Membranes solvent permeability coefficient

The skin permeability and the partition coefficients are free-raiergy-related quantities that can be modulated by the same set of solute descriptors of the chemicals. This offers a new strategy that the skin permeability does not have to be detomined directly for all of the chemicals or chemical mixtures rather, it can be predicted from the solute descriptors that can be determined via the measurement of their partition coefficients. The advantage of this strategy lies in the fact that the prame-ability measurement is affected by many experimental factors, and the experimental processes are difficult to be automated the determination of the membrane/solvent partition coefficients can be highly automated with the membrane-coated fiber technique. 

Bhanushali et al. [22] showed differences between porous UF polymer membranes and dense reverse osmosis/NF membranes. According to these authors, permeability can be correlated with the inverse of the solvent viscosity for UF membranes whatever the nature of the solvent. For reverse osmosis/NF membranes, a permeation model is proposed in which the flux relates to a solvent permeability coefficient, accounting for a number of solvent intrinsic parameters, like molar volume V , the viscosity p, the sorption value O, and to an intrinsic parameter of the membrane (the solid-vapor surface tension ysv). 

The solubility-diffusion theory assumes that solute partitioning from water into and diffusion through the membrane lipid region resembles that which would occur within a homogeneous bulk solvent. Thus, the permeability coefficient, P, can be expressed as... 

The transport of both solute and solvent can be described by an alternative approach that is based on the laws of irreversible thermodynamics. The fundamental concepts and equations for biological systems were described by Kedem and Katchalsky [6] and those for artificial membranes by Ginsburg and Katchal-sky [7], In this approach the transport process is defined in terms of three phenomenological coefficients, namely, the filtration coefficient LP, the reflection coefficient o, and the solute permeability coefficient to. 

Based on the 96-well format, OCT-PAMPA was proposed and has proved its ability to determine (indirectly) log Poet [87]. PAM PA is a method, first developed for permeability measurements, where a filter supports an artificial membrane (an organic solvent or phospholipids) [88, 89]. With this method, the apparent permeability coefficient (log P ) of the neutral form of tested compounds is derived from the measurement of the diffusion between two aqueous phases separated by 1-octanol layer (immobilized on a filter). A bilinear correlation was found between log Pa and log Poct> therefore log Poet of unknown compounds can be determined from log Pa using a calibration curve. Depending on the detection method used a range oflog P within —2 to +5 (with UV detection) and within —2 to +8 (with LC-MS detection) was successfully explored. This method requires low compound amounts (300 pi of 0.04 mM test compound) and, as for the previous method, samples can be prepared in DM SO stock solutions. For these experiments, an incubation time of 4h was determined as the best compromise in term of discrimination. The limitation of the technique lies in the lower accuracy values... 

The permeability coefficient is often treated as a pure materials constant, depending only on the permeant and the membrane material, but in fact the nature of the solvent used in the liquid phase is also important. From Equations (2.28) and (2.25), can be written as... 

The presence of the term y) makes the permeability coefficient a function of the solvent used as the liquid phase. Some experimental data illustrating this effect are shown in Figure 2.7 [11], which is a plot of the product of the progesterone flux and the membrane thickness, 7, against the concentration difference across the membrane, (cio — cif ). From Equation (2.28), the slope of this line is the permeability, P]. Three sets of dialysis permeation experiments are reported, in which the solvent used to dissolve the progesterone is water, silicone oil and poly(ethylene glycol) MW 600 (PEG 600), respectively. The permeability calculated from these plots varies from 9.5 x 10 7 cm2/s for water to 6.5 x 10 10 cm2/s for PEG 600. This difference reflects the activity term yj/ in Equation (2.28). However, when the driving force across the membrane is... 

If we consider a membrane having the same solute concentration on both sides, we have All 0 However, a hydrostatic pressure difference AP exists between the two sides, and we have a flow Jv that is a linear function of AP. The term Lp is called the mechanical filtration coefficient, which represents the velocity of the fluid per unit pressure difference between the two sides of the membrane. The cross-phenomenological coefficient Ldp is called the ultrafiltration coefficient, which is related to the coupled diffusion induced by a mechanical pressure of the solute with respect to the solvent. Osmotic pressure difference produces a diffusion flow characterized by the permeability coefficient, which indicates the movement of the solute with respect to the solvent due to the inequality of concentrations on both sides of the membrane. 

Determination of the diffusion coefficient by permeability experiments [42], when a liquid membrane is clammed between a feed and receiving phase, with a membrane solvent. At time t = 0, a carrier which is substituted with a chromophoric group is added to the feed phase ([cf]o). The carrier diffuses through the membrane and the increase of concentration in the receiving phase ([cj() is monitored by UV/Vis spectroscopy (dj as a function of time. The transport through the pores of the membrane is assumed to be rate limiting and Eq. (25) is derived ... 

Solvent characteristics that influence the diffusion and extraction are found to be viscosity ( )) and polarity ( ). For spherical solutes, the diffusion coefficient depends on the solvent according to the Stokes-Einstein relation (Eq. (22)). From this, it follows that the diffusion coefficient linearly increases with T/t). Hence, the permeability increases linearly with the reciprocal viscosity of the membrane solvent [95]. Figure 2.11 shows relation of the diffusion coefficient to the solvent viscosity. 

A good relationship has been established between the number of hydrogen bonds of small model peptides and their permeability coefficients, determined using Caco-2 cell monolayers (Burton et al. 1992). The method reflects the ability of the molecule to form hydrogen bonds with the surrounding solvent. The more bonds the molecule forms with water (luminal fluid), the less potential it has to diffuse into a lipid phase of a membrane. 

Yasuda et a .25 have developed die concept of a homogeneous solvent-swollen membrane in which thermally induced movement of segments of randomly coiled polymer molecules leaves an interstitial free volume available for solute transport. They concluded that the permeability characteristics of highly swollen systems cannot he represented try a single coefficient. Values of solute ned solvent permeabilities depend ou the conditions of mnesurement, in particular, the magnitude of diffusive flux relative to convnetive flux. 

This application is based on the hydrophilicity of the ion exchange membrane. Though it is not essential to use an ion exchange membrane,219 they show excellent performance in pervaporation for dehydration of organic solvents. Pervaporation is the separation of solvents on the basis of their different affinities for the membrane and different permeation speeds through the membrane phase. The system consists of a liquid mixture to be separated, which contacts one side of the membrane, and a gas phase to permeate under reduced pressure, which is on the other side of the membrane (Figure 6.37). Membrane performance is evaluated by a permeability coefficient (flux) and separation factor (selectivity coefficient). The permeability coefficient, Q, is the permeated solvent through the membrane per unit area and unit time (kgm 2 h1). When a mixed solvent composed of components A and B is separated, the separation factor, a, is defined as... 

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