Membrane dispersion forces

Interactions between crossed cylinders of mica in air, uncoated or coated with fatty acid monolayers, are described in J. N. Israelachvili and D. Tabor, "The measurement of van der Waals dispersion forces in the range 1.5 to 130 nm," Proc. R. Soc. London Ser. A, 331, 19-38 (1972). An excellent review of this and related work is given in J. N. Israelachvili and D. Tabor, Van der Waals Forces Theory and Experiment, Vol. 7 of Progress in Surface and Membrane Science Series (Academic Press, New York and London, 1973). Later reconciliation of theory and experiment required taking note of cylinder radius L. R. White, J. N. Israelachvili, and B. W. Ninham, "Dispersion interaction of crossed mica cylinders A reanalysis of the Israelachvili-Tabor experiments," J. Chem. Soc. Faraday Trans. 1, 72, 2526-36 (1976). 

In the above considerations, the hydrophobic portions of both the membrane polymer and the small molecules that enter the membrane are expected to interact in the hydrophobic microphases in the membrane. It therefore becomes useful to find a numerical measure of the miscibility of these hydrophobic portions of molecules. In the case of complete molecules, both small and polymeric, the solubility parameter concept has been useful in the past. This concept is related to the enthalpy change which occurs on mixing in regular solution theory as developed by Hildebrand and coworkers (10) and as used for polymer solution theory by Flory (11). The Hildebrand solubility parameter is a measure of the attraction between molecules of the same kind, including dispersion forces, polar forces, and hydrogen bonding... 

Le Chatelier s principle, 82 London dispersion force, 69 membrane potential, 76 micelle, 74... 

Polyelectrolyte molecules in highly dilute aqueous solutions exert strong electrical repulsions on each other. These repulsive forces are long range (proportional to l/r ) by comparison with normal dispersion forces (proportional to 1/r ), and as a consequence the intermolecular interactions persist down to the lowest measured concentrations. In osmotic-pressure measurements on polyelectrolytes, the Donnan membrane equilibrium must be satisfied and experimental results indicate that the second virial coefficient in the osmotic-pressure equation (p. 915) becomes very large. 

J. N. Israelachvili and D. Tabor, Van der Waals forces TTieory and experiment, Prog. Surface Membrane Sci. 7 1 (1973. See also J. N. Israelachvili and D. Tabor, The measurement of van der Waals dispersion forces in the range... 

Adsorption occurs as soon as the membrane snrface is in contact with the solution (macromolecnlar), when solnte molecules adsorb on the membrane surface due to chemical and physical interactions, for example, hydrophobic interactions (dispersion forces) and polar interactions (dipole-dipole and induced dipole). The nature of the membrane material, the type of solute, the solute concentration, ioific strength, and pH are parameters that determine the extent of adsorption [11]. 

PVC is a nonflammable and durable polymer formed from a vinyl chloride monomer. The C-Cl functional group in PVC is relatively polar, and nonspecific dispersion forces dominate the intermolecular interactions [2]. Consequently, PVC has an amorphous structure with a small degree of crystallinity. PVC is used as the polymer backbone in membranes because of its strength, inertness, and compatibility with a variety of carriers and plasticizers. Unlike CTA, PVC is also resistant to acid solutions since it is not prone to acid hydrolysis. 

Bostrom et al. emphasized the importance of dispersion forces regarding the specific ion effects on proteins and also regarding non protein-related Hofmeister effects. The charge on the protein lysozyme depends on the anion in salt solutions (Bostrom et al. 2003a), but only Cl and SCN (as their potassium salts) are compared. Phos-phatidylglycerol bilayers (Bostrom et al. 2006a) are affected by Cl , Br , and SCN (as their sodium salts) in this order. Even silica membranes show Hofmeister series effects, namely in pH measurements with a glass electrode in fairly concentrated... 

For all but spherically symmetrical molecules, van der Waals forces are anisotropic. The polarizabihties of most molecules are different in different molecular directions because the response of electrons in a bond to an external field will usually be anisotropic. A consequence of this effect is that the dispersion force between two molecules will depend on their relative molecular orientation. In nonpolar liquids, the effect is of minor importance because the molecules are essentially free to tumble and attain whatever orientation is energetically favorable. However, in sohds, hquid crystals, and polar media, the effect can be important in determining the relative fixed orientation between molecules, thereby affecting or controlling specific conformations of polymers or proteins in solution, critical transition temperatures in liquid crystals and membranes, and so on. Repulsive forces in polar molecules are also orientation dependent, and are often of greater importance in controlling conformations and orientations. 

Lipid bilayers (Section 23.6A) A two-layer noncovalent molecular assembly comprised primarily of phospholipids. The hydrophobic phospholipid tail groups of each layer orient toward each other in the center of the two-layered structure due to attractive dispersion forces. The hydrophilic head groups of the lipids orient toward the aqueous exterior of the bilayer. Lipid bilayers are important in biological systems such as cell membranes. 

In water, phosphoglycerides self-assemble into a lipid, bilayer (Figure 26.5). In this way, the hydrophobic tails avoid contact with water and interact with each other via London dispersion forces. The surface of the bilayer is polar and therefore water soluble. Lipid bilayers constitute the main fabric of cell membranes, where they function as barriers that restrict the flow of water and ions. Cell membranes enable cells to maintain concentration gradients—that is, the concentrations of sodium and potassium ions inside the cell are diflFerent from those outside of the cell. These concentration gradients are necessary in order for a cell to function properly. 

When two miscible phases have to be contacted by dispersing one in the other, traditionally conventional stirred mixers arc employed. One can also use porous membranes to mix two miscible phases. Zarkadas and Sirkar (2006) have employed porous hydrophobic hollow-fiber membranes to force either an isopropanol phase or an aqueous solution of an amino acid into the other phase. This was done to carry out antisolvent-based crystallization. Extraordinarily high rates of crystallization were achieved with much smaller crystal size. 

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