Section 5.20 Electrostatic

Electrostatic separator with motor, drive, switches but excluding rectifiers, transformers, air valve controls. FOB cost = 35 000 at a (solids nominal capacity, kg/s) 

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Fig. 3—Except lor electrodes in coalescing section, electrostatic treaters are simriar to other horizontal-type treaters.

In the following, the modelling of solute-solvent interactions at the interface will be discussed in Section Electrostatic and in Section Nonelectrostatic Interactions (cavitation, dispersion, and repulsion). Applications of the methods developed to the modelling of molecular properties at liquid surfaces will be described in the Section Energetics and Properties at Liquid-Gas and Liquid-Liquid Interface. 

As we are interested in reversible Janus micelles, i.e. non-centrosymmetric nanoparticles with compartmentalised shells (Fig. 1), complex coacervate core micelles are a rather natural choice. As described in the previous section, electrostatic interaction is a rather weak driving force as compared to hydrophobic interaction. C3Ms may thus form under full thermodynamic control. Although ABC triblock copolymers in selective solvents (poor solvent for B good solvent for both A and C) may also yield Janus micelles, they most frequently aggregate into micelles with a quenched rather than a dynamic nature, such that the aggregation number is fixed and no reversible association/dissociation is observed (on experimental time scales). 

Even when thermally stable, such molecules may behave as soft with respect to adsorption. Thermal stability and hardness, in fact, are not synonymous compared with alkanes, proteins have a lower thermal stability but a higher hardness. Protein hardness (which is manifested in the constancy of protein shape - a key functional factor) is imparted to proteins by a lot of thermally labile hydrogen bonds. Heating is not the unique way to destroy hardness. As discussed in the previous section, electrostatic interactions cleave the shape-preserving secondary bonds. Sufficiently large molecules with few intrachain hydrogen bonds may be soft because of the possibility to assume a lot of tautomeric configurations. That the adsorbates too may be soft impact severely the determination of surface areas. 

A special feature of molecular graphics is that it can generate various surfaces around the molecule, on which a variety of its properties can be displayed. For proteins the electrostatic potential (see O section Electrostatics ), water accessibility, and hydrophobicity (see O section Solvent-Accessible Surface ) patterns are most important. Commercial (see O section Molecular Mechanics ) and open source code software, like PyMOL (http //www.pymol.org/) and Jmol (http //jmol.sourceforge.net/) is available for molecular graphics. 

One fascinating feature of the physical chemistry of surfaces is the direct influence of intermolecular forces on interfacial phenomena. The calculation of surface tension in section III-2B, for example, is based on the Lennard-Jones potential function illustrated in Fig. III-6. The wide use of this model potential is based in physical analysis of intermolecular forces that we summarize in this chapter. In this chapter, we briefly discuss the fundamental electromagnetic forces. The electrostatic forces between charged species are covered in Chapter V. 

Most of the Langmuir films we have discussed are made up of charged amphiphiles such as the fatty acids in Chapter IV and the lipids in Sections XV-4 and 5. Depending on the pH and ionic strength of the subphase, electrostatic effects can become quite important. Here we develop the theoretical foundation for charged films with the Donnan relationship. Then we mention the influence of subphase pH on film behavior. 

Some electric properties of molecules are described in section Al.5.2.2 because the coefficients of the powers of Mr turn out to be related to them. The electrostatic, mduction and dispersion energies are considered m turn in section Al.5.2.3, section Al.5.2.4 and section Al.5.2.5, respectively. 

If the long-range mteraction between a pair of molecules is treated by quantum mechanical perturbation theory, then the electrostatic interactions considered in section Al.5.2.3 arise in first order, whereas induction and dispersion effects appear in second order. The multipole expansion of the induction energy in its fill generality [7, 28] is quite complex. Here we consider only explicit expressions for individual temis in the... 

The first term represents the forces due to the electrostatic field, the second describes forces that occur at the boundary between solute and solvent regime due to the change of dielectric constant, and the third term describes ionic forces due to the tendency of the ions in solution to move into regions of lower dielectric. Applications of the so-called PBSD method on small model systems and for the interaction of a stretch of DNA with a protein model have been discussed recently ([Elcock et al. 1997]). This simulation technique guarantees equilibrated solvent at each state of the simulation and may therefore avoid some of the problems mentioned in the previous section. Due to the smaller number of particles, the method may also speed up simulations potentially. Still, to be able to simulate long time scale protein motion, the method might ideally be combined with non-equilibrium techniques to enforce conformational transitions. 

The representation of molecules by molecular surface properties was introduced in Section 2.10. Different properties such as the electrostatic potential, hydrogen bonding potential, or hydrophobicity potential can be mapped to this surface and seiwe for shape analysis [44] or the calculation of surface autocorrelation vectors (refer to Section 8.4.2). 

Non-covalent interactions between molecules often occur at separations where the van der Waals radii of the atoms are just touching and so it is often most useful to examine the electrostatic potential in this region. For this reason, the electrostatic potential is often calculated at the molecular surface (defined in Section 1.5) or the equivalent isodensity surface as shown in Figure 2.18 (colour plate section). Such pictorial representations... 

A central multipole expansion therefore provides a way to calculate the electrostatic interaction between two molecules. The multipole moments can be obtained from the wave-function and can therefore be calculated using quantum mechanics (see Section 2.7.3) or can be determined from experiment. One example of the use of a multipole expansion is... 

How should a molecule be divided into groups In some cases there may appecir to be a chemically obvious way to define the groups, especially when the molecule is a polymer that is constructed from distinct chemical residues. A particularly desirable feature is that each group should, if possible, be of zero charge. The reason for this can be understood if we recall how the different electrostatic interactions vary with distance from Section 4.9.1 ... 

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