Solvent transport

The expansion and contraction of the polymer netwoik in conjunction with the sorption/desorption of solvent molecules and ions can be described in terms of mechanical work. This mechanical contribution should be considered in the calculation of the equilibrium electrode potential (see Chap. 5). The deformation coupled to the redox reaction is elastic in nature. A plastic deformation occurs when a neutral, diy film is immersed in electrolyte solution and electrolyzed. It has been observed for a range of neutral polymer films freshly deposited on metal substrates by solvent evaporation techniques that several potential sweeps are required for the films to become fully electroactive [2,19,126,195,196]. This phenomenon has been referred as the break-in effect (Fig. 6.19). 

As illustrated in the previous sections, the electrochemical properties of conducting polymer films are strongly influenced by polymer-ion interactions. These interactions are in turn influenced by the nature of the solvent and the solvent content of the film. Consequently, the electrochemical behavior of conducting polymer films can be highly solvent dependent. Films can even become electrochemically inactive because of lack of solvation.114,197 

In situ gravimetry on polyaniline has revealed a complicated dependence of solvent content and transport of the solvent (aqueous or nonaqueous) on pH.187 Two to three water molecules per electron can be inserted during oxidation in strongly acid solutions, while in propylene carbonate, less than one solvent molecule is involved. Propylene carbonate is first ejected from the film during oxidation, then inserted, and finally ejected again in the final stages. 

The rate of water transport through polyaniline has been measured by mass spectrometry.198 Consistent with the gravimetric results cited above, the permeability of the oxidized state was found to be much higher than that of the reduced state. 

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A reverse osmosis membrane acts as the semipermeable barrier to flow ia the RO process, aHowiag selective passage of a particular species, usually water, while partially or completely retaining other species, ie, solutes such as salts. Chemical potential gradients across the membrane provide the driving forces for solute and solvent transport across the membrane. The solute chemical potential gradient, —is usually expressed ia terms of concentration the water (solvent) chemical potential gradient, —Afi, is usually expressed ia terms of pressure difference across the membrane. 

Ionic liquids possess a variety of properties that make them desirable as solvents for investigation of electrochemical processes. They often have wide electrochemical potential windows, they have reasonably good electrical conductivity and solvent transport properties, they have wide liquid ranges, and they are able to solvate a wide variety of inorganic, organic, and organometallic species. The liquid ranges of ionic liquids have been discussed in Section 3.1 and their solubility and solvation in... 

The results from EQCM studies on conducting polymer films can be ambiguous because the measured mass change results from a combination of independent ion transport, coupled ion transport (i.e., salt transport), and solvent transport. In addition, changes in the viscoelasticity of the films can cause apparent mass changes. The latter problem can be minimized by checking the frequency response of the EQCM,174 while the various mass transport components can be separated by careful data analysis.175,176... 

Solvent Transport through the Membrane The fluid flux J... 

Solute Retention Retention is determined by the relative rates of solute and solvent transport through the membrane. The impact of operation on solute retention Rj = 1 — Cp/c can be evaluated from Eq. (20-73) by using the mass balance J, = Assuming high reten-... 

Marine lipids are important biological energy sources and have been used as tracers in food studies [395-398]. Some lipids, however, are pollutants [399, 400], and all lipids can potentially act as solvents, transporters, or sinks for pollutants [374,399,401,402]. 

The aforementioned view provides a rational for the distinct differences of the solvent transport coefficients of Nafion and solvated sulfonated pol-yarylenes (Figure 14b), which are generally less-separated and exhibit stronger polymer—solvent interactions. 

Another approach employed to establish the occurrence of a density nversion between the two solutions subsequent to boundary formation involves dialysis between the two solutions s0>. The dialysis membrane is impermeable to the polymer solutes but permeable to the micromolecular solvent, H20. Transfer of water across the membrane occurs until osmotic equilibrium involving equalization of water activity across the membrane is attained. Solutions equilibrated by dialysis would only undergo macroscopic density inversion at dextran concentrations above the critical concentration required for the rapid transport of PVP 36 0 50). The major difference between this type of experiment and that performed in free diffusion is that in the former only the effect of the specific solvent transport is seen which is equivalent to a density inversion occurring with respect to a membrane-fixed or solute-fixed frame of reference. Such restrictions are not imposed on free diffusion where equilibration involves transport of all components in a volume-fixed frame of reference. The solvent flow is governed specifically by the flow of the polymer solutes as described by Eq. (3) which, on rearrangement, gives... 

A part of the solvents transported into the water by the necessary -final cleaning of the hooks, the waste water is fed into the internal water cleaning system. Outsourcing the de-lacquering process is not feasible as the daily number of hooks for de-lacquering is up to 8,000 pieces. When outsourcing a multiple amount of hooks would be necessary and the transport effort would increase considerably as well. 

Transportation cost (supply of solvents, transportation of waste powder) ... 

Future developments that may facilitate ocean measurements from vessels or buoys include miniaturization of chromatographic equipment (so less solvent is needed per analysis), new solvent transport systems, such as electrokinetic transport, to reduce power requirements on the pumps, and more sensitive detectors for liquid chromatography. Certain combinations of very short columns and flow injection analysis are also promising for real-time studies. 

Solvent transport in organic polymer matrices is usually depicted as a two-step mechanism. The first step is the dissolution of the solvent in the superficial polymer layer. This process, which can be considered almost instantaneous in the case of water, creates a concentration gradient. The second step is the diffusion of the solvent in the direction of the concentration gradient. This process may be described by a differential mass balance (often called Fick s second law), which, in the unidimensional case, may be written as... 

If a solution is separated from pure solvent by a semipermeable membrane that allows solvent, but not solute, molecules to pass through, solvent will move into the solution in an attempt to equalize the concentrations on the two sides of the membrane. This solvent transport process is called osmotic flow. If there is no resisting force, solvent will continue to flow into the solution until the solvent reservoir is exhausted. However, if the solutions are arranged as shown in Figure 11-3, the incoming solvent will force the solution up the extension tube. The weight of this solution in the tube exerts a downward pressure that tends to oppose the flow of solvent. Eventually this pressure will exactly balance the force of solvent flow and equilibrium is attained. This equilibrium pressure is called the osmotic pressure. 

Osmotic distillation also removes the solvent from a solution through a microporous membrane that is not wetted by the liquid phase. Unlike membrane distillation, which uses a thermal gradient to manipulate the activity of the solvent on the two sides of the membrane, an activity gradient in osmotic distillation is created by using a brine or other concentrated solution in which the activity of the solvent is depressed. Solvent transport occurs at a rate proportional to the local activity gradient. Since the process operates essentially isothermally, heat-sensitive solutions may be concentrated quickly without an adverse effect. Commercially, osmotic distillation has been used to de-water fruit juices and liquid foods. In principle, pharmaceuticals and other delicate solutes may also be processed in this way. 

In all of these implementations, the required solvent transport is achieved by electroosmotic flow although some of the systems have the built in pressure driven flow ability. 

In the literature, several different notations for tj and Tj have been used. Today, the terms transport number and transference number are used for q side by side Staverman introduced the terms reduced electrical transport number for Ti and electrical transport number for tj. Scatchard called Ti a transference number and ti a transport number, while Agar " introduced the notation Washburn number if Ti is referred to one of the uncharged components. The solvent transference number A, which was introduced by C. Wagner is a reduced transference number with the reference system fixed to the sum of moles of all solvent components. Elektrische Losungsmitteliiberfuhrung , (electrolytic solvent transport) originates in the proposal of Nernst to discriminate between solvent molecules in the solvation shell of the ions and the free solvent. is a reduced transference number referred to the motion of the free solvent. Inspection of Eqs. (54) and (57) shows that Ti depends on the reference system used. This will be shown in the following section in more detail. 

The reduced transference number of the solvent is equal to the electrolytic solvent transport and may be calculated numerically only if t[ or njj are known from other experiments. With Eqs. (60), (61), (63) and (66) a relation between tp and t may be calculated ... 

The reduced transference numbers and are the Washburn numbers W2 and Wi introduced by Agar In his discussion the movement of a neutral solute is treated with respect to the solvent. Later on, Feakins used Washburn numbers to explain the solvent transport in mixtures of two solvent components when the solvent mole fraction is varied between 0 and 1. 

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