Fluorocarbons

The properties of fluid mixtures containing an aromatic fluorocarbon differ completely in many respects from those of mixtures containing an aliphatic or alicyclic fluorocarbon. It is therefore convenient to treat these two major classes of mixture separately. The desire to interpret these extreme differences in thermodynamic behaviour, at a molecular level, is one of the main reasons for the continuing interest of many physical scientists in such systems. 

Liquid Mixtures.— Aliphatic and alicyclic perfluorocarbons first became available in commercial quantities just over three decades ago. The impetus towards their large-scale production came from the war-time Manhattan Project when it was realized that the chemical stability and inertness of these materials, combined with their unexpectedly low solubility in most other common organic solvents, might prove useful in uranium isotopic separation processes based on 

In complete contradiction to the predictions of the existing liquid mixture theory it was found that these fluorocarbons were only partially miscible with hydrocarbons of comparable molar mass. The large volume of experimental work that resulted from attempts to interpret this unexpected behaviour of mixtures consisting of relatively simple non-polar molecules is covered comprehensively in Scott s 1958 review.  

Binary mixtures of non-aromatic fluorocarbons with hydrocarbons are characterized by large positive values of the major thermodynamic excess functions G , the excess Gibbs function, JT , the excess enthalpy, 5 , the excess entropy, and F , the excess volume. In many cases these large positive deviations from ideality result in the mixture forming two liquid phases at temperatures below rSpper. an upper critical solution temperature. Experimental values of the excess functions and of Tapper for a representative sample of such binary mixtures are given in Table 1. 

The values of and of TS for CF + CH4 shown in Table 1 were estimated by combining the measurements of G made by Thorp and Scott over the temperature range 105.5 to 110.5 K with the recent value of this function measured at 98 K by Simon and Knobler and are thus subject to considerable uncertainty. It is to be hoped that FT for this important binary system will be measured by a direct calorimetric method in the near future. 

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Liquid or solid films which reduce or prevent adhesion between surfaces solid-solid, solid-paste, solid-liquid. Waxes, metallic soaps, glycerides (particularly stearates), polyvinyl alcohol, polyethene, silicones, and fluorocarbons are all used as abherents in metal, rubber, food, polymer, paper and glass processing. 

It has been pointed out [138] that algebraically equivalent expressions can be derived without invoking a surface solution model. Instead, surface excess as defined by the procedure of Gibbs is used, the dividing surface always being located so that the sum of the surface excess quantities equals a given constant value. This last is conveniently taken to be the maximum value of F. A somewhat related treatment was made by Handa and Mukeijee for the surface tension of mixtures of fluorocarbons and hydrocarbons [139]. 

The extensive use of the Young equation (Eq. X-18) reflects its general acceptance. Curiously, however, the equation has never been verified experimentally since surface tensions of solids are rather difficult to measure. While Fowkes and Sawyer [140] claimed verification for liquids on a fluorocarbon polymer, it is not clear that their assumptions are valid. Nucleation studies indicate that the interfacial tension between a solid and its liquid is appreciable (see Section K-3) and may not be ignored. Indirect experimental tests involve comparing the variation of the contact angle with solute concentration with separate adsorption studies [173]. 

Most LB-forming amphiphiles have hydrophobic tails, leaving a very hydrophobic surface. In order to introduce polarity to the final surface, one needs to incorporate bipolar components that would not normally form LB films on their own. Berg and co-workers have partly surmounted this problem with two- and three-component mixtures of fatty acids, amines, and bipolar alcohols [175, 176]. Interestingly, the type of deposition depends on the contact angle of the substrate, and, thus, when relatively polar monolayers are formed, they are deposited as Z-type multilayers. Phase-separated LB films of hydrocarbon-fluorocarbon mixtures provide selective adsorption sites for macromolecules, due to the formation of a step site at the domain boundary [177]. 

Yamada S and Israelachvili J N 1998 Friction and adhesion hysteresis of fluorocarbon surfactant monolayer-coated surfaces measured with the surface forces apparatus J. Rhys. Chem. B 102 234-44... 

Solvophobicify (hydrophobicify witli respect to water) is most often exemplified as a linear or branched hydrocarbon chain. Fluorocarbon chains and siloxane chains are also hydrophobic. Many commercially important... 

New factors for tlie establislmient of multilayer stmctures are, for example, tire replacement of tire hydrocarbon chain by a perfluorinated chain and tire use of a subphase containing multivalent ions [29]. The latter can become incoriDorated into an LB film during deposition. The amount depends on tire pH of tire subphase and tire individual ion. The replacement of tire hydrocarbon by a rodlike fluorocarbon chain is one way to increase van der Waals interaction and tlierefore enlrance order and stability in molecular assemblies [431. 

Remarkably, such fluorocarbon monolayers show higher friction tlian tlieir hydrocarbon counteriDarts [441, altliough fluorocarbons are known to have tire lowest surface free energy of all organic materials. 

The lubricant properties of alkanethiols and fluorinated alkanes have been studied extensively by scanning probe techniques [163]. In agreement with experiments on LB monolayers it was found that the fluorocarbon monolayers show considerably higher friction than the corresponding hydrocarbon monolayers [164, 165 and 166] even though the fluorocarbons are known to have the lowest surface free energy of all organic materials. 

Laschewsky A, Ringsdorf H and Schmidt G 1985 Polymerization of hydroxocarbon and fluorocarbon amphiphiles in Langmuir-Blodgett multilayers Thin Soiid Fiims 134 153-72... 

Schneider J, Erdelen C, Ringsdorf H and Rabolt J F 1989 Structural studies of polymers with hydrophilic spacer groups. 2. Infrared-spectroscopy of Langmuir-Blodgett multilayers of polymers with fluorocarbon side-chains at ambient and elevated temperatures Macromolecules 22 3475-80... 

Figure C2.13.7. Change between polymerizing and etching conditions in a fluorocarbon plasma as detennined by tire fluorine-to-carbon ratio of chemically reactive species and tire bias voltage applied to tire substrate surface [36].

By far the largest use of hydrogen fluoride is in the manufacture of fluorocarbons which find a wide variety of uses including refrigerants, aerosol propellants and anaesthetics. Hydrogen fluoride is also used in the manufacture of synthetic cryolite, Na3AIFg, and the production of enriched uranium. 

In a search for fluorocarbons having anesthetic properties 1 2 dichloro 1 1 difluoropropane was subjected to photochemical chlorination Two isomeric products were obtained one of which was identified as 1 2 3 tnchloro 1 1 difluoropropane What is the structure of the second com pound" ... 

It has good weathering resistance and does not support combustion. It is resistant to most chemicals and solvents and has greater strength, wear resistance, and creep resistance than the preceding three fluorocarbon resins. 

More recently, solid supports made from glass beads or fluorocarbon polymers have been introduced. These supports have the advantage of being more inert than diatomaceous earth. 

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