Nonpolar molecules, diffusion

The semipermeability of the bilayer is evident when we again consider its highly nonpolar interior. Only nonpolar molecules will be able to cross this lipophilic barrier by a simple diffusion process. See Table 4. 

Sorption and Diffusion of Light Hydrocarbons and Other Simple Nonpolar Molecules in Type A Zeolites... 

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. 

Although the theoretical models presented in this paper are simple idealizations of complex systems, the theory provides a useful understanding of many aspects of the sorption and diffusion of simple nonpolar molecules in type-A zeolites and in H-chabazite. The extent to which such theories are applicable to other systems has not yet been investigated. 

In addition to movement through shunts, polar substances may diffuse through the outer surface of the protein filaments of the hydrated stratum corneum, while nonpolar molecules dissolve in and diffuse through the nonaqueous lipid matrix between the protein filaments. The rate of percutaneous absorption through this intercellular lipid pathway is correlated to the partition coefficient of the penetrant, as presented above in Fick s law. 

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 passive permeability of lipid membranes is another fluidity related parameter. In general, two mechanisms of membrane permeability can operate in the membrane (8). For many nonpolar molecules, the predominant permeation pathway is solubility-diffusion, which is a combination of partitioning and diffusion across the bilayer, both of which depend on lipid fluidity. In a few cases, such as permeation of positively charged ions through thin bilayers, an alternative pathway prevails (9, 10). It is permeation through transient pores produced in the bilayer by thermal fluctuations. This mechanism, in general, correlates with membrane fluidity. However, for model membranes undergoing the main phase transition, permeation caused by this mechanism exhibits a clear maximum near the phase transition point (11). 

Micelles are in dynamic equilibrium with their monomer surfactants. Two relaxation processes are related to this equilibrium, a fast one in the microsecond time domain associated with the exchange of individual monomers between the micelles and the bulk aqueous phase and a slower one on millisecond time-scale associated with the complete dissolution of the micelles into monomers [8], For example, the exit rate for the SDS anion from its micelle is about lO s, which is considered to be a diffusion-controlled process [8a]. Nonpolar molecules are usually attracted to the relatively hydrophobic inner core of micelles, whereas ionic reactants and products are either associated with the Stem and Gouy-Chapman layers or repelled from the micelles, depending on the sign of electrostatic interaction. For example, NMR studies show that nonpolar molecules such as benzene and naphthalene are... 

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... 

As discussed briefly in Section 4.1.2, the diffusion coefficients of the binary pairs D-j can be estimated from the kinetic theory of gases or from an appropriate correlation to a reasonable degree of accuracy, particularly for nonpolar molecules. The matrix of diffusion coefficients may therefore be calculated using Eq. 4.2.1 or, for a ternary system, directly from the relations derived below. 

Consider now the effect of presorbed polar molecules on the diffusivity of nonpolar molecules in host zeolites. Blockage of polyhedral cavities by strongly sorbed polar molecules is found experimentally to lead to a... 

Figure 5.13 also shows the self-diffusion coefficients measured for octamethylcyclotetrasiloxane, a large, heavy, approximately spherical nonpolar molecule. 

However, a variety of nonpolar molecules can be transported across biomembranes by simple diffusion. The molecules encounter little or no barrier at the bilayer-water interface and readily partition into the bilayer interior. The permeabilities of a variety of molecules have been found to correlate with their... 

NO is a colorless gas at room temperature and pressure (boiling point, -151.7°C at 1 atm). Its maximum solubility in water is similar to that of pure oxygen, 2-3 mM. It is a fairly nonpolar molecule which would be expected to freely diffuse through membranes. Certainly, one of the most unique and outstanding chemical features of NO is that it is a paramagnetic (radical) species. Using the most basic bonding description, the Lewis dot formalism, it is immediately evident that NO has an unpaired electron (Fig. 

Del Dg are the diffusion coefficients of the electron and the attaching molecule, respectively. is Avogadro s constant and (3 denotes an effective reaction distance which for electron attachment to a nonpolar molecule may be expressed as... 

In this chapter, you will be asked to design a synthesis for bombykol, the sex pheromone of the silk moth (bombyx mori) see page 191. Molecules of bombykol diffuse through open pores in the male moth s antenna. When bombykol binds to its receptor, an electrical charge is produced that causes a nerve impulse to be sent to the brain. Bombykol, however, is a nonpolar molecule (page 555) and has to cross an aqueous solution to get to its receptor. This problem is solved by the pheromone binding protein. The protein binds bombykol in a hydrophobic pocket and then carries it to the receptor. The area around the receptor is relatively acidic, and the decrease in pH causes the pheromone binding protein to unfold and release bombykol to the receptor. 

---