Caracterization of mixed micellar systems using 19F Nuclear Magnetic Resonance

MARÍA FLORENCIA TORRES; MARIANA ADELA FERNÁNDEZ

Córdoba, Argentina

Workshop; BRUKER Solid State NMR Workshop; 2009

FaMAF, UNC

Surfactants are versatile compounds widely in a variety of industrial and commercial applications. Especially, perfluorinated amphiphiles have exceptional properties, such as low solubility in water, and in polar and nonpolar organic solvents, high density, fluidity, compressibility, and high dielectric constants. They exhibit more surface-tension-lowering ability and chemical resistance than their corresponding hydrogenated ones and, in aqueous solutions, the critical micellar concentration (cmc) of perfluorinated surfactants is considerably lower than that of the corresponding hydrocarbon compounds.[1] These unique properties of perfluorinated materials are useful for different applications. Thus, they are used in blood substitute formulations,[2] oxygen and drug delivery systems,[3] adhesive formulations, cleaners, herbicides, and cosmetics,[4],[5] and in fire-fighting foams. Surfactants are usually used in mixtures of different amphiphilic molecules. Synergistic behavior has been observed in some mixed systems, which may be exploited to reduce the total amount of surfactant used in particular applications resulting in reduction of cost and environment impact.[6] Particularly, mixtures of perfluorinated and hydrocarbonated surfactants show special characteristics and a variety of possible applications. To improve the benefits of these mixtures it is necessary to understand the interactions between surfactants. The mixture formed by a non-ionic hydrocarbonated surfactant, Brij-35 (polyoxyethylene(23)lauryl ether) and an anionic perfluorinated surfactant, PFNA (perfluorononanoic acid) was studied by UV-Visible spectroscopy and fluorescence using different molecular probes. The cmc’s determined with both techniques differed significantly indicating that the different probes might be sensing different environments. Therefore, in this work we proposed the study of the same mixture using Nuclear Magnetic Resonance of 19F (19F NMR). This technique is especially useful for the analysis of perfluorinated amphiphiles since the chemical shifts in 19F NMR are highly sensitive to the environment. The surfactant environment should be very different according to whether it exists as a free monomer surrounded by water, or whether it is part of an aggregate surrounded by other amphiphilic molecules. In addition, the intramicellar environment is particularly different when mixed micelles are formed. The systematic study of this mixture was carried out changing the total surfactant concentration for each molar fraction studied. Changes in chemical shifts of the terminal trifluoromethyl group of the PFNA were analyzed. We chose this signal because this group is expected to be deeper in the micelle and consequently, it should suffer the bigger changes. We found that two aggregates are formed in each molar fraction studied. This observation suggests that UV-Visible spectroscopy and fluorescence sense different aggregates, which can be simultaneously seen with 19F NMR. These results are consistent with previous kinetic studies. [1] Mukerjee, P. Colloids Surf., A 1994, 84, 1. [2] Riess, J. G. Artif. Cells, Blood Substitutes Inmobilization Biotechnol. 1994,22, 215. [3] Patel, N.; Marlow, M.; Lawrence, M. J. J. Colloid Interface Sci. 2003, 258, 345. [4] Baran, J. R., Jr.; Decker, E. L.; Wilcox, H. N. J. Dispersion Sci. Technol. 2002, 23, 23. [5] Halpern, D. J. Fluorine Chem. 2002, 118, 47. [6] Mixed Surfactant Systems, Masahiko Abe and John F. Scamehorn Ed., Marcel Dekker, New York, 2005