Semi-permeable membrane permeability coefficient

In general, the flow of water due to a chemical potential gradient is called chemical osmosis. When compacted, clay can act as a semi-permeable membrane due to overlapping diffuse double layers. This means that the movement of solute particles is restricted across the membrane, while solvent is free to flow. To attain chemical equilibrium in case of an initial concentration gradient across the clay, water flows from low to high salt concentration. The degree of semi-permeability is described by the reflection coefficient a, which ranges from 0 (no osmosis) to 1 (no solute transport). 

The release rate (dM/dt) of a dmg from an osmotic pump can be described as Cd (dV/dt) where Cd is the dmg solubility in its reservoir compartment. The effective surface area, permeability coefficient, thickness, and osmotic reflection coefficient of the semi-permeable membrane used for the pump are 3.0 cm2, 0.7 10-4 cm2/day, 500 pm, and 0.8, respectively. Initially, the pump has a reservoir compartment with a dmg having Cd of 100 mg/ml, and the observed Ax is 100 atm. If we change the reservoir medium and osmotic agent to increase Cd of the dmg from 100 to 300 mg/ml and to increase Ax from 100 to 300 atm, by how much will the release rate of the dmg increase ... 

Equilibrium dialysis is used in a number of examples to analyse the ratio of lipid-bound to free analyte. Kramer et al. (1998) described the use of equilibrium dialysis by separating the liposome suspension and the water phase by a semi-permeable membrane. The analyte is dissolved in the water compartment of the system and diffuses into the liposome compartment. If equilibrium is reached, the remaining concentration of the analyte in the water compartment is determined by means of a quantification method (mainly HPLC or LCMS, fluorescence techniques) and the partition coefficient is calculated. Kramer et al. (1997) used a radio tracer substance as analyte to quantify the compound in both compartments using liquid scintillation counting. 

Transport of the gaseous species from the donor to the acceptor stream depends on several parameters, e.g., temperature, ionic strength, surface tension, contact time between the solutions and the semi-permeable medium, the characteristics of this medium (active surface, porosity, thickness), the partition coefficient between the fluid and the membrane,... 

The reflection coefficient (a,-) and osmotic pressure coefficient (Op) would only be the same for either a fully permeable membrane or an ideal semi-permeable membrane. The reflection coefficient (Op) can be determined from either empirical relationships with osmotic pressure coefficient based on back-analyses of the experimental results obtained for a given drilling fluid-shale system or theoretical equations based on thermodynamics (van Oort et al., 1995 Zeynaly-Andabily et al., 1996). 

The amount of Cr(III) that reacted with PAAM in the IMC samples was determined by measuring the amount of unreacted chromium which was separable from the crosslinked polymer. The amount of unreacted chromium was determined by equilibrium dialysis ,where the unreacted chromium was separated from the crosslinked polymer with a semi-permeable membrane. At dialysis equilibrium, the unreacted chromium which was free to move across the membrane should have the same activity on both sides of the membrane. The activity coefficients of free chromium ions on both sides of the membrane have been previously determined to be essentially... 

The reflection coefficient refers to the propensity of the semi-permeable membrane for the diffusion of drug. Ideally the semi-permeable membrane should only allow the diffusion of aqueous fluid into the tablet core and not the subsequent diffusion of the dissolved drug in the reverse direction, Under these circumstances, the reflection coefficient should be 1 and can therefore be eliminated from Equation 27. 

From Equations 28 and 29 it may be concluded that zero-order release may be obtained if the permeability characteristics of the semi-permeable membrane are controlled, namely the area, thickness, the diffusion coefficient of water across the membrane, and a saturated concentration of drug is maintained within the tablet core (by the correct choice of drug salt and inclusion of excipients). In the original OROS system the semi-pcrmcable membrane was composed of cellulose acetate and the osmotic pressure of the core was controlled by the inclusion of salts. If a non-saturaled drug solution is present in the tablet core non zero-order release occurs (a.47, a. 159). 

C and D being the concentration of the analyte in the lumen, and its diffusion coefficient, respectively. Rq is the radius of the lumen, as also shown in Figure 3. At the start, we consider no analyte to be present in the lumen, and the semi-permeable membrane itself does not form an extra obstacle for diffusive transport. Then, the initial and boundary conditions become... 

Here the permeability of the membrane to the solute is defined in terms of reflection coefficients aQ and for osmosis and filtration respectively. When (To = 1, then perfect semi-permeabihty results. in Eq. (4) is the diffusive permeabihty of the membrane, while (Cj) is the average composition of the solute in the membrane. 

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