly constrained conditions. One could use the relative dissociation rates of (H20)4Cr( X-0H)2Cr(0H2)4] (aq) and (en)(H20)2Cr( i-0H)2Cr(H20)2(en) (aq) complexes, for example, to gauge the effect that coordination of two -NH2 ligands to a metal site has on the dissolution rates of a Cr(0H)3(s) mineral if amine ligands could be induced to adsorb in a structurally simple manner (11,12). 

The effect of metal-ion concentration on the stoichiometry of surface complexes was studied by in situ attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR) spectroscopy, monitoring the binding of Cd + to a carboxylate-terminated SAM [29]. Specific... 

Surface complexation models attempt to represent on a molecular level realistic surface complexes e.g., models attempt to distinguish between inner- or outer-sphere surface complexes, i.e., those that lose portions of or retain their primary hydration sheath, respectively, in forming surface complexes. The type of bonding is also used to characterize different types of surface complexes e.g., a distinction between coordinative (sharing of electrons) or ionic bonding is often made. While surface coordination complexes are always inner-sphere, ion-pair complexes can be either inner- or outer-sphere. Representing model analogues to surface complexes has two parts stoichiometry and closeness of approach of metal ion to... 

Quantitative experiments have been performed to find the stoichiometry of oxygen complexes on surfaces. Oxygen also adsorbs reversibly on a Fe(II) porphyrin attached to the imidazole groups of a silica gel that is adequately treated to give a 1 1 stoichiometry of the Fe-02 adduct (320). 

This example illustrates the qualitative nature of information that can be gleaned from macroscopic uptake studies. Consideration of adsorption isotherms alone cannot provide mechanistic information about sorption reactions because such isotherms can be fit equally well with a variety of surface complexation models assuming different reaction stoichiometries. More quantitative, molecular-scale information about such reactions is needed if we are to develop a fundamental understanding of molecular processes at environmental interfaces. Over the past 20 years in situ XAFS spectroscopy studies have provided quantitative information on the products of sorption reactions at metal oxide-aqueous solution interfaces (e.g., [39,40,129-138]. One... 

For simplicity, in this equation, we have assumed that activities are equal to concentrations and brackets refer to activities. C is a units conversion constant = Vy m relating void volume Vy (mL) in the porous media and the mass m (g) of the aquifer material in contact with the volume Vy, is the formation constant for an aqueous uranyl complex, and the superscripts i, j, k describe the stoichiometry of the complex. The form that the sorption binding constant takes is different for the different sorption models shown in Figure 4 (e.g., see Equation (5)). Leckie (1994) derives similar expressions for more complex systems in which anionic and cationic metal species form poly dentate surface complexes. Equation (7) can be derived from the following relationships for this system ... 

In the area of interfacial charging at the solid/liquid interface of metal oxide aqueous suspensions, the "surface complexation or site binding concept is commonly used [3-20]. This concept is characterised by consideration of specific ionic reactions with surface groups, rather than assuming simple binding of ions to the surface or their accumulation at the interface (adsorption). In the past decade several different models were introduced on the basis of the surface complexation model (SCM) they differ in the assumed structure of the electrical interfacial layer (EIL) and in the proposed mechanisms and stoichiometries of surface reactions leading to surface charge. 

Modeling divalent metal ion sorption requires estimation of the proton stoichiometry (the number of protons released per metal ion sorbed), the type of surface complex (inner or outer sphere) formed, and the formation constants for each reaction selected. Table 7-2 presents a list of various reactions that may be incorporated into the TLM. Because a variety of combinations of different sorption reactions and constants may fit various aspects of the sorption data equally well (see, e.g., Westall Hohl, 1980 Hayes et al., 1991 Katz Hayes, 1995a), protocols are needed to insure the best choice of reactions and a more universally accepted set of guidelines to allow reproducibility from one laboratory to another. The strategy used in modeling Co(II) sorption to a-Al203 involved ... 

If one OH group is attached to one surface site, the stoichiometry of the complex is 1 1. These groups are commonly designated in the Hterature as type I hydroxyls (see Figure 2.3, left two images). When the bond is essentially ionic the OH groups are Hnear (type la), whereas covalent bonds lead to tilted (bent) structures (Ib). In the ionic model, the proton can be seen as... 

In addition to the issues associated with reactions between two materials, high processing temperatures can also affect the stoichiometry of the complex piezoelectric materials as highly volatile compounds (such as Pb, Na, and K) can evaporate from the surface of the material with a resultant change in functional properties [II]. This is particularly important for MEMS scale devices were the surface area to volume ratio is very high meaning that any loss of stoichiometry in... 

Metal carbonyls, such as Fe(CO)s and other organometallic complexes have been reported to react preferentially with metallic rather than with oxidic surfaces to the corresponding metals. Though the thus performed decomposition of suitable organometallic complexes ensures the formation of exclusively bimetallic particles, the composition of the individual particles may still vary. Preparation procedures based on the deposition of heteronuclear complexes are therefore much more attractive. The fixed stoichiometry of the complexes favours the desired uniform composition of the individual supported particles. Polynuclear metal carbonyls have proved to be very suitable for the preparation of bimetallic... 

Stoichiometry of surface reactions between organometallic complexes and surfaces should be determined, inter alia, by quantitative analysis of gaseous products, surface metal concentration, and quantitative extraction of surface complexes. 

The transfection mechanism of plasmid-chitosan complexes as well as the relationship between transfection activity and cell uptake was analyzed by using fluorescein isothiocyanate-labeled plasmid and Texas-Red-labeled chitosan. Several factors affect transfection activity and cell uptake, for example the molecular mass of chitosan, stoichiometry of complex, seriun concentration and the pH of the transfection medium. The level of transfection with plasmid-chitosan complexes was found to be highest when the molecular mass of chitosan was 40 or 84 kDa, the ratio of chitosan nitrogen to DNA phosphate was 5, and serum at pH 7.0 was 10%. Plasmid-chitosan complexes most likely condense to form large aggregates (5-8 p,m), which absorb to the cell surface. After this, plasmid-chitosan complexes are endocytosed, and accumulate in the nucleus [97]. 

Simultaneous measurements of both the amount of butane evolved and the quantity of grafted organometallic fragments at increasing time of reaction allows the stoichiometry of the surface complexes Pts[SnBux] / to be determined (Fig. 7). Furthermore, extrapolation to a low amount of Sn grafted shows that a [BusSnJ Pts species is formed, which then rapidly evolved towards more dealkylated species and finally to Pts[SnBu]jy. 

The stoichiometry of these reactions can be controlled by modulating the concentration of hydroxyl groups on the surface of silica. When starting with the tetra-alkyl complex, subsequent reaction with an alcohol R OH (Equation(4)) is necessary this generally occurs under conditions mild enough to maintain the anchoring bond SiO—M. 

The second approach (Equation(3)) has a number of advantages over the first one (Equation(2)). The alkyl complexes are more reactive than the related alkoxides, the latter being for group 4 elements generally associated into dimers or trimers 48 also, reaction (2) liberates an alcohol which may further react with the surface of silica, whereas the alkane ( Equation(3)) is inert. It was demonstrated by various spectroscopic techniques and elemental analysis that with a silica dehydroxylated at 500 °C under vacuum, the stoichiometry of reaction (3) corresponds to n = 1.45,46 Moreover, a better control of the surface reaction was achieved with the procedure represented in Equation(3). 

At least for ethylene hydrogenation, catalysis appears to be simpler over oxides than over metals. Even if we were to assume that Eqs. (1) and (2) told the whole story, this would be true. In these terms over oxides the hydrocarbon surface species in the addition of deuterium to ethylene would be limited to C2H4 and C2H4D, whereas over metals a multiplicity of species of the form CzH D and CsHs-jD, would be expected. Adsorption (18) and IR studies (19) reveal that even with ethylene alone, metals are complex. When a metal surface is exposed to ethylene, selfhydrogenation and dimerization occur. These are surface reactions, not catalysis in other words, the extent of these reactions is determined by the amount of surface available as a reactant. The over-all result is that a metal surface exposed to an olefin forms a variety of carbonaceous species of variable stoichiometry. The presence of this variety of relatively inert species confounds attempts to use physical techniques such as IR to char-... 

The data of Loukidou et al. (2004) for the equilibrium biosorption of chromium (VI) by Aeromonas caviae particles were well described by the Langmuir and Freundlich isotherms. Sorption rates estimated from pseudo second-order kinetics were in satisfactory agreement with experimental data. The results of XAFS study on the sorption of Cd by B. subtilis were generally in accord with existing surface complexation models (Boyanov et al. 2003). Intrinsic metal sorption constants were obtained by correcting the apparent sorption constants by the Boltzmann factor. A 1 2 metal-ligand stoichiometry provides the best fit to the experimental data with log K values of 6.0 0.2 for Sr(II) and 6.2 0.2 for Ba(II). 

Fig. 28.1. Results (symbols) and simulations (lines) of an experiment at 25 °C by Liger et al. (1999 their Fig. 6) in which uranyl was oxidized by ferrous iron in the presence of nanoparticulate hematite, which served as a catalyst. Vertical axis is amount of NaHCCE-extractable uranyl, which includes uranyl present in solution as well as that sorbed to the nanoparticles in the experiment, nearly all the uranyl was sorbed. Broken line shows results of a simulation assuming uranyl forms a single surface complex, >Fe0U020H, which is catalytically active solid line shows simulation in which a non-catalytic site of this stoichiometry is also present. Inset is an expanded view of the first few hours of reaction.

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