Physicochemical form, effect

The growing interest in selenium speciation is due to its toxic and/or essential properties that depend on the actual physicochemical form of the element. Strong evidence exists on the adverse health effects related to selenium deficiency. It was also demonstrated that supplementation with different selenium forms might help to remove the symptoms of deficit. Selenium has been found in the active site of different antioxidant enzymes, protecting cells against the action of free radicals. The species-dependent cancer-preventive properties are well documented, yet the specific biological functions have not been ultimately elucidated. The list of small selenium compounds includes inorganic forms (two stable valence states), volatile alkylated species, and... 

Quantifying the influence of physicochemical form on metal uptake from solids is an important prerequisite to developing realistic models of metal dynamics in animals. In this paper -we statistically assess the effect of selected chemical characteristics of estuarine sediments on the concentration of Zn in bivalves which ingest those sediments while feeding. Our objective is to determine what proportion of the variation in Zn concentrations of deposit-feeding bivalves from natural systems may be explained by the physicochemical partitioning of Zn in the sedimentary food of the animals. 

Effect of the Physicochemical Form of Trace Metals on Their Accumulation by Bivalve Molluscs... 

Bivalve molluscs effectively concentrate many trace elements (1-13). They are filter feeders and as such maintain a flow of water through their gills for feeding, respiration, and the removal of metabolic wastes. Trace metals occur in many physicochemical forms in water, and thus can enter animals by their ingestion of living and nonliving particulate material suspended in the water and from the sorption of substances dissolved in the water. We know neither the rates of accumulation nor the effects of physicochemical form on accumulation of many of the critical elements in most animals. 

Differences in concentration factors between stable and radioactive isotopes may result also from other factors. If more than one chemical form of the element are present in the environment, they may be subject to different turnover and concentration processes. Seasonal effects could result if different physicochemical forms dominate during parts of the year. 

Effect of Physicochemical Form. The physicochemical form of the element may affect the quantities accumulated. To assess the... 

The goal of the work reported here was to devise a theory that predicts the polar/nonpolar phase separation as a favorable thermodynamic process. In addition, the effects of each of the physicochemical forms on the thermodynamic and structural characteristics of the biphasic material were sought. The current molecular model was not developed to predict mechanical or thermal behavior of ionomers. Hence, properties like glass transition and melt temperatures, storage, and shear moduli cannot be determined from the current model. We must also stress that this modeling work is still in its infancy. As such, it has used several tenuous assumptions that must be tested. The formalism is a start, but not the end, to devising a comprehensive treatment of ionomeric structure. 

Sikov M, Mahlum D. 1976. Influence of age and physicochemical form on the effects of 239Pu on the skeleton of the rat. In Webster S, ed. The health effects of plutonium and radium. Salt Lake City, UT J.W. Press, 33-47. 

The method that was developed builds on computed values of physicochemical effects and uses neural networks for classification. Therefore, for a deeper understanding of this form of reaction classification, later chapters should be consulted on topics such as methods for the calculation of physicochemical effects (Section 7.1) and artificial neural networks (Section 9.4). 

This reaction data set of 626 reactions was used as a training data set to produce a knowledge base. Before this data set is used as input to a neural Kohonen network, each reaction must be coded in the form of a vector characterizing the reaction event. Six physicochemical effects were calculated for each of five bonds at the reaction center of the starting materials by the PETRA (see Section 7.1.4) program system. As shown in Figure 10,3-3 with an example, the physicochemical effects of the two regioisomeric products arc different. 

Deposition commonly reflects a combination of physicochemical processes and localized effects. It may occur through fouling as a result of contamination by process materials, perhaps plus scaling from the supersaturation of dissolved salts, and coupled with some active under-deposit corrosion. As a consequence, deposits forming within a boiler are almost never single mineral scales but typically consist of a variable mix of scale and corrosion debris, chemical treatment residuals, process contaminants, and the like. 

The surface-phase layers will difier in character depending on the stractures of metal and oxide. On certain metals (zinc, cadmium, magnesium, etc.), loose, highly porous layers are formed which can attain appreciable thicknesses. On other metals (aluminum, bismuth, titanium, etc.), compact layers with low or zero porosity are formed which are no thicker than 1 pm. In a number of cases (e.g., on iron), compact films are formed wfiicfi fiave a distorted lattice, owing to the influence of substrate metal stracture and of the effect of chemical surface forces. The physicochemical and thermodynamic parameters of such films differ from tfiose of ordinary bulk oxides. Because of the internal stresses in the distorted lattice, such films are stable only when their thickness is insignificant (e.g., up to 3 to 5 nm). 

Conversely, in a membrane model, acetylcholine showed mean log P values very similar to those exhibited in water. This was due to the compound remaining in the vicinity of the polar phospholipid heads, but the disappearance of extended forms decreased the average log P value somewhat. This suggests that an anisotropic environment can heavily modify the conformational profile of a solute, thus selecting the conformational clusters more suitable for optimal interactions. In other words, isotropic media select the conformers, whereas anisotropic media select the conformational clusters. The difference in conformational behavior in isotropic versus anisotropic environments can be explained considering that the physicochemical effects induced by an isotropic medium are homogeneously uniform around the solute so that all conformers are equally influenced by them. In contrast, the physicochemical effects induced by an anisotropic medium are not homogeneously distributed and only some conformational clusters can adapt to them. 

In summary, the effect of pH on the dissolution rate of a drug from an oral dosage form depends on (a) the pH of the GI fluids, a patient variable (b) the acid or base strength of the drug, a pharmaceutical variable as well as (c) the physicochemical properties of the dosage form, another pharmaceutical variable. Furthermore, by intentionally designing the dosage form such that it buffers the diffusion layer, we can control a patient variable by a pharmaceutical variable. 

The physicochemical characteristics of the active ingredient in relation to the dosage form and the suitability for its intended purpose was discussed in several EPARs, particularly relating to the solubility characteristics and absorption from the gut. The compression characteristics were also mentioned in some EPARs. The possible effects of different polymorphs or evidence that only a single polymorph is used are addressed as appropriate. Different amorphous or crystalline forms are also discussed. Where affecting the dosage form, selection properties such as unpleasant taste or smell are mentioned. 

A mass of evidence seems to confirm that the mixing rate of radiocarbon in the atmosphere is rapid, and that with respect to its radiocarbon content the atmosphere can be considered as a homogeneous entirety. The contamination of samples with matter from an extraneous source can nevertheless invalidate this assumption. Two types of contamination can be differentiated physicochemical contamination and mechanical intrusion. There are two forms of physicochemical contamination. One is due to the dilution of the concentration of radiocarbon in the atmosphere by very old carbon, practically depleted of radiocarbon, released by the combustion of fossil fuel, such as coal and oil. The other is by the contamination with radiocarbon produced by nuclear bomb tests during the 1950s and later in the twentieth century. The uncertainties introduced by these forms of contamination complicate the interpretation of data obtained by the radiocarbon dating method and restrict its accuracy and the effective time range of dating. 

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