Organic molecules, diffusion

Free apertures in second channel system are too small for organic molecules to diffuse readily, making the channel system of mordenite essentially monodimensional. 

Amberlite IRA-402 1.3 1.07 Lower cross-linkage than IRA-400 better diffusion rate with large organic molecules. 

In the previous discussion it has been assumed that the vapour is a pure material, such as steam or organic vapour. If it contains a proportion of non-condensable gas and is cooled below its dew point, a layer of condensate is formed on the surface with a mixture of non-condensable gas and vapour above it. The heat flow from the vapour to the surface then takes place in two ways. Firstly, sensible heat is passed to the surface because of the temperature difference. Secondly, since the concentration of vapour in the main stream is greater than that in the gas film at the condensate surface, vapour molecules diffuse to the surface and condense there, giving up their latent heat. The actual rate of condensation is then determined by the combination of these two effects, and its calculation requires a knowledge of mass transfer by diffusion, as discussed in Chapter 10. 

Hyde, PD Ediger, MD, NMR Imaging of Diffusion of Small Organic Molecules in Silk Fibroin Gel, Macromolecules 24, 620, 1991. 

Certain microbes synthesize small organic molecules, ionophores, that function as shuttles for the movement of ions across membranes. These ionophores contain hy-drophihc centers that bind specific ions and are surrounded by peripheral hydrophobic regions this arrangement allows the molecules to dissolve effectively in the membrane and diffuse transversely therein. Others, Hke the well-smdied polypeptide gramicidin, form channels. 

Multidimensional and heteronuclear NMR techniques have revolutionised the use of NMR spectroscopy for the structure determination of organic molecules from small to complex. Multidimensional NMR also allows observation of forbidden multiple-quantum transitions and probing of slow dynamic processes, such as chemical exchange, cross-relaxation, transient Over-hauser effects, and spin-diffusion in solids. 

More simply, in the early regions of the tubule (proximal tubule and Loop of Henle), Na+ ions leave the lumen and enter the tubular epithelial cells by way of passive facilitated transport mechanisms. The diffusion of Na+ ions is coupled with organic molecules or with other ions that electrically balance the flux of these positively charged ions. In the latter regions of the tubule (distal tubule and collecting duct), Na+ ions diffuse into the epithelial cells through Na+ channels. 

The experimental data bearing on the question of the effect of different metals and different crystal orientations on the properties of the metal-electrolyte interface have been discussed by Hamelin et al.27 The results of capacitance measurements for seven sp metals (Ag, Au, Cu, Zn, Pb, Sn, and Bi) in aqueous electrolytes are reviewed. The potential of zero charge is derived from the maximum of the capacitance. Subtracting the diffuse-layer capacitance, one derives the inner-layer capacitance, which, when plotted against surface charge, shows a maximum close to qM = 0. This maximum, which is almost independent of crystal orientation, is explained in terms of the reorientation of water molecules adjacent to the metal surface. Interaction of different faces of metal with water, ions, and organic molecules inside the outer Helmholtz plane are discussed, as well as adsorption. 

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