Diffusion through gels

One of the earliest, and still most frequently used, descriptions of diffusion through gels is due to Ogston (Equation 4-28). Although Ogston developed this expression for the diffusion of particles in polymer solutions, it is equally applicable to particle diffusion through gels. Similarly, the partition coefficient (the ratio of concentration within the gel to concentration within an external aqueous solution) can be estimated as ... 

Another compound with formula Er2(bdc)3 (H20)6 has also been reported (Deluzet et al., 2003). Contrary to the two previous ones, it has been obtained by slow diffusion through gel. Its crystal structiue is three-dimensional and can be described as the succession of erbium ion planes linked by terephtalato organic groups (see fig. 14). 

Two main synthesis methods have been used in order to design lanthanide-containing coordination polymers, namely the diffusion through gels and hydrothermal conditions. We would like to briefiy described both and try to highlight their respective advantages and drawbacks. 

Glucose permeability was determined the amount of glucose diffused through gel covered polycarbonate membrane in 0.9% NaCl at 20 minute intervals detected by a Beckman Glucose Analyzer. 

The urease is incorporated into a polyacrylamide gel which is allowed to set on the bulb of the glass electrode and may be held in position by nylon gauze. Preferably, the urease can be chemically immobilised on to bovine serum albumin or even on to nylon. When the electrode is inserted into a solution containing urea, ammonium ions are produced, diffuse through the gel and cause a response by the ammonium ion probe ... 

Kokufuta, E Jinbo, E, A Hydrogel Capable of Facilitating Polymer Diffusion through the Gel Porosity and Its Application in Enzyme Immobilization, Macromolecules 25, 3549, 1992. Kresge, CT Leonowicz, ME Roth, WJ Vartuli, JC Beck, JS, Ordered Mesoporous Molecular Sieves Synthesized by a Liquid-Crystal Template Mechanism, Nature 359, 710, 1992. 

Numerous models have been proposed to interpret pore diffusion through polymer networks. The most successful and most widely used model has been that of Yasuda and coworkers [191,192], This theory has its roots in the free volume theory of Cohen and Turnbull [193] for the diffusion of hard spheres in a liquid. According to Yasuda and coworkers, the diffusion coefficient is proportional to exp(-Vj/Vf), where Vs is the characteristic volume of the solute and Vf is the free volume within the gel. Since Vf is assumed to be linearly related to the volume fraction of solvent inside the gel, the following expression is derived ... 

The fourth type of mechanism is exclusion although perhaps inclusion would be a better description. Strictly, it is not a true sorption process as the separating solutes remain in the mobile phase throughout. Separations occur because of variations in the extent to which the solute molecules can diffuse through an inert but porous stationary phase. This is normally a gel structure which has a small pore size and into which small molecules up to a certain critical size can diffuse. Molecules larger than the critical size are excluded from the gel and move unhindered through the column or layer whilst smaller ones are retarded to an extent dependent on molecular size. 

Large coils diffuse through the porous gel via the broadest chaimels, thus following the shortest way, and they arrive as the first at the end. Smaller coils are able to follow all side paths and have a longer travelling time. 

Start the electrophoresis immediately when all samples, marker protein mixtures, or references are applied, because molecules diffuse through the soft stacking gel and the pH jump between stacking gel and separation gel, which is important for separation power, drops down. 

As seen, diffusion in nonporous gel membranes differs from that in macro-porous or microporous membranes. Various theories based on solute diffusion through the macromolecula r free volume in the membrane have been proposed. It is clear from these theories that structural parameters of the polymer network such as degree of swelling, molecular weight between crosslinks, and crystallinity in addition to factors such as solute size and solvent free volume play important roles in this type of transport. 

With ion exchangers as catalysts for olefin hydration, special attention was paid to transport problems within the resin particles and to their effects on the reaction kinetics. In all cases, the rate was found to be of the first order with respect to the olefin. The role of water is more complicated but it is supposed that it is absorbed by the resin maintaining it in a swollen state the olefin must diffuse through the water or gel phase to a catalytic site where it may react. The quantitative interpretation depends on whether the reaction is carried out in a vapour system, liquid-vapour system or two-phase liquid system. In the vapour system [284, 285], the amount of water sorbed by the resin depends on the H20 partial pressure it was found at 125—170°C and 1.1—5.1 bar that 2-methyl-propene hydration rate is directly proportional to the amount of sorbed water... 

After polymer skin formation, gel bubbles form on the device surface because the skin layer prevents any further outflow of water and pressure to expel water is exerted on the surface from the inside after shrinking [5], It is proposed that pressure is induced within the gel in the process of shrinking. This pressure may induce the outward convection of water. In the on state, drug is released from the surface by diffusion through the hydrated gel matrix. However, in the process from the on state to the off state upon increasing temperature, the drug must be released not only by diffusion but also by convective transport [9]. 

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