Monitoring Ternary Systems of C 12 E 5 /Water/Tetradecane via the Fluorescence of Solvatochromic Probes Graham Hungerford,* Elisabete M. S. Castanheira, and M. Elisabete C. D. Real Oliveira Departamento de Fı ´sica, UniVersidade do Minho, 4710-057 Braga, Portugal M. da Grac ¸ a Miguel and Hugh D. Burrows Departamento de Quı ´mica, UniVersidade de Coimbra, 3004-535 Coimbra, Portugal ReceiVed: August 7, 2001; In Final Form: December 23, 2001 In this work ternary systems of the nonionic surfactant C 12 E 5 (C 12 H 25 (OCH 2 CH 2 ) 5 OH), tetradecane, and water were probed by the use of steady state and time-resolved fluorescence. Microemulsions, which were rich in oil, rich in water, and also both continuous in oil and water, were monitored via the application of solvatochromic fluorescent probes. The hydrophobic laser dye Nile Red was used to report from the side of the oil-surfactant interface, while Prodan, which is soluble in each of the constituent solvents, was used to provide a global picture. To complement these, the hydrophilic dye 4-(2-(dimethylamino)phenyl)-1-methyl- pyridinium iodide was used to report from the side of the water-surfactant interface. The results show the partitioning of the probes into different environments within the ternary systems as evident from the solvatochromic shift in the emission spectra, uncovering similarities between the water-rich and bicontinuous regions in terms of polarity. Estimates for the polarity of the environments in which the fluorophores are solubilized are made in terms of the E T(30) parameter scale. The time-resolved kinetics of some of the probes have been found to be viscosity dependent, which brought out similarities between the bicontinuous and oil-rich regions along with the observation of the effect of solvent relaxation in these ternary systems. Introduction Among the different fluorescence techniques available for studying microemulsions the use of solvatochromic probes is attractive, as a single probe can provide information concerning the range of environments found in these systems. Our interest has been in uncovering information relating to ternary systems involving the nonionic polyoxyethylene surfactant C 12 E 5 (C 12 H 25 (OCH 2 CH 2 ) 5 OH). This is a neutral surfactant consisting of a polyoxyethylene headgroup and an alkyl chain tail. The phase behavior of ternary systems of this surfactant in the presence of an alkane and water have been well studied, 1-9 and these systems have, intriguingly, been found under certain conditions of composition and temperature to form phases that are simultaneously continuous in both water and oil. 10-12 This is of both practical and theoretical importance, and although evidence exists for these phases, 1-3,11 further experimental data are required to fully explain their exact structure. We have previously made use of the excimer-forming properties of pyrene to monitor such systems. 13 Another possible route to elucidate further information is the use of solvatochromic probes. These have found many applications in areas of biology 14 and in the study of micellar systems. 15-17 The interest in their usage stems from the fact that their peak emission wavelength is highly dependent on the dielectric constant of their environment. Hence, in microheterogeneous media a probe molecule partitioned between different environments is capable of reporting on both simultaneously and an initial study has been performed on the so-called bicontinuous region in C 12 E 5 ternary systems. 18 Two probes of particular note are Prodan 19 and Nile Red. 20 The former has found usage in numerous applications, including as a probe for proteins 21 and micellar media. 15-17 The origin of the solvatochromic properties of Prodan has been a matter of debate with discussion of the presence of a twisted intra- molecular charge state. Although it exhibits charge transfer behavior, solvation equilibria have also been used to describe its properties. 22 This probe also has a wide range of solubility, making it suitable to access a wide range of sites within microheterogeneous media. In the ternary system we are interested in studying it was found to be soluble in all three component solvents. The laser dye Nile Red has also proved useful, especially when employed as an extrinsic protein label. 23,24 This probe has a hydrophobic nature (low solubility and fluorescence in water) and therefore has demonstrated value to study biological membranes, 20 with their hydrocarbon-like environments. Its spectroscopic properties in the confinement of micelles 25,26 and γ-cyclodextrin 27 have also been investigated. Both the steady state and time-resolved fluorescence properties of this dye have been found to be highly dependent on its environment. 28,29 As well as its usage in probing microhetero- geneous media Nile Red has been employed to measure solvent polarity and compared to Reichardt’s pyridinium betaine dye E T(30) scale. 30 Because of these properties, these two probes appear well suited for use with the C 12 E 5 ternary system. To complement Prodan and Nile Red, we chose the hydro- philic dimethylaminostibazolium dye 4-Di-1-ASP (pyridinium, 4-(2-(dimethylamino)phenyl)-1-methyl, iodide). This dye has also been reported as having polarity- and viscosity-dependent spectroscopic behavior 31 and has been used in work related to dye-induced photoelectric effects in bilayer membranes. 32 Thus, * Corresponding author. Fax: 00351-253-678981. E-mail: graham@ fisica.uminho.pt. 4061 J. Phys. Chem. B 2002, 106, 4061-4069 10.1021/jp013047v CCC: $22.00 © 2002 American Chemical Society Published on Web 03/29/2002