Solvent penetration

Similarly, polymers dissolve when a solvent penetrates the mass and replaces the interchain secondary bonds with chain-solvent secondary bonds, separating the individual chains. This cannot happen when the chains are held together by primary covalent cross-links. Thus, linear and branched polymers dissolve in appropriate solvents, whereas cross-linked polymers are insoluble, although they may be swelled considerably by absorbed solvent. 

As the name implies, these stains are sprayed on and require Httie if any wiping. The solvent itself penetrates into the pore and allows the pigment and a small amount of binder to remain on the surface. These stains usually are composed of an oil-type vehicle and a combination of earth pigments reduced in a combination of aHphatic and aromatic hydrocarbons such as naphtha and toluene. The solvent system itself plays a big role in the appearance of the stain owing to the varying degrees to which solvents penetrate. Restrictions on the use of certain aromatic hydrocarbons have affected the manner in which these stains work. 

Studies have shown that the pore size distribution can influence liquid penetration into tablets [28,29,52]. The accessibility of water vapor to tablet components has been related to chemical stability, especially in the case of hydrolyzable drugs [28]. Solvent penetration, which can affect tablet disintegration and dissolution, has been related to pore size [52]. 

Defatting is an example of a commonly used sample pretreatment process where the analyte is insoluble in non-polar solvents. Lipids interfere in many analytical processes and so are removed at the start of the analysis, if possible, by washing with a non-polar solvent, such as hexane. Removal of fat may also assist in subsequent solvent penetration for extraction of the analyte. 

In contrast to the 02 generator, the polymer used in the O2 sensor must be highly gas permeable but solvent impenetrable. Solvent penetration will alter the probe properties and make calibration dependent on environment. Furthermore, good solvents for the probe will leach the probe and destroy the sensor. Again, it is important that the support dissolve the probe well and not greatly quench the luminescence. 

Morphology based on chemical environment can be probed using F NMR spectroscopy because the chemical shifts of F atoms in the side chains are considerably separated from those in the backbone. Conformational dynamics as affected by domain-selective solvent incorporation are reflected in the widths of static F peaks. These conformational motions, in turn, can influence the migration of solvent penetrants. 

The effects of exposure of organic solids to particular solvents such as pyridine on their conformational stability can also be Interpreted In terms of the structural features discussed above. How small nucleophilic molecules disrupt Inter- and Intramolecular polar Interactions In coals thereby relaxing the structural matrix and allowing further solvent penetration has been extensively discussed by Peppas (e.g. 11,12), Larsen (1,13) and Marzec (14-16) and their colleagues. Indeed the extent to which exposure to a polar solvent such as pyridine destabilizes a material s molecular structure Is a measure of the extent to which the stability of the material depends on polar Interactions. 

Table III lists the contribution to the overall irradiated film solubility rate from the enhanced solvent penetration resulting from radiation exposure. The values for Sr(IF) and Sr(IP) in Table III were calculated using the equations shown in Figure 2. The % contribution to the overall solubility rate, 100 [Sr(IF)—Sr(IP)]/Sr(IF), shown in Table III increased as the radiation dose increased. This behavior is expected since the number and possibly the size of the voids created in the film would be expected to increase as the radiation dose increased.

Contribution to of Irradiated PBS Film From Enhanced Solvent Penetration ... 

The intermicellar potential, deduced from Baxter model, decreases with the number of carbon atoms (25). This has been explained in term of solvent penetration bulk solvent molecules having a small number of carbon atoms penetrate easily in the surfactant alkyl chains, which are then well solvated. This induces a decrease in the intermicellar interactions. The increase in the number of carbon atoms of the bulk solvent induces a decrease in the solvation of the surfactant and then an increase in the intermicellar attractive potential. 

Distortment of active site geometry by solvent penetration... 

Both Mangenot and Raison, Fenson and Fordham have established from their microscopic studies on swelling and solution of nitrocellulose fibres that the surface of the fibres consists of a thin coating of a distinctively less soluble substance than the interior of fibres. Fenson and Fordham assumed that this was a superficial layer that had been partially denitrated in consequence of drastic stabilization processes. Therefore in the instance of a nitrocellulose of 11% N a solvent penetrates into the fibres through the damaged points of this layer. 

To dearly distinguish between these two modes of solvent penetration of the gel, we immersed poly(acrylamide-co-sodium methacrylate) gels swollen with water and equilibrated with either pH 4.0 HQ or pH 9.2 NaOH solution into limited volumes of solutions of 10 wt % deuterium oxide (DzO) in water at the same pHs. By measuring the decline in density of the solution with time using a densitometer, we extracted the diffusion coefficient of D20 into the gel using a least squares curve fit of the exact solution for this diffusion problem to the data [121,149]. The curve fit in each case was excellent, and the diffusion coefficients obtained were 2.3 x 10 5cm2/s into the ionized pH 9.2 gel and 2.4 x 10 5 cm2/s into the nonionized pH 4.0 gel. These compare favorably with the self diffusion coefficient of D20, which is 2.6 x 10 5 cm2/s, since the presence of the polymer can be expected to reduce the diffusion coefficient about 10% in these cases [150], In short, these experiments show that individual solvent molecules can rapidly redistribute between the solution and the gel by a Fickian diffusion process with diffusion coefficients slightly less than in the free solution. 

The physical structure of the stationary phase depends on the compatibility of the solvent with the bonded n-alkyl chains. Compatible nonpolar solvents tend to promote extension of the chains, allowing full penetration by the solvent. Conversely, fairly polar solvents tend to promote collapse of the chains upon each other, allowing negligible solvent penetration. The stationary phase therefore has the ability to adjust itself to maintain a relatively nonpolar character (113). Retention on monomeric bonded phases with octyl (C8) or longer chains are dominated by a partitioning-like mechanism (114). 

The extraction methods for aflatoxins are based on the solubility of these toxins in organic solvents, mainly chloroform, methanol, acetone, benzene, and acetonitrile. For more complex matrices, the addition of diatomaceous earth or citric acid is required. From matrices of vegetable origin, water is usually added in the extraction step, since it facilitates solvent penetration into substrates, improving the percentage of extraction of the toxin. 

Extraction is a process in which the solvent penetrates the dried herb and dissolves the soluble components of the plant material. These then diffuse out into the free solvent surrounding the plant material particles. This process eventually reaches a state of equilibrium (Theoretically, complete equilibrium is only achieved at infinite time and the approach to completion is an asymptotic curve Figure 12.4.) In practice, about 90% of available solids come into solution within 24 horn s for a typical leaf herb at ambient temperature, although harder material, such as some dried woody roots and barks, will take rather longer to achieve a satisfactory degree of extraction. 

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