Fluorinated surfactants in solution

The solubilities of fluorinated surfactants are related to the unusual properties of the fluorine atom and the C—F bond. Fluorine is the most electronegative element and is very difficult to polarize. Fluorine can form a very stable bond with hydrogen or carbon (see Section 3.1). The rigidity of the C— F bond causes stiffening of the perfluoroalkane chain and limits interactions with other molecules. Because of their small size, fluorine atoms can shield the perfluorinated carbon atom without steric stresses. Perfluoromethyl or perfluoromethylene groups therefore form compounds with very weak intermolecular forces. As a consequence of weak interactions, perfluoroalkanes are insoluble in common organic solvents. Perfluo-roalkanes are more hydrophobic than hydrocarbons, evidenced by solubility data CF4 is seven times less soluble in water than CH4 [1,2]. Water is almost 7 times less soluble in perfluoroheptane than in heptane on a equal weight basis [3] and 25 times more on a molar basis. 

The solubility of fluorinated surfactants depends on the hydrophile of the surfactant, in addition to the structure of the fluorinated group. The solubility of perfluoroalkanoic acids in water, like the solubility of nonfluorinated alkanoic acids, decreases with increasing chain length. At 25°C, perfluorohexanoic acid and shorter-chain perfluoroalkanoic acids are miscible with water in all proportions. However, perfluorooctanoic acid and perfluorodecanoic acid are only slightly soluble in water [4]. 

The solubility of alkali metal salts of peifluorinated alkanoic acids in water decreases with increasing chain length. Some metal salts of perfluoroacetic and perfluorobutyric acids are soluble in water and in organic solvents as well 

The perfluowalkanesulfonic acids having one to six carbon atoms are colorless oils which fume in moist air and form monohydrates [6]. The C2 and C3 acids are miscible with water in all proportions. The solubility of perfluoroalka-nesulfonic acids in water decreases with increasing chain length and the C7 and Cg acids are only moderately soluble in water. Their surface activity increases with decreasing solubility in water. 

The solubilities of the salts of pejfluoroalkanesulfonic acids depend on the nature of the counterion. The sodium and potassium salts of Ci, C2, and C3 per-fluoroalkanesulfonic acids are soluble in water, but their solubilities decrease with increasing chain length [6]. The tetraethylammonium salt of perfluorooctanesul-fonic acid is more soluble in water than the potassium salt and is soluble in organic solvents such as alcohols, dimethyl formamide (DMF), or dimethyl sulfoxide (DMSO) as well [7]. 

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In general, the concentration of a fluorinated surfactant in solution can be determined by conventional volumetric or spectroscopic methods used for hydrocarbon-type surfactants [1-5], In addition to the functional groups utilized for the analysis of hydrocarbon-type surfactants, the fluorine content is a unique feature useful for the determination of fluorinated surfactants. If the fluorinated surfactant is the only fluorine-containing species in a solution or a substrate, then the fluorine content indicates the concentration of the fluorinated surfactant. 

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