Diffusion polarization, solution

In multicomponent systems, large solutes with lower diffusivity polarize more and exclude smaller solutes from the membrane surface, decreasing their passage. Operation at the knee of the flex curve reduces this effect. 

The most popular and versatile bonded phase is octadecylsilane (ODS), n-C18H37, a grouping that is non-polar and used for reverse phase separations. Octylsilane, with its shorter chain length, permits faster diffusion of solutes and this results in improved peak symmetry. Other groups are attached to provide polar phases and hence perform normal phase separations. These include cyano, ether, amine and diol groups, which offer a wide range of polarities. When bonded stationary phases are used, the clear distinction between adsorption and partition chromatography is lost and the principles of separation are far more complex. 

The BRB is anatomically separated into an inner and outer blood barrier. The RPE is a tight, ion-transporting barrier and paracellular transport of polar solutes across the RPE from the choroid is restricted. This is reflected by the transepithelial electrical resistance (TEER) of the cell layers. It has been reported that the choroidal TEER ( 9 ohm cm2) is less than 10% the total resistance of isolated bovine RPE-choroid (100-150 ohm cm2). Passive RPE diffusion has been shown to be a function of lipophilicity. The endothelium of the retinal vessels represents the inner BRB and offers considerable resistance to systemic penetration of drugs. 

The BBBs to macromolecules and most polar solutes are created by the formation of tight junctions between the cerebral endothelial cells, the choroid plexus epithelial cells, and the cells of the arachnoid membrane. These tight junctions are a key feature of the BBB and effectively abolish any aqueous paracellular diffusional pathways between the endothelial cells from the blood plasma, or somatic ECF, to the brain ECF. This removal of the paracellular pathway efficiently prevents the free diffusion of polar solutes from blood to brain [11],... 

It is the presence of these tight junctions that occludes the aqueous paracellular diffusional pathway between the endothelial cells, and blocks the free diffusion of macromolecules, polar solutes, and ions from blood plasma into the ECF of the brain. It is this impediment to the movement of ions that results in the high in vivo electrical resistance of the BBB, of approximately 1800 fl cm2 [17]. This high electrical resistance or low conductance of the potential paracellular pathway emphasizes the extreme effectiveness of the tight junctions in occluding this pathway by effectively reducing the movement of ions. The radius of a sodium... 

The relatively high kq values for K2 and the observed dependence of kq on the dielectric permittivity (polarity) of the solvent (Table 2) show that in polar solutions, the charge transfer mechanism (3) becomes crucial in the quenching process. This is probably explained by the redox properties of the dye favorable for the charge transfer mechanism. Correlation of kq with the solvent viscosity is not observed, which indicates the absence of contribution of diffusion of reactants to the quenching process (the quenching occurs in the kinetic regime). 

Sub-/supercritical fluid chromatography is essentially NP chromatography with the added advantage that the lower viscosity and higher diffusivity of the mobile phase results in higher column efficiencies allowing for rapid resolutions. The columns employed are the same as those utilized in conventional NP chromatography. Carbon dioxide is the most commonly used nonpolar eluent but requires a more polar modifier such as an alcohol for the elution of polar solutes. The modifier increases the polarity of the mobile phase and... 

The differences between the temperature dependence for permeation of the polar permeants and corticosterone suggest two distinct mechanisms of diffusion through HEM. Based upon comparisons with studies of model systems, the temperature dependence of the polar solutes approaches expecta-... 

The importance of diffusion polarization in corrosion results from the observation that in many situations, the current density of the reduction reaction is large enough to place it under diffusion control. Two important examples are the depletion of hydrogen ions in the solution adja-... 

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