Mobile component

In the special case that A and B are similar in molecular weight, polarity, and so on, the self-diffusion coefficients of pure A and B will be approximately equal to the mutual diffusivity, D g. Second, when A and B are the less mobile and more mobile components, respectively, their self-diffusion coefficients can be used as rough lower and upper bounds of the mutual diffusion coefficient. That is, < D g < Dg g. Third, it is a common means for evaluating diffusion for gases at high pressure. Self-diffusion in liquids has been studied by many [Easteal AIChE]. 30, 641 (1984), Ertl and Dullien, AIChE J. 19, 1215 (1973), and Vadovic and Colver, AIChE J. 18, 1264 (1972)]. 

Electrodecantation or electroconvec tion is one of several operations in which one mobile component (or several) is to be separated out from less mobile or immobile ones. The mixture is introduced between two vertical semipermeable membranes for separating cations, anion membranes are used, and vice versa. When an electric field is apphed, the charged component migrates to one or another of the membranes but since it cannot penetrate the membrane, it accumulates at the surface to form a dense concentrated layer of particles which will sink toward the bottom of the apparatus. Near the top of the apparatus immobile components will be relatively pure. Murphy [J. Electrochem. Soc., 97(11), 405 (1950)] has used silver-silver chloride electrodes in place of membranes. Frilette [J. Phys. Chem., 61, 168 (1957)], using anion membranes, partially separated and Na, ... 

One-layer systems. One-layer systems might easily overcome most of the above-mentioned problems. Such materials show predominantly ionic conduction in the as-prepared state but behave as electrodes in that the concentration of the mobile component is increased and decreased by the charging process in the vicinity of the two electronic leads. 

The type of disorder may be determined by conductivity measurements of electronic and ionic defects as a function of the activity of the neutral mobile component [3]. The data are commonly plotted as Brouwer diagrams of the logarithm of the concentration of all species as a function of the logarithm of the activity of the neutral mobile component. The slope is fitted to the assumption of a specific defect-type model. 

Ubiquinone or Q (coenjyme Q) (Figure 12-5) finks the flavoproteins to cytochrome h, the member of the cytochrome chain of lowest redox potential. Q exists in the oxidized quinone or reduced quinol form under aerobic or anaerobic conditions, respectively. The structure of Q is very similar to that of vitamin K and vitamin E (Chapter 45) and of plastoquinone, found in chloroplasts. Q acts as a mobile component of the respiratory chain that collects reducing equivalents from the more fixed flavoprotein complexes and passes them on to the cytochromes. 

Functionally and strucmrally, the components of the respiratory chain are present in the inner mitochondrial membrane as four protein-lipid respiratory chain complexes that span the membrane. Cytochrome c is the only soluble cytochrome and, together with Q, seems to be a more mobile component of the respiratory chain connecting the fixed complexes (Figures 12-7 and 12-8). 

By measuring the proton relaxation times, and T,p, it is possible to estimate the mobility of polymer chains within the cell wall (11). Proton spin relaxation editing (PSRE) is a method of expressing these results. It separates the components seen in a conventional CP-MAS C spectra into low-mobility and intermediate-mobility components. If PSRE is applied to a experiment (12) the mobility of the... 

The term chemistry in interphases was first introduced in the field of reverse-phase chromatography [41], In 1995 Lindner et al. transferred the concept to the area of transition metal catalysis [42] and in a recent review the concept is explained in detail [43], The interphase is defined as a region within a system in which the stationary and a mobile component penetrate on a molecular level without the formation of a homogeneous mixture. In these regions the reactive centre on the stationary phase... 

The expression d In a/d In c is known as the thermodynamic factor and is a special case of the Wagner factor (or thermodynamic enhancement factor) which plays an important role for the kinetic properties of electrodes. This term indicates the deviation from ideality of the mobile component. For ideal systems this quantity becomes 1 and comparison with Pick s first law yields... 

The chemical diffusion coefficient may be expressed by the diffusivity of the mobile species i, the transference number and the variation of the activity of the mobile component as a function of its concentration according to Eqns (8.26) and (8.27), if the transference numbers of other ionic species are negligibly small. 

Is the electrochemical method not suitable for the determination of the effective diffusion coefficient of the mobile component inside the membrane ... 

Meade (1966) shows that claystones have a porosity decreasing to 0% at 1 Km depths and sandstones, 20% porosity at the same depth. Manheim (1970) shows that ionic diffusion rates in sediments are 1/2 to 1/20 that of free solutions when the sediments have porosities between 100 - 20%. It is evident that the burial of sediments creates a very different physical environment than that of sedimentation. As a result of reduced ionic mobility in the solutions, a different set of silicate-solution equilibria will most certainly come into effect with the onset of burial. The activity of ions in solution will become more dependent upon the chemistry of the silicates as porosity decreases and the system will change from one of perfectly mobile components in the open sea to one approaching a "closed" type where ionic activity in solution is entirely dictated by the mass of the material present in the sediment-fluid system. Although this description is probably not entirely valid even in rocks with measured zero porosity, for practical purposes, the pelitic or clayey sediments must certainly rapidly approach the situation of a closed system upon burial. 

Although gibbsite and kaolinite are important in quantity in some soils and hydrothermal deposits, they have diminishing importance in argillaceous sediments and sedimentary rocks because of their peripheral chemical position. They form the limits of any chemical framework of a clay mineral assemblage and thus rarely become functionally involved in critical clay mineral reactions. This is especially true of systems where most chemical components are inert or extensive variables of the system. More important or characteristic relations will be observed in minerals with more chemical variability which respond readily to minor changes in the thermodynamic parameters of the system in which they are found. However, as the number of chemical components which are intensive variables (perfectly mobile components) increases the aluminous phases become more important because alumina is poorly soluble in aqueous solution, and becomes the inert component and the only extensive variable. 

Eh and activity of the mobile components, the degrees of freedom due to these variables are... 

When the maximum number of possible phases are present, f 0 and therefore c = 1. When the number of inert components is one, a multicomponent phase such as glauconite must be precipitated from the perfectly mobile components and one which is immobile or inert. In the instance of sediment-sea water reactions alumina seems a good candidate for the immobile component, due to its low solubility at normal sea water conditions. 

Figure 17. Proposed phase relations where K is a mobile component and Al, Fe are immobile components at about 20°C and several atmosphere water pressure for aluminous and ferric-ferrous mica-smectite minerals. Symbols are as follows I illite G = non-expanding glauconite Ox = iron oxide Kaol = kaolinlte Mo montmorillonite smectite N nontronitic smectite MLAL aluminous illite-smectite interlayered minerals Mlpe = iron-rich glauconite mica-smectite interlayered mineral. Dashed lines 1, 2, and 3 indicate the path three different starting materials might take during the process of glauconitization. The process involves increase of potassium content and the attainment of an iron-rich octahedral layer in a mica structure.

Figure 36. Representation of the zeolite-clay mineral assemblages found in a systeirf at 25°C and atmospheric pressure where Na is an intensive variable (perfectly mobile component) whereas A1 and Si are extensive variables or inert components of the system. G = gibbsite kaol = kaolinite Mo = montmorillonite Si = amorphous silica Anal = analcite.

Thus the solution always contains Mg in approximately constant abundance, which makes it effectively a perfectly mobile component. The same is true for H+ since pH changes little after precipitation of the sepiolite even though the reaction consumes (OH). The experimental system is then "open" with respect to these two components. A determination of the solubility product constant of a natural iron-calcium-aluminous sepiolite confirms generally the above results (Christ, et al , 1973). 

---