Other nonpolar compounds

Benzene is a typical nonpolar compound that, like other nonpolar compounds, has a low solubility in water. It has a characteristic odor which most people find unpleasant. It was widely used in academic labs as a solvent, but that use has been largely discontinued since it was found that benzene may be carcinogenic. 

When water is mixed with benzene or hexane, two phases form neither liquid is soluble in the other. Nonpolar compounds such as benzene and hexane are... 

The hydrophilicity of nonionic surfactants can be characterized numerically as their hydrophile-lipophile balance (HLB). An HLB value of 3-6 indicates that the compound is a likely W/O emulsifier 7-9, a wetting agent 8-13, an O/W emulsifier 13-15, a detergent and 15-18, a solubilizer (of oil or other nonpolar compounds) in water. The HLB values of some common compounds are presented in Table 34.12.170 An HLB value of 8.0 is shown in Table 34.12 for lecithin, but manufacturers are able to supply modified lecithins with values of2-12. 

In choosing the solvent the chemist is guided by the dictum like dissolves like. Even the nonchemist knows that oil and water do not mix and that sugar and salt dissolve in water but not in oil. Hydrocarbon solvents such as hexane will dissolve hydrocarbons and other nonpolar compounds, and hydroxylic solvents such as water and ethanol will dissolve polar compounds. Often it is difficult to decide, simply by looking at the structure of a molecule, just how polar or nonpolar it is and therefore which solvent would be best. Therefore, the solvent is often chosen by experimentation. 

Oils, such as cooking oil, do not mix with water. An oil is nonpolar, and water is polar. However, paint thinner is soluble with the oil in oil-based paints. Both the paint and paint thinner are nonpolar. Polar compounds tend to dissolve in other polar compounds, and nonpolar compounds tend to dissolve in other nonpolar compounds. 

It is well known that water is a good solvent for ions and polar molecules. On the other hand, it is a poor solvent for nonpolar molecules such as hydrocarbons. Perhaps surprisingly, the insolubility of hydrocarbons and other nonpolar compounds is not due to a positive enthalpy effect. The enthalpy of mixing hydrocarbons with water is foimd to be either very small or negative. Therefore, the positive value for AG must arise from a negative entropy change. This effect, called the hydrophobic effect, assumes importance in many processes, including solubilization and the adsorption of compounds at interfaces. 

Ahphatic hydrocarbons and other nonpolar compounds are thought to be incorporated or solubilized in the micelle interiors (Figure 4.22a). Molecules having some surface activity, such as alcohols, distribute themselves among the surfactant molecules, as shown in Figure 4.22b. This simation is closely related to formation of micelles from mixtures of surfactants, which was discussed previously. Rather polar substances may even occupy positions at the micelle surface (Figure 4.22c). 

Interactions between nonpolar compounds are generally stronger in water than in organic solvents. At concentrations where no aggregation or phase separation takes place, pairwise hydrophobic interactions can occur. Under these conditions, the lowest energy state for a solute molecule is the one in which it is completely surrounded by water molecules. However, occasionally, it will also meet other solute molecules, and form short-lived encounter complexes. In water, the lifetime of these complexes exceeds that in organic solvents, since the partial desolvation that accompanies the formation of these complexes is less unfavourable in water than in organic solvents. 

Dichlorine monoxide is the anhydride of hypochlorous acid the two nonpolar compounds are readily interconvertible in the gas or aqueous phases via the equilibrium CI2 O + H2 0 2H0Cl. Like other chlorine oxides, CI2O has an endothermic heat of formation and is thus thermodynamically unstable with respect to decomposition into chlorine and oxygen. Dichlorine monoxide typifies the chlorine oxides as a highly reactive and explosive compound with strong oxidhing properties. Nevertheless, it can be handled safely with proper precautions. 

The solubility of organomercury compounds depends primarily on the nature of the X group nitrates and sulfates tend to be salt-like and relatively water-soluble, whereas chlorides are covalent, nonpolar compounds of low water solubility. Methyl mercury compounds tend to be more volatile than other organomercury compounds. 

This is a relatively new technique that is used for PCBs and other nonpolar, volatile and semi-volatile organic compounds. Typically, a small aliquot of soil sample (0.5-20 g) is used for the extraction. Soil samples are extracted with one or more organic solvents using microwave energy at elevated temperature (100-115 °C) and pressure (50-175 psi). This method uses less solvent and takes significantly less time than Soxhlet extraction but is limited to thermally stable compounds. 

Traditional electrophoresis and capillary electrophoresis are competitive techniques as both can be used for the analysis of similar types of samples. On the other hand, whereas HPLC and GC are complementary techniques since they are generally applicable to different sample types, HPLC and CE are more competitive with each other since they are applicable to many of the same types of samples. Yet, they exhibit different selec-tivities and thus are very suitable for cross-validation studies. CE is well suited for analysis of both polar and nonpolar compounds, i.e. water-soluble and water-insoluble compounds. CE may separate compounds that have been traditionally difficult to handle by HPLC (e.g. polar substances, large molecules, limited size samples). 

From the characteristics of the methods, it would appear that FD-MS can profitably be applied to poly-mer/additive dissolutions (without precipitation of the polymer or separation of the additive components). The FD approach was considered to be too difficult and fraught with inherent complications to be of routine use in the characterisation of anionic surfactants. The technique does, however, have a niche application in the area of nonpolar compound classes such as hydrocarbons and lubricants, compounds which are difficult to study using other mass-spectrometry ionisation techniques. 

Polar compounds and compounds that ionize can dissolve readily in water. These compounds are said to be hydrophilic. In contrast to hydrophilic substances, hydrocarbons and other nonpolar substances have very low solubility in water because it is energetically more favorable for water molecules to interact with other water molecules rather than with nonpolar molecules. As a result, water molecules tend to exclude nonpolar substances, forcing them to associate with themselves in forming drops, thereby minimizing the contact area between... 

On the other hand, the increase in temperature decreases the inter-molecular interaction (hydrogen bonding) between water molecules, which lessens the squeezing-out effect for nonpolar solutes. At the supercritcal state, water exhibits an antiaqueous property. For example, water at high temperatures exhibits considerable, and sometimes complete, miscibility with nonpolar compounds. 

Intuitively, one would expect a volume contraction on forming a strongly bonded compound from the elements. Indeed, Richards 190, 191) regarded heats of formation as heats of compression. The fractional volume contraction, AV = (molecular volume - 2 atomic vol-ume)/2(atomic volume), has been related to formation heats for NaCl or CsCl type structures 151). Even nonpolar compounds in the condensed state cohere in close-packed arrays. The packing density of difluorine, derived from the ratio of the van der Waals envelope to the molecular volume, is especially low, and a larger contraction would be expected for fluorides than for other halides. This approach has yet to be systematically examined. 

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