Organic chemicals permeation

It is presently unclear how molecular size affects the membrane permeability of simple hydrophobic organic chemicals. A size restriction to membrane permeation may exist, but it appears to be greater than the proposed 0.95 nm and it may be erroneous to view it as a sharp "cut-off."... 

Figure 9.1 illustrates the processes of chemical uptake and elimination via the respiratory surface for fish. These processes result from the combination of the water ventilation rate, the rate of chemical permeation across the membranes of the respiratory surface, and the rate of distribution of the chemical within the organism. 

A lack of gill membrane permeation for uptake from water has been proposed for large molecules which have an effective cross section larger than 0.95 nm [79] or which exceed a length of 4.3 nm [60], although this limiting value may be species-dependent [43,80]. Hydrophobic organic chemicals which are big molecules may thus show a very low bioaccumulation behaviour. Limited uptake will thus lead to an actual low BCR... 

MI-PTR-MS The MI-PTR-MS platform was first explored by Alexander and coworkers [190]. A membrane of polydimethylsiloxane (PDMS) was employed in the MI-PTR-MS as an interface that can penetrate some organic chemicals easily while others may show little or no permeation. With the MI-PTR-MS, the fundamental properties like diffusion and partition coefficients have been determined for different chemical compounds both in nonpolar and polar species [190,191]. If the analyte-membrane interaction is stronger, then the smaller diffusion coefficient and larger partition coefficient will be obtained. The effect of the temperature in the range of 25-65°C on the diffusion and partition coefficient has also been investigated. The detection limit of the MI-PTR-MS system was in the order of tens of ppt, and its response was linear for more than four orders of magnitude. This study demonstrated that MI-PTR-MS has the ability to measure VOCs dissolved in aqueous solutions [191]. 

Correlations between the bioaccumulation tendency of organic chemicals in aquatic organisms and 1-octanol/water partition coefficients show a loss of linearity for very hydrophobic compounds. In order to establish the possible cause(s) of this phenomenon, the roles of metabolism, exposure time, membrane permeation, lipid solubility, and bioavailability on the bioconcentration potential of chemicals, are discussed. Data are presented showing their relative importance. It is concluded that, although insufficient experimental data presently exist which can conclusively establish the cause(s), reduced lipid solubility and reduced bioavailability are the most likely factors contributing to the loss of linear correlation for non-metabolizing chemicals. 

Scholars of American political history note the opportunity crises present for policy innovation, and in the synthetic organic chemicals industry, American policymakers faced the challenge of creating laws and institutional mechanisms to build technical and market expertise in one of the most complicated science-based industries of the early twentieth century. Policymakers often drew on familiar tools, particularly the tariff laws, but some policies reflected newer trends in society, including the growing faith in information and data associated with the Progressive Era. In other cases, policymakers experimented and tried alternatives to suit the immediate task at hand. Trial and error permeated the halting steps to mobilize for... 

Yan et al. (1994) prepared thin Pd membranes using the metal-organic chemical vapour deposition (MOCVD) method in the macropores of an a-alumina support tube. The best Pd membrane was obtained under a pressure of 100-120 Pa inside the reactor and a heating rate of 10°C/min, at 300°C. The H2 permeability was equivalent to that of the membrane prepared by Uemiya et al. (1991d), and the selectivity was higher than 1000 at a permeation temperature of 100-300°C. The H2 permeability was proportional to the first order of the H2 partial pressure, suggesting that the diffusion of dissolved H2 was not rate-determining. H2 embrittlement was restrained at a temperature as low as 100°C, and the membrane was resistant to abrasion in spite of its thinness. 

Pervaporation has been commercialized for two appHcations. The first and most developed is the separation of water from concentrated alcohol solutions. GFT of Neunkirchen, Germany, the leader in this field, installed their first important plant in 1982. More than 100 plants have been installed by GFT for this appHcation (90). The second appHcation is the separation of small amounts of organic solvents from contaminated water (91). In both of these apphcations, organics are separated from water. This separation is relatively easy, because organic compounds and water, due to their difference in polarity, exhibit distinct membrane permeation properties. The separation is also amenable to membrane pervaporation because the feed solutions are relatively nonaggressive and do not chemically degrade the membrane. 

Experiments on absorption and permeation of flavorants have been carried out in high nitrile barrier containers with a number of organic compounds which represent a variety of chemical functionalities. Table... 

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