Membrane permeability modeling water permeation

When investigating the effects of water on transdermal permeation, animal skin may yield results markedly different to human data. For example, hairless mouse skin is unsuitable for modeling human stratum corneum regarding hydration effects the murine skin, when hydrated for 24 h, became grossly more permeable than human skin membranes [8]. Thus water effects on skin permeability obtained using animal models need cautious assessment. 

The rate of transmembrane diffusion of ions and molecules across a membrane is usually described in terms of a permeability constant (P), defined so that the unitary flux of molecules per unit time [J) across the membrane is 7 = P(co - f,), where co and Ci are the concentrations of the permeant species on opposite sides of membrane correspondingly, P has units of cm s. Two theoretical models have been proposed to account for solute permeation of bilayer membranes. The most generally accepted description for polar nonelectrolytes is the solubility-diffusion model [24]. This model treats the membrane as a thin slab of hydrophobic matter embedded in an aqueous environment. To cross the membrane, the permeating particle dissolves in the hydrophobic region of the membrane, diffuses to the opposite interface, and leaves the membrane by redissolving in the second aqueous phase. If the membrane thickness and the diffusion and partition coefficients of the permeating species are known, the permeability coefficient can be calculated. In some cases, the permeabilities of small molecules (water, urea) and ions (proton, potassium ion) calculated from the solubility-diffusion model are much smaller than experimentally observed values. This has led to an alternative model wherein permeation occurs through transient hydrophilic defects, or pores , formed by thermal fluctuations of surfactant monomers in the membrane [25]. 

There have been several models for the permeation mechanism of ionic molecules. All models propose the ion permeation with a guide of water molecules. For example, the water wire is widely believed to help a proton transport across the membrane. Investigating the water wire is still a challenging subject for MD simulations, though a few studies have been reported [61, 62]. However, permeability of neutral small molecules across the bilayers can now be evaluated by MD simulations. Here, we demonstrate an evaluation of water permeability across DPPC and DPhPC bUayers. 

Since spontaneous water permeation across the lipid membranes is a rare event in the simulation time range, the permeation rate cannot be straightforwardly measured via ordinal MD simulation. Therefore, we need an alternative approach to assess the water permeability across the lipid membranes. According to the inhomogeneous diffusion model proposed by Marrink and Berendsen [63], permeability coefficient, P, is derived by... 

Passive Transport. Transport by simple diffusion This mode of transport is available for apolar molecules. Permeation is predominantly governed by partitioning of the substrate between the lipid and water. The membrane simply acts as a permeability barrier small molecules pass more easily than large ones. The transport is explained in terms of a simple diffusion model involving three steps passage of the substrate from the exterior into the membrane, diffusion through the membrane, and passage out of the membrane. 

Transport by facilitated diffusion A large number of molecules and ions were shown to permeate membranes considerably faster than expected from their lipid-water partitioning behavior. This led to the recognition of additional transport mechanisms. Systematic investigations of permeability rates in membranes, reconstituted membranes, and membrane models as functions of the temperature of the nature and concentration of the permeant in the absence and in the presence of additives, suggested three different facilitated passive transport mechanisms ... 

After experimental work, the value of the activation energy (EA) and the reference permeability (Qref) for a PVA-based (from GKSS) pervaporation membrane and mixture in use have been obtained (see tablel), and the model for the pervaporation process with and without heat integration has been successfully validated. The absolute error between the model and the experimental permeate fluxes is under 8% (see fig.4), and almost insignificant for the water purity in the permeate side (under 0.5%) obtaining a permeate stream with up to 99.9% in water. 

Recently, we have examined solute permeation through hydrogel membranes in an effort to develop models which describe in detail the transport phenomena with particular emphasis on the role of water in this process. These studies have utilized p-HEMA and its copolymers, and both hydrophobic and hydrophilic solutes (7., i). It was determined that p-HEMA and its copolymers are permeable to both hydrophobic and hydrophilic solutes. 

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