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Along with the amount of time the chemical was on the material and the analysis of the test material, one can determine the cumulative permeation of the test chemical. The following table gives examples of the calculated permeability coefficient of certain gases through a silicone membrane.
The SI unit for permeability is the square metre (m 2). A practical unit for permeability is the darcy (d), or more commonly the millidarcy (md) (1 d ≈ 10 −12 m 2). The name honors the French Engineer Henry Darcy who first described the flow of water through sand filters for potable water supply. Permeability values for most materials ...
Likewise, it is used to calculate lipophilic efficiency in evaluating the quality of research compounds, where the efficiency for a compound is defined as its potency, via measured values of pIC 50 or pEC 50, minus its value of log P. [27] Drug permeability in brain capillaries (y axis) as a function of partition coefficient (x axis) [28]
Fick's first law relates the diffusive flux to the gradient of the concentration. It postulates that the flux goes from regions of high concentration to regions of low concentration, with a magnitude that is proportional to the concentration gradient (spatial derivative), or in simplistic terms the concept that a solute will move from a region of high concentration to a region of low ...
The darcy is referenced to a mixture of unit systems. A medium with a permeability of 1 darcy permits a flow of 1 cm 3 /s of a fluid with viscosity 1 cP (1 mPa·s) under a pressure gradient of 1 atm/cm acting across an area of 1 cm 2. Typical values of permeability range as high as 100,000 darcys for gravel, to less than 0.01 microdarcy for ...
Equation 1: = / / The drug apparent permeability (P app) is calculated by normalizing the drug flux (j) over the initial concentration of the API in the donor compartment (c 0) as: Equation 2: = / Dimensionally, the P app represents a velocity, and it is normally expressed in cm/sec. The highest is the permeability, the highest is expected to ...
The n-octanol-water partition coefficient, K ow is a partition coefficient for the two-phase system consisting of n-octanol and water. [1] K ow is also frequently referred to by the symbol P, especially in the English literature. It is also called n-octanol-water partition ratio. [2] [3] [4]
Electrical potential surface of paracetamol showing polar areas in red and blue. The polar surface area (PSA) or topological polar surface area (TPSA) of a molecule is defined as the surface sum over all polar atoms or molecules, primarily oxygen and nitrogen, also including their attached hydrogen atoms.