Article pubs.acs.org/Langmuir
Kinetics of Photocontrollable Micelles: Light-Induced Self-Assembly and Disassembly of Azobenzene-Based Surfactants Revealed by TRSAXS Reidar Lund,*,† Geoffrey Brun,‡ Eloïse Chevallier,§ Theyencheri Narayanan,∥ and Christophe Tribet‡,§ †
Department of Chemistry, University of Oslo, P.O. Box 1033, Blindern, N-0315 Oslo, Norway Ecole Normale Supérieure-PSL Research University, Département de Chimie, Sorbonne Universités - UPMC Univ Paris 06, CNRS UMR 8640 PASTEUR, 24 rue Lhomond, 75005 Paris, France § PSL Research University, Ecole Super. Phys. & Chim. Ind. ESPCI, Sci & Ingn Matiere Molle, ParisTech, CNRS UMR 7615, 10 Rue Vauquelin, F-75231 Paris 05, France ∥ ESRF-The European Synchrotron, 71 Avenue des Martyrs, F-38043 Grenoble, France ‡
S Supporting Information *
ABSTRACT: The kinetics of micelles involving photosensitive surfactants is still not well understood. In this work, we unravel the mechanistic pathways involved in the micelle formation and dissolution of photocontrollable micelles. We focus on the fast self-assembly processes of photosensitive cationic azobenzene-containing surfactants (AzoTMA) that display a change in hydrophobicity induced by a reversible cis−trans conformational transition upon exposure to light. By combining both in situ time-resolved small-angle X-ray scattering (SAXS) and light scattering, we characterized the detailed structure and phase behavior of AzoTMA in mixtures of water and dimethylformamide (DMF). Time-resolved synchrotron SAXS with monochromatic light as a trigger enabled us to observe the nonequilibrium formation and dissolution process of micelles (demicellization) directly on the nanoscale with a time resolution starting from milliseconds. The structural results show that in pure water UVlight illumination leads to a 12% reduction of the aggregation number of the micelles and more than a 50% increase in the critical micelle concentration (CMC). Close to the CMC, adjusted by the addition of DMF, UV light illumination leads to a complete dissolution of the micelles, while shining blue light reverses the process and leads to the reformation of micelles. The UVtriggered dissolution follows a two-step mechanism; the first and rapid (second time scale) release of unimers is followed by a slower decomposition of the micelles (over tens of seconds) as a result of an increase in temperature due to optical absorption. Similarly, the reverse process, i.e., micelle formation, occurs rapidly upon photoconversion to trans conformers under blue light, and micelles are disrupted at long exposure time due to the optical absorption and corresponding increase in temperature. Interestingly, the coexistence of unimers with regular micelles is found at all times, and no other transient assemblies could be detected by SAXS.
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INTRODUCTION The self-assembly of amphiphilic molecules is essential to a wide range of processes relevant to practical applications such as foaming, wetting, emulsification, solubilization, and detergency. In emerging technologies, out-of-equilibrium, dynamic changes of amphiphilic assemblies are exploited to achieve control of microflows and the manipulation of complex fluids1,2 or motion of microparticle,3 including control of interfacial movements,4 film coalescence,5 dewetting,6,7 and so forth. Progress in these new technologies depends critically on the availability of methods providing fast and spatially resolved modulation of assemblies, which could be realized by means of © XXXX American Chemical Society
dynamically switchable amphiphilic properties. Optical control is an attractive route to accomplishing this goal and can be achieved by using surfactant molecules possessing a lightresponsive hydrophilic/hydrophobic balance. Other stimuliresponsive surfactants are based on redox8 or magnetic-fieldsensitive responses.9 It is well known that the self-assembly and equilibrium properties of light-sensitive amphiphilic molecules can be accurately tuned and optimized by the adjustment of Received: December 25, 2015 Revised: February 26, 2016
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DOI: 10.1021/acs.langmuir.5b04711 Langmuir XXXX, XXX, XXX−XXX
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Scheme 1. Light-Induced trans/cis Conformation and Associated Micelle Formation of Azobenzene-Based Surfactant AzoTMA
external parameters (temperature, salt, pH, etc.),10 but the response time of these systems has rarely been studied. Understanding and controlling the dynamics of these stimulisensitive surfactant systems are nevertheless of fundamental importance in solution where the kinetics of the responses would determine the temporal and spatial resolution in competition with the diffusion and half-life time of lightactivated molecules. Practical control depends on rapid kinetics of stimulation, self-assembly, or surface coverage by surfactants, i.e., on their rate of ordering. For instance, light-switchable surfactants have been shown to permit large (>20 mN/m) and reversible changes in the surface tensions of liquids on short time scales (second),11,12 which is afforded via transient contrasted dynamics of interfacial interaction to control droplet detachment,6 Marangoni flows,13 film coalescence,5 and so forth. A common strategy to design surfactants possessing robust, reversible light sensitivity takes advantage of azobenzene groups that change their polarity according to the absorption of light. Azobenzene undergoes photoconversion between polar and apolar cis/trans isomers under diverse solvent conditions, including those in water and in micellar environments. Photosurfactants based on 4-alkyl,4′-alkoxy phenylazobenzene derivatives were optimized by tailoring their alkyl chain lengths and neutral head (oligoethyleneoxide11) or cationic head (trimethylammonium14−16). Maximal photovariation of the critical micellar concentration (CMC) upon cis/trans conversion is reached with butyl or propyl side chains. Here we studied a member of these optimal amphiphiles, abbreviated simply as AzoTMA, whose structure is shown in Scheme 1. The investigation of the kinetic behavior of the phototriggered selfassembly/dissolution of AzoTMA (or similar azosurfactants) is only in its infancy. Previous studies have been essentially focused on the equilibrium structure of micelles studied by NMR and neutron scattering.14,17,18 Structural studies with systematic exploration during a slow (>hours) evolution of the cis/trans isomer ratio reported the formation of intermediate assemblies corresponding to morphological transitions between equilibrium assemblies19 or oligomeric premicellar aggregates that were tentatively ascribed to a weak stacking interaction between the aromatic moieties of the azobenzene below the
CMC.14,20 The existence of stacking and premicellar clusters obviously adds complexity compared to the situation for nalkyl-based surfactants.12 In addition, kinetics on fast (1 g/L, the intensity of light is rapidly decreased with the increasing depth of penetration in the samples (DO436 nm ≈ 5 as calculated from the spectra of dilute AzoTMA linearly extrapolated to 6 g/L concentration and through a 1-mm-thick optical path length). Accordingly, cis/trans conversion kinetics are slowed. This is shown in Figure 9a, which depicts the TR-SAXS scattering curves obtained under constant illumination of blue light.
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CONCLUSIONS An in situ study of the kinetics associated with the self-assembly of photosensitive surfactant AzoTMA has been performed by light scattering and X-ray scattering. Carefully tuning the CMC of the system by adding DMF enabled us to follow both the dissolution and reformation of micelles upon shining UV and blue light, respectively. Synchrotron SAXS, with a temporal resolution below 100 ms, combined with theoretical modeling directly revealed the nanostructure of the micellar mixture and changes over time. This allowed for a quantification of intermediate compositions along the phototriggered kinetic pathway. The data revealed that upon UV illumination the ratedetermining step (here a few seconds) is the photoconversion of trans to cis unimers leading to the gradual dissolution of the micelles. These micelles were reformed within 2 to 3 s by shining blue light. However, because of the absorption of light and energy dissipation, light-induced heating dominated at times longer than a few seconds and contributed to the redissolution of the micelles. The prevailing trans unimers could be subsequently self-assembled into micelles during cooling in the quiescent dark state. Studies of a faster phototriggered transition, though technically feasible by applying higher light intensities, e.g., with lasers, would be accordingly hampered by the lack of control of temperature in capillaries. Finally, under the present experimental conditions, all intermediate, initial, and final steps (during, after, and before irradiation) were adequately described by a simple two-state model assuming the coexistence of unimers and micelles. It is concluded that AzoTMA micellization and demicellization processes are rapid on the ≤1 s time scale and that the presence of premicellar and/or intermediate clusters can be neglected. From this point of view, AzoTMA resembles conventional cationic alkyl-trimethylammonium surfactants, but it significantly differs from neutral azobenzene-containing surfactants that in their cis form showed premicellar clusters (Nagg ≈ 10), as reported by Lee et al.14 In conclusion, through a systematic study where we combined scattering techniques and theoretical modeling, we were able to obtain quantitative insight into the kinetics of light-controlled micellization. This information should help in the design of systems based on photosurfactants for future technologies requiring on demand and in situ spatial control, for instance, for light-triggered delivery, control of flow in microfluidic devices, and dynamic lithography.
Figure 9. TR-SAXS data of the 6 g/L AzoTMA solution in 17% DMF: (a) under blue light illumination and (b) for relaxation in the dark after turning off the light. Solid lines correspond to model fits.
The data in Figure 9 exhibit peculiar behavior where a pronounced change in the shape is visible at intermediate times. The forward intensity increases as a function of time, and after a few seconds, there is a pronounced change from the flat scattering of unimers to more micellar-like scattering. After approximately 10−20 s, the intensity then changes to an almost flat scattering distribution. This suggests that upon blue light illumination micelles are initially formed and subsequently dissolved upon prolonged exposure. This can be understood by a combined effect of trans conversion and heating effects. As shown in Figure S2 in the SI, the temperature in the capillary increased rather dramatically up to almost 7 °C under blue light. The temperature variation under exposure to blue light is much more pronounced than under UV irradiation (about a 2 °C increase; see the SI) and leads to a significant increase in the CMC. In addition, because of the local heating in the capillary, the scattering contrast and the solvent background scattering are expected to change. These data are very challenging to analyze using a simple scattering model. Nevertheless, as H
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b04711. Thermal relaxation of AzoTMA in water/DMF, temperature measured in a SAXS capillary filled with an AzoTMA solution in water/DMF, light-scattering intensity as a function of temperature for various concentrations of AzoTMA, radius of AzoTMA micelles as determined by dynamic light scattering, effective volume fraction determined from the structure factor describing the interparticle interactions as a function of time for AzoTMA in DMF, and variation upon exposure to light of the absorbance spectra of AzoTMA in water/ DMF (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS R.L. is grateful for financial support through the SYNKNØYT program of the Norwegian Research Council (grant numbers 218411 and 228573) . C.T. and G.B. greatly acknowledge support by the Fondation pour la Recherche Médicale (grant number DCM20111223066). We are grateful to the ESRF for the allocation of beam time on the ID02 instrument. Access to pendant drop tensiometry was kindly provided by Prof. Damien Baigl (ENS).
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