Orientation forces, electrostatic

We shall now consider the case where the intermolecular forces are caused by electrostatic orientation forces e.g, dipole-dipole). Two limiting cases have to be considered. In the first the electrostatic interactions occur mainly between molecules of the same substance. An example is furnished by solutions of methanol in carbon tetrachloride. If we designate the alcohol by A, then we have (c/. also chapter XXV)... 

Substitution of Equation (3.62) into Equation (3.60) gives the relative zero shear viscosity. When the shear rate makes a significant contribution to the interparticle interactions, the mean minimum separation can be estimated from balancing the radial hydrodynamic force, Fhr, with the electrostatic repulsive force, Fe. The maximum radial forces occur along the principle axes of shear, i.e. at an orientation of the line joining the particle centres to the streamlines of 6 = 45°. This is the orientation shown in Figure 3.19. The hydrodynamic force is calculated from the Stokes drag, 6nr 0au, where u is the particle velocity, which is simply... 

The Ion-Dipole Model. In this model ion-dipole forces are the principal forces in the ion-water interaction. The result of these forces is orientation of water molecules in the immediate vicinity of an ion (Fig. 2.11). One end of the water dipole is attached electrostatically to the oppositely charged ion. The result of this orienting force is that a certain number of water molecules in the immediate vicinity of the ion are preferentially oriented, forming a primary hydration shell of oriented water molecules. These water molecules do not move independently in the solution. Rather, the ion and its primary water sheath is a single entity that... 

If both molecules are polar, then opposite charges on the two are drawn together by electrostatic forces. These forces are called orientation forces because the two molecules must be oriented such that the opposite charges are close enough for attraction. These forces are of major importance in polar substances. 

When one molecule is polar and the other nonpolar, the polar molecule induces a dipole in the nonpolar one. The two dipoles are again attracted by electrostatic forces, in this case designated as induction forces. These forces are considerably weaker than orientation forces but stronger than the attractive forces between nonpolar molecules. 

Streaming Current and Electroviscosity, Fig. 2 Fluid charge distributions, electrostatic field orientations, force orientations, and intensities for positive and negative surface charges... 

According to Ludema and Tabor sthe effect of tan 6 on unlubricated sliding for glassy polymers is less evident than those for rubbery polymers. Thus, surface effect (van der Waals or electrostatics ) could be more readily detectable for glassy polymers than for rubbery polymers. Since van der Waals forces consist of dispersion forces, orientation forces, induction forces, and hydrogen bonding, we may then write F3... 

Protein adsorption has been studied with a variety of techniques such as ellipsome-try [107,108], ESCA [109], surface forces measurements [102], total internal reflection fluorescence (TIRE) [103,110], electron microscopy [111], and electrokinetic measurement of latex particles [112,113] and capillaries [114], The TIRE technique has recently been adapted to observe surface diffusion [106] and orientation [IIS] in adsorbed layers. These experiments point toward the significant influence of the protein-surface interaction on the adsorption characteristics [105,108,110]. A very important interaction is due to the hydrophobic interaction between parts of the protein and polymeric surfaces [18], although often electrostatic interactions are also influential [ 116]. Protein desorption can be affected by altering the pH [117] or by the introduction of a complexing agent [118]. 

Protems can be physisorbed or covalently attached to mica. Another method is to innnobilise and orient them by specific binding to receptor-fiinctionalized planar lipid bilayers supported on the mica sheets [15]. These surfaces are then brought into contact in an aqueous electrolyte solution, while the pH and the ionic strength are varied. Corresponding variations in the force-versus-distance curve allow conclusions about protein confomiation and interaction to be drawn [99]. The local electrostatic potential of protein-covered surfaces can hence be detemiined with an accuracy of 5 mV. 

Unlike the forces between ions which are electrostatic and without direction, covalent bonds are directed in space. For a simple molecule or covalently bonded ion made up of typical elements the shape is nearly always decided by the number of bonding electron pairs and the number of lone pairs (pairs of electrons not involved in bonding) around the central metal atom, which arrange themselves so as to be as far apart as possible because of electrostatic repulsion between the electron pairs. Table 2.8 shows the essential shape assumed by simple molecules or ions with one central atom X. Carbon is able to form a great many covalently bonded compounds in which there are chains of carbon atoms linked by single covalent bonds. In each case where the carbon atoms are joined to four other atoms the essential orientation around each carbon atom is tetrahedral. 

Fig. 3. NAMD 1 employs a modular, object-oriented design in which patches communicate via an encapsulated communication subsystem. Every patch owns an integrator and a complete set of force objects for bonded (BondForce), nonbonded (ElectForce), and full electrostatic (DPMTA) calculations.

The range of systems that have been studied by force field methods is extremely varied. Some force fields liave been developed to study just one atomic or molecular sp>ecies under a wider range of conditions. For example, the chlorine model of Rodger, Stone and TUdesley [Rodger et al 1988] can be used to study the solid, liquid and gaseous phases. This is an anisotropic site model, in which the interaction between a pair of sites on two molecules dep>ends not only upon the separation between the sites (as in an isotropic model such as the Lennard-Jones model) but also upon the orientation of the site-site vector with resp>ect to the bond vectors of the two molecules. The model includes an electrostatic component which contciins dipwle-dipole, dipole-quadrupole and quadrupole-quadrupole terms, and the van der Waals contribution is modelled using a Buckingham-like function. 

Similarly, examples of barriers arising largely from simple steric hindrance can be found, as for instance in the hindered diphenyls.35 On the other hand there are many arguments suggesting that this is not the important force in ethane and similar molecules. It would be difficult to understand the relatively slow fall in barrier from ethane to methyl silane to methyl germane on a van der Waals repulsion basis. Furthermore, the small effect of substituting F, Cl, or Br on one end would also seem mysterious. The equilibrium orientation in propylene is opposite to the predictions of one of the quantitative van der Waals theories. Finally, the apparently small effect of bending back the C—H bonds is not in accord with either the electrostatic or van der Waals pictures. 

---