Langmuir 2003, 19, 6579-6581
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Photoresponsive Microemulsions Julian Eastoe,* Margarita Sanchez-Dominguez, Hannah Cumber, Gary Burnett, and Paul Wyatt School of Chemistry, University of Bristol, Bristol, BS8 1TS, United Kingdom
Richard K. Heenan ISIS-CLRC, Rutherford Appleton Laboratory, Chilton, Oxon, OX11 0QX, United Kingdom Received June 4, 2003 Photoresponsive microemulsions have been made by addition of a photodestructible surfactant, sodium 4-hexylphenylazosulfonate (1), to AOT-stabilized water-in-heptane phases. Exposure of these samples to UV light led to changes in phase stability and droplet size, owing to breakdown of 1. Small-angle neutron scattering was used to follow this UV-induced shrinkage of the water nanodroplets: a maximum volume decrease was found to be in the order of 60%. Proton NMR of an UV-irradiated microemulsion shows that around 90% of the added 1 decomposes, to yield a mixture of 4-hexylphenol (∼37%) and hexylbenzene (52%).
Introduction A significant limitation for applications of microemulsions in areas such as organic synthesis, pharmaceuticals, and nanotechnology is the difficulty of isolating products from the highly stable microemulsion. Several methods have been used to affect phase separation of microemulsions: pervaporation,1 addition of electrolytes,2-5 addition of solvent,6 ultrafiltration,3 low or high temperature, pH change, and saturation with water.5 In this communication, we report a novel approach to trigger microemulsion destabilization: if a fraction of the photodestructible surfactant 1 (Scheme 1) is introduced in a typical microemulsion formulation (heptane/AOT/water), its breakdown can be induced by the use of UV light. This kind of photosurfactant has been used previously in emulsion polymerization.7 In related water-in-oil (w/o) microemulsions, isothermal UV-triggered percolation has been induced by means of trace levels of light-sensitive solubilizates.8,9 On the basis of current literature, the present study represents the first example of photoinduced breakdown of a microemulsion. Azosulfonates are photodestructible since they react with UV light in an irreversible way. Scheme 1 shows possible photolysis pathways for the surfactant sodium 4-hexylphenylazosulfonate 1. It has been proposed10 that the photoproduct yield ratio 2:3 depends on the microen* To whom correspondence should be addressed. E-mail:
[email protected]. Tel: + 44 117 9289180. Fax: + 44 117 9250612. (1) Aouak, T.; Moulay, S.; Hadj-Ziane, A. J. Membr. Sci. 2000, 173, 149. (2) Keiser, B. A.; Varie, D.; Barden, R. E.; Holt, S. L. J. Phys. Chem. 1979, 83, 1276. (3) Yang, C. Z.; Hadjiev, D.; Aurelle, Y.; Cotteret, J. Recents Prog. Genie Procedes 1992, 6, 407. (4) Desnoyers, J. E.; Quirion, F.; Hetu, D.; Perron, G. Can. J. Chem. Eng. 1983, 61, 672. (5) Hazbun, E. A.; Schon, S. G. U.S. Patent Application US4770670 (1988). (6) Ingelsten, H. H.; Beziat, J. C.; Bergkvist, K.; Palmqvist, A.; Skoglundh, M.; Hu, Q.; Falk, L. K. L.; Holmberg, K. Langmuir 2002, 18, 1811. (7) (a) Mezger, T.; Nuyken, O.; Meindl, K.; Wokaun, A. Prog. Org. Coat. 1996, 29, 147. (b) Dunkin, I. R.; Gittinger, A.; Sherrington, D. C.; Wittaker, P. J. Chem. Soc., Perkin Trans. 2 1996, 1837. (8) Wollf, T.; Hegewald, H. Colloids Surf., A 2000, 164, 279. (9) Ness, D.; Cichos, U.; Wolff, T. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 1372.
Scheme 1. Photolysis of Sodium 4-Hexylphenylazosulfonate
vironment: in a bulk aqueous phase below the critical micelle concentration (cmc) the ionic mechanism would yield photoproduct 2 (4-hexylphenol), whereas in hydrophobic environments such as micelles, or in the present case w/o droplets, photoproduct 3 (hexylbenzene) would be favored. It is expected that surface activity, and hence the ability to reduce interfacial tension between water and oil, would be modified after irradiation, since the headgroup of the surfactant is destroyed. The concept is that the double-chain AOT favors negative curvature, whereas single-chain 1 promotes positive curvature: the mean curvature of the stabilizing layer may be increased by mixing 1 with AOT and then reduced again by photolyzing the azosulfonate component. Furthermore, for surfactant mixtures it is well-known that composition is a key factor controlling stability of microemulsions [e.g., ref 11]: if part of the surfactant were destroyed, the sample would be located in a different region of the phase diagram thereby inducing phase separation. With well-defined mixtures of like-charged single-chain (1) and double-chain (AOT) surfactants, the mole ratio X12 defines composition, and as documented elsewhere11,12 this may be used as a (10) Dunkin, I. R.; Gittinger, A.; Sherrington, D. C.; Wittaker, P. J. Chem. Soc., Perkin Trans. 2 1996, 1837. (11) Bumajdad, A.; Eastoe, J.; Griffiths, P.; Steytler, D. C.; Heenan, R. K.; Lu, J. R.; Timmins, P. Langmuir 1999, 15, 5271. (12) X(%) ) 100{[1]/([AOT} + [1])} therefore giving the single-chain surfactant level in terms of mole % of the total surfactant present. Hence, for a fixed total [AOT] + [1] ) 0.10 mol dm-3, as used here, increasing X represents replacement of the double-chain AOT (favors negative curvature) with the single-chain 1 (favors positive curvature). See ref 11.
10.1021/la0349830 CCC: $25.00 © 2003 American Chemical Society Published on Web 07/02/2003
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control variable for phase behavior and interfacial curvature. On this basis, several water-in-oil microemulsions containing from X ) 5% to 10% of 1 of a total surfactant concentration of 0.10 mol dm-3 were prepared. Experimental Section Photosurfactant 1 (sodium 4-hexylphenylazosulfonate) was synthesized as described elsewhere:7 mp, 159-162 °C; literature, 155-157 °C.7 Elemental analysis: found C 49.51, H 5.68, N 9.60, S 10.71%; requires C12H17N2 O3SNa C 49.30, H 5.90, N 9.6, S 11.00%. δH (CD3OD): 7.84 (2H, d, ArH), 7.40 (2H, d, ArH), 2.72 (2H, t, ArCH2CH2), 1.67 (2H, m, ArCH2CH2), 1.36 (6H, m, 3×CH2), and 0.90 (3H, t, CH3). νmax/cm-1 (KBr): 3054w, 2959m, 2927s, 2872m, 2854s, 1605w, 1513m, 1466m, 1419w, 1284m, 1235s, 1159m, 1067s, 1016w, 954w, 897w, 846m, 833m, 760m, 722w, and 648s. The Krafft temperature (Tk ) 25 °C) was determined by electrical conductivity, and the cmc by surface tension using a Kruss K10 Du Nouy Ring instrument (cmc at 30 °C ) 2.25 × 10-2 mol dm-3). These measured properties compare favorably with literature values.7 The water-in-oil microemulsions were prepared containing AOT sodium salt (Sigma, 99% min, recrystallized from CHCl3 and purified over activated charcoal before use), n-heptane (Lancaster, 99% min), D2O (Fluorochem, 99.9%), and photosurfactant 1. There is hence a mixture of 1 and AOT, with a composition defined by X(%) ) 100{[1]/([AOT] + [1])}; the total concentration of surfactant {[1] + [AOT]} was 0.10 mol dm-3. The mole ratio X was kept below 10% so that the microemulsions prior to irradiation were all single phases at 40 °C. The initial dispersion ratio prior to UV-irradiation w ([water]/[surfactant]) was kept constant at w ) 40. Microemulsions were irradiated for 120 min in quartz cuvettes (1 cm path length) with a 100 W high-pressure Hg lamp (LPS220/250 power supply from PTI; lamp housing from HI-TECH Scientific and Mercury short arc photo optic lamp HBO 103 W/2), which was polychromatic to maximize the photon flux. Samples were thermostated so that the initial nonirradiated microemulsions were single phases. The reactions were monitored by UVvis spectroscopy. The identity and ratio of photoproducts 2:3 were confirmed by 1H NMR (JEOL delta 270) with samples containing deuterated solvents and standards, which were the n-octyl analogues of 2 and 3 (Aldrich). In the high-δ region of interest, the aromatic proton signals from these standards are expected to be indistinguishable from those for n-hexyl chain decay products 2 and 3. Small-angle neutron scattering (SANS) experiments were carried out on the time-of-flight LOQ instrument at ISIS, U.K., as previously reported [e.g., ref 11]. D2O water droplets were contrasted against the h-heptane and h-surfactants. To ensure all samples were initially well inside the single-phase region, SANS experiments were carried out at an appropriate temperature (40 °C for X ) 0, 5, and 7.5%, but 25 °C with X ) 10%). SANS data were analyzed using the interactive program FISH.13 The model employed here was Schultz polydisperse spheres with an attractive structure factor to account for weak interactions. The polydispersity distribution width was fixed at 0.20, which is characteristic of w/o microemulsions stabilized by AOT.14 The Porod approximation was also employed. Both of these models are described elsewhere.15
Results (a) Sample Appearance and Droplet Size. Figure 1 shows the destabilization seen for these systems upon irradiation with UV light: initially, a clear and monophasic dispersion was formed; after around 10 min of irradiation cloudiness appeared (b) and eventually an aqueous phase with some emulsified oil phase-separated, as shown in the bottom part of the cell in Figure 1c. (13) Heenan, R. K. FISH Data Analysis Program; Rutherford Appleton Laboratory; Report RAL-89-129; CCLRC: Didcot, U.K., 1989. (14) Nave, S.; Eastoe, J.; Heenan, R. K.; Steytler, D.; Grillo, I. Langmuir 2000, 16, 8741. (15) Zemb, T.; Lindner, P. Neutron, X-Ray and Light Scattering: Introduction to an Investigative Tool for Colloidal and Polymeric Systems; Elsevier Science: New York, 1991; p 19.
Figure 1. Photoinduced destabilization of AOT/water/heptane microemulsions containing surfactant 1 at X ) 7.5%. Photographs were taken at 25 °C, where sample a was initially single phase.
Figure 2. SANS data for a w/o microemulsion containing X ) 10% of photosurfactant 1 before (b) and after (O) irradiation; plot inset showing Porod analysis, using the same symbology. Table 1. Fitted Droplet Core Radius ((2 Å) as a Function of X% 1, before and after Irradiation X% 1
core radius, before irradiation/Å
core radius, after irradiation/Å
% volume shrinkage
0 5 7.5 10
51.2 54.8 52.4 50.2
51.1 39.9 38.4 35.8
61 61 63.7
On the basis of Figure 1, the nanodroplet cores should shrink with irradiation; this was confirmed by SANS. Effects of photosurfactant 1 concentration and UV on droplet size are summarized in Table 1. The droplet core becomes smaller as the concentration of 1 is increased, whereas the relative effect of droplet shrinkage after irradiation remains fairly constant. Figure 2 shows data for a sample with X ) 10%: the scattering intensity decreases after irradiation as expected for a reduction in droplet size. The fitted lines correspond to mean core radii of 50.2 and 35.8 Å for nonirradiated and irradiated samples, respectively. Analysis by the Porod approximation (Figure 2 inset) shows a shift in Qmax to higher Q and was in reasonable agreement with these values, giving 49.1 Å (nonirradiated) and 41.5 Å (irradiated) for the D2O droplet radius. The reduction in intensity for Q > 0.10 Å-1 is consistent with a lower total surface area,14 which would be expected if surfactant 1 and water were eliminated from the dispersion. The fact that there is a similar effect for a sample containing 5% as for the one with 10% of 1 can be explained in terms of a different cloud point: these concepts are enlarged on in ref 11. (b) Photolysis Pathway. To achieve a better understanding of the breakdown mechanism, separate microemulsions were made with deuterated solvents containing pure surfactant 1, 4-octylphenol (for 2), and octylbenzene
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further irradiation, suggesting an equilibrium composition has been reached. Therefore, in the hydrophobic reverse micellar or w/o environment, a radical mechanism in Scheme 1 may dominate. Since almost 90% of 1 present has been destroyed, the effective w value changes from 40 to 44, inducing phase separation of part of the water. In addition, hexylbenzene is adding to the oil phase; this is equivalent to destroying surfactant but adding oil. However, the phenol is also surface active and is likely to coadsorb, and further experiments will clarify the role of this photoproduct.
Figure 3. Partial 1H NMR spectra of microemulsions containing (a) photosurfactant 1, (b) 4-octylphenol (analogue of 2), (c) octylbenzene (similar to 3), and (d) an irradiated microemulsion with 1 at X ) 7.5%.
(analogue of 3), all at a doping level equivalent to X ) 7.5%. Figure 3 shows 1H NMR spectra in the high-δ aromatic region for these microemulsions. Analysis of the spectrum from the postirradiated sample, which is Figure 3d, indicates 52.4% hexylbenzene 3, 37.0% hexylphenol 2, and 10.6% of unreacted 1. This spectrum was obtained after 120 min of UV exposure and did not change after
Summary A novel approach to microemulsion destabilization has been presented. The use of UV light to induce water separation represents a straightforward method, as there is no need for addition of extra components or modification of the conditions, which in real life systems could produce undesirable effects. In addition, by regulating the intensity of the light used it could be possible to control the rate of the microemulsion destabilization as required. It is hoped that this concept can be applied to areas such as the recovery of nanoparticles from microemulsions. Acknowledgment. M. S.-D. is grateful to the Mexican organization CONACYT (National Council of Science and Technology, Grant No. 151737) for a scholarship. We also acknowledge CLRC for allocation of beam time at ISIS and grants toward consumables and travel. LA0349830
