Water penetration coefficient

Plasticizers may also change properties of a polymer surface from l drophobic to hydrophilic which affects permeability of Itydrophilic penetrants. Decreased hydropho-bidty is recpiired for PVC material to increase its receptivity of water-based coatings. Combinations of plasticizers and metal salts were used to induce hydrophilic properties. ° Rubber print rolls also require to have hydrophilic properties to hold film of water-based ink and this was achieved by use of a special plasticizer. Increase in hpophillicity of plasticizer causes linear increase in water penetration coefficient and water flux across membranes obtained from PVC plasticized with various plasticizers. Compatibility of a polymer and a plasticizer was found to affect staining resistance. Compatible plasticizers were less likely to dissolve stains and help them to penetrate into material. 

The importance of lipophilicity to bitterness has been well established, both directly and indirectly. The importance of partitioning effects in bitterness perception has been stressed by Rubin and coworkers, and Gardner demonstrated that the threshold concentration of bitter amino acids and peptides correlates very well with molecular connectivity (which is generally regarded as a steric parameter, but is correlated with the octanol-water partition coefficient ). Studies on the surface pressure in monolayers of lipids from bovine, circumvallate papillae also indicated that there is a very good correlation between the concentration of a bitter compound that is necessary in order to give an increase in the surface pressure with the taste threshold in humans. These results and the observations of others suggested that the ability of bitter compounds to penetrate cell membranes is an important factor in bitterness perception. 

The correlation (or lack of correlation) of other physiochemical characteristics has not yet been established. For instance, are all surfactants irritants Can one classify severity by the size of the molecule Can octanol water partition coefficients predict irritation potential does a propensity to partition out of the ocular fluid mean that a compound presents more of an irritation hazard than one which is more water soluble Theoretically, these data should reflect the ability of a compound to penetrate the eye and cause an irreversible lesion. However, until definitive data are available, physical and chemical parameters will probably have limited utility in an overall assessment of irritation. 

Absorption of some highly ionized compounds (e.g., sulfonic acids and quaternary ammonium compounds) from the gastrointestinal tract cannot be explained in terms of the transport mechanisms discussed earUer. These compounds are known to penetrate the Upid membrane despite their low Upid-water partition coefficients. It is postulated that these highly lipophobic drugs combine reversibly with such endogenous compounds as mucin in the gastrointestinal lumen, forming neutral ion pair complexes it is this neutral complex that penetrates the Upid membrane by passive diffusion. 

The substance with higher water partition coefficient values can penetrate... 

Many bixxer compounds contain both hydrophobic and hydrophilic sites which can alter cell membranes through penetration. There is a correlation between bitter intensity and hydrophobicity-solubility indexes such as fee octanol/water partition coefficient, lo (7). Penetration may directly affect cAMP phosphodiesterase as part of fee transduction process (see below). A bitter receptor protein may be involved wife certain bitters, such as specific structural requirements wife fee bitter tasting dipeptides and denatonium salts (27). The latter is used in some consumer products to avoid accidental ingestion. A receptor mechanism is also supported by fee existence of a genetic "taste blindness" for some bitter materials (see below). 

According to McFarland [26], aquatic toxicity can be considered the result of penetration of toxicant into biophases and its interaction with one or more biochemical sites of action. Thus, he and others have postulated that toxicity is a function of the ability of the chemical to enter biophases and its ability to react with cellular compounds. Bioavailability of chemicals in fish has been shown to be related to chemical flux across fish gills [27], an identified exposure pathway. Flux across fish gills is in turn related to the ability of the chemical to partition between organic and aqueous phases, which is usually correlated with the its octanol-water partition coefficient (logPo/w) [28]. It is therefore not surprising that the acute toxicity of narcotic chemicals has been shown to be related to their propensity to accumulate in the membranes, and hence their logPe/w [29]. 

Emollients are often added to cream formulations to modify either the characteristics of the pharmaceutical vehicle or the condition of the skin itself to promote penetration of the active ingredient to act either locally or systemically. The stratum corneum, being keratinized tissue, behaves as a semipermeable artificial membrane, and drug molecules can penetrate by passive diffusion. The rate of drug movement depends on the drug concentration in the vehicle, its aqueous solubility, and the oil/ water partition coefficient between the stratum corneum and the product s vehicle. Commonly used emollients include glycerin, mineral oil, petrolatum, isopropyl pal-mitate, and isopropyl myristate. 

In ISFETS utilizing polymeric ion-selective membranes, it has been always assumed that these membranes are hydrophobic. Although they reject ions other than those for which they are designed to be selective, polymeric membranes allow permeation of electrically neutral species. Thus, it has been found that water penetrates into and through these membranes and forms a nonuniform concentration gradient just inside the polymer/solution interface (Li et al., 1996). This finding has set the practical limits on the minimum optimal thickness of ion-selective membranes on ISFETS. For most ISE membranes, that thickness is between 50-100 jttm. It also raises the issue of optimization of selectivity coefficients, because a partially hydrated selective layer is expected to have very different interactions with ions of different solvation energies. 

Other fatty acids as absorption enhancers have been reported. Ogiso et al. [112] demonstrated that lauric acid (C12) produced the largest increase in permeation rate, penetration coefficient, and partition coefficient of propranolol. Onuki et al. [113] reported that docosa-hexaenoic acid (DHA) has a strong insulin permeability enhancement effect and little toxicity, compared to oleic acid and eicosapentaenoic acid (EPA) using a water-in-oil-in-water (W/O/W) multiple emulsion with no or little mucosal damage. 

The stratum corneum has been estimated to contribute 1000 times the diffusional resistance to chemical penetration as the layers beneath it, except for extremely lipid-soluble compounds with tissue/water partition coefficients greater than 400. As in most other epithelial tissues, the two other layers of the skin (dermis and subcutaneous tissue) offer little resistance to penetration. Once a substance has penetrated the outer epithelium, these tissues are rapidly traversed. This may not be true for highly lipid-soluble compounds, because the dermis may function as an additional aqueous barrier preventing a chemical that has penetrated the epidermis from being absorbed into the blood. 

If we assume that the solute penetrates only through the water-filled channels in the membrane (with volume fraction c/jw), then the solute-water frictional coefficient is close to that of free diffusion D°... 

Cornea The cornea is an important mechanical barrier protecting the intraocular tissues. It is considered to be the main pathway for ocular penetration of topically apphed drugs. However, due to its unique structure, with the hydrophilic stroma sandwiched between the highly lipophilic epithelium and the less lipophilic endothelium, the penetration of compounds through the cornea depends on their n-octanol-water partition coefficient. Only drugs with a partition coefficient between 10 and 100 that show both lipid- and water-soluble properties can readily pass through the cornea. 

The polarity of insecticides has been regarded as an important factor for cuticular penetration. As mentioned earlier, the typical insect cuticle should be considered a two-phase system, the outer layer (epicuticle) having hydrophobic properties and the inner layers (procuticle) having hydrophilic properties. Thus, whether the insecticide is lipid soluble or water soluble, its tendency to move through the cuticle as a whole depends on whether it can pass through the hydrophobic or hydrophilic barrier, whichever the case may be. The efficiency of such movement will probably depend on the oil-water partition coefficient of the insecticide, the nature of the surfactant or solvent—which may be a part of the insecticide formulation—and the nature of the cuticle itself (Terriere, 1982). 

In the beginning of the last century, it was observed that the permeability of a substance across a cell membrane is proportional to the relative partition coefficient of the permeating substance between phases of oil and water. In later years, it was found that the product of the permeability coefficient P and the square root of the molecular weight of a permeant shows a better correlation with the partition coefficient than does permeability alone. This correlation has lead to the idea that permeation is limited not only by the hpid solubility of the permeant but also by a screen-like property of the membrane because small molecules penetrate faster than would be predicted from their oil-water partition coefficients. For larger molecules, however, one would expect a product of permeability with the cube root, rather than the square root, of the molecular weight to be more closely correlated with the oil-water partition coefficient. Likewise, the apparent partition coefficient to the membrane plays a major role for the transport processes across the membrane. In strict physico-chemical terms, a membrane is not a phase, but it is most common to treat it as such anyway. 

Most structure-permeation evaluations of transdermal transport indicate the strong relationship between a chemical s permeability coefficient across the skin (Aip) and its lipophilicity. Potts and Guy analyzed a diverse database of skin penetration data, comprising 91 compounds and developed a simple relation indicating that Kp depended only on lipophilicity, as measured by the chemical s octanol-water partition coefficient (P), and molecular weight (MW) ... 

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