Extremely Hydrophobic Compounds

NPC has been used in the analysis hydrophobic compounds such as polyaromatic hydrocarbons [31-33]. An interesting example of an application of NPC involving extremely hydrophobic compounds was recently offered by Liu and 

---

A totally different rationale has to be applied to the transport of hydrocarbons. These extremely hydrophobic compounds seem to reside in the interior of the lipophorin particles. A mechanism involving uptake and degradation of the lipoprotein might be possible for the transport of hydrocarbons to the epidermal cells. This type of mechanism might be important in certain stages of insect development, when the lipoprotein could deliver amino acids and other lipid components necessary for the construction of the cuticle. A similar process may also exist for the delivery of carotenoids and sterols. 

If the above solvent search reaches the solvent system of n-hexane/methanol/water (2 1 1), which is suitable for the most hydrophobic compounds, and a more hydro-phobic solvent system is required, one may reduce the amount of water from the above solvent system and/or replace methanol with ethanol. Some useful solvent systems for the extremely hydrophobic compounds are M-hexane/ethanol/water (6 5 2) and w-hexane/methanol (2 1). On the other hand, if the solvent search reaches the solvent systems of w-butanol/water or ferf-butyl methyl ether/butanol/acetonitorile/water (2 2 1 5), which are suitable for the most hydrophilic compounds, and a still more hydrophilic solvent system is required, the above solvent system may be modified by the addition of acid or salt trifluoroacetic acid (TFA) or ammonium acetate has been successfully used. 

Figure 3.11 Liquid-liquid equilibrium data (points) for water-benzene and water-perfluorobenzene. Notice the much lower solubilities in water of perfluorobenzene compared to benzene, which explains the higher interfacial tensions (57.2 mN m for perfluorobenzene-water versus 35 mN m for benzene-water). Fluorocarbons are extremely hydrophobic compounds having very weak van der Waals forces. As in Figure 3.9, the lines are modelling results using the CPA equation of state. Reprinted from Tsivintzelis and Kontogeorgis (2012), with permission from American Chemical Society...

The elution order for neutral species in MEKC depends on the extent to which they partition into the micelles. Hydrophilic neutrals are insoluble in the micelle s hydrophobic inner environment and elute as a single band as they would in CZE. Neutral solutes that are extremely hydrophobic are completely soluble in the micelle, eluting with the micelles as a single band. Those neutral species that exist in a partition equilibrium between the buffer solution and the micelles elute between the completely hydrophilic and completely hydrophobic neutrals. Those neutral species favoring the buffer solution elute before those favoring the micelles. Micellar electrokinetic chromatography has been used to separate a wide variety of samples, including mixtures of pharmaceutical compounds, vitamins, and explosives. 

These species are also unusual iu that they are extremely hydrophobic anions which form very strong conjugate acids. This unique combination of features leads to a number of potential uses such as the extraction of organic compounds from extremely dilute solutions and the isolation of metal cations, including the quantitative separation of radionucUdes, eg, Cs (192). 

Quinones are among the best-behaved organic compounds to undergo redox reactions in aqueous solutions. There are a reasonably large number of synthetic and natural products containing the quinone moiety, and many are excellent candidates for selective determination by LCEC. Unfortunately, some of the most important of these compounds are extremely hydrophobic due to the presence of long hydrocarbon side chains, and are therefore quite difficult to study by reversed-phase LC. 

There is widespread acknowledgment that it is extremely difficult to predict BCF values accurately for very hydrophobic compounds. Macdonald et al. (2002) have commented that for compounds with log Kow values greater than about 6, fitting any equation in this region is very adventurous. For compounds with log Kow > 8, they recommend the use of expert judgment. 

In normal-phase chromatography, polar stationary phases are employed and solutes become less retained as the polarity of the mobile-phase system increases. Retention in normal-phase chromatography is predominately based upon an adsorption mechanism. Planar surface interactions determine successful use of NPC in separation of isomers. The nonaqueous mobile-phase system used in NPC has found numerous applications for extremely hydrophobic molecules, analytes prone to hydrolysis, carbohydrates, and sat-urated/unsaturated compounds. In the future, with the advent of new stationary phases being developed, one should expect to see increasingly more interesting applications in the pharmaceutical industry. 

Elemental sulfur is an extremely hydrophobic material and the same holds for sulfur-rich compounds R-S -R with hydrophobic organic terminal groups R like alkyl or aryl. However, strongly hydrophihc groups Hke -SOs" are able to turn these hydrophobic substances into hydrophihc materials even at very high sulfur contents. Sulfane disulfonate anions S (S03)2, better known as polythionate anions, are such species which quahfy for amphiphilic behavior in water. These ions exist as a long homologous series with n... 

In case of hydrophobic compounds, which strongly adsorb to particles, e.g. pentabro-modiphenylether or 5 and 6 ring polycyclic aromatic hydrocarbons, special care is required to ensure complete extraction of the particle bound fraction. Separate analysis of SPM and of the liquid would be a good option. If it can be justified, for example, by consideration of contaminant partitioning, analysis of the SPM fraction as surrogate for whole water might be appropriate. Nevertheless, in water bodies with extremely low SPM content (<3mg/L) the dissolved fraction of those contaminants contributes significantly to the total concentration, and hence, has to be taken into account. 

---