Interfacial adsorption systems

Following Section 3.3.7.4, the separation hictor in interfacial adsorption where both bulk phases are fluids is considered first For air-water systems containing two surface active solutes, i = 1,2 (both being nonelectrolytes), the separation factor i2 has been defined as 

This phenomenon will also allow the creation of a solid phase having a certain desired level of impurity or dopant once a certain amount of impurity level is introduced first into the melt Doped gallium arsenide is an example of such a material in the semiconductor industry. Table 4.1.7 illustrates some of the dopants and their partition coefficients between the bulk crystal and the melt 

This result indicates that in a closed vessel containing two bulk phases, namely air and water, the selectivity between two nonionic surfactants distributed between the bulk liquid and the gas-liquid interfacial phase will be determined by the ratios of the slopes of the decrease of with the concentration of each individual surfactant Although this ratio can be easily different from 1, it may not be very large. When, however, the surfactants are ionic, significant selectivity can be achieved. The expressions for this type of system are studied in Section 5.2.5. 

qf is the number of moles of i per unit mass of adsorbent when adsorbed from a pure gas i at the same surface pressure k and temperature T as the mixture. 

The determination of the activity coefficients yj for the adsorbed phase requires accurate experimental data at constant temperature and pressure for the entire range of gas-phase compositions (Myers and Prausnitz, 1965). However, if the adsorbed solution phase is considered ideal, i.e. y,o. = 1, then 

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Another unique and specific feature of the interfacial reaction is the formation of aggregate of dye molecules, metal complexes, and other solvophobic molecules. As reported in many interfacial adsorption systems, the saturated interfacial concentration of usual molecules is of the order of 10 10mol/cm2, which can be attained even under an extremely low bulk phase concentration. This means that the liquid-liquid interface is ready to be saturated to form a two-dimensionally condensed state for the adsorbate. In solvent extraction process of metal ions, we used to find formation of some precipitate at the interface, which is called crud. The study of the interfacial aggregate is therefore important to know the real interfacial reaction as met in the industrial solvent extraction where rather concentrated solutes have to be treated. 

The previous three types of liquid-solid equilibria considered equilibrium between two bulk phases we will now briefly look at two types of interfacial adsorption systems where the two bulk phases are either fluid-fluid or fluid-solid. Consider first the interfacial equilibrium relation for a nonelectrolytic surface active solute in an air-water system (a fluid-fluid system). If the surface active solute i is such that the interfacial tension decreases linearly with the surfactant concentration C,i as... 

Gas-solid and liquid-solid based interfacial adsorption systems... 

Another interfacial adsorption system, where one of the bulk phases is a solid, involves adsorption of solutes from a solvent onto the surface of solid adsorbents. Such liquid-solid adsorption systems are frequently used in... 

One of the most attractive roles of liquid liquid interfaces that we found in solvent extraction kinetics of metal ions is a catalytic effect. Shaking or stirring of the solvent extraction system generates a wide interfacial area or a large specific interfacial area defined as the interfacial area divided by a bulk phase volume. Metal extractants have a molecular structure which has both hydrophilic and hydrophobic groups. Therefore, they have a property of interfacial adsorptivity much like surfactant molecules. Adsorption of extractant at the liquid liquid interface can dramatically facilitate the interfacial com-plexation which has been exploited from our research. 

The heptane water and toluene water interfaces were simulated by the use of the DREIDING force field on the software of Cerius2 Dynamics and Minimizer modules (MSI, San Diego) [6]. The two-phase systems were constructed from 62 heptane molecules and 500 water molecules or 100 toluene molecules and 500 water molecules in a quadratic prism cell. Each bulk phase was optimized for 500 ps at 300 K under NET ensemble in advance. The periodic boundary conditions were applied along all three directions. The calculations of the two-phase system were run under NVT ensemble. The dimensions of the cells in the final calculations were 23.5 A x 22.6 Ax 52.4 A for the heptane-water system and 24.5 A x 24.3 A x 55.2 A for the toluene-water system. The timestep was 1 fs in all cases and the simulation almost reached equilibrium after 50 ps. The density vs. distance profile showed a clear interface with a thickness of ca. 10 A in both systems. The result in the heptane-water system is shown in Fig. 3. Interfacial adsorption of an extractant can be simulated by a similar procedure after the introduction of the extractant molecule at the position from where the dynamics will be started. 

When a heptane solution of 5-Br-PADAP and an aqueous solution of Ni " " were stirred, the ligand in the organic phase was continuously consumed according to the complexation, but there was no extraction of the complex. The complex formed was completely adsorbed at the interface. On the other hand, in a toluene system the complex was extracted very slowly (Fig. 6). The complexation mechanism in the two solvent systems could be analyzed by taking into account the interfacial adsorption of the ligand. The next equation was derived for the initial rate of the consumption of HLq in the heptane system ... 

FIG. 6 Measurements of the interfacial adsorptivity and the extraction rate from the absorbance changes of the organic phase by means of the high-speed stirring method, (a) Heptane-water system... 

In the ion-association extraction systems, hydrophobic and interfacially adsorbable ions are encountered very often. Complexes of Fe(II), Cu(II), and Zn(II) with 1,10-phenanthro-line (phen) and its hydrophobic derivatives exhibited remarkable interfacial adsorptivity, although the ligands themselves can hardly adsorb at the interface, except for protonated species [19-21]. Solvent extraction photometry of Fe(II) with phen is widely used for the determination of trace amounts of Fe(II). The extraction rate profiles of Fe(II) with phen and its dimethyl (DMP) and diphenyl (DPP) derivatives into chloroform are shown in Fig.9. In the presence of 0.1 M NaC104, the interfacial adsorption of phen complex is most remarkable. The adsorption of the extractable complex must be considered in the analysis of the extraction kinetic mechanism of these systems. The observed initial rate r° shows the relation... 

The oscillations observed with artificial membranes, such as thick liquid membranes, lipid-doped filter, or bilayer lipid membranes indicate that the oscillation can occur even in the absence of the channel protein. The oscillations at artificial membranes are expected to provide fundamental information useful in elucidating the oscillation processes in living membrane systems. Since the oscillations may be attributed to the coupling occurring among interfacial charge transfer, interfacial adsorption, mass transfer, and chemical reactions, the processes are presumed to be simpler than the oscillation in biomembranes. Even in artificial oscillation systems, elementary reactions for the oscillation which have been verified experimentally are very few. 

The oscillation at a liquid liquid interface or a liquid membrane is the most popular oscillation system. Nakache and Dupeyrat [12 15] found the spontaneous oscillation of the potential difference between an aqueous solution, W, containing cetyltrimethylammo-nium chloride, CTA+CK, and nitrobenzene, NB, containing picric acid, H" Pic . They explained that the oscillation was caused by the difference between the rate of transfer of CTA controlled by the interfacial adsorption and that of Pic controlled by the diffusion, taking into consideration the dissociation of H Pic in NB. Yoshikawa and Matsubara [16] realized sustained oscillation of the potential difference and pH in a system similar to that of Nakache and Dupeyrat. They emphasized the change of the surface potential due to the formation and destruction of the monolayer of CTA" Pic at the interface. It is... 

The catalytic role of die interface was recognized in various liquid/liquid extraction systems. Interfacial adsorption of reactants was the key step in the interfacial catalysis in the extraction of metal ions. The interfacial ligand-substitution mechanism has great importance in the kinetic synergism. Some essential guidelines proposed here are highly useM, not only in solvent extraction but also in interfacial synthesis. 

Fig. 2. The tertiary structure of secretory phospholipases A a-Carbon traces of (A) the class I Naja naja atra venom sPLAa (White et al., 1990), (B) the class II human nonpancreatic sPLAa (Scott el al, 1991), and (C) the class III bee venom enzyme (Scott et al, 1990b). The location of the primary calcium ion is indicated by the large black sphere, the position of the cocrystallized transition-state analog is shown in A and C, and the side chains of His-48 and Asp-99 are shown in B. The face of the enzyme that corresponds to the proposed interfacial adsorption surface is inscribed. The residues in A and B are labeled according to the common numbering system (Renetseder et al., 1985).

Chemical formulas of crystalline salts whose ions tend to be specifically adsorbed do not reflect complex solution chemistry of these salts. Apparently the adsorption systems are simple, but in fact the solutions are multicomponent systems, and the coexisting species can substantially differ in their affinities to the surface. The speciation in the interfacial region can be completely different from that in bulk solution. Many analytical methods do not distinguish between particular species, and the results representing overall sorption behavior of all species involving the element of interest are obtained. Fortunately, some results obtained by means of spectroscopic methods can be resolved into pieces of information regarding... 

The term structural-mechanical barrier was for the first time introduced by P. A. Rehbinder [2,46-48]. This is a strong factor of stabilization of colloidal systems related to the formation of interfacial adsorption layers of low and high molecular weight surfactants which lyophilize interfaces. The structure and mechanical properties of such adsorption layers are able to ensure very high stability of dispersion medium interlayers between dispersed particles. 

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