Cohesive energy ionic compounds

To this point, we have considered the interaction of nonionic surfactants within the framework of the mathanatical model. The activity and character of anionics in emulsification is complicated by the ionization steps which an anionic surfactant may take when exposed to salt solutions. For instance, in a dialkyl metallic salt, there are three compounds which may exist in various concentrations, depending ipon the ionic strength of the salt solution which, in turn, would exhibit, at least, three different HLB nuiiibers. To address the problem of generating Cohesive Energy Density parameters for the anionic hydrophiles, certain standardized assunptions... 

An early use of Equation (5.40) was by Phillips and van Vechten. They applied it to derive a scale of percent ionic character. Their assumptions were such that a continuous scale was generated. While such a scale may be questioned for solids, there was also an important conclusion the structures of the AB compounds depended entirely on the ionicity. Low ionicity led to CN4, and high ionicity to CN6 or 8. This is consistent with our earlier analysis based on cohesive energies. 

Our immediate concern is whether Eg, or possibly E, serves as a suitable measure of chemical hardness just as (/ — A) does for molecules. Examination of the data in Table 5.5 shows that it does. There is a good correlation btween Eg and the cohesive energy, as long as related solids are compared. That is, the 4-4 compounds show Eg falling just as AE coh does. The alkali halides also are correlated with each other, but not with the 4-4 cases. In the 2-6 examples, we can compare the CN6 compounds with each other, but not with the CN4 cases, which have their own relationship. The 3-5 solids form their own family for CN4, but there are no Eg data for the ionic 3-5 cases, which probably belong to a different family. 

Dioxane, ethylene glycol, water-soluble esters, and short-chain alcohols at high bulk phase concentrations may increase the CMC because they decrease the cohesive energy density, or solubility parameter, of the water, thus increasing the solubility of the monomeric form of the surfactant and hence the CMC (Schick, 1965). An alternative explanation for the action of these compounds in the case of ionic surfactants is based on the reduction of the dielectric constant of the aqueous phase that they produce (Herzfeld, 1950). This would cause increased mutual repulsion of the ionic heads in the micelle, thus opposing micellization and increasing the CMC. 

Table 6 demonstrates that separate optimizations of the basis sets for the bulk and the isolated atoms or ions is mandatory for obtaining the cohesive energies of ionic compounds. As a general rule, variationally equivalent basis sets are to be used for the bulk and the atoms or ions, rather than equal basis sets. 

The underestimation of the cohesive energy is a general feature of HF, as can be seen in Table 8, where AE is reported for a series of different simple crystalline compounds, which includes three alkaH halides with increasing ion size, an ionic oxide with stronger electrostatic interactions caused by divalent ions (MgO), a covalent system (Si), and a metal (Be). 

Our calculation is based on a semiempirical classical theory because we use (15.22) and experimental values for ro and n. A more rigorous theory, however, would not improve the result much. That is because the major part of cohesive energy of ionic compounds is due to the Coulomb interaction of the ions which can be considered as electric charges at lattice sites. 

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