Fluorescence probing of mixed reverse micelles formed with AOT and nonionic surfactants in n-heptane

Fluorescence probing of mixed reverse micelles formed with AOT and nonionic surfactants in n-heptane

Colloids and Surfaces A: Physicochemical and Engineering Aspects 139 (1998) 21–26 Fluorescence probing of mixed reverse micelles formed with AOT and ...

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Colloids and Surfaces A: Physicochemical and Engineering Aspects 139 (1998) 21–26

Fluorescence probing of mixed reverse micelles formed with AOT and nonionic surfactants in n-heptane Daojun Liu, Jiming Ma *, Humin Cheng, Zhenguo Zhao Department of Chemistry, Peking University, Beijing 100871, China Accepted 27 November 1997

Abstract The nature of the aqueous core of mixed reverse micelles formed with AOT and nonionic surfactants in n-heptane was investigated by fluorescence techniques. The emission properties of 1,8-anilinonaphthalenesulfonic acid (ANS) and tris(2,2∞-bipyridine)ruthenium dichloride hexahydrate (Ru(bpy)2+) were found to be extremely sensitive to the 3 solubilized water content. The fluorescence intensity decreases with increasing water content while the position of the emission maximum l shifts to longer wavelengths. The l of the fluorescence probe also depends on the added max max nonionic surfactant content and their EO chain length. The microstructure of the waterpool and the effect of added nonionic surfactants were studied by the fluorescence analysis of the two fluorescence probes from the viewpoint of their different location in mixed reverse micelles. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Mixed reverse micelles; Fluorescence probe; AOT; Nonionic surfactants

1. Introduction Static structures of sodium bis(2-ethylhexyl ) sulfosuccinate (AOT ) reverse micelle, capable of solubilizing relatively large amounts of water in the core, have been well characterized by a variety of techniques such as laser light scattering [1–3], small-angle X-ray (or neutron) scattering [4,5], fluorescence probing [6–12], NMR spectroscopy [13,14], infrared spectroscopy [15–17], and conductivity measurements [18,19] etc. Among these methods, fluorescence probing is a usual and indirect procedure for determining the structure of micelles and microemulsion droplets. The solubilizates which have solvent-sensitive fluorescence spectra are utilized to characterize the state of water in reverse micelles. Hasegawa et al. [6–8] have measured the microviscosity in the waterpool * Corresponding author. 0927-7757/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0 9 2 7 -7 7 5 7 ( 9 8 ) 0 0 19 6 - 4

of AOT reverse micelle as a function of W , with 0 the help of a viscosity-sensitive fluorescence probe, auramine O, and have reported that the waterpool is highly viscous in the lower W -region; with 0 increasing W , viscosity rapidly decreases below 0 W =10 and then gradually decreases until the 0 micellar solution becomes turbid above W =50. 0 Wong et al. [9] have determined the W depen0 dence of the quantum yield and l values for max 1,8-anilinonaphalenesulphate (ANS ) in AOT reverse micelles. It was observed that the apparent polarity (and hence dielectric constant) of the solubilized water increased rapidly until W =12 0 and more slowly thereafter. The water-in-oil microemulsions formed with AOT have been studied by analyzing the luminescence decay profiles of tris(2,2∞-bipyridine)ruthenium dichloride hexahydrate (Ru(bpy)2+) in the presence of varying 3 quencher, Fe(CN )3+, concentration [10]. The 6 information about the micellar droplet concen-

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tration, the surfactant aggregation number, and the exchange rate of solubilizates between droplets has been obtained. The fluorescence properties of several pyrene and naphthalene derivatives were also used to characterize AOT reverse micelles [11,12]. The present paper focuses on the fluorescence probing of mixed reverse micelles formed with AOT and nonionic surfactants by using two kinds of polarity-sensitive fluorescence molecules, the anionic ANS and the cationic Ru(bpy)2+. 3 2. Experimental 2.1. Materials Sodium bis(2-ethylhexyl )sulfosuccinate (AOT, Sigma); Brij52, Brij56, Brij58 (polyoxyethylene (2) cetyl ether, polyoxyethylene (10) cetyl ether, polyoxyethylene (20) cetyl ether, Aldrich); Brij30 (polyoxyethylene (4) lauryl ether, Acros Organics). All the surfactants were used without further purification. n-Heptane was A.R. grade ˚ molecular sieves and and was dried with 4 A redistilled. 1,8-anilinonaphthalenesulfonic acid (ANS, Acros Organics) and tris(2,2∞-bipyridine)ruthenium dichloride hexahydrate (Ru(bpy)2+, 3 Acros Organics) were used as supplied. Deionized and distilled water was used throughout the experiment.

3. Results and discussion 3.1. The fluorescence behavior of cationic probe Ru(bpy)2+ in mixed reverse micelles 3 The molar ratio of water to total surfactants is defined as W . Steady state fluorescence spectra 0 of Ru(bpy)2+ measured at different W values in 3 0 reverse micelles of AOT/H O/heptane are shown 2 in Fig. 1. In reverse micelles with low water content, Ru(bpy)2+ displays very intense fluores3 cence with a maximum intensity at l =595 nm max at W =0.5. This fluorescence is drastically reduced 0 by increasing water content with a concomitant red shift of l . Similar behavior is exhibited by max Ru(bpy)2+ in mixed reverse micelles formed with 3 AOT and nonionic surfactants. Separate plots of fluorescence intensity as well as l values of max Ru(bpy)2+ in mixed reverse micelles formed with 3

2.2. Methods The total concentration of surfactants was 0.1 mol l−1. The same volume of aqueous solutions of Ru(bpy)2+ or ANS were injected into mixed 3 reverse micelles formed with AOT and nonionic surfactants in n-heptane. The final probe concentrations of Ru(bpy)2+ and ANS were maintained 3 at 1.5×10−5 and 1.8×10−6 mol l−1, respectively. For samples with higher water content, a certain amount of water was injected continuously. The mixtures were shaken at room temperature to form optically transparent solutions. Static fluorescence measurements were carried out with a RF-540 spectrofluophotometer (Shimadzu). The excitation wavelength was 460 nm and 365 nm respectively when Ru(bpy)2+ and ANS were used as fluores3 cence probe.

Fig. 1. Effect of W on the fluorescence spectra of 0 Ru(bpy)2+ (1.5×10−5 mol l−1) in AOT/H O/n-heptane 3 2 reverse micelles. Water contents (from top to bottom) were W =0.5, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 8.0, 10.0, 14.0. 0

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Fig. 2. Variation of fluorescence intensity and l of fluoresmax cence probe Ru(bpy)2+ with W in 0.09 M AOT+0.01 M 3 0 Brij58/H O/n-heptane mixed reverse micelles. 2

AOT and Brij58 versus W are presented in Fig. 2. 0 It is obvious that the changes in fluorescence intensity and l are most significant in a region max where the water content of the mixed reverse micelles is relatively small, and approaches a plateau at larger water content. Ru(bpy)2+ is known as a molecular probe in 3 which l increases with increasing the polarity max of its microenvironment [20], it seems reasonable then to explain the data in Fig. 2 in terms of variation in the effective polarity of the reverse micellar interior. At lower water content in the reverse micelles, the polarity of the microenvironment where Ru(bpy)2+ is localized is weak, and 3 the polarity of the microenvironment of Ru(bpy)2+ increases by adding more water and 3 reaches a plateau value at greater water content. It is shown in Fig. 2 that l of Ru(bpy)2+ at max 3 the plateau is still considerably smaller than that in bulk water (625 nm), which can be explained by the fact that the Ru(bpy)2+ molecules locate 3 at the surfactant–water interface owing to the electrostatic attractions with the sulfonate anionic groups of AOT, where the polarity is lower than that in bulk water. The influence of the content and polyoxyethylene chain ( EO chain) length of the added nonionic surfactants on the l of Ru(bpy)2+ is shown in max 3 Fig. 3. Blue shift of l occurs with the increase max

Fig. 3. Variation of l of Ru(bpy)2+ with W in mixed max 3 0 reverse micelles.

of either the EO chain length of nonionic surfactant when the ratio of AOT to nonionic surfactant is fixed (Fig. 3A), or the nonionic surfactant content when the nonionic surfactant is definite ( Fig. 3B). It can be easily inferred that the polarity of the microenvironment near sulfonate groups at the surfactant–water interface decreases with the increase of the nonionic surfactant content and their EO chain length. Another phenomenon, shown in Fig. 3, is that there exist intersections between the l –W curve of the AOT reverse max 0 micelle and those of the mixed reverse micelles formed with AOT and nonionic surfactants, which indicates that the addition of nonionic surfactants into AOT reverse micelles has a different influence on the polarity of the surfactant–water interface when the water content is changed. In the case of the lower water content, the l of Ru(bpy)2+ is max 3

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increased and hence the polarity of the interface is increased by adding nonionic surfactant, whereas the addition of the nonionic surfactant at larger water content leads to the blue shift of l of max Ru(bpy)2+ and the decrease of the polarity of the 3 interface region. The W where the l approaches plateau is 0 max defined as Wp , and the Wp of Ru(bpy)2+ in mixed 0 0 3 reverse micelles shifts to a larger W value with 0 the increase of the EO chain length of nonionic surfactant, but moves to a smaller W value with 0 increased nonionic surfactant content, as evidenced in Fig. 3. The effect of nonionic surfactant content and the EO chain length in Wp will be further 0 discussed in Section 3.3. 3.2. The fluorescence behavior of anionic probe ANS in mixed reverse micelles The anionic fluorescence probe 1,8-anilinonaphthalenesulfonic acid (ANS) in mixed reverse micelles exhibits fluorescence behavior similar to that of Ru(bpy)2+. Steady state fluorescence 3 spectra of ANS measured at different W values 0 in mixed reverse micelles of AOT /H O/heptane 2 are shown in Fig. 4. In AOT reverse micelles with low water content, ANS also displays very intense fluorescence with a maximum intensity at l =451 nm at W =0.5. This fluorescence is max 0 drastically decreased by increasing water content with a concomitant red shift of l until a certain max W value, and thereafter it varies gradually and 0 approaches a plateau at larger water content. ANS is also a polarity-sensitive probe, and the l –W curve can also be explained by the variamax 0 tion of the effective polarity of the waterpool in reverse micelles. The influence of the content and EO chain length of the added nonionic surfactant on the fluorescence behavior of ANS in mixed reverse micelles is shown in Fig. 5. It can be seen in Fig. 5 that the influence of nonionic surfactant on the fluorescence behavior of ANS is not as strong as that of Ru(bpy)2+. The addition of nonionic sur3 factant always increases the polarity of the microenvironment where the ANS molecules are located throughout the range of water content, there is no obvious intersection between the l –W curve of max 0

Fig. 4. Effect of W on the fluorescence spectra of ANS 0 (1.8×10−6 mol l−1) in AOT/H O/n-heptane reverse micelles. 2 Water contents (from top to bottom) were W =0.5, 1.0, 2.0, 0 3.0, 4.0, 5.0, 6.0, 8.0, 10.0, 14.0, 18.0.

the AOT reverse micelle and that of the mixed reverse micelles formed with AOT and nonionic surfactants. When the ratio of AOT to nonionic surfactant is fixed, a slight red shift of l is max observed by adding nonionic surfactant in AOT reverse micelles, and the length of EO chain has little effect on the l (Fig. 5A). However, for a max particular nonionic surfactant, an obvious red shift of l occurs with increasing nonionic surfactant max content at lower water content ( Fig. 5B). An apparent feature that can be seen in Fig. 5 is that the l of ANS in mixed reverse micelles with max different surfactant composition approaches similar values at the larger water content, and this l value is still considerably smaller than that in max bulk water (l =510 nm), indicating that the max effective polarity of the waterpool is smaller than that of the aqueous bulk phase even at larger water content.

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Fig. 5. Variation of l of ANS with W in mixed reverse max 0 micelles.

Again, as evidenced in Fig. 5, the change of the molar ratio of AOT to nonionic surfactants has some influence on the value of Wp where the 0 l of ANS approaches plateau. The Wp shifts max 0 to a smaller W value when the nonionic surfactant 0 content is increased, but is hardly affected by the variation of the EO chain length of nonionic surfactants when the molar ratio of AOT to nonionic surfactants is fixed. 3.3. Comparison and discussion of the fluorescence behavior of Ru(bpy)2+ and ANS 3 An important point worthy of note is the location of the probes in the waterpool of reverse micelles. A cationic probe, Ru(bpy)2+, will be 3

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localized at the vicinity of the anionic sulfonate groups owing to the electrostatic attraction. Therefore, the Ru(bpy)2+ fluorescence reflects the 3 microenvironment near the surfactant–water interface. An anionic probe, ANS, will be localized in the core of the waterpool owing to the electrostatic repulsion with the sulfonate anionic groups of AOT, and its fluorescence reflects the microenvironment of the core of the waterpool. It is then easy to understand why the fluorescence behavior of Ru(bpy)2+ is strongly associated with the sur3 factant composition of the system, as shown in Fig. 3, while the fluorescence behavior of ANS is hardly affected by the surfactant composition at larger water content, as shown in Fig. 5. In the mixed reverse micelles with lower water content, the ANS molecules are located at the vicinity of the polar groups of surfactants owing to steric confinement, and hence the surfactant composition has some influence on its fluorescence behavior; with increases in the water content, the ANS molecules gradually leave the surfactant–water interface, and the l approaches a similar value max no matter what the surfactant composition is. It can be concluded from the above discussion that the microenvironment that can be probed in reverse micelles depends on the kind of the fluorescence probe used. Based on the different locations at which Ru(bpy)2+ and ANS probe molecules exist, it is 3 reasonable to assume that the Wp of Ru(bpy)2+ 0 3 (Wp ) corresponds to the completion of the 0, Ru hydration of the polar group of surfactant, and the Wp of ANS (Wp ) is relevant with the 0 0, ANS appearance of the free water in the waterpool of the reverse micelles. The effects of nonionic surfactant content and their EO chain length on Wp 0, Ru and Wp in Figs. 3 and 5 can be interpreted 0, ANS according to this assumption. Since it has been reported that the hydration capacity of sulfonate groups of AOT is stronger than that of the EO chain of nonionic surfactant [1,20], it follows that, in the case of the ratio of AOT to nonionic surfactants being fixed, the increase of the EO chain length of nonionic surfactants can give rise to the increase of the hydration capacity of the polar group of mixed surfactants, and hence the Wp value increases; whereas when 0

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the EO chain length of the nonionic surfactant is kept constant, the increase of its content can result in the decrease of the total hydration capacity of the polar group of the mixed surfactants and the Wp shifts to a smaller W value. When ANS is 0 0 used as fluorescence probe, the Wp value is 0, ANS hardly affected by the EO chain length of nonionic surfactant, indicating that the onset of the appearance of free water is mainly determined by the molar ratio of AOT to nonionic surfactants. It is found ( Figs. 3 and 5) that the Wp is 0, ANS smaller than Wp , especially in the case of the 0, Ru longer EO chain of nonionic surfactant, which indicates that at the appearance of free water, the hydration of the polar group of surfactant still does not complete. At lower water content, or W Wp , thereafter the 0 0, Ru hydration of the polar group of surfactant is complete, and all the added water is in the free state. From the Ru(bpy)2+ and ANS fluorescence 3 analysis, the following conclusion can be reached. When the molar ratio of AOT to nonionic surfactant is fixed, the EO chain length of nonionic surfactants has little effect on Wp value where free 0 water appears in mixed reverse micelles. The molar ratio of AOT to nonionic surfactant plays a major role in determining the appearance of free water, and the increase of the content of nonionic surfactant will make the free water appear at smaller W values. However, both the molar ratio of AOT 0 to nonionic surfactant and the EO chain length of the nonionic surfactant have significant effects on W where the hydration of polar group of surfac0 tant is complete. Increasing the EO chain length of the nonionic surfactant and decreasing the

molar ratio of the nonionic surfactant to AOT leads to the upwards shift of the W value. 0 Acknowledgment This project (29733110) is supported by the National Natural Science Foundation of China.

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