Permeability thickness, definition

In an effort to optimize the solvent-containing passive sampler design, Zabik (1988) and Huckins (1988) evaluated the organic contaminant permeability and solvent compatibility of several candidate nonporous polymeric membranes (Huckins et al., 2002a). The membranes included LDPE, polypropylene (PP), polyvinyl chloride, polyacetate, and silicone, specifically medical grade silicone (silastic). Solvents used were hexane, ethyl acetate, dichloromethane, isooctane, etc. With the exception of silastic, membranes were <120- um thick. Because silicone has the greatest free volume of all the nonporous polymers, thicker membranes were used. Although there are a number of definitions of polymer free volume based on various mathematical treatments of the diffusion process, free volume can be viewed as the free space within the polymer matrix available for solute diffusion. 

Polyethylene terephthalate) (PET), with an oxygen permeability of 8 iiiuol/(ius-GPa), is not considered a barrier polymer by die old definition however, it is an adequate barrier polymer for holding carbon dioxide in a 2-L bottle for carbonated soft drinks. The solubility coefficients for carbon dioxide are much larger than for oxygen. For the case of the PET soft drink bottle, the principal mechanism for loss of carbon dioxide is by sorption in the bottle walls as 500 kPa (5 atm) of carbon dioxide equilibrates with the polymer. For an average wall thickness of 370 pm (14.5 mil) and a permeabdity of 40 nmol/(m-s-GPa), many months are required to lose enough carbon dioxide (15% of initial) to be objectionable. 

Alternative definitions for the permeability coefficients have been utilized in Eqs. (12.13) and (12.14) for the purpose of explicitly representing the dependence on membrane thickness h and viscosity x. They are related to their counterparts in Eqs. (12.9) and (12.10) ... 

A closer inspection of the definition of membrane permeability suggests additional methods to correlate the permeability with other properties of the diffusing species. The permeability, called or P, is related to the equilibrium partition coefficient, K, the diffusion coefficient in the membrane, D, and the thickness of the membrane, L ... 

How would a 40 A thick layer of lipid molecules comprise a barrier to solute diffusion Consider now Fig. 2, which represents a lipid phase separating two aqueous phases and acting as a permeability barrier between those two phases. The net flux of molecules from side 1 to side 2 of this barrier is v and, from the definition of permeability coefficient given above, is given by... 

Two different definitions of water vapor permeability are used in the BS and ASTM standards, and this can cause confusion. BS 3177 and BS 2782 define the water vapor transmission rate as the mass transfer rate of water vapor per unit area (g m 24h) (see Section 1). but they call it "permeability," When the material is homogeneous and the transfer rate is inversely proportional to thickness. BS 3177 defines an "equivalent permeability" as the "permeability" multiplied by the thickness of the test piece in thousandths of an inch (g mil m 24h). 

The ASTM standards adopt definitions that are consistent w ith the equivalent definitions for gas transmission. Water vapor transmission rate is the mass transfer rate of water vapor per unit area (g nr 24h). Permeance is the ratio of the water vapor transmission rate to the difference in vapor pressure between the surfaces of the test piece measured in mm of mercury this unit is known as the metric perm (g nr 24h mmHg). This is equivalent to the gas transmission rate. Permeability is the product of the permeance and the thickness of the test piece, assuming that the permeance is inversely proportional to thickness for homogeneous materials this unit is known as the perm-centimetre (g cm nr 24h mmHg). Since the adoption of SI units, the water vapor permeability may also be expressed in the units of microgram meter per newton hour (pgm N h or pgm m Pa h ). 

The chloroplasts in green algae and plants are the seat of photosynthessi and producers of glucose-l-P. They are disc-shaped partides 3 to 10 (x in diameter and 1 to 2 p in thickness. It is possible to isolate them from leaves and show that they are bounded by a definite membrane which is semi-permeable. The chloroplasts contain a number of gratia whose diameter varies, according to the type of cell, from 0 2 to 2 (x. A chloroplast contains from 10 to 100 gratia imbedded in a protein matrix. The electron microscope reveals that they have a laminar structure. They contain 33 to 50% protein and also contain lipides. The chloroplasts are auto-reproductive and can divide. 

Permeability is defined as the product of the diffusion constant and the solubility coefficient. Water vapor permeability (WVP] is defined as the time rate of water vapor transmission through unit area of flat material with a definite unit thickness induced by unit vapor pressure difference between two specific surfaces, and under indicated temperature and humidity conditions. (ASTM E 96, Standard Test Method for Water Vapor Transmission of Materials]. 

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