Nonpolar solids

The liquids discussed in the preceding chapters form different kinds of solid phases single crystals, polycrystalline solids, or glasses. Their electronic properties are influenced by the crystal structure, the presence of traps, and by the electrode/sample interface. In the following three sections, we shall discuss some electronic properties of nonpolar solids which are related to the corresponding liquid phase. The book concludes with a brief section on polymeric liquids. 

---

Nonpolar solids are usually soluble in nonpolar solvents. 

The sorption of water vapor onto nonhydrating crystalline solids below RHq will depend on the polarity of the surface(s) and will be proportional to surface area. For example, water exhibits little tendency to sorb to nonpolar solids like carbon or polytetrafluorethylene (Teflon) [21], but it sorbs to a greater extent to more polar materials such as alkali halides [34-37] and organic salts like sodium salicylate [37]. Since water is only sorbed to the external surface of these substances, relatively small amounts (i.e., typically less than 1 mg/g) of water are sorbed compared with hydrates and amorphous materials that absorb water into their internal structures. 

At present the term RPC is mostly associated with the use of a nonpolar solid stationary phase and a polar eluent. Consequently the name RPC en-... 

A. Preferential Adsorption from Solution by Polar and Nonpolar Solids... 291... 

Figures la and lb are typical curves found for the heat of immersion of polar solids in water (and are also foimd for polar and nonpolar solids in organic liquids). An example of Fig. la is found in the immersion of chrysotile asbestos having known and increasing amounts of physically adsorbed water on its surface (S ). The linear relationship between the heat of wetting and the volume adsorbed up to about a monolayer is significant and indicates surface homogeneity since the heat evolved is proportional to the amount of bare surface present. In accord with this finding, the isosteric heat values calculated from adsorption isotherms increased with coverage to a maximum near the monolayer as expected for adsorption on a surface possessing nearly uniform sites.

The heat values are markedly higher for the polar solid immersed in polar liquids they also vary considerably with the functional group of the liquid. For Graphon, however, the heats are almost unaffected by the structural features of the wetting liquid. This nonpolar solid, despite the presence of a small amount of hydrophilic sites on its surface 0), interacts with the liquids primarily through London dispersion forces. Because of the additive nature of these forces, each adsorbed molecule tends to lie flat on such a surface 40). In the case of a polar molecule the functional group is oriented somewhat away from the nonpolar surface toward the liquid. 

Separations as Influenced by Temperature. Variations in temperature were also investigated as a means of probing the selection of more refined reversed-phase conditions for the chromatography of rhGH. It is generally recognized that both the adsorption of proteins onto the nonpolar solid... 

FIGURE 4.29 Adsorption isotherms of nonionic surfactants on the polar and nonpolar solid surfaces. [Graph reconstructed from data by T. F. Tadros, Solid/Liquid Dispersions, Academic Press, New York, 1987.]... 

Figure 18 A portion of the X-ray structure of the koilate formed between the mono fused koiland 23 and hexadiyne 33 as connector. The directional 1-D network is obtained by a single translation in a noncentric fashion of two different assembling cores defined by the inclusion of the connector into the two differentiated cavities of the koiland (a) and a portion of the structure showing the centrosymmetric packing of consecutive directional koilates leading to a nonpolar solid (b). For the sake of clarity, the carbon atoms of the connector are coloured in grey and the hydrogen atoms are not represented.

Supercritical fluids have the ability to dissolve nonpolar solids, and this is what makes them useful for various applications, especially cleaning, though the fact that solids can dissolve in gases remains counterintuitive to most scientists. A number of attempts have been... 

Nonpolar solids such as naphthalene, CjQHg, do not dissolve appreciably in polar solvents such as water because the two substances do not attract each other significantly. This is true despite the fact that crystal lattice energies of solids consisting of nonpolar molecules are much less negative (smaller in magnitude) than those of ionic solids. Naphthalene dissolves readily in nonpolar solvents such as benzene because there are no strong attractive forces between solute molecules or between solvent molecules. In such cases, the increase in disorder controls the process. These facts help explain the observation that like dissolves like. ... 

Reversed-phase solid-phase extraction (SPE) involves the partitioning of organic solutes from a polar mobile phase, such as water, into a nonpolar solid phase, such as the C-18 sorbent (Fig. 4.1). Partitioning involves the interaction of the solute within the chains of the stationary phase, which may be a C-18 hydrocarbon, C-8 hydrocarbon, or the polymeric sorbents (such as styrene-divinylbenzene). The word hydrophobic mechanism is commonly... 

Healy, T.W. and Fuerstenau, D.W., The isoelectric point/point-of zero-charge of interfaces formed by aqueous solutions and nonpolar solids, liquids, and gases, J. Colloid Interf. Sci., 309, 183, 2007. 

Polymers have much smaller effective dipole moments in the solid phase than in dilute solutions. The dipole moments of polar and nonpolar solid polymers thus differ much less from each other than the dipole moments of polar and nonpolar polymers in dilute solution or of liquids of polar and nonpolar simple molecules. 

Coleman W.M. (1997) A study of the behavior of polar and nonpolar solid-phase microextraction fibers for the extraction of Maillard reaction products. J. Chromatogr. Sci. 35, 245-58. 

In contrast to liquids, two different volumes of a solid phase can not be merged together upon contact. Since the mobility of molecules within solid phases is low, the differences in the bulk and surface structure of these volumes can not disappear spontaneously. Thus, even at the closest contact possible, the real physical interface having its own characteristic value of the specific surface free energy a is present between the two solid phases. For the two solid crystals, a is referred to as the specific surface energy of the grain boundary, agb. For nonpolar solids -1/2 Umo, (b) is less than the surface energy, a, i.e. -Umo[ (b)= = 2a-a. ... 

Fig. III-7. The orientation of surfactant molecules adsorbed at different interfaces a - nonpolar solid/surfactant aqueous solution b - polar solid/surfactant solution in non-polar liquid (oil phase) [4]...

This is the most interesting result of the calculation the values obtained using Cq and Cl from Table 8-4 are also listed in Table 8-4. These values are measurable in nonpolar solids by X-ray techniques, which give ( = 0.63 + 0.04 for both silicon and germanium (Segmuller and Neyer, 1965). Values found by Martin (1970) are a few percent higher than ours and are probably more reliable. We will therefore use Martin s values, or values estimated in terms of his, in our discussion of piezoelectricity. We may also substitute Eq. (8-28) back into Eqs. (8-25) and (8-27) to obtain the total energy per atom pair ... 

The overall effect of solvent polarity on the solubility of naphthalene follows the same general solubility rule in liquid extractions that like dissolves like . Naphthalene is a nonpolar solid and is most soluble in supercritical ethane. Carbon dioxide behaves as a nonpolar solvent but less so because of its quad-rupole moment [11]. Fluoroform is the most polar solvent because of the elec-... 

A full discussion of why nonpolar substances are insoluble in water requires the thermodynamic arguments that we shall develop in Chapters 4 and 15. However, the points made here about intermolecular interactions will be useful background information for that discussion. For the moment, it is enough to know that it is less favorable thermodynamically for water molecules to be associated with nonpolar molecules than with other water molecules. As a result, nonpolar molecules do not dissolve in water and are referred to as hydrophobic ( water-hating, from the Greek). Hydrocarbons in particular tend to sequester themselves from an aqueous environment. A nonpolar solid leaves undissolved material in water. A nonpolar hquid forms a two-layer system with water an example is an oil shck. The interactions between nonpolar molecules are called hydrophobic interactions or, in some cases, hydrophobic bonds. 

---