Nonpolar molecules, diffusion small

The results of experimental studies of the sorption and diffusion of light hydrocarbons and some other simple nonpolar molecules in type-A zeolites are summarized and compared with reported data for similar molecules in H-chabazite. Henry s law constants and equilibrium isotherms for both zeolites are interpreted in terms of a simple theoretical model. Zeolitic diffusivitiesy measured over small differential concentration steps, show a pronounced increase with sorbate concentration. This effect can be accounted for by the nonlinearity of the isotherms and the intrinsic mobilities are essentially independent of concentration. Activation energies for diffusion, calculated from the temperature dependence of the intrinsic mobilitieSy show a clear correlation with critical diameter. For the simpler moleculeSy transition state theory gives a quantitative prediction of the experimental diffusivity. 

In contrast to dipole-dipole forces, London Dispersion interactions are much weaker in nature since they involve nonpolar molecules that do not possess permanent dipole moments. The only modes for molecular attraction are through polarization of electrons, which leads to the creation of small dipole-dipole interactions and mutual attractive forces. Since electron polarization occurs much more readily for electrons farther from the nucleus, this effect is more pronounced for molecules that are larger with a greater number of electrons, especially positioned on atoms with a high atomic number, consisting of more diffuse orbitals. These induced dipole forces are responsible for the liquefaction of gases such as He and Ar at low temperatures and pressures. The relative strength of London Dispersion forces is described by Eq. 3 ... 

How does a polar molecule or ion in the water outside a cell pass through the nonpolar interior of the ceU membrane and enter the cell Some nonpolar molecules like O2 are small enough to enter and exit the cell by diffusion. Polar molecules and ions, on the other hand, may be too large or too polar to diffuse efficiently. Some ions are transported across the membrane with the help of molecules called ionophores. 

The lipid membrane, as a whole, shows a unique combination of fluidity and rigidity. In terms of the solubility and the diffusion of small nonpolar molecules, the membrane behaves very much like an oil drop. In contrast, the translational diffusion... 

The diffusion coefficients of nonpolar molecules in zeolites can be changed greatly by the addition of controlled amounts of small polar molecules. These are sorbed very strongly and are immobile at the temperature of the subsequent runs with the nonpolar sorbates. Moderated diffusion was studied first in 1954 (11) for Ho, O2, N2, Ar, and C2H6 in crystals of mordenite and chabazite. The moderators were H2O, NH.3, and CH3NH2. These measurements were extended subsequently to O2, N2, and Ne diffusing in Na-, (Ca,Na)-, and (K,Na)-A moderated with controlled amounts of NH3 (28). 

Equilibration of adsorbate with a zeolite may be extremely rapid for small nonpolar molecules in open zeolites (a few minutes). Small polar molecules which stick where they hit e.g., water) may be so strongly sorbed that in a bed of powder the redistribution is very slow (equilibrium pressure very small). It helps to raise the temperature, hold the system at the high temperature in the presence of the sorbate, and then slowly cool to the desired low temperature. Molecules whose physical dimensions are as large or a little larger than the windows in the crystals may be sorbed very slowly indeed (by activated diffusion). In extreme cases, weeks may be needed. 

The velocity of diffusion across a membrane depends on the size of the respective molecule and its relative solubility vdthin the lipid phase. Small nonpolar molecules exhibit good lipid solubility and have a rather high velocity of diffusion. Uncharged polar molecules such as H 2O or CO2, which have a rather low lipid solubility, may also cross a membrane by passive diffusion. In contrast, charged molecules and ions... 

Hydrogen is a colorless, odorless, tasteless gas (Table 14.1). Because H2 molecules are nonpolar, they can attract each other only by London forces. Each molecule has only two electrons, and hence only a very small instantaneous electric dipole therefore these forces are so weak that hydrogen does not condense to a liquid until it is cooled to 20 K. Because of these weak inter molecular forces, it has only low solubility in many liquids, particularly polar liquids. Furthermore, H2 molecules are so small and move at such a high average speed that molecules of hydrogen gas diffuse more rapidly than those of any other substance. 

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