Macromolecule potential surface, effective

The effective single macromolecule potential surface, U(R), consists of three physically distinct contributions ... 

The results obtained demonstrate competition between the entropy favouring binding at bumps and the potential most likely to favour binding at dips of the surface. For a range of pairwise-additive, power-law interactions, it was found that the effect of the potential dominates, but in the (non-additive) limit of a surface of much higher dielectric constant than in solution the entropy effects win. Thus, the preferential binding of the polymer to the protuberances of a metallic surface was predicted [22]. Besides, this theory indirectly assumes the occupation of bumps by the weakly attracted neutral macromolecules capable of covalent interaction with surface functions. 

Ohshima, H Kondo, T, Electrophoretic Mobility and Donnan Potential of a Large Colloidal Particle with a Surface Charge Layer, Journal of Colloid and Interface Science 116, 305, 1987. O Neil, GA Torkelson, JM, Modeling Insight into the Diffusion-Limited Cause of the Gel Effect in Free Radical Polymerization, Macromolecules 32,411, 1999. 

Enzymes are structurally complex, highly specific catalysts each enzyme usually catalyzes only one type of reaction. The enzyme surface binds the interacting molecules, or substrates, so that they are favorably disposed to react with one another (fig. 1.15). The specificity of enzyme catalysis also has a selective effect, so that only one of several potential reactions takes place. For example, a simple amino acid can be used in the synthesis of any of the four major classes of macromolecules or can simply be secreted as waste product (fig. 1.16). The fate of the amino acid is determined as much by the presence of specific enzymes as by its reactive functional groups. 

Specific coimter-ion effects are critical to biological function, in determining forces between individual sub-units of macromolecules and in the consequent shapes they take up. How much these effects can be attributed to physics and how much to specific chemistry can only be revealed by a reanalysis of all data in the light of the new theories of molecular forces. Until that reanalysis is done, present experimental inferences on binding surface potential and charge remain phenomenological curve fitting. 

Several types of diffraction by crystals are now studied. Neutron diffraction can be used with great effectiveness to give information on molecular structure. These results complement those from X-ray diffraction studies, because there are different mechanisms for the scattering of X rays and of neutrons by the various atoms. X rays are scattered by electrons, while neutrons are scattered by atomic nuclei. Neutron diffraction is important for the determination of the locations of hydrogen atoms which, because of their low electron count, are poor X-ray scatterers. Electron diffraction, while requiring much smaller crystals and therefore being potentially useful for the study of macromolecules, produces diffraction patterns that are more complicated. Their interpretation is hampered by the fact that the diffracted electron beams are rediffracted within the crystal much more than are X-ray beams. This has limited the practical use of electron diffraction in the determination of atomic arrangements in crystals to studies of surface structure. 

In conclusion, the versatility and power of FFF are not restricted to its ability to effect high-resolution separations and sizing of particles and macromolecules. FFF can also be used to probe the surface properties of colloidal samples. Such studies have great potential to provide detailed insight into the nature of adsorption phenomena. 

Figure 7.3 Representation of the conditions at a negative surface, with a layer of adsorbed positive ions in the Stern plane. The number of negative ions increases and the number of positive ions decreases (see upper diagram) as one moves away from the surface, the electrical potential becoming zero when the concentrations are equal. The surface potential, and the potential at the Stern plane, are shown. As the particle moves, the effective surface is defined as the surface of shear, which is a little further out from the Stern plane, and would be dependent on surface roughness, adsorbed macromolecules, etc. It is at the surface of shear that the zeta potential, is located. The thickness of double layer is given by 1 /k.

The absolute value of the zeta potential decreases until a plateau is reached at a certain polyanion concentration. A contrary effect is obtained in the case of adding a polycation. A partial stabilization of the kaolin particles can be realized due to an adsorption of anionic charged macromolecules at the edges of the kaolin platelets. By adding a polycation to the kaolin dispersion, an adsorption at the negative basal surface becomes possible, and the iep of the particles is reached very quickly at low PEI concentrations. A further addition of PEI leads to an increase of the zeta potential while flocculation was observed. This is because the adsorption of cationic polymer can cause a face-to-face association that can generate polymer-kaolin complexes. 

Water-soluble cellulose derivatives themselves adsorb onto solid particles and may for instance affect the suspension properties of these insolubles. The mechanisms involved are quite complex and depend on the polymer concentration. At low concentrations macromolecules influence the electrophoretic mobility and the flocculation of the particles. At higher concentrations, surface coverage by the adsorbed polymer is sufficient to prevent particle-particle interaction and thus to stabilize the suspension sterically. As an example, the effect of NaCMC (among other polymers) on the zeta potential, flocculation and sedimentation properties of sulfadimidine has been investigated by Kellaway and Najib [115,116],... 

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