Intermolecular interactions energetics

The conformational dynamics of chain segments near the head groups is more restricted than that of those far from the micellar core [8]. Moreover, to avoid the presence of energetically unfavorable void space in the micellar aggregate and as a consequence of the intermolecular interactions, surfactant molecules tend to assume some preferential conformations and a staggered position with respect to the micellar core [9]. A schematic representation of a reversed micelle is shown in Figure 1. 

These differences reflect the conformations of (+)- and meso-isomers as they sit at the air-water interface. What is much harder to elucidate is the effect of stereochemistry on intermolecular interactions. How does changing the stereochemistry at one chiral center affect interactions between diastereomers Ab initio molecular orbital calculations have been used to address the problem of separating stereochemically dependent inter- and intra-molecular interactions in diastereomeric compounds (Craig et al., 1971). For example, diastereomeric compounds such as 2,3-dicyanobutane exhibit significant energetic dependence on intramolecular configuration about their chiral centers. So far, however, little experimental attention has been focused on this problem. 

Figures 51(A-C) give the compression and expansion cycles for the two isomers of the C-15 6,6 -A diacids at 25, 30 and 35°C. At 30°C, the energetics of compression of the second eluting C-15 6,6 -A diacid are similar to those of the second eluting C-12 6,6 -A diacid at 25°C. Raising the temperature causes a weakening of the intermolecular interactions in the monolayer spread...

It has been shown by Harvey et al. (1989) that incorporation of palmitic acid into a monolayer spread from stearoylserine methyl ester (SSME) breaks up intermolecular SSME interactions. The palmitic acid acts as a two-dimensional diluent. Figures 52(A-C) give the Yl/A isotherms for mixtures of FE and SE C-15 6,6 -A with palmitic acid. Dilution of the monolayer cast from the second eluting isomer with 15 mol% palmitic acid separates the diacid molecules from one another on the water surface and perhaps allows for the expression of their stereochemically dependent conformations. The mixed film (15% palmitic acid/85% C-15 6,6 -A) expands at low II and behaves in much the same manner as the single-component monolayer (C-15 6,6 -A) behaves at 30°C. Addition of 15 mole% palmitic acid into a monolayer cast from the FE C-15 diacid has little effect on its energetics of compression, indicating a stronger intermolecular interaction afforded by its stereochemically dependent conformation at the air-water interface. 

If multiple peaks are present, the separation power of the system to separate them can be expressed by calculating the selectivity, which is related to several of the derived parameters, but most critically to the difference in the energetics of the intermolecular interactions. 

The word energetics, rather than thermochemistry, was adopted in figure 1.2 and in the book title to emphasize that most of the methods displayed do not involve the experimental determination of heat. Furthermore, the use of energetics avoids the traditional link between thermochemistry and calorimetry (which is semantically correct because thermo is the Greek designation for heat ). The word molecular, on the other hand, stresses that this book will be mainly concerned with single molecules. Properties like enthalpies of phase transition, which depend on intermolecular interactions, are very important data in their own right, but the methods used to derive them will not be comprehensively covered. 

The N,N -diphenylguanidinium (dpg+) has been foimd to adopt different conformations both in aqueous solutions [9] and in several salts that are being reviewed in this paper. The conformation of dpg+ is very sensitive to the counter-ion, and this effect has been the subject of ab-initio quantum mechanical and molecular mechanics calculations [10]. Stabilization of a particular conformation depends critically on intermolecular interactions with the solvent, since the energetic cost of rotation of the phenyl rings is much lower than typical solvation energies. 

These transformations arise from the energetical stability caused by intramolecular or intermolecular hydrogen bond (HB) interactions. Thus, by the balance of intramolecular and intermolecular HB interactions in polypeptide blends, it is expected that the strength of intermolecular interaction in the blends is different from those in homopolypeptides then new conformations can be formed by intermolecular HB interactions that do not exist originally in homopolypeptides. There are many studies on intermolecular HB interactions in homopolypeptides and copolypeptides in the solid state, but to the best of our knowledge there is little study on intermolecular HB interactions in polypeptide blends except for our previous studies. 

The above properties (a and b) are interpreted by Cohen and Schmidt (21) on the basis of a detailed crystallographic study of photochromic and thermochromic anils. They conclude that photochromic crystals involve structures in which the central portion of adjacent molecules are essentially isolated from one another, so that, to a first approximation the energetics are that of an isolated molecule. On the other hand, when the alignment of the molecular dipoles is such as to give strong intermolecular interactions then the transition to the quinoid form requires much less energy and can occur thermally. For crystals in which thermochromism occurs, the photochemical isomerization is still possible but the reverse reaction is so rapid that no buildup of color is observed. In fact, fluorescence measurements on the thermochromic 5 -chlorosalicylidene-aniline (Fig. 5) indicate that photochemical isomerization precedes the luminescence process via the photochromic route ... 

The local partial adsorption isotherms O j( P ) of various adsorption centers, which are needed for Eq. (A.5), are calculated by the system of equations from Refs. [80,89]. These equations take into account energetic non-uniformity of lattice sites and intermolecular interactions at a distance of R c.s.,... 

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