Polarity, liquid membranes

HFCLM-based Perstraction Processes. Perstraction, the combination of permeation and extraction, is selective permeation of a species from a liquid feed through a membrane into a strip or sweep liquid which selectively extracts the permeating species (Sirkar (30) has a brief review). Cahn and Li (31) report separation of toluene and n-heptane through an aqueous ELM and kerosene as the strip liquid. Papadopoulos and Sirkar (32) explored the separation of a 2 vol% isopropanol-n-heptane mixture through a highly polar liquid membrane in a HFCLM... 

Liquid membranes can be constituted by liquid chiral selectors used directly [170] or by solutions of the chiral molecules in polar or apolar solvents. This later possibility can also be an advantage since it allows the modulation of the separation con-... 

As in the 1,2-dichloroethane case too, transient EMF and SHG responses to KSCN were observed for the nitrobenzene membranes without ionic sites. This suggests that here too not only SCN but also K ions are transferred into the nitrobenzene phase. Salt extraction into the bulk of the organic phase, in analogy to similar observations previously reported for neutral ionophore-incorporated liquid membranes without ionic sites [55], was indeed independently confirmed by atomic absorption spectrometry. Figure 15 shows the concentration of K in nitrobenzene equilibrated at room temperature with a 10 M aqueous solution of KSCN as a function of equilibration time. The presence of the ion exchanger TDDMA-SCN efficiently suppresses KSCN extraction into the organic phase but in its absence a substantial amount of KSCN enters the nitrobenzene phase. The trends of the EMF and the SHG responses are therefore very similar in spite of the different polarities of the plasticizers. 

In the past decade, several novel solvent-based microextraction techniques have been developed and applied to environmental and biological analysis. Notable approaches are single-drop microextraction,147 small volume extraction in levitated drops,148 flow injection extraction,149 150 and microporous membrane- or supported liquid membrane-based two- or three-phase microextraction.125 151-153 The two- and three-phase microextraction techniques utilizing supported liquid membranes deposited in the pores of hollow fiber membranes are the most explored for analytes of wide ranging polarities in biomatrices. This discussion will be limited to these protocols. 

In the body of a liquid, intermolecular forces pull the molecules in all directions. At the surface of the liquid, the molecules pull down into the body of the liquid and from the sides. There are no molecules above the surface to pull in that direction. The effect of this unequal attraction is that the liquid tries to minimize its surface area. The minimum surface area for a given quantity of matter is a sphere. In a large pool of liquid, where sphere formation is not possible, the surface behaves as if it had a thin stretched elastic membrane or skin over it. The surface tension is the resistance of a liquid to an increase in its surface area. It requires force to break the attractive forces at the surface. The greater the intermolecular force, the greater the surface tension. Polar liquids, especially those that utilize hydrogen bonding, have a much higher surface tension than nonpolar liquids. 

Organic-soluble ester is brought to the reactor with the organic feed solution and freely permeates the immobilized organic liquid membrane to reach the catalyst enzyme. The ester is then hydrolyzed. The alcohol and acid products of hydrolysis are much more polar than the ester and, as such, are water soluble but relatively organic insoluble. These products diffuse to the aqueous permeate solution. The membrane both provides an active site for the reaction and separates the products of reaction from the feed [38]. 

Electroosmosis — (also called electroendosmosis and endosmosis) The movement of a polar liquid through a capillary tubing or porous solid driven by an electrical potential difference. First described by F. F. Reuss in 1809. In fuel cells, electroosmosis causes protons moving through a proton exchange membrane (PEM) to drag water molecules from one side (anode) to the other (cathode). This phenomenon is utilized for the dessica-tion of different objects, e.g., walls of buildings. 

The study of electron transfer (ET) at the polarized oil (0)/water (W) (or liquid/ liquid) interface is useful for understanding not only certain catal)rtic reactions in two-phase systems (e.g., liquid membranes, microemulsions, micelles, etc.) but also energy conversion processes occurring at biomembranes. In 1979, Samec etal. [1,2] reported, as the first example, an ET between ferrocene (Fc) in nitrobenzene (NB) and Fe(CN)6 in W ... 

The extreme sensitivity of the visible absorption spectrum to small changes in the surrounding medium has made this betaine dye a useful molecular probe in the study of micellar systems [298, 299, 443-445], mieroemulsions and phospholipid bilayers [299], model liquid membranes [300], polymers [301, 446], organic-inorganie polymer hybrids [447], sol-gel matrices [448], surfaee polarities [449-451], and the retention behaviour in reversed-phase liquid chromatography [302]. Using polymer membranes with embedded betaine dyes, even an optical alcohol sensor has been developed [452]. 

Internal Phase Composition As with the continuous phase, the internal phase properties also influence the properties of the ELM. Ionic strength, pH, and the presence of organic species will impact on the stability of the ELM. Emulsion liquid membranes work on the basis that the polar substances (usually high concentrations of acid or base) contained in the internal phase are impermeable to the membrane phase. However, the presence of the surfactant can cause the uptake of these compounds by the formation of reverse micelles [97]. 

Several studies have been concerned with the penetration of liquids into latewood and earlywood (JJ, 16-23). Under atmospheric pressure, the penetration of nonpolar liquids into softwood latewood may be caused, in part, by capillary action in the very small lumens and passage through unaspirated pit membranes. In aspirated earlywood this penetration would not occur. Penetration of nonpolar liquids may also be through drying checks in the thick latewood cell walls. As the temperature and pressure of the liquid are raised, penetration of polar liquids in earlywood would be expected to increase because of softening of the pit structure and displacement of the pit membrane. Because the cell wall of earlywood is thinner than that of latewood, penetration into earlywood walls would be quicker and facilitated by swelling. Incrustation occurs in the pit membranes of southern pine latewood (24) this would retard liquid penetration. 

Way, Noble and Bateman (49) review the historical development of immobilized liquid membranes and propose a number of structural and chemical guidelines for the selection of support materials. Structural factors to be considered include membrane geometry (to maximize surface area per unit volume), membrane thickness (<100 pm), porosity (>50 volume Z), mean pore size (<0.1)jm), pore size distribution (narrow) and tortuosity. The amount of liquid membrane phase available for transport In a membrane module Is proportional to membrane porosity, thickness and geometry. The length of the diffusion path, and therefore membrane productivity, is directly related to membrane thickness and tortuosity. The maximum operating pressure Is directly related to the minimum pore size and the ability of the liquid phase to wet the polymeric support material. Chemically the support must be Inert to all of the liquids which It encounters. Of course, final support selection also depends on the physical state of the mixture to be separated (liquid or gas), the chemical nature of the components to be separated (inert, ionic, polar, dispersive, etc.) as well as the operating conditions of the separation process (temperature and pressure). The discussions in this chapter by Way, Noble and Bateman should be applicable the development of immobilized or supported gas membranes (50). 

Liquid membrane technology has been applied to a great extent for separation of mixtures of saturated and aromatic hydrocarbons. Investigations reveal that the LSM process offers potential for dearomatization of petroleum streams like naphtha and kerosene to meet product specifications for naphtha cracker feedstock and aviation kerosene, respectively [25, 63, 85, 144-146]. The separation is based on a simple permeation technique and occurs due to the difference in solubility and diffusivity of permeating species through the membrane. Kato and Kawasaki [70] conducted studies on the enhancement of hydrocarbon permeation by the use of a polar additive like sulfolane or triethyl glycol. Sharma et al. [147] enhanced the selectivity of the membrane by several orders with the addition of a carrier. Chakraborty et al. [85] used cyclodextrins to enhance the separation factor and removal efficiency of aromatic compound. 

The stability of a SILM based on [bmim ][BF ] supported in a nylon membrane has been also analysed in other organic solvents, such as n-hexane//eri-butyl methyl ether and n-hexane/dimethyl sulphoxide [29]. The SEM-EDX study of the membranes after continnons operation showed that the stability of the supported liquid membrane increases with the decrease of the polarity of the solvent used. 

These relations can be used as rough estimates of steric rejection, if the solute and membrane pore dimensions are known. The derivation is based on a strictly model situation (see Figure 1) and a long list of necessary assumptions can be written. Apart from the simplified geometry (hard sphere in a cylindrical pore), it was also assumed that the solute travels at the same velocity as the surrounding liquid, that the solute concentration in the accessible parts of the pore is uniform and equal to the concentration in the feed, that the flow pattern is laminar, the liquid is Newtonian, diffusional contribution to solute transport is negligible (pore Peclet number is sufficiently high), concentration polarization and membrane-solute interactions are absent, etc. 

The fluorescence properties of DCM, 4-cyanomethylene-2-methyl-6-p-dimethyl aminostyryl-4H pyran, show that there is thermal equilibrium between the cis- and trans-isomers. There is no observable aggregation of this dye except in liquid membranes. Polarized absorption and emission spectra of stilbazolium merocyanines and the properties of pyrylium and thiopyrylium high efficiency laser dyes are topics covered other related publications. 

Figure 9.22A illustrates the purely diffusion-controlled process, in which the effects of boundary layers and interfacial reaction rates are negligible. In this case, the concentrations of the complex at the interfaces are the equilibrium concentrations. Figure 9.22B illustrates the partially boundary-layer-controlled case. Here, prior to steady state, the permeant diffuses across the membrane faster in the feed-side boundary layer and accumulation of permeant in the product-side boundary layer. The consequence of this concentration polarization is a reduction in the net concentration gradient across the membrane, and a reduced flux compared with the diffusion-controlled case. The last case is that of partially reaction-rate-controlled flux, illustrated by the concentration profile in Figure 9.22C. Here, either the permeant initially diffuses away from the feed interface faster than it can be replenished by the interfacial reaction, or the dissociation reaction is not fast enough to prevent accumulation of the complex at the product interface. Again, the net result is a decrease in the concentration gradient compared with that in the purely diffusion-controlled case. In all three cases, the flux is proportional to the slope of the concentration profile across the liquid membrane.

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