Penetration of molecules into

In 1909 McBain reported that the uptake of hydrogen by carbon appeared to occur in two stages a rapid process of adsorption appeared to be followed by a slow process of absorption into the interior of the solid. McBain coined the term sorption to cover both phenomena. In recent years it has been found convenient to use sorption when it is not possible to make a clear distinction between the stages of uptake, and also to use it to denote the penetration of molecules into very narrow pores (Barrer, 1978). 

Trace amounts of polar impurities can contaminate crystallized products, and very small amounts of impurities can be responsible for producing intensely colored products. Polar impurities can be removed by stirring a solution of the product with 1—2 wt% of activated carbon relative to the solute, adsorbing these impurities to the finely divided solids. Impurities are trapped in the pores of the activated carbon by van der Waals attractive forces. There are three categories of pore sizes macroporous (1000—100,000 A), mesoporous (100—1000 A), and microporous (<100 A). Viscous solvents slow the penetration of molecules into the pores [18], and polar solvents are generally more effective than nonpolar solvents for treatment [19], A filter aid (Section II.D.) is often used to avoid slow filtrations in removing activated carbon. After filtration, the filtrate is processed to the product. 

As mentioned earlier, penetration enhancers are extremely important for specific and rationally tailored delivery of pharmaceuticals. One novel class of such compounds are cell penetration enhancers (CPEs), which are unique peptides, first recognized in biological systems such as HIV, herpes enzymes, some fly [99] and frog species [100], and more [101]. These peptides have the ability to cause a perforation in the membranes of living cells thereby enhancing the penetration of molecules into the cells and target specific molecules to specific cells or nuclei [102, 103]. 

Gryns (1896), Hedin (1897), and especially Overton (1900) looked at the permeability of a wide range of different compounds, particularly non-electrolytes, and showed that rates of penetration of solutes into erythrocytes increased with their lipid solubility. Overton correlated the rate of penetration of the solute with its partition coefficient between water and olive oil, which he took as a model for membrane composition. Some water-soluble molecules, particularly urea, entered erythrocytes faster than could be attributed to their lipid solubility—observations leading to the concept of pores, or discontinuities in the membrane which allowed water-soluble molecules to penetrate. The need to postulate the existence of pores offered the first hint of a mosaic structure for the membrane. Jacobs (1932) and Huber and Orskov (1933) put results from the early permeability studies onto a quantitative basis and concluded molecular size was a factor in the rate of solute translocation. 

The dentin-adhesive interface has been studied using a Raman microprobe technique [199], which shows the formation of resin-reinforced dentin and the penetration of resin into dentin substrate to a depth of 5-6 microns. Further study of the interface showed that only small molecules such as MMA, 4-MET (hydrolyzed 4-META) or oligomers infiltrated the dentin, and that all of the resin in the dentin originated from the monomer solution [200]. SEM and TEM studies of the ultrastructure of the resin-dentin interdiffusion zone showed a 2 micron zone with closely packed collagen fibrils running parallel to the interface [201]. 

The morphological structure of the fiber determines the pathway that dyes take during dyeing and is critical for the rate and extent of dye uptake. In some way, the dye has to penetrate the more or less hydrophobic layer on the fiber surface, formed by the epicuticle and the exocuticle. The strong swelling capacity of the intercellular cement is important for the penetration of dyes into the fiber. Only then are the sulfur-rich keratins also penetrated by the dye molecules. In general terms, Fick s law can be applied to the diffusion phenomena [46],... 

Normal Micelles - Solubilizate Probes. The addition of a probe molecule, usually bearing a C=0 group, to a micelle has been used to asses die solubilization site of the probe (67) and to infer the extent of penetration of water into micelles (68,69). The basis of such studies is the well known decrease in the 0=0 band frequency upon hydrogen bond formation (70 -73). Two important concepts must be addressed, however, when using probes in studies of micelles the solubilization site of the probe (micelle core or palisade layer) and the possibility of probe-induced changes in the micelle. 

There are some (Heinze, 1996) to whom the polaron explanation of the ionic introduction of electronic conductivity in organic compounds is specious The roughness factor of 400 would limit the degree of penetration of ions into the interstices of the polymer. However, Li+ or even CIOJ is of course much smaller than the test molecules (large dye molecules) which are generally used to probe the real area. Thus, one might conceive of a model of the polymer that is all fibers, the intercalation being all pervasive. It is obvious that an Li+ ion adsorbed on the surface of a fiber will promote an electron that may indeed be free to move under a field, i.e., to conduct. 

Only a few laser scanning confocal microscopy (LSCM) studies have examined the passive permeation pathways of molecules across the skin. Cullander and Guy [36] showed that calcein, a multiply charged fluorophore, penetrates minimally into the SC of hairless mouse skin (HMS). Similar studies by Turner et al. [37a] have confirmed this observation. Indeed, it is the hydrophilic, charged nature of calcein that prevents its facile partitioning into the lipophilic intercellular spaces of the SC. Although some penetration of calcein into the SC intercellular domains, and into the pilary canal of the hair follicles, is observed, the total passive epidermal transport of calcein was negligible. 

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