Solubility stagnant layer

M is the time-dependent undissolved (solid) drug amount present in the donor compartment S is the area available for permeation K, is the drug dissolution constant Cj is the drug solubility Vr is the receiver compartment volume Ai, A3, and A2 are, respectively, the thicknesses of the first and the second stagnant layers and the membrane Cjo and Mg are, respectively, the initial drug concentration and undissolved drug amount in donor compartment, while K2X, K22, and K r are partition coefficients defined as follows ... 

Principles and Characteristics Supercritical fluid extraction uses the principles of traditional LSE. Recently SFE has become a much studied means of analytical sample preparation, particularly for the removal of analytes of interest from solid matrices prior to chromatography. SFE has also been evaluated for its potential for extraction of in-polymer additives. In SFE three interrelated factors, solubility, diffusion and matrix, influence recovery. For successful extraction, the solute must be sufficiently soluble in the SCF. The timescale for diffusion/transport depends on the shape and dimensions of the matrix particles. Mass transfer from the polymer surface to the SCF extractant is very fast because of the high diffusivity in SCFs and the layer of stagnant SCF around the solid particles is very thin. Therefore, the rate-limiting step in SFE is either... 

The simple kinetics for uptake of soluble substrate of the bacteria in a biofilm is traditionally described by a combination of mass transport across the water/biofilm interface, transport in the biofilm itself and the corresponding relevant biotransformations. Transport through the stagnant water layer at the biofilm surface is described by Fick s first law of diffusion. Fick s second law of diffusion and Michaelis-Menten (Monod) kinetics are used for describing the combined transport and transformations in the biofilm itself (Williamson... 

Contaminant volatilization from subsurface solid and aqueous phases may lead, on the one hand, to pollution of the atmosphere and, on the other hand, to contamination (by vapor transport) of the vadose zone and groundwater. Potential volatihty of a contaminant is related to its inherent vapor pressure, but actual vaporization rates depend on the environmental conditions and other factors that control behavior of chemicals at the solid-gas-water interface. For surface deposits, the actual rate of loss, or the pro-portionahty constant relating vapor pressure to volatilization rates, depends on external conditions (such as turbulence, surface roughness, and wind speed) that affect movement away from the evaporating surface. Close to the evaporating surface, there is relatively little movement of air and the vaporized substance is transported from the surface through the stagnant air layer only by molecular diffusion. The rate of contaminant volatilization from the subsurface is a function of the equilibrium distribution between the gas, water, and solid phases, as related to vapor pressure solubility and adsorption, as well as of the rate of contaminant movement to the soil surface. 

The parameter refers to the thickness of a stagnant diffusion medium layer at the surface of the solid, where the drug concentration reacBgat steady-state dissolution. Indeed, it is preferable to consideCsto be the solubility ofthe drug in the diffusion layer, since it is the maximum concentration possible in that layer that controls the dissolution rate. Nevertheless, on the basis of this equation, it can still be seen that if the solubility in the dissolution medium was increased, the dissolution rate would also increase. 

Higuchi [26] assumed that ein equilibrium exists between the solute and solution at the solid-liquid interface eind that the rate of movement of the drug into the bulk is governed by the diffusion of free solute and solubilized drug across a stagnant diffusion layer. Drug solubilized in micelles will have a lower diffiision coefficient than the free solute molecules. This means that the effect of surfactant on the dissolution rate will be related to the dependence of dissolution rate on the diffusion coefficients of the species emd not on their solubilities as suggested by equation (2.19). Thus, the rate... 

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