Diffusion coefficients of solvents

Diffusion coefficient of solute in the membrane, m /s Diffusion coefficient of solvent in the membrane, m /s Quanhty difference Surface porosity... 

Solvents produce different effects than do corrosive chemicals. Both silica and carbon black filled natural rubbers were more resistant to solvents than unfilled rubber. Also, the cure time was important, indicating that the bound rubber plays a role in the reduction of a solvent sorption. The diffusion coefficient of solvents into rubbers decreases with longer cure times and higher fillers loadings. Polychloroprene rubber swollen with solvent has a lower compression set when it is filled with carbon black. 

This loss in density of P(MMA) on exposure to high energy radiation should result in a very large (several orders of magnitude) increase in the diffusion coefficient of the developer into the P(MMA) matrix. It is well known that relatively small increases in the free volume, e.g., polymers heated from below to above Tg is accompained with a large increase in the diffusion coefficient of solvents in the polymer matrix (5 6). For example, Kambour (13) reported very large increases in the diffusion coefficient of ethanol in crazed poly carbonate. 

Numerical factor for the diffusion coefficient of solvent in polymer, Eq. (1)... 

The advantage of computer simulations is the possibility of obtaining transport data that cannot or can only barely be measured. It is possible in this way to simulate diffusion coefficients of solvent molecules in the ionic solvation shells and to compare them with those of the bulk solvent molecules and with those of the ions, or to study transport coefficients at different time scales. 

In our previous works [14, 15, 16, 17, 18], the pulsed-high-field-gradient spin-echo (PHFGSE) H NMR method was used for determining the diffusion coefficient of solvent, probe molecules, and probe polymers in a gel and rodlike polypeptides in the thermotropic liquid-crystalline state. The great strength of the field... 

Feng, X., and Burns, C.M. (2001) Measurements of partition, diffusion coefficients of solvents in polymer membranes using rectangular thin-channel column inverse gas chromatography (RTCCIGC)./. Membr. Sci., 188,... 

The same drag coefficient determines s and Dp, leading to Dp/Dpo s/sq, implying that Eq. 9.18 should also govern probe diffusion. Indeed, Bu and Russo(22) show that their measurements agree with Eq. 9.18. As seen above, in probe polymer systems Dp/Dpo depends strongly on M, with a M. This dependence is inconsistent with the physical model invoked by Langevin and Rondelez in their ansatz for their equation. However, the diffusion coefficients of solvent molecules and other very small probes do appear to be independent of M, so Eq. 9.18 may have a limited domain of validity. 

The value of coefficient depends on the composition. As the mole fraction of component A approaches 0, approaches ZJ g the diffusion coefficient of component A in the solvent B at infinite dilution. The coefficient Z g can be estimated by the Wilke and Chang (1955) method ... 

Umesi, N.O. (1980), Diffusion coefficients of dissoived gases in iiquids -Radius of gyration of solvent and solute . M.S. Thesis, The Pennsylvania State University, PA. 

Diffiusion Coefficient. The method of Reference 237 has been recommended for many low pressure binary gases (238). Other methods use solvent and solute parachors to calculate diffusion coefficients of dissolved organic gases in Hquid solvents (239,240). Molar volume and viscosity are also required and may be estimated by the methods previously discussed. Caution should be exercised because errors are multiphcative by these methods. 

The Stokes-Einstein equation has already been presented. It was noted that its vahdity was restricted to large solutes, such as spherical macromolecules and particles in a continuum solvent. The equation has also been found to predict accurately the diffusion coefficient of spherical latex particles and globular proteins. Corrections to Stokes-Einstein for molecules approximating spheroids is given by Tanford. Since solute-solute interactions are ignored in this theory, it applies in the dilute range only. 

FIG. 16 Reduced self-diffusion coefficients of SPM model ions in pores of different sizes. The zero solvent packing represents the RPM model. 

Polymer gels and ionomers. Another class of polymer electrolytes are those in which the ion transport is conditioned by the presence of a low-molecular-weight solvent in the polymer. The most simple case is the so-called gel polymer electrolyte, in which the intrinsically insulating polymer (agar, poly(vinylchloride), poly(vinylidene fluoride), etc.) is swollen with an aqueous or aprotic liquid electrolyte solution. The polymer host acts here only as a passive support of the liquid electrolyte solution, i.e. ions are transported essentially in a liquid medium. Swelling of the polymer by the solvent is described by the volume fraction of the pure polymer in the gel (Fp). The diffusion coefficient of ions in the gel (Dp) is related to that in the pure solvent (D0) according to the equation ... 

Pure PHEMA gel is sufficiently physically cross-linked by entanglements that it swells in water without dissolving, even without covalent cross-links. Its water sorption kinetics are Fickian over a broad temperature range. As the temperature increases, the diffusion coefficient of the sorption process rises from a value of 3.2 X 10 8 cm2/s at 4°C to 5.6 x 10 7 cm2/s at 88°C according to an Arrhenius rate law with an activation energy of 6.1 kcal/mol. At 5°C, the sample becomes completely rubbery at 60% of the equilibrium solvent uptake (q = 1.67). This transition drops steadily as Tg is approached ( 90°C), so that at 88°C the sample becomes entirely rubbery with less than 30% of the equilibrium uptake (q = 1.51) (data cited here are from Ref. 138). 

To summarize, the hydration status of the drug molecule and other components of a pharmaceutical formulation can affect mass transport. Solubility of drug crystals in an aqueous or nonaqueous solvent may depend on the presence or absence of moisture associated with the drug. Hydration may also determine the hydrodynamic radii of molecules. This may affect the frictional resistance and therefore the diffusion coefficient of the drug molecules. Diffusion of drugs in polymeric systems may also be influenced by the percent hydration of the polymers. This is especially tme for hydrogel polymers. Finally, hydration of... 

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