Orientation interaction

This orientation interaction thus varies inversely with the sixth power of the distance between dipoles. Remember, however, that the derivation has assumed separations large compared with d. 

For structures with a high curvature (e.g., small micelles) or situations where orientational interactions become important (e.g., the gel phase of a membrane) lattice-based models might be inappropriate. Off-lattice models for amphiphiles, which are quite similar to their counterparts in polymeric systems, have been used to study the self-assembly into micelles [ ], or to explore the phase behaviour of Langmuir monolayers [ ] and bilayers. In those systems, various phases with a nematic ordering of the hydrophobic tails occur. 

As has been noticed by Gelbart and Gelbart [7], the predominant orientational interaction in nematics results from the isotropic dispersion attraction modulated by the anisotropic molecular hard-core. The anisotropy of this effective potential comes from that of the asymmetric molecular shape. The coupling between the isotropic attraction and the anisotropic hard-core repulsion is represented by the effective potential... 

On the basis of these observations, an interesting formation of nanostructures consisting of SWNTs was probably achieved by magnetic force, magnetic orientation, interaction of induced magnetic moment of SWNTs due to strong magnetic fields, and self-assembly of SWNTs due to hydrophobic interaction in aqueous solution and so on [46, 48]. 

Currently, only a handful of examples of unique protein carboxylate-zinc interactions are available in the Brookhaven Protein Data Bank. Each of these entries, however, displays syn coordination stereochemistry, and two are bidentate (Christianson and Alexander, 1989) (Fig. 5). Other protein structures have been reported with iyw-oriented car-boxylate-zinc interactions, but full coordinate sets are not yet available [e.g., DNA polymerase (Ollis etal., 1985) and alkaline phosphatase (Kim and Wyckoff, 1989)]. A survey of all protein-metal ion interactions reveals that jyw-carboxylate—metal ion stereochemistry is preferred (Chakrabarti, 1990a). It is been suggested that potent zinc enzyme inhibition arises from syn-oriented interactions between inhibitor carboxylates and active-site zinc ions (Christianson and Lipscomb, 1988a see also Monzingo and Matthews, 1984), and the structures of such interactions may sample the reaction coordinate for enzymatic catalysis in certain systems (Christianson and Lipscomb, 1987). 

Despite the dominance of the a-helical motif as a recognition unit, the /3-sheet motif has been used in a limited number of cases. In these cases (three are known) two chains in the anti-parallel orientation interact with half-sites in adjacent large grooves of the DNA (see fig. 30.28 for an example). (The /3-sheet motif obviously works in these cases de-... 

Figure 16 A schematic representation of double lattice model for oriented interactions between molecules / and j (Hu et at, 1991b).

T] surface fraction of a segment of polymer that can participate in oriented interaction function defined by Equation (54)... 

The IEF methodology, combined with an effective mean-field modelling of short-range orienting interactions, can also provide good predictions of the permittivity of nematics and its temperature dependence. Indeed, a realistic account of the cavity shape allows us to calculate the permittivity purely on the basis of the structure of the constituting... 

There are two possible molecular orientations in the sc phase, here descriptively denoted the P (pentagon) and H (hexagon) orientations. This notation arises because a double bond on one molecule can be oriented toward a carbon-atom pentagon or a hexagon on its neighbor [9-11]. Quite advanced theoretical models have been developed for the orientational interactions and dynamics [9-11,31], but to a first approximation we can assume simply that the relative fraction/of P oriented molecules is... 

In the model, the uniform contribution (and thus, the doublet splitting) is proportional to the overall average orientation . The interaction parameter u characterises the strength of orientational interactions between segments (0 < u < 1). Thus, for a given deformation ratio X, the spectrum contains one constant splitting and a distribution of additional shifts, which is clearly seen in Figure 15.4. 

To emphasise the role of orientational interactions in the appearance of the doublet in 2H NMR spectra, model PDMS networks have been swollen to various extents with a good solvent of PDMS [53]. Figure 15.10 shows the slopes P=A/(X2-X 1) obtained in a PDMS network swollen with chloroform. The swelling process results in stretching... 

The uniaxial contribution to the stress-induced orientation (the second term in Equation 15.4) has been attributed mainly to orientational interactions between chain segments. When guest molecules are introduced in a rubber matrix, these interactions take place between network chain segments and guest molecules as well. This effect has been recognised earlier, and may be used to study indirectly the behaviour of the matrix. Two types of guest molecules have been used for this purpose. 

The orientation of the swelling agent (solvent or free chains) has to be taken into account in the analysis of the stress-optical behaviour of swollen networks. Specifically, the segment polarisability (relative to the network chains or to the diluent chains), as currently derived from stress-optical coefficients [33], may not be representative of intrinsic properties of isolated chains. Short-range orientational interactions between the probe molecules and network chains (and between the chains of the matrix itself) must be considered in the interpretation of opticoelastic properties of swollen (and dry) rubbers [67]. 

Table 2.4 lists the individual contributions or partial polarities for the solutes that appear in table 2.2. From this table, it is clear that a distinction can now be made between molecules of similar overall polarity. Much of the cohesive energy of toluene is due to dispersion interaction, whereas dipole orientation is more important in ethyl acetate. Orientation interaction is of more relevance in methylene chloride than it is in dioxane, which shows a considerable contribution from induction interaction. 

Van der Waals interactions are noncovalent and nonelectrostatic forces that result from three separate phenomena permanent dipole-dipole (orientation) interactions, dipole-induced dipole (induction) interactions, and induced dipole-induced dipole (dispersion) interactions [46]. The dispersive interactions are universal, occurring between individual atoms and predominant in clay-water systems [23]. The dispersive van der Waals interactions between individual molecules were extended to macroscopic bodies by Hamaker [46]. Hamaker s work showed that the dispersive (or London) van der Waals forces were significant over larger separation distances for macroscopic bodies than they were for singled molecules. Through a pairwise summation of interacting molecules it can be shown that the potential energy of interaction between flat plates is [7, 23]... 

It is indeed thermodynamically necessary that in mixtures in which the (temperature dependent) orientation interaction plays a part the entropy is not given by the (temperature independent) ideal entropy of mixing (or Gibbs term) given above. 

The equation 18.214 expresses the deformation energy of interaction of two dipoles and forces of this type do not depend to a first approximation on the temperature. For the calculation of the energy of orientation interaction it is possible to proceed in the same manner as in the deduction of the Debye equation (see above) but it is also possible to proceed directly from equation 18.214. 

---