J . Phys. Chem. 1987,91, 1450-1454
1450
which could prevent this interaction and thus influence the stabilization of the Chl cation.
and anisotropy measurements. This study was supported by a NATO grant.
Acknowledgment. Thanks are due to Pr. B. Valeur for his experimental contribution concerning Chl fluorescence lifetime
Regisby NO.PVS, 77951-49-6; VC6, 56343-76-1; VC,, 56343-81-8; VC,, 73645-29- 1; Chl, 479-61 -8.
Mlcelles In Perfluorlnated Surfactant Solutions Camillo La Mesa Department of Chemistry, Universitd della Calabria, Arcavacata di Rende, Cosenza, Italy
and Bianca Sesta* Dipartimento di Chimica, Universitd di Roma, "La Sapienza", Roma, Italy (Received: April 7, 1986)
Thermodynamic and transport properties of lithium and sodium perfluorononanoatesin water have been studied by different experimental techniques, with particular attention paid to the volumetric properties of micelles and to their sizes and shapes. Despite the large volume changes upon micellization, the micelle aggregation numbers are low (about 20 units) and depend slightly on the overall surfactant concentration. By examination of the bulk and surface properties of perfluorinated surfactants on thermodynamic grounds, it becomes evident that these properties are related to the weak contributions to the hydrophobic effect of perfluoromethylene chains.
Introduction Perfluorinated surfactants have interesting peculiarities which make them suitable for many practical purposes, like the formation of fire extinguishing fluids, wetting agents, etc. Their properties have been widely studied in the past, particular attention being focused on the surface activity efficiency,' on the nonideality of mixing with hydrocarbons: and on spectroscopic properties, mostly studied by fluorine N M R experiment^.^^^ More recently, systematic studies have been reported to account for their properties on sound thermodynamic grounds5 and to get further insight on the occurrence of "defective" lyotropic mesophases at high surfactant concentratiom6 In regard to the properties of perfluorinated salts in the micellar region, not much is known about micellar size and shape nor about some thermodynamic properties. In this paper we report several data on thermodynamic and transport properties of lithium and sodium perfluorononanoate aqueous solutions. Experimental Section
Materials. Perfluorononanoic acid was a high purity product (Riedel). Its melting point temperature is 77 "C, in agreement with previous literature findings? It was dissolved in ethanol and titrated with lithium or sodium hydroxide solutions, the end point of reaction being checked by phenolphthalein indicator. The resulting salts, LiPFN or NaPFN, respectively, were desiccated and twice purified by precipitation from mixed solvents (ethanol-acetone, chloroform-methanol, or hexane-butanol) and vacuum-dried at 80 "C for 2 days. Their purity was checked by determination of the melting point temperatures. Furthermore, no surface active impurities could be detected by surface tension data of their aqueous solutions, Figure 1. (1) Tadros, T. J . Colloid Interface Sci. 1980, 74, 196. (2) (a) Shinoda, K.; Nomura, T. J . Phys. Chem. 1980, 84, 365. (b) Mukerjee, P.; Yang, A. Y. S. J . Phys. Chem. 1976, 80, 1388. (3) Muller, N.; Birkhahn, R. H. J. Phys. Chem. 1%7, 71, 957: 1968, 72,
583.
(4) Ulmius, J.; Lindman, B. J . Phys. Chem. 1981, 85, 4131. ( 5 ) Mukerjee, P.; Handa, T. J . Phys. Chem. 1981, 85, 2298. ( 6 ) Boden, N.; Jackson, P. H.; McMuller, K.; Holmes, M. C. Chem. Phys. Lett. 1979, 65, 476. (7) Fontell, K.; Lindman, B. J . Phys. Chem. 1983, 87, 3289.
TABLE I: Melting Point, P , ("C), Krafft Point, K, ("C), and critical Micellar Concentration, cmc (mol L-I), of Perfluorononanoic Acid and of Its Lithium and Sodium Salts HPFN
LiPFN
NaPFN
77 59.3-61.1" 776 71-72' 246 244-245' 283-284 282f
43.8 48.3"
8 X lo4 2.8 X
