Pure Dipolar Substances

The approximation outlined above forms the central idea of all the existing theories of small orientational forces such as that of Barker [1951, 1953, 1954], Pople [1952, 1953, 1954], Cook and Rowlinson [1953], Rowlinson [1954], Rowlinson and Sutton [1955] and Balescu [1955]. In the following sections, after having outlined the effects in pure polar substances, we shall give a treatment of non-central perturbations of this t3 pe in solutions. The method due to Balescu [1955] is a ssmthesis of Rowlinson s theory [1954] and the average potential theory of solutions presented in the Ch. IX-XI. 

We outline here a development for pure substances which we shall later apply to solutions. 

We superpose on the Lennard-Jones potential of two molecules the electrostatic interaction energy of their dipole moments. Then their total potential energy will be 

We now calculate Sd- (For more details, see Bottcher [1952] and Hirschfelder, Curtiss and Bird [1954]). It is weU known that the electrostatic potential of a point dipole is 

This formula havingbeendeducedforpoint dipoles is rigorously valid only for very large distances. However, considering Fig. 14.2.2 one easily verifies by calculating the exact and the approximate potentials 

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Introduction 287. 2. Pure Dipolar Substances 291. 3. General Perturbation Calcula -... 

The binary and ternary radial correlation parameters occurring in equations (221) and (222) are defined by (151) and (152), respectively. Fluctuations of molecular fields. As in the case of pure liquids, fluctuations of molecular fields in solutions contribute to the effects und consideration. In particular, they lead to non-zero values of the constant in non-dipolar substances. For instance, if the molecules of the various components are axially symmetric and if they possess permanent quadrupole moments, we obtain for the binary correlations, in a first approximation ... 

Obviously, in strongly dipolar substances, the purely dipolar constant will be the predominant one and consequently this treatment of gas theory results in a negative saturation effect. Numerical values of molar non-linear electric field constants (259) are listed in Table 15 for some of the more important non-dipolar as well as dipolar molecules. 

It might be useful in some cases to raise the dielectric constant of mixed solvents by addition of suitable substances and it is known that dipolar molecules such as amino acids do so in pure water. These amino acids are virtually insoluble in nonpolar solvents but they dissolve readily in aqueous salt solutions and in most mixed solvents according to their highly polar structure. Most of what is known about their dielectric behavior concerns aqueous solutions, in which they were studied up to concentrations near saturation. 

Dipolar filter techniques (see Section 3) are becoming more common for examining regions of different mobility in polymers.87 In one particular study,88 a dipolar filter is used initially to select 1H spins from mobile regions of coreshell latex systems. Dipolar filters of increasing strength are then applied so as to obtain a characteristic decay curve for each sample. This enabled the mobility of the various components in the system to be compared with the respective pure substances, so that the effect of mixing could be assessed. 

Forces other than ionic and covalent bonds are also known to hold the particles of some pure substances together in the solid and liquid states. These forces include metallic bonds, dipolar attractions, hydrogen bonds, and dispersion forces. The strength of the predominant force acting in a substance is indicated by the melting and boiling points of the substance. 

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