Light-Responsive Hydrophobic Association of Azobenzene-Modified

In solutions of amphiphilic molecules or polymers containing suitable chromophores, exposure to light can be used to achieve photoresponses, such as ...
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Langmuir 2007, 23, 94-104

Light-Responsive Hydrophobic Association of Azobenzene-Modified Poly(acrylic acid) with Neutral Surfactants† S. Khoukh,‡ R. Oda,§ Th. Labrot,§ P. Perrin,‡ and C. Tribet*,‡ Laboratoire de Physico-chimie des Polyme` res et des Milieux Disperse´ s, UniVersite´ Pierre et Marie Curie, CNRS UMR 7615, 10 rue Vauquelin, F-75005 Paris, France, and Laboratoire Mole´ cules, Biomole´ cules et Objets Supramole´ culaires, Institut Europe´ en de Chimie et Biologie (IECB), 2 rue Robert Escarpit, 33607 Pessac Cedex, France ReceiVed June 14, 2006. In Final Form: August 3, 2006 Photoresponsive association between azobenzene-modified poly(acrylic acid)s (AMPs) and the nonionic surfactants tetraethylene glycol monododecyl ether and octadecyl ether (C12E4 and C18E4) has been achieved in dilute aqueous solution. The binding was investigated by (i) spectrophotometry that probes the polarity close to the azobenzene chromophore, (ii) capillary electrophoresis to obtain the amount of C12E4 bound per polymer chain, and (iii) pressurearea curves of Langmuir films to obtain information on the adsorption of AMP at the water-C18E4 interface. Increasing hydrophobicity of AMP (with increasing degree of modification with azobenzene side-groups) tightened the association with C12E4 in the dark. Exposure to UV light rapidly converted the azobenzene to their more polar cis isomer, which in turn weakened the association with surfactant. Almost complete photorelease of bound C12E4 was obtained with the optimal structure of AMP. Adsorption on large interfaces is much less sensitive to light. The possible origin of the photoresponse is analyzed in terms of AMP affinity for surfactant assemblies and azobenzene penetration in the hydrophobic core of micelles. We propose that the photoswing of polarity is amplified by the binding to small micelles because of the small number of anchors involved. A few azobenzene anchors afford tight binding in the dark, but also detach more easily than the whole AMP chain upon photoisomerization.

Introduction In solutions of amphiphilic molecules or polymers containing suitable chromophores, exposure to light can be used to achieve photoresponses, such as precipitation, aggregation, and selfassembly.1-3 At the molecular scale, light typically causes the isomerization or the lysis of the chromophore, which in turn triggers mesoscopic up to macroscopic changes. The macroscopic properties triggered by light reflect changes in the polarity, hydrophobicity, and/or conformation of the chromophores. In the field of surfactants, the molecular swings between their conformations affect not only the polarity but also the shape of the amphiphiles, which controls the self-assembly and adsorption onto interfaces.4,5 In the field of polymers, the polarity of chromophore side-groups acts on monomer-monomer interactions, that is, on the average quality of a solvent.2,6 Reversible photoisomerization can be obtained with azobenzene derivatives, including in water, to achieve highly targeted cycles of material properties. The practical requirement of the transparency of samples limits the amount of chromophores in systems. Ideally, their concentration should be submillimolar to maintain a sample absorbance † Part of the Stimuli-Responsive Materials: Polymers, Colloids, and Multicomponent Systems special issue. * Corresponding author. E-mail: [email protected]; phone: (33)1 40 79 47 45; fax: (33)1 40 79 46 40. ‡ Universite ´ Pierre et Marie Curie and CNRS. § Institut Europe ´ en de Chimie et Biologie (IECB).

(1) Eastoe, J.; Vesperinas, A. Soft Matter 2005, 1, 338-347. (2) Irie, M. AdV. Polym. Sci. 1993, 110, 49-65. (3) Feringa, B. L.; Van Delden, R. A.; Koumura, N.; Geertsema, E. L. Chem. ReV. 2000, 100, 1789-1816 (4) Eastoe, J.; Dominguez, M. S.; Wyatt, P.; Beeby, A.; Heenan, R. K. Langmuir 2002, 18, 7837-7844. (5) Shang, T. G.; Smith, K. A.; Hatton, T. A. Langmuir 2003, 19, 1076410773. (6) Mamada, A.; Tanaka, T.; Kungwatchakun, D.; Irie M. Macromolecules 1990, 23, 1517-1519.

below 1-2 OD units and to afford rapid responses. A tremendous sensitivity to molecular switches is therefore required to achieve workable macroscopic responses. Most photoresponsive solutions of polymers or surfactants developed to date cannot accordingly be suitable for practical use that would require a typical concentration of several weight percent of polymer or surfactant additive with a concentration of photochrome less than 1 mM.1,2,4-8 The molar fraction of photochrome-bearing molecules in current surfactant- and polymer-based systems remains relatively high, typically above 10 mol %, which hampers the development of applications. Here, we propose the consideration of a new mode of response, based on the recognition-like binding of micelles into hydrophobically modified polymer chains. Ideally, polymer/surfactant complexes can typically involve one polymer chain and one micelle containing 100-200 surfactants. If a step of amplification of the photoresponse critically depends on the formation of such complexes, the corresponding amount of photochrome falls down to a few groups per polymer chain, a composition that would be compatible with the above constraints of applications. Hydrophobically modified polymers (HMs) containing alkyl side-chains on a hydrophilic backbone exhibit affinity toward surfactants, as they offer attraction between polymer-linked hydrophobes and the hydrophobic core of micelles.9,10 Increasing the density of hydrophobes enhances binding. Mixed micelles are formed with typically 1-10 hydrophobe side-groups of HMs entering in a micelle.11,12 Accordingly, a few transient-links should markedly affect the (7) Howley, C.; Marangoni, D. G.; Kwak, J. C. T. Colloid Polym. Sci. 1997, 275, 760-768. (8) Lee, C. T.; Smith, K. A.; Hatton, T. A. Macromolecules 2004, 37, 53975405. (9) Iliopoulos, I. Curr. Opin. Colloid Interface Sci. 1998, 493-498. (10) Winnik, F. M.; Regismond, S. T. A. Colloids Surf., A 1996, 118, 1-39. (11) Piculell, L.; Guillemet, F.; Thuresson, K.; Shubin, V.; Ericsson, O. AdV. Colloid Interface Sci. 1996, 63, 1-21. (12) Winnik, F. M.; Ringsdorf, H.; Venzmer, J. Langmuir 1991, 7, 905-911.

10.1021/la061714b CCC: $37.00 © 2007 American Chemical Society Published on Web 09/27/2006

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Scheme 1. Structures of the Azobenzene-Modified AMPs: Polymers with a Weight-Average Molecular Weight of 370 000 g/mol, Randomly Grafted with Azobenzene Chromophores (τ ) 0.6, 1, 2, 3.3, and 6.7 mol %)a

a The stable apolar isomer (trans form) can be reversibly converted into the polar cis isomer by exposure to UV light. The stationary cis/trans ratio only depends on the wavelength (∼4:1 at 365 nm; ∼1:4 at 436 nm).

affinity of HMs for micelles: isomerization of a few side-groups would trigger the binding/release of micelles on HM chains. We used azobenzene-modified poly(acrylic acid)s (AMPs) as model HMs that can exhibit light-responsive polarity of their hydrophobic side-groups (Scheme 1).13 We report here an analysis of AMPs binding in dilute solutions onto nonionic surfactants. Focusing the investigation on a dilute regime simplified the analysis of the origin of response to light because binding mainly occurs on isolated AMP chains. Photoresponses can, however, be achieved in similar systems at higher concentrations, specifically in the semidilute regime, with additional complexity due to entanglements and multichain association.14 We resort to complementary techniques to obtain data on the association at different scales: (i) binding of an azobenzene group to surfactant molecules, (ii) binding of micelles into AMPs, and (iii) binding of an AMP macromolecule at the surfactant-water interface. Namely, we made use of spectrophotometry, capillary electrophoresis, and the Langmuir balance technique. UV-visible spectra provided a sensitive signature of azobenzene binding in an apolar environment. Electrophoresis gave access to the separation of bound micelles from unbound ones. Langmuir balance allowed us to investigate AMP binding on a flat layer of surfactants. The samples were exposed to UV and blue light to investigate the effect of cis-trans isomerization of the azo dye. They comprised a neutral surfactant n-dodecyl-tetraethylene glycol (C12E4) or n-octadecyl-tetraethylene glycol (C18E4) and AMPs, that is, poly(acrylic acid)s of varying degree of (random) modification with phenylazophenylamido-alkylamine (different hydrophobic spacers between the backbone and the azobenzene; cf. Scheme 1). (13) Zimmerman, G.; Chow, L.; Paik, U. J. Am. Chem. Soc. 1958, 80, 35283531. (14) Pouliquen, G.; Porcar, I.; Tribet, C.; Amiel, C. Polymer Gels: Fundamentals and Applications; Bohidar, H., Dubin, P., Osada, Y., Eds.; ACS Advances in Chemistry Series; American Chemical Society: Washington, DC, 2001; Vol. 833, Chapter 18, pp 262-289.

This investigation focused on enhancing the current understanding of photochrome-containing macrocomplexes between HMs and surfactants. Achieving a high magnitude of photoresponse would add to the current wide range of applications of amphiphilic polymer/surfactants (gels, emulsions, etc.) the possibility of being triggered by light. Efficient and rapid responses to light would accordingly find an increasing number of applications in pharmaceutics, cosmetics, and agriculture. Experimental Section Materials. Nonionic tetraethylene glycol monododecyl ether surfactant (C12E4) was purchased from Nikkol (Japan). C18E4 surfactant was obtained from Fluka, and NaCl was purchased from Aldrich. The synthesis and characterization of the AMPs is described in ref 15. Briefly, random copolymers were obtained from a poly(acrylic acid) parent chain (Polysciences, Inc., Warrington) by the quantitative coupling of azobenzene-containing amines in N-methyl-pyrrolidone. The poly(acrylic acid) backbone has a weightaverage molecular weight of 370 000 g/mol and a polydispersity index of about 6. The structure of AMP is shown in Scheme 1. Their degrees of modification were determined by (1) H1 NMR spectroscopy and (2) UV-visible spectrophotometry using the absorption coefficient 2.32 × 104 L/mol/cm for the azo dye.15 The polymers are noted τC6azo, with τ being the corresponding degree of modification (round values of τ were consistent with a typical experimental uncertainty of 5-10%). Solutions of AMPs and C12E4 were prepared by stirring in the dark for 24 h in water (Milli-Q water, Millipore) and possibly 20-300 mM NaCl. UV-Visible Spectrophotometry. Absorbance measurements were carried out at 20 °C, with a UV-visible Hewlett-Packard 8453 spectrophotometer in a wavelength range between 200 and 700 nm, using a double-compartment quartz cell. The first part of the cell was filled with a 300 mM NaCl aqueous solution or 20 mM boric acid-NaOH buffer, pH 9.2. The second compartment contained the same volume of an aqueous solution of C12E4 in the same buffer or (15) Pouliquen, G.; Tribet, C. Macromolecules 2006, 39, 373-383.

96 Langmuir, Vol. 23, No. 1, 2007 NaCl solution. UV-absorption spectra were recorded through the two compartments, before and after their mixing. Prior to measurements, the samples were kept in the dark for 24 h. During the experiments, the samples were vertically and continuously irradiated at 436 nm with a 250 W mercury arc lamp (Oriel) equipped with the appropriate interference filter (band-pass ( 10 nm) to maintain a stationary fraction of 80% of the trans isomer of azobenzene, irrespective of perturbation by the beam of the spectrophotometer. The differential absorption spectra of the samples were obtained by subtracting the spectrum of polymer with no C12E4 to the spectrum in the presence of C12E4. Frontal Analysis Continuous Capillary Electrophoresis. Experiments were carried out with a Beckman P/ACE system MDQ instrument equipped with a diode array multiwavelengths UVvisible detector (Beckman Instruments Fullerton, CA) operating at 20 kV and 20 °C and fitted with bare silica capillaries of 75 µm × 60 cm; effective length: 50 cm (Chromoptic, France). The capillary was flushed daily with 0.1 M NaOH, followed by a water rinse, and finally was allowed to equilibrate with the running buffer (20 mM boric acid-NaOH, pH 9.2). Sample solutions were prepared by mixing a C12E4 stock solution (20 g/L, 55 mM) containing 1 mM pyrene as the micelle marker, with a polymer stock solution (3 g/L) in the running buffer. The C12E4 concentration range was 0.1-0.5 g/L, and the polymer concentration range was 0.1-0.4 g/L. The solutions were kept at room temperature for at least 2 h prior to measurements. Frontal analysis 16 was carried out using the continuous electrokinetic injection mode against the running buffer, with a positive voltage and constant pressure (0.3 Psi) applied to the inlet. The first analysis was done on a dark-adapted sample, which was after the run exposed to 365 nm UV light for 15 min (1 mW/cm2) before being reanalyzed immediately after irradiation. The concentration of free C12E4 was determined from absorbance in the electropherograms, using a calibration curve constructed with samples of known concentrations of C12E4 with no polymer. Langmuir Monolayer Experiments. Monolayer experiments at the air-water interface were carried out with a computer controlled Langmuir trough (Nima, Coventry, 70 × 5 cm). The C18E4 amphiphile formed a stable monolayer after spreading from a dilute chloroform solution onto a water subphase (NaCl 0.3 M with or without polymer). It was slowly compressed by a moving barrier, with the compression speed of the barrier set to 5 cm2/min. The pressure was measured using a filter paper Wilhelmy plate. To irradiate the whole film on the water surface, a laboratory-built multiband UV 365/436 nm lamp (4 × 8 W) was fixed at a 10 cm distance over the film balance. The irradiation time was fixed to 30 min, and the light intensity at the air-water interface measured by a photodiode was about 6 mW/cm2 at 365 nm. For the spreading solution, 1 mg of C18E4 was dissolved in 10 mL of CHCl3. Diluted polymer solutions (in NaCl 0.3 M) were prepared and kept in the dark for 24 h before experiments.

Results Isotherms of C12E4 Binding by Capillary Electrophoresis. The binding of C12E4 micelles into AMPs was investigated by capillary electrophoresis, which provides a simple means to separate neutral (unbound) micelles from charged (anionic) complexes. Under conditions of continuous electrokinetic injection,16 the formation of a sharp zone of free C12E4, dragged by the injection flow, forms before the zone of polymer/surfactant complexes, the motion of which is retarded by the counter-flow electrophoretic mobility. Because of the small volume of the zone of pure C12E4 (nanoliters), the composition of the sample (1 mL) is not affected, and binding equilibrium is not modified. From the absorbance of pyrene in each zone, and assuming equal concentration of pyrene in bound and free micelles, we determined the binding isotherms represented in Figures 1 and 2. Actually, (16) Gao, J. Y.; Dubin, P. L.; Muhoberac, B. B. Anal. Chem. 1997, 69, 29452951.

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Figure 1. Binding isotherms in the dark of C12E4 into 2C6azo polymer obtained from capillary electrophoresis. Samples contained 0.1-0.5 g/L C12E4 in 20 mM boric acid-NaOH buffer, pH 9.2 and polymer 2C6azo at: 0.1 g/L (white triangles); 0.2 g/L (gray triangles); and 0.4 g/L (black triangles). Repeatability of measurements are on the order of the size of the symbols.

the micelles were found to bear a weak negative charge that does not interfere with separation (not shown). We begin in the following with binding data obtained with dark-adapted samples and azobenzene molecules in their alltrans conformation. In all cases, the amount of polymer-bound C12E4 increases almost linearly with [C12E4]free above its critical micelle concentration (CMC; 0.02 g/L). Although the method of detection cannot be applied for [C12E4]free below the CMC (insolubility of pyrene), the isotherms show that the sharper increase of the amount of bound surfactant obviously occurs at the lowest C12E4 concentrations, supporting a tight initial association followed by anticooperative binding. In the experimental window, the bound C12E4-polymer does not depend on polymer concentration (Figures 1 and 2A), which is consistent with binding on isolated chains and the absence of contribution of multipolymer complexes. The surfactant-polymer reaches values exceeding 4 g/g C12E4-3.3C6azo, and 3 g/g C12E42C6azo at the highest [C12E4]free, which corresponds to molar amounts beyond 35-40 molecules of bound C12E4 per azobenzene on average. Part of the azobenzene groups may remain unbound, and the actual binding ratio of [C12E4]bound/[azobenzene]bound is likely to be above 40 mol/mol. To consider the effect of the degree of modification of AMPs, we can compare the bound surfactant under two extreme conditions. First, the amount of bound surfactant at a fixed 0.1 g/L [C12E4]free reflects binding above the CMC at the same fixed activity of the surfactant. Second, linear extrapolation of the binding density down to 0.02 g/L [C12E4]free gives an indication of the stoichiometry corresponding to tight complexes formed in equilibrium with the surfactant at the onset of free micelle formation. Both values reported in Table 1 show that the weight of bound surfactant per chain increases markedly upon increasing the density of the azobenzene side-groups. At 0.1 g/L [C12E4]free, the bound C12E4-azobenzene values formulated in molar amounts increase from ∼10 mol/mol to beyond 40 mol/mol with increasing hydrophobicity of the AMPs from 0.6 to 3.3 mol % C6azo. At the CMC, we obtained much lower values and weaker variations of these molar ratios: 6-12 mol/mol with a typical uncertainty of 2 mol/mol. It is now recognized that, at low concentrations, hydrophobically modified polymer/surfactant complexes comprised bound micelles, in which the aggregation number is close

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Figure 2. Effect of polymer structure and UV irradiation (365 nm) on the binding isotherms of C12E4 into AMPs in 20 mM boric acid-NaOH, pH 9.2. (A) Binding to 1C6azo in the dark (closed symbols) and after exposure to UV light (open symbols) at polymer concentrations of 0.2 g/L (black diamonds) or 0.4 g/L (gray diamonds). (B) Binding to 1C6azo (0.2 g/L) with the addition of 20 mM NaCl in the buffer in the dark ([) or after UV exposure (]). (C) Binding to 2C6azo (0.2 g/L) in the dark (gray triangle) or after UV exposure (white triangle). (D) Binding to 3.3C6azo (0.1 g/L) in the dark (9) and after UV irradiation (0). Table 1. C12E4-AMP Values (g/g) Obtained from Electrophoresisa

AMP

dark-adapted at 0.1 g/L C12E4 free

dark-adapted extrapolated to CMC

UV-adapted at 0.1 g/L C12E4 free

UV-adapted extrapolated to CMC

0.6C6azo 1C6azo 1C6azo in 20 mM NaCl 1C6azo in 100 mM NaCl 2C6azo 3.3C6azo

0.25 ( 0.05 0.3 ( 0.05 0.55 ( 0.05 0.9 ( 0.1 2.1 ( 0.1 >5

0.12 ( 0.02 0.22 ( 0.02 n.d.b 0.25 1.0 ( 0.1 ∼1 - 3 g/g

∼0.05c ∼0.05c ∼0.05c ∼0.05c 0.2 ( 0.05 2.5 ( 0.3

∼0.05c ∼0.05c n.d. n.d. 0.1 ( 0.05 0.8 ( 0.2

a Ratio of the bound surfactant to the total polymer concentrations in equilibrium with C12E4 free at 0.1 g/L or 0.02 g/L (CMC). Values were extrapolated from the isotherms determined by capillary electrophoresis, with samples maintained either in the dark or pre-exposed to UV light (365 nm) to reach their stationary state. b n.d.: above 100 mM, the complexes between 1C6azo and C12E4 were not soluble. c On the order of the uncertainty in measurements

to or below the corresponding value for free micelles.9,10,11,17 From the molar ratio given above, the smallest micelles typically formed by CnEm surfactants (i.e., aggregation number of 100 surfactants18) would accordingly interact with 8-16 azobenzene chromophores on average. The binding stoichiometry falls down (17) Mizusakia, M.; Yusa, S.; Kawanishia, S.; Morishima, Y. Polymer 2002, 43, 5865-5871. Noda, T.; Hashidzume, A.; Morishima, Y. Macromolecules 2001, 34, 1308-1317.

to 2-10 azobenzenes per 100 surfactant molecules upon increasing the free C12E4 concentration. Micelles of C12E4, even at low surfactant concentrations and a temperature of 20 °C, are thought to be elongated and show a transition toward oblate (18) Vasilescu, M.; Caragheorgheopol, A.; Caldararu, H. AdV. Colloid Interface Sci. 2001, 89, 169-194. Nagarajan, R. Surface Science Series: Structure Performance Relationships in Surfactants; Ueno, M., Ed.; M. Dekker, Inc.: New York, 1997; Vol. 70, Chapter 1, pp 44-57.

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assemblies above 20 °C.18 The actual number of azobenzenes per C12E4 micelle will be more extensive than 2-10. In the whole range of experimental concentrations, exposure to low intensity UV light (365 nm, ∼1 mW/cm2) triggered within less than 2 min the formation of predominantly cis isomers of the azobenzene side-groups (Scheme 1, data not shown).19 Photoisomerization markedly affected the binding isotherms, as shown in Figure 2. Most typically in the experimental window, an important fraction of bound C12E4 was released upon irradiation, irrespective of the AMP sample. An optimal structure of the AMPs exists, however, in terms of the magnitude of the effect of light. The degree of binding in the dark into AMPs with the lower azobenzene grafting degree (0.6 mol %) does not differ by more than a factor of 2 from binding under UV, essentially because a small amount of micelles is bound, even in darkadapted samples. A similar factor lower than 2 (and even close to 1 at the CMC) is found for relative binding in the dark and under UV to 3.3C6azo (Figure 2D). In contrast, the latter case corresponds to a permanently high binding, and C12E4-AMP is higher than 1-2 g/g even under UV exposure. At an intermediate grafting density (1C6azo, 2C6azo), the magnitude of binding can be high in the dark and low under UV, with typical variation by a factor above 10 upon irradiation. At the optimal composition of 1C6azo (Table 1), selective binding in the dark and almost complete release under UV persists despite a marked enhancement in the dark-adapted affinity with increasing salt concentration. Therefore, obtaining light-triggered affinity appears to be essentially controlled by the composition of AMPs and an optimal density of the chromophore on the order of 1 mol %. It is important to note that complexes that were dissociated by exposure to UV reformed upon irradiation with blue light, or day-long incubation in the dark. Index of Azobenzene Association by Spectrophotometry. The isotherms obtained by electrophoresis give the average number of bound surfactants per chain, irrespective of the fraction of bound chromophore. Part of the azobenzene in a chain can remain in contact with water or plunge more or less deeply in a micellar environment. To investigate the actual binding degree of azobenzene, we used their UV-visible absorption as a good reporter of their solvation state. As represented in Figure 3A, the main absorption band of azobenzene chromophores shows a marked red-shift and some increase in intensity (hyperchrome effect) upon transfer from water into a less polar solvent 20 that could be compared to the interior of a micelle. Red-shift and increase of the peak were similarly obtained upon supplementation of a solution of AMP with C12E4. Accordingly, we interpret the spectral distortions as the signature of the partition of a fraction of polymer side-groups into micelles. Figure 3B gives the corresponding differential spectra of samples with and without C12E4, at fixed AMP concentration. The presence of isosbestic points, at ∼400 and 347 nm respectively, is consistent with two types of environment around the azo-dyes. The solvation of the chromophores in water coexists in the presence of surfactants with a second state that corresponds to a single environment detected when the azobenzene interacts with hydrophobic aggregates. Importantly, the isosbestic points and the wavelength of maximum sensitivity to surfactant (372 nm) did not depend on AMP composition (0.6C6azo-3.3C6azo). Their invariance reflects a similar environment of the bound azobenzene, irrespective of the degree of modification of AMPs. The variation of absorbance determined at the wavelength of maximum (19) Morishima, Y.; Tsuji, M.; Kamachi, M.; Hatada, K. Macromolecules 1992, 25, 4406-4410. (20) Hutchings, M. G.; Gregory, P.; Campbell, J. S.; Strong, A.; Zamy, J. P.; Lepre, A.; Mills, A. Chem.sEur. J. 1997, 3, 1719-1727.

Khoukh et al.

Figure 3. UV-visible absorption spectra of the chromophores phenylazophenyl-amidopentylamine (H2N-C6azo) in different solvents, or azobenzene-modified polymers in aqueous solutions. (A) C6azo chromophore (40 µM) in the dark in (s) pure water; (O) toluene and ([) n-dodecane. (B) 1C6azo polymer (0.6 g/L) in 300 mM NaCl and increasing C12E4 concentration under continuous irradiation at 436 nm. The difference in polymer spectra in the presence and in the absence of C12E4 is plotted vs wavelength. Arrows indicate the isosbestic points.

sensitivity to surfactant (i.e., 372 nm) provides an index of azobenzene binding (ζ ) ∆OD/[azobenzene]). In Figure 4A, ζ shows a rapid, and almost linear increase with increasing [C12E4], and reaches a plateau at high concentrations of surfactant. This shape is representative of all samples tested, with other examples shown in Figure 5. The plateau value ζmax depends significantly on the degree of modification of AMPs, and appears to also be sensitive to the ionic strength (Figure 4A,B). Differences in ζmax betray different solvations of the chromophore, or differences in the fraction of azobenzene that could remain unbound even in excess surfactants. An effect of salt on the strength of association was expected because of the polyelectrolyte nature of AMPs and the corresponding variation of intrachain repulsion with ionic strength. Works by several groups have shown that Coulombic effects can control the penetration of hydrophobes in micellar cores.8,10,11,21

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Figure 5. Index of transfer of azobenzene between water and micelles as a function of C12E4 concentration (cf. Figure 4) for different polymers τC6azo in 300 mM NaCl under continuous irradiation (436 nm). The total azobenzene concentration was in the range 1618 µM for (9) 3.3C6azo; (2) 2C6azo; ([) 1C6azo; (b) 0.6C6azo and (/) 6.7azo. Lines are guides to the eye. The error in measurements reaches (0.0005 µM-1 at high C12E4 concentrations due to the incipient turbidity of the samples. Table 2. Index of Binding Obtained from Spectrophotometry

0.6C6azo 1C6azo 2C6azo 3.3C6azo 6.7azo

Figure 4. Index of transfer of azobenzene between water and micelles obtained from the absorbance at 372 nm in 2C6azo solutions under permanent exposure to blue (436 nm) light, at varying NaCl concentrations. ∆OD ) (absorbance in the presence of C12E4 absorbance with no surfactant); [azo] is the concentration of azobenzene photochrome in µM units. (A) 0.06 g/L 2C6azo and various C12E4 concentrations (2) in 300 mM NaCl or (4) in 20 mM sodium borate buffer, pH 9.2. Lines are to guide the eye. (B) Same index vs square root of [NaCl] in mM, at 0.1 g/L 2C6azo in the presence of 0.03 g/L C12E4 (gray triangle) or in the presence of 0.1 g/L C12E4 (black triangle).

Because the surfactant is neutral, the effect of ionic strength on ζmax points to the high sensitivity of the formation of complexes to intra-AMP Coulombic repulsions. Intrachain repulsion controls the swelling degree of single chains. Multipoint association onto a micelle brings at small distances, on the order of the curvature of the micelle-water interface, the anionic monomers attached to bound azobenzene, which adds an energy penalty to binding. From this point of view, the Coulombic hindrance to association at low ionic strength shall decrease continuously with increasing salt until it finally vanishes when the Debye length falls below the radius of a small micelle (in the case of elongated micelles, a similar argument applies to the cross-section of micelles), that is, ∼2 nm. Experimental variations in ζ with the square root of ionic strength show an initial linear increase in Figure 4B, and, (21) Yoshida, K.; Morishima, Y.; Dubin, P. L.; Mizusaki, M. Macromolecules 1997, 30, 6208-6214.

ζmax at 20 mMb

ζmax at 300 mMc

[C12E4]tot-AMP at the onset of the plateau in 20 mM borate (g/g)d

n.d.e 0.0004 0.0012 0.001 0.0015

0.0012 ( 0.0002 0.002 ( 0.0005 0.004 ( 0.0002 0.005f 0.003 ( 0.0002

n.d. 0.4 ( 0.1 1.0 ( 0.2 1.5 ( 0.2 n.d.

a ζmax is given here in µM-1 and corresponds to the magnitude of the plateau-absorbance (at 372 nm) reached at high [C12E4] and is normalized by the concentration of azobenzene chromophore in the solution. bIn 20 mM boric-acid NaOH buffer, pH 9.2. cSame buffer and 300 mM NaCl. d Gives the weight ratio [C12E4]/[AMP] at the intercept of the plateau regime with the initial increase of ζ with increasing C12E4. en.d.: no binding detected, or ζmax always below 0.0004 µM-1. f Because of turbidity, the maximum [C12E4] considered was 0.005 g/L.

as expected, a gradual leveling off at high ionic strength (with the maximum salt concentration corresponding here to a Debye length of ∼0.6 nm). Similar enhancement of association with increasing ionic strength is also effective at low [C12E4], below the saturation plateau of ζmax. To compare the effect of AMP hydrophobicity on ζmax, we consider only the set of data obtained at 300 mM, the larger experimental ionic strength. At lower salt concentrations, the value of ζmax obtained with AMP 0.6C6azo and 1C6azo were actually close to the error of measurement and did not afford reasonable analysis, though the trend was essentially similar to that observed at 300 mM NaCl. The values of ζmax increase markedly with the density of azobenzene (Table 2). For 3.3C6azo and 2C6azo, ζmax approaches the value obtained in pure dodecane (0.0051 µM-1). Probing a polarity similar to that of dodecane points to a deep penetration of the azobenzene in the hydrophobic core of micelles. The fraction of unbound azobenzene must therefore be negligible in the case of 3.3C6azo and 2C6azo, and 300 mM NaCl. In contrast, the lower ζmax obtained with 1C6azo and 0.6C6azo can be ascribed to partial binding or association

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in a more polar environment. It is, however, surprising that the binding site of azobenzene in a micelle depends markedly on polymer composition because, at low degree of modification of AMPs, no significant change is expected for both the Coulombic intrachain repulsions and the local steric effects. In the homologous set of AMPs considered here, more than 97% of the monomers are anionic. The high charge density together with a screening length below 2 nm certainly controls intrachain repulsions, irrespective of the degree of modification of the AMPs. Similarly, steric hindrance between AMPs and micelles of C12E4 is expected to depend essentially on local structure, that is, the structure of the chromophores and possibly a few neighboring monomers that do not differ significantly between 3.3C6azo and 0.6C6azo. To interpret the variation of ζmax together with the absence of variation of the isosbestic points in difference spectra (see above), we assumed accordingly that the environment of the bound chromophores belonging to 1C6azo and 0.6C6azo was presumably similar to that probed with 3.3C6azo. The differences in ζmax would reflect in this condition that a significant fraction of the side-groups in 1C6azo and 0.6C6azo is not bound at all. The polymer modified with no spacer (6.7azo) also shows very weak binding of the chromophore and a ζmax well below 0.005 µM-1, even at high NaCl concentrations. The absence of spacer between the polymer backbone and the chromophore might render difficult a deep penetration of the chromophore in apolar regions of micelles. Both binding to the surface of micelles and the presence of unbound azobenzene can explain the low value of ζmax in the case of 6.7azo. Adsorption on a Flat Interface. To investigate the attachment of a whole AMP chain onto surfactant/water interfaces, we turned to layers of C18E4 deposited at the air-water interface. The low solubility of C18E4 in water affords excellent control of the density of molecules at the air-water interface that was not achievable with soluble C12E4. Slow compression of the layer was operated beginning at low surface density (∼300 Å2/molecule), where the surface tension was almost that of pure water. The continuous increase of surface pressure with decreasing area was indicative of the formation of a dense two-dimensional layer (Figure 6A, and ref 22). Importantly, exposure of the sample to permanent UV irradiation did not affect the compression curve in the absence of polymer (not shown). In the presence of AMP (1C6azo in Figure 6) in the water subphase, the pressure-area curves began similarly from zero pressure at high surface area, but increased more rapidly upon decreasing the area. In addition, higher initial polymer concentrations in the subphase led to higher pressures (Figure 6B). Higher pressure at a fixed area per C18E4 points to the presence of additional molecules, that is, AMP adsorbed at the interface. Compression isotherms were obtained under different exposures to light, or in the dark with light applied for 30 min before compression (it was checked by spectrometry on an aliquot of the subphase that the cis-trans isomerization was completely achieved). Within the uncertainty limit, the same isotherm was obtained at 0.08 g/L 1C6azo, irrespective of the wavelength of light. In contrast, at a polymer concentration of 0.32 g/L, the pressure was significantly higher under UV exposure than under visible light. The magnitude of the UV-triggered increase in pressure, however, remained below the increase in pressure due to the presence of AMP in water. Altogether, the results obtained show that 1C6azo chains are always adsorbed at the interface, irrespective of their degree of cis-trans isomerization, although modulation of the structure and interaction in the layer can occur at high polymer concentrations. Forming a clear view of the (22) Meier, W.; Ramsden, J. J. J. Phys. Chem. 1996, 100, 1435-1438.

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Figure 6. Pressure-area (π-A) isotherms of C18E4 at the airwater interface at 20 °C: (A) with no polymer (-) and in the presence of 0.08 g/L 1C6azo in the 300 mM NaCl subphase in the dark (O) or under UV irradiation (9); (B) with no polymer (-) and in the presence of 0.32 g/L 1C6azo in the 300 mM NaCl subphase in the dark (O), under blue irradiation (4), or under UV irradiation (9). The arrows indicate the collapse points. Compression speed of the barrier: 5 cm2/min. (C) Pressure response of the interface held at a constant area (∼130 Å2/C18E4 molecule) upon irradiation with a multiband UV lamp at 365 or 436 nm. Subphase composition of 1C6azo is 0.32 g/L in 300 mM NaCl.

PhotoresponsiVe Association of AMPs with CnEm

origin of light-triggered variation of pressure would, however, be premature because several opposite phenomena can coexist and result in a complex network of interaction at the interface.23 For instance, it is expected that the apolar trans-azobenzene enters more deeply than the cis isomer in the interface, which in turn should increase the pressure. But interactions between nonadsorbed segments of the adsorbed chains also play a role.24 The balance between Coulombic repulsion and hydrophobic attraction between chain segments protruding toward the water phase might be affected by photoisomerization. Decreasing the polarity of AMP loops exposed to blue light (or dark-adapted) leads to the expected decrease in pressure, due to the reenforcement of attraction between the loops in solution. We note in addition that the direction of photoresponse depends on the equilibration time of the layer. To test the importance of relaxation dynamics in the surfactant layer, we measured the pressure at a fixed area per C18E4 (the compression barrier is kept motionless). Slow relaxation of the layer was revealed by the decrease in pressure during ∼1 h after the immobilization of the barrier (not shown). Exposure to UV light during relaxation abruptly decreased (within a couple of minutes) the pressure, whereas blue light increased it (Figure 6C). The magnitude of the photoinduced pressure jumps was below 2 mN/m and was much smaller than the relaxation by ∼5 mN/m occurring here over 3 h. However, similar amplitudes of variation of surface tension have been reported with azobenzene containing surfactants, and attributed to tighter adsorption of apolar isomers.1,4,25 The present data show that the (rather weak) effects of light and even the direction of the pressure variations critically depend on the kinetics of adsorption and layer dynamics. The threshold area at collapse points (arrows in Figure 6) gives additional information related to the behavior at high pressure, that is, of a dense solidlike layer of surfactant. The presence of azobenzene in the layer was detected by an increase in this threshold area (from 48 to 65 Å2 per C18E4), which is consistent with the interpretation of pressure increase. In comparison with the latter approximately +20 Å2, the effect of irradiation remains weak and typically close to an uncertainty of 2-3 Å2. The lack of response to light of the collapse point suggests that cis-trans isomerization does not significantly affect the fraction of azobenzene forming tight interaction with the interface. This conclusion does not preclude the idea that the orientation and the depth of penetration of the azobenzene vary with its isomer form. However, the magnitude of photoresponse on flat interfaces clearly appears to be much weaker in comparison to the high selectivity obtained with dispersed surfactants, and some AMPs remain adsorbed even under their cis form. Consensus Model of Binding. The isotherms obtained for (i) the binding of azobenzene in a micellar environment and (ii) the binding of C12E4 in AMPs enable us to analyze the stoichiometries of association. First, the invariance of the AMP concentration of the isotherm from electrophoresis points to the formation of macrocomplexes containing a single polymer chain in dilute solutions. To estimate the amount of bound surfactant in terms of a number of micelles per chain, we need the order of magnitude of the degree of polymerization (DP) and of the aggregation number (Nagg) of bound micelles. Nagg ) 100 corresponds to the smallest value of the aggregation number of free C12Em micelles.18 Because of the formation of elongated micelles, the actual Nagg (23) Staples, E.; Tucker, I.; Penfold, J.; Warren, N.; Thomas, R. K. J. Phys.: Condens. Matter 2000, 12, 6023-6038. (24) Regismond, S. T. A.; Gracie, K. D.; Winnik, F. M.; Goddard, E. D. Langmuir 1997, 13, 5558-5562. (25) Faure, D.; Gravier, J.; Labrot, Th.; Desbat, B.; Oda, R.; Bassani, D. M. Chem. Commun. 2005, 1167-1169.

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for C12E4 is certainly larger. Bound AMPs, however, form a highly charged corona on the micelles, which favors highly curved interfaces. Consequently, binding is likely to significantly decrease the micelle size.10,11 Measurements of the hydrodynamic radii of AMP/C12E4 complexes were consistent with the micelle size being much smaller than the AMP radius of ∼20-30 nm (Supporting Information). We used the aggregation number of 100 as an order of magnitude to illustrate the typical features of the system with a commonly encountered size of micelle. The number average molecular weight of the parent poly(acrylic acid) corresponds to the degree of polymerization, DPn ) 700. In a chain of 700 monomers, the 0.6C6azo and 3.3C6azo contain ∼4 and 21 azobenzene groups respectively, which seems to be the lower limit compatible with the 8-16 photochromes per 100 surfactants measured on average (results of electrophoresis and Table 1 above). With a polydispersity index of ∼6, most of the chains actually have a DPn larger than 700, which implies that either longer chains bind several micelles of 100 surfactants, or one chain can interact with longer micelles. Accordingly, the estimate of binding stoichiometry using DPn ) 700 gives an illustration of the smallest possible structure of the macrocomplexes. The binding of longer chains to elongated micelles is not commented on for the sake of simplicity. The picture obtained nevertheless remains qualitatively valid since micelle size is smaller than the polymer coil. The sizes of the complexes did not significantly differ from those of the polymer coil (hydrodynamic radius of AMP ∼ 25 nm), as determined by dynamic light scattering, which points to bound micelles markedly smaller than 20 nm (Supporting Information). A more detailed investigation of the actual number and size of the assemblies would refine the present picture, and deserve further investigation. From the values in Table 1, the typical number of bound (small) micelles per chain in equilibrium with 0.1 g/L C12E4 varies from 0.5 to 10 upon increasing the density of C6azo from 0.6% to 3.3%. Recalling that, for the presence of chains much longer than DP ) 700, we conclude that the formation of macrocomplexes could correspond to multimicellar binding in isolated chains. The average number of chromophores per micelle varies according to the data in Tables 1 and 2. We focus this paragraph on the conditions of low ionic strength, which enables one to gather data from electrophoresis and spectrometry, and different conditions of exposure to light. In the case of predominantly trans isomers, we note first the similar range of C12E4-AMP values corresponding respectively to bound C12E4 at 0.02 g/L [C12E4]free (Table 1) and total C12E4 at the onset of saturation of spectral distortion (Table 2). Both cases point to compositions typical of tight association. The azobenzene/micelle ratio obtained at these compositions is 19-16 mol/mol for 0.6C6azo and 1C6azo, and ∼8 mol/mol for 2C6azo and 3.3C6azo. Accounting for the partial binding of azobenzene, as revealed by the low value of ζmax obtained at low salt concentrations (Table 2), the actual number of hydrophobic anchors per micelle would accordingly reach only a few units. Assuming that ζmax ) 0.0051 µM-1 at full binding, the data in Table 2 are ascribed to about 20% effective association of the photochromes for 2C6azo and 3.3C6azo. This fraction is even lower for polymers 1C6azo and 0.6C6azo. The order of magnitude of ∼2 azobenzene per (small) micelle is obtained under conditions of tight binding and a [C12E4]free close to the CMC. The increase of C12E4 bound by a factor of ∼2 at higher concentrations in the samples (Table 1) would thus reach the saturation of one azobenzene per micelle. On the other hand, ζmax may reflect partial penetration of the chromophore in micelles, and the number of chromophore in

102 Langmuir, Vol. 23, No. 1, 2007 Scheme 2. Illustration of the Proposed Mechanisms of AMP Binding onto a Micelle of C12E4 (A) or at the Air-Water Interface with C18E4 (B)a

a Chromophore groups (gray ellipsoids) and distances between chromophores are not drawn to scale; micelle size corresponds to the smallest assemblies formed by CnEm surfactant (Nagg ∼ 100), although C12E4 can form elongated micelles as suggested on the right. Their typical number in a micelle with Nagg ∼ 100 and their trans-cis (kinked ellipses) fractions, however, match with experimental results.

contact with micelles would, under this assumption, be more extensive and reach ∼2.6 at high C12E4 concentrations (3.3C6azo and 2C6azo). Recalling that we essentially seek orders of magnitude because Nagg is not known, the compositions discussed above match relatively well with a model of a limited number of contacts between the AMP chain and one micelle, as shown in Scheme 2. Exposure to UV reduces the bound surfactant by a factor, which when measurable (2C6azo and 3.3C6azo), was in the range of 2-10 (Table 1). The chromophores switch predominantly to their cis form in this condition, but 20% are left in their trans form.19 Under UV light, AMP with 3.3 mol % azobenzene contains about 0.7% trans-azobenzene. The C12E4-3.3C6azo value under UV (0.8 g/g) can accordingly be compared with the ratio measured with 0.6C6azo in the dark (0.12 g/g), that is, with similar fractions of trans-azobenzene in both chains. The higher value obtained with UV-adapted 3.3C6azo points to some contribution of the cis isomer to affinity. The total number of azobenzenes per bound micelle goes up to 16-80 under these conditions, that is, values on the order of the total number of azobenzene in a chain. But complexes with 3.3C6azo and 2C6azo under UV typically contain 3-6 trans-azobenzenes left in a chain that binds one micelle. The order of magnitude of a few trans isomers per bound micelle is still valid under UV light. Because values markedly below one micelle per chain are obtained for 1C6azo and 0.6C6azo, it is likely that polydispersity contributes significantly to remnant binding. Presumably, the number of trans chromophores per chain in UV-adapted 1C6azo and 0.6C6azo do not afford binding in the shortest chains or in chains having a degree of modification lower than average.

Discussion In the literature of the photoresponsive aggregation of amphiphiles, two classes of systems may be distinguished depending on the nature of the chromophore-bearing amphiphile: polymer or molecular surfactant. To our knowledge, however, when the fraction of chromophores in either amphiphile

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is decreased below a practical threshold of ∼10 mol %, both classes of systems show a rapid narrowing of the experimental window (T, concentrations, etc.) compatible with maintaining photoresponses. This behavior differs markedly from the optimal AMP (1C6azo), which not only contains an extremely low mole fraction of chromophores, but also shows photoassociation with C12E4 over a much larger range of both AMP and surfactant concentrations. In our opinion, no mechanism proposed to account for the response to light of photosurfactants or photopolymers is consistent with our results. To clarify the molecular origin and specificity of the photoresponses of AMP-micelle association, we first summarize the origin of the narrow range of response in the conventional classes of systems, and consider in a second part of this discussion the association between nonresponsive micelles and AMPs. Despite the common molecular behavior of their photochromecontaining moieties, the various photoresponsive systems differ in their modes of amplification of the phenomenon up to the macroscopic scale. Response in dilute solutions of polymers is essentially based on the triggered solubility of the chains, swinging from good to poor solvent conditions with exposure to light. Extensive works by Irie and colleagues2,6 have shown that the maximum of response to light is achieved in the vicinity of marginal solubility of the chains. In copolymers combining solvophilic and solvophobic monomers (one of the monomers being a photochrome), the fraction of photochrome monomers in the chains continuously affects the average solubility behavior. Accordingly, an optimal composition fulfills the condition of marginal solubility. Nevertheless, the contribution of photoisomerization to solubility varies in proportion to the fraction of chromophores in the chains. Consequently, polymers with low photochrome density are not affected by light, or their response is limited to extremely narrow conditions of solvent and temperature. Such a mechanism also applies to block copolymers capable of self-assembly, with one nonresponsive block remaining solvophilic while another block contains the photochromes.26 The surfactant-based response adds self-assembling constraints to the photovariation of solubility, namely the effect of molecular shape. Extensive works by the groups of Eastoe1 and Hatton5 have shown that a maximum effect on the hydrophobichydrophilic balance can be achieved when the chromophore is located at hinge positions in the molecules. Irrespective of how sensitive the assembly of pure surfactant can be made, mixing the photosurfactants with nonresponsive ones averages the response in these systems. Typically, the magnitude of response and the concentration window required to obtain some effect of light decrease continuously with the fraction of photosurfactant in samples.1,27 In practice, light-responsive micelles must contain above 10 mol % of chromophores. Finally, in both surfactantbased and polymer-based aggregation, the experimental window compatible with response to light is maximized if all the amphiphilic moieties can bear one photochrome group. A priori, the association between nonresponsive micelles and AMP is hydrophobic in nature and, accordingly, shows sensitivity to polarity and shape variations of the photochromes (e.g., the importance of the spacer group between the azobenzene and the polymer backbone). The fundamental differences with the systems described above lies in the coupling of polymer behavior to the size and/or curvature of the complexes. Teneous effects were achieved when AMP is bound on flat interfaces as compared to almost recognition-like behavior in colloid dispersions (micelles) (26) Wang, G.; Tong, X.; Zhao, Y. Macromolecules 2004, 37, 8911-8917. (27) Bonini, M.; Berti, D.; Di Meglio, J. M.; Almgren, M.; Teixeira, J.; Baglioni, P. Soft Matter 2005, 1, 444-454.

PhotoresponsiVe Association of AMPs with CnEm

of surfactants having similar headgroups and similar hydrophobic cores in terms of polarity. The essential role of the size and/or curvature of the assemblies formed in AMP/C12E4 samples point to an additional amplification scale that is not encompassed by the hydrophobic/hydrophilic balance of the molecular photochrome moiety. In consistency with the importance of geometry, AMPs with extremely low chromophore density (1-2 mol %) show an ample magnitude of response, even at degrees of photochrome that do not usually afford significant photovariation of the polymer polarity and solubility (see paragraph above). An interpretation based exclusively on hydrophilic/hydrophobic balance also fails to capture the preservation of response over a large window of compositions of the polymer/surfactant complexes and with large variations of salt concentration that markedly modify AMP solubility (at the highest experimental ionic strength, insolubility of AMP/micelle complexes was reached). Naturally, the polarity of monomers still tune the affinity of AMP for both micelles and C18E4 layers, and some variation in affinity is achieved by cis-trans isomerization. To interpret the lack of light-triggered polymer desorption on C18E4 flat layers, while micellar systems show sharp UV-triggered dissociation, we point out that the relative size of polymer and micelles is an important issue. In addition to hydrophobically driven affinity, the binding of a chain into a micelle is constrained by the necessity to adapt polymer conformation to the micelle size. At first, a somewhat obvious effect of small size is that, the smaller the micelle size, the higher the probability of releasing the micelle. The probability of dissociation upon weakening the hydrophobicity of side-groups of AMP increases rapidly with decreasing the number of bound chromophores to a few units per bound micelle. In contrast, on a flat layer of surfactants, multiple anchoring can take place along the whole length of the chain, and, accordingly, once adsorbed, the complete desorption of the chain becomes very unlikely. A second consequence of small micelle size is that, in addition to a general solubility/ hydrophobicity variation, bound segments of the AMP chain on a micelle interact in a range of distances that must be commensurate to micellar size. The formation of small micellelike clusters into a polymer chain has been described by parameters strongly dependent on the size of these clusters. Polymer segments are brought in close vicinity by the self-association of hydrophobic side-groups, and intersegment repulsion balances the growth of the hydrophobic cluster.11,28 From this point of view, response to irradiation would be achieved when the repulsion between the water-soluble parts of bound segments balances the affinity of their azobenzene anchor plunging in the micelle core. Tight hydrophobic association of a few azobenzene “anchors” in a micelle may induce a collapse of the polymer chain, although hydrophilic segments remain finally separated, and repel each other at distances on the order of the micelle radius. To match with the typical radius of direct micelles (or with micelle crosssections in the case of elongated assemblies), the optimal density of chromophore side-groups should correspond, on average, to a distance of typically 1-3 nm between chromophores in the polymer coil. In addition, higher densities and lower distances between photochrome anchors increase the probability of double or multiple attachments of azobenzene at the end of a polymer segment (the presence of short blocks with a high density of azobenzene). This sort of mutual binding, which may be compared with the behavior of gemini surfactants, is expected to significantly tighten the association and would hamper simultaneous dissociation of all azobenzenes in the block. Interestingly, at the (28) Borisov, O. V.; Zhulina, E. B. Macromolecules 2005, 38, 2506-2514. Borisov, O. V.; Halperin, A. Curr. Opin. Colloid Interface Sci. 1998, 3, 415-421.

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optimal 1 mol % degree of modification, the distance between chromophores should be on the order of the radius of a 100monomer-long chain, that is, a few nanometers. The whole AMP chain with 1000 monomers has a hydrodynamic radius of 25 nm on average (Supporting Information). The essential features of the model of AMP association on surfactants are summarized in Scheme 2, accounting for our data to fix the typical number of bound chromophores to ∼2-3 per 100 surfactants (this typical aggregation number of 100, although not applicable to the elongated micelles of C12E4, nevertheless provides a good idea of the number of molecules that fill the volume of a 2-3 nm sphere). We specifically illustrated the following qualitative conclusion: flat layers will bind AMPs under conditions of weak, but multiple, interactions with cis isomers that correspond to almost complete dissociation of small micelles having a radius smaller than the AMP chains. A small size of assemblies affords recognition-like behavior due to the binding of a few azobenzene dyes per micelle and, as a consequence, imparts the micellar complexes with extremely high sensitivity to light.

Conclusion Putting together the results converges to a consistent picture of the associations and the origin of photoresponse. Association is favored by increasing AMP hydrophobicity, and decreasing intrachain Coulombic repulsion. The highest degree of binding was obtained (i) by using a spacer between the backbone of AMP and the azobenzene, (ii) at a maximum modification degree of the set of AMPs experimentally considered (only 3.3 mol % azo-chromophore here), and (iii) at a maximum salt concentration. The corresponding samples showed the maxima of both the amount of surfactant bound per AMP chain and the fraction of azobenzene side-groups plunging in an apolar environment (presumably the core of the micelles). Weakening the affinity can result in partial penetration of the azobenzene in micelles or the presence of unbound azobenzene even in equilibrium with excess (free) micelles of C12E4. Most of these features were not surprising, and AMPs showed expected behaviors for amphiphilicpolymer/surfactant systems. Optimal conditions can accordingly be obtained by balancing the trends of azobenzene to bind into micelles with the hydrophilicity of the AMP backbone. Under optimal conditions, almost complete dissociation of the complexes between AMP and the surfactants occurs upon a limited variation of polarity, such as those resulting from cis-trans photoisomerization. The novelty of AMP-micelle mixed systems is reflected by their exceptional high magnitude of response and the recognition-like behavior obtained. Achieving responses at unprecedented low fractions of chromophores (∼1mol % of the monomers) not only enables one to significantly decrease the concentration of chromophore below millimolars for practical use, but also points to a new origin of amplification of the cis-trans photoconversion. We suggest that the limited number of bound chromophores per micelle is a key parameter in achieving photoresponsive association. Responses are obtained when typically less than 10 chromophores (down to 1-2) are actually interacting with 100 surfactant molecules and when surfactants form dispersed micelles rather than large assemblies on the surfaces. For practical developments of light-triggered systems, using as little chromophore as possible provides low absorbance and rapid responses in solution. Because conventional surfactant can be used here in combination with acrylic polymers easily synthesized, the principle of photoresponse should be readily adapted for specific applications. Controlling and tuning (with a clean trigger such

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as photons) the interaction between simple surfactants and polymers encompasses a vast potential of development with recognized importance, including emulsion properties29 and interfacial behaviors,4 gelation and viscosity control,8,15 and the dispersion/release of compounds.30

Supporting Information Available: Size distribution curves obtained by dynamic light scattering with 2C6azo/C12E4 samples. This material is available free of charge via the Internet at http://pubs.acs.org.

(29) Khoukh, S.; Perrin, P.; de Berc, F. B.; Tribet, C. ChemPhysChem 2005, 6, 2009-2012.

(30) Orihara, Y.; Matsumura, A.; Saito, Y.; Ogawa, N.; Saji, T.; Yamaguchi, A.; Sakai, H.; Abe, M. Langmuir 2001, 17, 6072-6076.

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