Photoswitching of Azobenzene-Based Reverse Micelles above and at

Department of Physical Chemistry, University of Chemistry and Technology, Prague, Technická 5, 16628 Prague 6, Czech Republic. Langmuir , 2017, 33 (9...
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Photoswitching of Azobenzene-Based Reverse Micelles above and at Subzero Temperatures As Studied by NMR and Molecular Dynamics Simulations Lenka Filipová,†,‡ Miriam Kohagen,§ Peter Štacko,† Eva Muchová,§ Petr Slavíček,*,§ and Petr Klán*,†,‡ †

Department of Chemistry and ‡RECETOX, Faculty of Science, Masaryk University, Kamenice 5, 62500 Brno, Czech Republic § Department of Physical Chemistry, University of Chemistry and Technology, Prague, Technická 5, 16628 Prague 6, Czech Republic S Supporting Information *

ABSTRACT: We designed and studied the structure, dynamics, and photochemistry of photoswitchable reverse micelles (RMs) composed of azobenzene-containing ammonium amphiphile 1 and water in chloroform at room and subzero temperatures by NMR spectroscopy and molecular dynamics simulations. The NMR and diffusion coefficient analyses showed that micelles containing either the E or Z configuration of 1 are stable at room temperature. Depending on the water-to-surfactant molar ratio, the size of the RMs remains unchanged or is slightly reduced because of the partial loss of water from the micellar cores upon extensive E → Z or Z → E photoisomerization of the azobenzene group in 1. Upon freezing at 253 or 233 K, E-1 RMs partially precipitate from the solution but are redissolved upon warming whereas Z-1 RMs remain fully dissolved at all temperatures. Light-induced isomerization of 1 at low temperatures does not lead to the disintegration of RMs remaining in the solution; however, its scope is influenced by a precipitation process. To obtain a deeper molecular view of RMs, their structure was characterized by MD simulations. It is shown that RMs allow for amphiphile isomerization without causing any immediate significant structural changes in the micelles.

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CTAB/water/chloroform-d system.7 According to this model, the reverse micelles are formed after the structural reorganization of linear premicellar aggregates. Only a few studies have been performed to investigate reverse micelles at subzero temperatures. Several works showed that water is expelled from the micellar water core (water shedding) at subzero temperatures,8−10 including a CTAB/water/chloroform-d11 system. This phenomenon can be suppressed by the fast cooling of reverse micelle solutions at sufficiently low temperatures.11,12 Amorphous ice encapsulated via the hydrogen-bonded polar heads of AOT molecules was reported to form for relatively small

INTRODUCTION Reverse micelles (RMs) are self-organized assemblies of surfactant molecules (amphiphiles) in nonpolar solvents in which the polar heads of amphiphiles are oriented toward the water cores, whereas the outer shell is composed of their hydrophobic chains. Many applications of reverse micelles, such as microreactors for chemical and biochemical reactions,1 drug delivery vehicles,2 biocatalysis,3 the stabilizing of metastable proteins,4 and the synthesis of nanoparticles,5 have been presented in the literature. The nature and behavior of reverse micelles under different conditions have been extensively investigated over the past few years. The mechanism of multistep formation of reverse micelles in nonpolar solvent, known as Eicke’s association model,6 was recently experimentally validated in a micellization study of a © XXXX American Chemical Society

Received: December 13, 2016 Revised: February 14, 2017

A

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purification. Chloroform-d (99.8%, Sigma-Aldrich) was used for the preparation of the solutions of RMs and was stored in amber bottles over flame-dried molecular sieves (3 Å). Preparation of Reverse Micelle Solutions. The RM solutions = 0.05 were prepared by the direct weighing of amphiphile 1 (canalytical 1 mol dm−3; this concentration was sufficiently above the critical micelle concentration; see the following text) into NMR tubes, followed by the addition of chloroform-d. Water was subsequently added to this solution analytical = 4. to adjust the water-to-surfactant molar ratio of x = canalytical water /c1 The tubes were closed with a plastic cap and sealed with parafilm. The mixture was vigorously agitated, and then the samples were allowed to equilibrate for 12 h before the measurements started. A sealed capillary tube containing a solution of tetramethylsilane or dichloromethane in chloroform-d was added as an internal NMR standard. The water loading (wobs water,core) of RMs was determined by the integration of the water signal against that of the internal standard before and after irradiation. The concentrations of RMs in the solutions are calculated as the concentrations of amphiphile 1. Photochemical Experiments. A homemade reactor with two removable diode plates was used for the irradiation of the RM solutions in NMR tubes. Each diode plate contained 16 commercially available LEDs emitting at λmax = 375 or 530 nm. (Their spectra are shown in Figures S1 and S2.) Freezing of Reverse Micelle Solutions. The RM solutions in NMR tubes were cooled in a cooling bath (ethanol/dry ice) at 253 or 233 K and were allowed to equilibrate for 1 h. In one set of experiments, the samples were placed in a precooled NMR spectrometer at the given temperature, and 1H NMR spectra were recorded; subsequently, the samples were warmed to 303 K. In another set of experiments, the RM solutions were prepared in the same way and irradiated at the given temperature, and the 1H NMR spectra were recorded. Molecular Dynamics (MD) Simulations. Classical molecular dynamics simulations were carried out for RM systems containing 49 water and 12 surfactant molecules embedded in 10 663 chloroform molecules. Water was described by the SPC/E model.40 The force field of chloroform comprising point charges and Lennard-Jones interactions was taken from the literature.41 The force field for the surfactant molecules was derived in the following way. The force field for the azobenzene unit39 is based on the parameters of the GROMOS 45a3 force field.42 Because we were interested in modeling both the E and Z configurations as well as switching between them, we used the average values for the bond lengths and angles of different configurations.39 The parameters for the trimethylammonium group were taken from the GROMOS 45a3 force field.42 The partial charges for the remaining atoms were determined from a restrained electrostatic potential (RESP) fit performed with the NWChem quantum chemistry code.43 The RESP fit was carried out on a geometry of the E form of the amphiphilic cation embedded in chloroform, represented as a dielectric continuum. We calculated the electrostatic potential of the molecule; the partial charges that were already known39 were kept constant, and the rest of the partial charges were fitted accordingly. The B3LYP functional and a 6-31G* basis set were used in this calculation. The parameters for the bromide ion were defined according to the literature.44 The mixed Lennard-Jones parameters were derived from self-parameters using the geometric average. The force field used is summarized in Table S1. The force field described above lacks electronic polarization, which can, however, seriously affect the outcome of the MD simulations, especially for the simulation of electrolytes with low charge density.45 The polarizable force fields are, on the other hand, impractical for long MD simulations. Therefore, we applied a thrifty approach to include the electronic polarization, employing the so-called electronic continuum correction (the ECC model).46,47 In this approach, the charges are scaled by a factor of 1 , where εel is the optical part of the dielectric

micelles (150 water molecules in the core) at 180 K, whereas the disintegration of larger micelles in the size range of 10−500 nm to release larger ice structures was observed.13 Recently, Suzuki and Yui demonstrated that the combination of a small sample cell volume and rapid cooling prevents water shedding; metastable cubic ice was detected in AOT reverse micelles with a radius below 2.1 nm.14 In this work, we aimed to construct switchable reverse micelles based on an azobenzene-containing photoswitch. Azobenzene and its derivatives are chromophores that undergo reversible E,Z photoisomerization. The absorption spectrum of azobenzene is characterized by two absorption bands, the π,π* and n,π* transitions in the UV region and visible region, respectively.15 Differences in the absorption spectra of both isomers enable us to select the irradiation wavelength to obtain either the E or Z isomer preferentially in the photostationary state.16 Azobenzene derivatives have been used in many applications, such as molecular switches,17 molecular machines,18 surface-modified materials,19 protein probes,20 and nanotubes.21 The photostationary concentration ratio of the azobenzene isomers can be affected by competing thermal Z → E isomerization; the activation energy for this reaction is ∼96 kJ mol−1.22 When the azobenzene chromophore is incorporated into the structure of an amphiphile, photoresponsive micelles can be formed. Many such applications have been reported, for example, the formation of light-responsive micelles studied by atomic force microscopy23 and scanning electron microscope 24 techniques, light-switchable vesicles,25 and block copolymer vesicles dissociated and reformed by light.26 Azobenzene amphiphiles have also been used in the targeted disruption of giant unimolecular vesicles27 or for thermo- and phototriggered aqueous microgels,28 erasable multilayer thin films,29 aggregation and disaggregation of DNA conjugates,30 and photoresponsive nanostructures for drug delivery.31 In this work, we investigate the properties and photoresponse of azobenzene-based reverse micelles in chloroform using 1H NMR and absorption spectroscopy. The aim of this investigation was to address differences in the size and dynamics of the water pool and micelles themselves occurring upon E,Z photoisomerization of the amphiphile azobenzene units in the temperature range of 303−233 K. The experiments on the azobenzene-modified micelles are complemented with molecular dynamics (MD) simulations. The MD simulations can provide molecular-level insight into the structure and stability of the micelles.32−36 Azobenzene is an archetypal molecular switch, and its photoisomerization was studied previously at the ab initio molecular dynamics level.37 However, the modeling of azobenzene photoswitching in complex systems, such as reverse micelles, at the full quantum chemical level is beyond the scope of current computational chemistry. The problem can be circumvented within the QM/MM scheme in which the chromophore is treated quantum mechanically while the environment is treated at the molecular mechanics (MM) level.38 The photoisomerization of azobenzene can be fully described even at the MM level using separate force fields for the E and Z forms of azobenzene; these force fields then represent two diabatic states that can be switched.39 With this approach, we can model such complex systems as the azobenzene-containing RMs, albeit at the price of losing the details of nonadiabatic transitions.

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εel

constant (2.09 for chloroform). We scaled the charges of the bromide and headgroup ions by a factor of 0.692. The water charges were not scaled because they were originally fitted to the experiment and have to be considered to be effective charges. The simulations with the scaled atomic charges were successfully used for modeling simple electrolyte solutions48 as well as more complex systems, such as ionic liquids.49−51

MATERIALS AND METHODS

Materials. The reagents and solvents of the highest purity available (Sigma-Aldrich, Fluka, or Penta) were used without additional B

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Langmuir Scheme 1. Synthesis of Amphiphile 1

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The scaling of charges was also used previously for systems containing RMs.11 The input structure was built as follows. First, a single molecule of each of the amphiphiles (in either the Z or E configuration) was built with the Avogadro code,52 the inverse micelle around the water core was constructed with Open Babel,53 and the ions and chloroform were inserted with the respective tools into the Gromacs 4.6.3 program package.54 Each box contained 49 water molecules, 12 amphiphilic molecules, and 10 663 chloroform molecules. The ratio of the water and amphiphile molecules was set to correspond to the experimental value of x = 4. The MD simulations were performed in the Gromacs 4.6.3 program package.54 The time step for the propagation was set to 1 fs, and 3D periodic boundary conditions were employed. Simulations were carried out under a constant pressure of 1 bar, controlled by the Parrinello− Rahman barostat55 with a coupling constant of 0.5 ps, and constant temperature of 300 K, controlled by the velocity rescaling thermostat56 with a coupling time of 0.5 ps. Most of the simulations were performed at 300 K, although the RM structure was also studied at a lower temperature (233 K). For the simulations at lower temperatures, we used the TIP4P2005 model57 because it is in better agreement with the experimental phase diagram of pure water at low temperatures57 compared to the SPC/E water model.40 The total simulation length was 100 ns, of which the first 30 ns was left for equilibration. Constraints for all bonds were applied using the Lincs algorithm.58 The electrostatic and van der Waals interactions were truncated at 1.4 nm, and the long-range electrostatic interactions were accounted for by the particle mesh Ewald method.59 For the van der Waals interactions, the dispersion correction for the energy and pressure was used. The MSMS (maximal speed molecular surface) code60 was used to calculate the surface and volume of the water pool in the RMs. To investigate the influence of interconversion between the Z and E isomers, we adapted the force field in the same way as was presented in the work of Hamm and co-workers:61 The double-minimum potential for the central C−NN−C dihedral angle was replaced by singleminimum potentials to force the molecule to attain either the E or Z configuration. The simulations subsequently started either with a singleminimum potential in the E configuration which switched to the Z configuration by replacing the potential or vice versa. In this case, the simulations were performed in the NVE ensemble. The time step was decreased to 0.5 fs,62 and the calculations were performed with double precision to avoid an energy drift in Gromacs 4.5.3.54 The simulation lengths were set to 250 ps in all cases because this time is sufficient for the system to equilibrate after the switching. We performed 10 simulations for switching from the E to Z isomer and vice versa; in this case, all molecules of amphiphile 1 were switched. We also performed simulations in which only a smaller number of amphiphile 1 molecules in RMs were switched from the E to the Z configuration or vice versa.

RESULTS AND DISCUSSION Synthesis. The preparation of amphiphile 1 started with Oalkylation of p-nitrophenol 2 with 1-bromooctane to produce 3, followed by the reduction of the nitro group catalyzed by palladium on carbon in a hydrogen atmosphere to give aromatic amine 4 (Scheme 1). In the next step, a diazonium salt, prepared by the oxidation of amine 4 with sodium nitrite, was coupled with phenol to construct a diazo moiety in 5. The hydroxyl group in 5 was then alkylated using 1,8-dibromooctane to form 6. The final amphiphile, 1, was obtained via the substitution reaction with trimethylamine (59% yield over five steps). In addition, several analogous amphiphiles possessing different alkyl chain lengths on both sides of the azobenzene moiety were synthesized (Scheme S2); however, our attempts to use them for the formation of micelles were unsuccessful because of their low solubility in chloroform. Photoisomerization of 1. It has already been shown that the thermal stability of the sterically hindered, thermodynamically less stable Z isomers of azobenzene is influenced by the phenyl ring substitution.63,64 Unlike Z-azobenzenes substituted with an electron-donating group on one phenyl ring and an electron-withdrawing group on the other one that undergo efficient thermal Z,E photoisomerization,65,66 4,4′-dialkoxyazobenzenes exhibit a very slow isomerization (the lifetime of the Z isomers in solutions is on the order of hours at 20 °C).67 Therefore, we selected a 4,4′-dialkoxyazobenzene derivative (1) for this work. The E,Z photoisomerization of 1 was studied using absorption and NMR spectroscopy. Figure 1 shows the absorption spectrum of pure E-1 (blue line) in chloroform. It possesses a strong absorption band at λmax = 360 nm, which is slightly bathochromically shifted compared to that of unsubstituted azobenzene (λmax = 320 nm),68 and it represents the symmetry-allowed π,π* transition. A much weaker band at λmax ≈ 450 nm corresponds to the symmetry-forbidden n,π* transition.69 Upon irradiation at 375 nm and 20 °C, E-1 was converted to the Z isomer, possessing two bands at λmax = 310 and 454 nm assigned to the π,π* and n,π* transitions, respectively69 (Figure 1, red line) in a very good chemical yield. The resulting photostationary-state (PSS) concentration ratio [Z-1]/[E-1] was 96:4, as determined by 1H NMR using the signals of phenyl-ring hydrogens (Figure 2). C

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incorporated into RMs. Therefore, the maximum wobs water,core (E-1 RMs) value is ∼4. In addition, the downfield chemical shift of δwater,core with an increasing wobs water,core was observed. We found that the chemical shifts of the amphiphile E-1 hydrogens were not affected by the change in the amphiphile concentration. However, a distinct difference in the chemical shifts was observed for the water signals; the signal moved downfield with increased surfactant concentration (Figure 3). At a low

Figure 1. UV−vis absorption spectra of E-1 in chloroform (c = 3 × 10−5 M) before (blue line) and after (red line) irradiation at 375 nm and 20 °C.

Figure 3. Selected 1H NMR spectra of E-1 in CDCl3/water (x = 4) obtained for different concentrations of E-1. The signals for water hydrogens (δwater) are marked by an asterisk.

amphiphile concentration (5 mM), the chemical shifts of water, δwater, were found at 1.56 ppm, which is a typical value for water dissolved in CDCl3.70 At higher concentrations of E-1, a downfield chemical shift of up to 4.00 ppm reflects the formation of hydrogen bonds in a growing water pool inside the aggregates.71 In freshly prepared samples (before equilibration can occur7), δwater was found at 4.79 ppm and can be assigned to the bulk water signal.70 Figure 4 shows a plot of chemical shifts δwater vs 1/canalytical for 1 an E-1/water/chloroform-d system at x = 4, which was used to

Figure 2. 1H NMR spectra of a CDCl3 solution of E-1 (c = 25 mM): (i) before irradiation (pure E isomer, blue asterisks), (ii) upon irradiation at 375 nm (Z-1 is formed, red asterisks), and (iii) upon subsequent irradiation at 530 nm (reverse isomerization gives E-1, blue asterisks).

Subsequently, reverse Z,E isomerization was accomplished upon irradiation using visible light (530 nm) to give the PSS ratio of [E-1]/[Z-1] = 95:5 (determined using the 1H NMR signal areas of the aromatic ring hydrogens, Figure 2). The forward/ reverse photoisomerization cycle was repeated five times; no side products were formed during the process. Determination of the Critical Micelle Concentration. Initially, we determined the maximum water loadings (wobs water, core), that is, the largest amounts of water that RMs composed of either E-1 or Z-1 amphiphiles can incorporate (excess water added to the solution leads to its phase separation). The critical micelle concentration (CMC) can be determined by different methods, such as NMR, isothermal titration calorimetry, conductometry, and fluorescence measurements. In this study, the CMC was determined using 1H NMR spectroscopy. A set of samples with a different concentration of amphiphile E-1 and a constant concentration of water (x = 4) were prepared and allowed to equilibrate for 24 h at 20 °C. The water-to-surfactant molar ratio x was selected on the basis of the plot of wobs water,core vs x (Figure S16), where wobs is the ratio of micellar-core water water,core and amphiphile E-1 concentrations determined by 1H NMR NMR (cNMR ). At x ≤ 4, the wobs water,core/c1 water,core values show a linear increase until w = x, when a maximum amount of water is

Figure 4. Plot of δwater vs 1/canalytical for x = 4 for a 1/water/chloroform-d 1 system.

determine the CMC using the mass action (close association) model.7,72,73 The data in this graph provide two straight lines with different slopes, and their intersection point at canalytical = 28 1 ± 0.7 mM corresponds to the CMC. This value is close to that of ∼40 mM reported for cetyltrimethylammonium bromide (CTAB) in a CTAB/water/chloroform-d system,7 which has a D

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Langmuir Table 1. Irradiation of a 1/Water/Chloroform-d Systema at 303 Kb λirr/nm

entry

c

1 2 3 4

dark 375 530 530e

[E-1]/[Z-1]

δwater,core/ppm

wobs water,core

D/1010 m2 s−1

Rh/nm

100:0 4:96d 95:5d 99:1e

2.86 ± 0.03 2.31 ± 0.06 2.40 ± 0.07 2.67 ± 0.04e

3.95 ± 0.07 2.73 ± 0.07 2.81 ± 0.09 3.75 ± 0.05e

3.9 ± 0.4 4.5 ± 0.4 4.4 ± 0.3 4.1 ± 0.03e

1.11 ± 0.12 0.96 ± 0.09 0.99 ± 0.07 1.07 ± 0.08e

∼4. bcanalytical = 50 mM, x = 4. The amount of 1 (1H NMR) remained constant during all experiments. cA freshly prepared sample 1 kept in the dark. The concentration ratio at the PSS. eThe sample containing 96% Z-1 RMs (entry 3) was equilibrated overnight in the dark.

a obs wwater,core(initial)

d

When the Z-1 RM solution (entry 2) was irradiated at λirr = 530 nm, where the Z-1 isomer preferentially absorbs, for ∼1 h, almost complete reverse photoisomerization ([E-1]/[Z-1] = 95:5; Table 1, entry 3; Figure S20) was observed. The δwater,core and wobs water,core values remained approximately the same, and the signal for bulk water (∼4.7 ppm) was absent. Therefore, no more water was released from RMs during this reverse photoisomerization process. However, when the Z-1 RM sample (entry 3) was left to stand in the dark for 10 h, the δwater,core, wobs water, core, D, and Rh values (entry 4) returned to essentially the same values adjusted in the beginning of the experiment (entry 1), and thus this separated water was fully incorporated back into RMs (Figure S21). Such a slow, most likely diffusion-driven process has also been observed in the case of the formation of CTAB RMs from CTAB and phase-separated water in chloroform, which proceeds with a rate constant of 8.1 × 10−6 s−1.11 In the next step, we tested the photoisomerization of smaller RMs with wobs water,core ∼2.8 (Table 1, entry 2) that exhibited exceptional stability during reverse Z → E photoisomerization = 50 mM, wobs (entry 3). Thus, E-1 RMs (canalytical 1 water,core = 2.8) were prepared, and E → Z or Z → E photoisomerization of 1 did not cause any water shedding, thus the size of the micelles remained intact (Table 2; Figures S26−S28).

molecular length and geometry comparable to those of E-1. Therefore, the E-1 concentration of 50 mM, sufficiently above the determined CMC, was used in RM solutions studied in this work. In addition, we performed the same experiments with RMs containing Z-1 amphiphiles, and the maximum wobs water,core value was found to be 2.8. Properties and Photoisomerization of RMs at 303 K As Studied by 1H NMR. The physicochemical properties of a 1/ water/chloroform-d system at 303 K and their changes triggered by the photoisomerization of 1 were studied by 1H NMR. RM = 50 mM, x = 4, E-1) were prepared in NMR solutions (canalytical 1 tubes and were allowed to equilibrate at 303 K for 12 h. Such a time is sufficient for equilibration and the incorporation of water into the core of reverse micelles (upon equilibration, the corresponding 1H NMR spectrum does not change with time). A single NMR signal of incorporated water, δwater,core = 2.86 ppm, and the water loading, wobs water, core = 3.95, were found (Table 1, entry 1; Figure S17), which indicate7,11 that water is present only in the micellar core. This RM solution was irradiated at λirr = 375 nm, where the E-1 isomer predominantly absorbs (Figure 1), for 2 h at this temperature, and changes in the concentrations of the E- and Z-1 isomers in RMs and the δwater,core values were determined. When the PSS was reached ([E-1]/[Z-1] ratio = 4:96), irradiation was stopped (Table 1, entry 2; Figure S18). The δwater,core value shifted slightly upfield to ∼2.3 ppm, and wobs water,core dropped to ∼2.7, indicating that approximately 30% of the micellar core water was lost from RMs. Indeed, a broad NMR signal at ∼4.6 ppm corresponding to this amount of water and assigned to bulk water70 released to chloroform (a thermodynamically driven loss of water from RM cores at subzero temperatures is referred to as water shedding8−10) appeared (Figure S18). The higher diffusion coefficients D and the corresponding decreased hydrodynamic radii Rh also confirmed that the micellar size decreased. In addition, these values were compared to those of much smaller premicellar aggregates (D = that is substantially lower 6.6 × 10−10 m2 s−1) formed at canalytical 1 (10 mM, x = 4) than that of the CMC to demonstrate that our smaller assemblies were still RMs. When this sample was left to stand for 3 h in the dark, the bulk-water signal at ∼4.6 ppm disappeared from the spectrum because water phase separation (as a top layer on the chloroform solution) occurred; however, partial reverse Z → E isomerization was observed ([E-1]/[Z-1] = 22:88; not shown). Although the Z-1 derivative in a chloroform solution relatively quickly and spontaneously reverts to its E isomer in the dark (k = (4.96 ± 0.10) × 10−3 s−1, as determined by UV−vis spectroscopy), a thermal reverse isomerization of Z-1 incorporated into RMs is over 2 orders of magnitude slower (k = (1.33 ± 0.02) × 10−5 s−1, τ1/2 = 14.4 h, as determined by 1H NMR). Therefore, we repeated the same experiment but the [E1]/[Z-1] ratio was kept constant (5:95) under steady-state irradiation at 375 nm. The resulting RMs had the same parameters as those in entry 2, only the bulk-water signal disappeared (Figure S19).

Table 2. Irradiation of a 1/Water/Chloroform-d Systema at 303 Kb entry 1 2 3

λirr/nm dark 375 530

c

[E-1]/[Z-1]d

δwater,core/ppm

wobs water,core

100:0 4:96d 93:7d

2.31 2.14 2.24

2.86 2.84 2.83

(initial) ≈ 2.8. bcanalytical = 50 mM, x = 2.8. The amount of 1 1 ( H NMR) remained constant during all experiments. cA freshly prepared sample kept in the dark. dThe concentration ratio at the PSS.

a obs wwater,core 1

It has been shown on several occasions that the photoisomerization of azobenzene-containing amphiphiles induced the self-assembly and disassembly of micelles, for example, in DNA-based micelles,30 photosensitive lipid vesicles,27 and multilamellar vesicles.25 In this work, the fluctuations of some parameters, such as δwater,core, diffusion coefficient D, micellar hydrodynamic radii (Rh) values, and the number of micelleincorporated amphiphile molecules (Table 1), invoked upon a nearly complete forward and reverse photoisomerization of 1 in a 1/water/chloroform-d system irradiated at 303 K, were relatively small or absent, depending on the RM size. A small decrease in both δwater,core and Rh in larger micelles reflects the shrinking of the micellar cores due to partial water shedding but not micellar destruction. The δwater,core values above ∼2.2 ppm are still above the value typical for linear premicellar aggregates (1.8 ppm).7 An Rh value of approximately 1 nm is surprisingly small for a micellar system but certainly larger than that of linear (premicellar) E

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Langmuir aggregates formed below the CMC.6 A water/counterion arrangement in the core should also affect some of the parameters, and this question has been addressed by molecular simulations shown later. The water core of RMs is composed of an interfacial layer between the surfactant headgroups and the water pool containing counterions.74,75 Depending on its size, the water pool can have properties of bulklike water. However, the interfacial water molecules (also present near the biological membranes) exhibit behavior markedly different from that of bulk water.76,77 They have restricted mobility and lack the normal hydrogen-bonded structure of bulk water.74 We conclude that our micellar system is quite stable, especially when the wobs water,core ratio is close to ∼2.8 (Table 2), and that a partial water shedding process observed during E → Z photoisomerization can be controlled by the RM size. Different RM sizes may lead to different packing and micellar size/surface properties,78 such as different arrangements of amphiphile chains, which can then be reflected in the (in)stability of the water core. Properties of RMs at 300 K: An MD View. The methods of molecular dynamics can provide further details on the microscopic structure of RMs that are otherwise difficult to access experimentally. We used the MD simulations to model the structure of RMs, describe the allocation of each of the components of RMs, and study the stability of both forms. One of the main questions of this work was whether any significant structural changes in the RMs occur with the photoisomerization of 1. Figure 5 shows two representative

more polarizable anions, in this case, heavier halide anions. Such an enhanced surface concentration of ions was reported before for surfaces, water droplets, and importantly also for CTAB reverse micelles.11 It is assumed that the surface preference of bromide ions is caused by enthalpic effects. The surface solvation of the bromide ion leaves the favorable water−water interactions intact at a price of decreased entropy of the system. The balance is shifted toward bulk solvation for smaller anions such as chloride and even for the bromide ion bearing a full charge of −e. The inclusion of the electronic polarization leads to an effective decrease in the charge on the bromide ion, and thus the bromide ions are expelled onto the surface.81 Within the ECC simulations, the diameter of the water pool containing bromide ions is ∼1 nm. This value is consistent with the experimentally measured hydrodynamic radii of the whole micelle. The headgroup of amphiphile molecule 1 does not penetrate the water pool. Here again, the nonpolarizable calculations predict a structure with the headgroup being more solvated. (See the position of the trimethylammonium nitrogen atom, Figure 6C.) The structural characteristics of RMs based on our MD simulations are consistent with previous experimental studies, suggesting the formation of the Stern layer on the headgroup/water pool interface.82 However, within the ECC model, the structural parameters describing the water pool or the positions of ions in the water pool are similar for both E-1 and Z-1 RMs. The polar part of the reverse micelle remains almost unaffected by the amphiphile isomerization. Although the experimental results suggest that Z-1 RMs should have a somewhat reduced water pool size, the driving force for the reduction is apparently not too strong. The position of the nonpolar tail of the micelle is different for the two isomers of 1. This can be inferred from Figure 6D, where the end-to-end distributions of the two molecules are presented. The Z isomer is bent, which is demonstrated by shorter end-toend distances. The E isomer has a larger distance between the ends; that is, the amphiphilic molecule is relatively straight. Therefore, the E-based micelles have a more hairy look. We can ask why the different shape of the micelle is not reflected by the measured hydrodynamic radius, which is (within the error bars) the same for both forms of RMs and approximately equal to the radius of the water−bromide core. The hydrodynamic radius represents a hard sphere diffusing with the same diffusion constant as for the real object. However, the reverse micelles are by no means hard spheres. The tails coil and recoil and do not cover the whole surface of the water core. The object becomes less dense with increasing distance from the center of mass of RM. The hydrodynamic properties are therefore controlled by the heavy and compact core. It follows from our previous simulations of CTAB-based reverse micelles11 that the micelles can appear to be highly nonspherical. The nonsphericity of the water pool is visualized here by displaying semiaxes a, b, and c that are related to the principal moments of inertia:

Figure 5. Representative snapshots of RMs with the E (left) and Z (right) configurations of 1. Bromide ions are colored pink; oxygen atoms, red; hydrogen atoms, white; carbon atoms, cyan; and nitrogen atoms, blue.

snapshots of RMs composed of 49 water molecules and 12 molecules of 1 in its E and Z configurations. At first glance, no significant difference is apparent in the structure of the water pool, whereas the overall shape of the micelle is different for the two isomers. The micelles were stable on the time scale of the simulation. However, the dissociation of a single water molecule from the micelle into the chloroform box was observed on several occasions. This suggests that the equilibrated micelles will indeed have a very small size, which is in good agreement with our experimental results. The structure of RMs is characterized quantitatively in Figure 6. We first focused on the structure of the water pool. The ECC model used in this study shows a clear surface preference for bromide ions. The water molecules form a compact, more or less spherical droplet, with the bromide ions forming an interface between the water pool and 1. The polar headgroup is therefore in direct contact with the bromide counterion that is further hydrated by water and resembles the structure of anion-exchange resins.79,80 Such a structure is in fact in stark contrast to findings in nonpolarizable simulations where bromide ions are dissolved in the water pool, and thus it occupies a larger volume. Clearly, the electronic polarization plays a decisive role in the position of F

I1 =

1 M (a 2 + b 2 ) 5

I2 =

1 M (a 2 + c 2 ) 5

I3 =

1 M (b 2 + c 2 ) 5 DOI: 10.1021/acs.langmuir.6b04455 Langmuir XXXX, XXX, XXX−XXX

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Langmuir

Figure 6. Radial distribution functions for bromide ions (A), water (B), and nitrogen atoms (C) of the trimethylammonium headgroup of 1 with respect to the center of mass of the water pool of RMs. Intramolecular end-to-end distance distributions for E-1 and Z-1 are shown in panel D.

Table 3. Sphericity Parameters of the Water Pool in RMs form (model)

aa

ba

ca

sb

Z-1 (ECC) Z-1 (nonpolarized) E-1 (ECC) E-1 (nonpolarized)

1.0 ± 0.1 1.13 ± 0.09 1.0 ± 0.1 1.2 ± 0.2

0.78 ± 0.07 0.93 ± 0.06 0.79 ± 0.07 0.95 ± 0.07

0.61 ± 0.06 0.76 ± 0.06 0.61 ± 0.06 0.77 ± 0.07

1.9 ± 0.2 2.5 ± 0.1 1.9 ± 0.2 2.6 ± 0.1

a

Average values of the semiaxes lengths (a, b, c). bAverage values of the ratio of the solvent-excluded surface to surface calculated from the SESrelated volume (s) for the water pool.

Table 4. Changes in the RM Properties Induced by Freezinga amphiphile c

E-1

Z-1d

T/Kb

δwater,core/ppm

wobs water,core

D/1010 m2 s−1

Rh/nm

c(1)/mMe

303 253 233 303 253 233

2.86 ± 0.03 2.68 ± 0.11 2.81 ± 0.09 2.31 ± 0.06 2.12 ± 0.12 2.21 ± 0.07

3.95 ± 0.07 3.48 ± 0.10 3.79 ± 0.09 2.73 ± 0.07 2.11 ± 0.11 2.36 ± 0.08

3.9 ± 0.4 4.5 ± 0.2 4.0 ± 0.1 4.5 ± 0.4 4.7 ± 0.4 4.8 ± 0.4

1.11 ± 0.12 0.96 ± 0.04 1.08 ± 0.03 0.96 ± 0.09 0.92 ± 0.08 0.94 ± 0.08

50 15 13 50 50 50

a analytical c1 b

= 50 mM, x = 4; all data were determined from the NMR measurements obtained immediately after the samples were warmed to 303 K. The lowest temperature to which the RM samples were exposed. cRMs composed of E-1 (100%). dRMs initially composed of Z-1 ([Z-1]/[E-1] = 96:4); see Table 1. eThe concentration of 1 in the solution at the given temperature shown in the second column (1H NMR).

This ratio decreases for a more spherical object, with a lower surface-to-volume ratio. Table 3 shows some distortion of the cluster, which is, nevertheless, not very large. It is generally comparable to the shape of the water pool as seen in the CTABcontaining RMs.11 Changes in RM Properties Induced by Freezing at 253 and 233 K. The effects of temperature on the physicochemical properties of a 1/water/chloroform-d system, where 1 is in either the E or Z configuration, were studied in the temperature range

The three semiaxes would be identical for a spherical object. Another quantitative criterion is based on calculating the solventexcluded surface (SES) and the corresponding volume. We can define a ratio s as

s=

SSES Ssphere

where SSES is the SES and Ssphere is the surface calculated from the SES-related volume considering the spherical shape of the object. G

DOI: 10.1021/acs.langmuir.6b04455 Langmuir XXXX, XXX, XXX−XXX

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Langmuir Table 5. Photochemistry of a 1/Water/Chloroform-d System at Subzero Temperaturesa entry 1 2 3 4 5 6

T/Kb 303 253 233 303 253 233

[E-1]/[Z-1] only [E-1] 7:93 70:30 4:96d 95:5 95:5

c

δwater,core/ppm

wobs water,core

D/1010 m2 s−1

Rh/nm

c(1)/mMe

2.87 ± 0.07 2.42 ± 0.10 2.60 ± 0.08 2.31 ± 0.06 2.27 ± 0.07 2.35 ± 0.04

3.95 ± 0.07 2.68 ± 0.09 3.45 ± 0.05 2.73 ± 0.07 2.59 ± 0.10 2.51 ± 0.08

3.9 ± 0.4 3.9 ± 0.2 4.2 ± 0.2 4.5 ± 0.4 4.0 ± 0.2 4.1 ± 0.2

1.11 ± 0.12 1.11 ± 0.06 1.03 ± 0.05 0.96 ± 0.09 1.08 ± 0.06 1.06 ± 0.05

50 50 13 50 1.5 n.d.

a analytical c1

= 50 mM, x = 4; all data were determined from the 1H NMR measurements obtained upon sample warming to 303 K. bThe temperature during irradiation (E-1 and Z-1 RMs were irradiated at 375 and 530 nm, respectively). cRMs initially composed of E-1 (100%). dRMs initially composed of Z-1 ([Z-1]/[E-1] = 96:4). eThe concentration of 1 in the solution after irradiation at the given temperature as shown in the second column (NMR).

of 303−233 K by 1H NMR. Initially, the solutions of RMs (canalytical = 50 mM, x = 4; E-1) in NMR tubes were allowed to 1 equilibrate for 12 h at 303 K (Table 4). The solutions were then cooled to either 253 or 233 K in a cooling bath and were allowed to equilibrate for 1 h. (No changes in the 1H NMR spectra were observed beyond this time period.) The frozen samples were immediately inserted into an NMR spectrometer precooled to the same temperature. In the case of E-1 RMs, the NMR spectra taken at subzero temperatures showed the signals of 1, but those of water nearly (253 K, Figure S22) or entirely (233 K, Figure S23) disappeared; therefore, it precluded the calculation of wobs water,core. It has already been shown that, contrary to the amphiphile signals, the water core NMR signal in CTAB RMs in chloroform disappears upon cooling to 233 K (the water core is frozen) and that RMs retain their original size and composition upon warming the sample back to 303 K.11 However, in the present case, the NMR signal intensities of 1 considerably decreased at subzero temperatures, and only 30 and 27% of E-1 remained in the solution at 253 and 233 K, respectively. Simultaneously, a yellow precipitate was formed over the whole volume of the solution and was clearly perceptible to the naked eye. The equilibration time of 1 h mentioned above was in fact needed to reach equilibrium between RMs in the solution and precipitated RMs. When the sample was rapidly warmed to 303 K (before any possible phase-separated water could be incorporated back into the RMs; see above), the precipitate immediately dissolved. The initial concentration of 1 was restored, and the δwater,core and wobs water,core values were lower only by approximately 10 and 5% for 253 and 233 K, respectively, compared to those set before freezing. Thus, only a very small amount of water was released from the RMs during the cooling process (Table 4). Indeed, the sizes of the RMs did not change after the freeze−thaw cycle was finished because the Rh values (1.11 vs 1.08 nm, Table 4) remained within experimental error. We assume that the core is already substantially rigid to prevent water shedding at 233 K. In addition, a higher viscosity of chloroform as 233 K83 might also contribute to the stabilization of RMs. The behavior of RMs composed primarily of the Z-1 isomer (96%; prepared by irradiation of E-1 RMs at 375 nm to the PSS at 303 K; see Table 1) at subzero temperatures was also studied. This Z-1/water/chloroform-d system (wobs water,core ≈ 2.7) was exposed to the same cooling−warming cycles as in the preceding experiments (Table 4, Figures S24 and S25). In a 303 → 253 → 303 K temperature cycle, wobs water,core decreased from 2.73 ± 0.07 to 2.11 ± 0.11 ppm because of water shedding (33%). Interestingly, contrary to E-1 RMs, the concentration of Z-1 remained the same at subzero temperatures (Table 4), and thus no RM precipitation occurred. In a 303 → 233 → 303 K cycle, water

shedding was less pronounced (15%). The corresponding diffusion coefficient D and hydrodynamic radius Rh values correlate very well with the changes in reverse micelles size inferred from δwater,core and wobs water,core. Photoisomerization in RMs at Subzero Temperatures. The photochemical behavior of reverse micelle solutions composed of both E and Z isomers of 1 was also studied at subzero temperatures. First, the E-1 or Z-1 RMs solutions (canalytical = 50 mM, x = 4; Z-1 RMs were prepared photochemi1 cally at 303 K, see Table 4) in NMR tubes were allowed to equilibrate at 303 K for 10 h (Table 5, entry 1). The solutions were then cooled to either 253 or 233 K in a cooling bath and were allowed to equilibrate for 1 h (a precipitate appeared; Table 4). Then the samples were irradiated at 375 or 530 nm in the cases of E-1 (entries 2 and 3) and Z-1 (entries 4 and 5) RMs, respectively, until a PSS state was reached (typically in 2−8 h, when no changes in the NMR spectra were observed). After irradiation was stopped, an NMR tube was inserted into a precooled NMR spectrometer, and the 1H NMR spectrum was recorded. Afterward, the samples were rapidly warmed to 303 K, and 1H NMR spectra were taken again before equilibration of the sample could occur (to prevent any potential phase separation of the released water, Table 5). Upon irradiation of E-1 RMs at 253 K (Figure S29), the PSS ratio of [E-1]/[Z-1] measured for RMs that remained in the solution (the concentration of RMs, c(1), varied during irradiation because of RMs precipitation) was ∼3:97. Upon warming the sample to 303 K, all precipitated RMs were redissolved to give the final 7:93 ratio (Table 5, entries 2 and 3), which is only slightly different from that obtained by the photolysis of the same sample at 303 K (4:96; Table 1). Upon irradiation of E-1 RMs at 233 K (with ∼75% of RMs precipitated), the PSS ratio of [E-1]/[Z-1] in RMs in the solution again reached the [E-1]/[Z-1] ratio of ∼3:97; however, when the precipitated RMs were redissolved at 303 K, we found that only incomplete isomerization of E-1 ([E-1]/[Z-1] = 70:30) occurred despite extensive irradiation (Table 5, entry 3, Figure S30). Although the size (Rh) of RMs remaining in the solutions was essentially unaffected during irradiation, the water content (wobs water,core) and shift (δwater,core) decreased at 253 K, whereas these changes were negligible at 233 K. The more efficient water shedding at 253 K is consistent with the already-discussed behavior of E-1 RMs (Table 4). In contrast, photochemical experiments with Z-1 RMs started with samples where all RMs were dissolved. Thus, the irradiation of Z-1 RMs at 530 nm at both 253 and 233 K gave micelles with almost complete conversion of the amphiphile configuration ([E-1]/[Z-1] = 95:5; Table 5, entries 4 and 5, Figure S31 and S32), but RMs precipitated during irradiation to a large extent; H

DOI: 10.1021/acs.langmuir.6b04455 Langmuir XXXX, XXX, XXX−XXX

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form. In our simulations, all 1 molecules were isomerized at the beginning of the simulations. This represents an extreme case in which all of the azobenzene units isomerize at the same time, and thus the system experiences a rather large flow of kinetic energy from the chromophore. It is in contrast to the experimental procedure described in the experimental section, when the chromophores isomerize relatively slowly in sequence. Figure 8 shows the end-to-end distances of the amphiphile 1 molecules as an average of 10 simulations of the Z → E (Figure 8A) and E → Z (Figure 8B) isomerization. After the isomerization is complete, the average end-to-end distances (distance between the trimethylammonium headgroup nitrogen and carbon atom on the other end, rN−Cter) of the amphiphiles rapidly adjust to the new arrangement on a time scale of picoseconds. In the case of the Z → E isomerization, the average distances increase, and for the E → Z isomerization, the average distances decrease. The dynamics are relatively slow as a result of the dense environment of chloroform as a solvent and the large size of the micelles. The average temperature of the amphiphile 1 molecules also reflects a similar trend; e.g., it can be used as one of the possible descriptors of the process. Immediately after the conversion takes place, the kinetic energy of 1 increases because of the huge amount of excess energy as compared to the thermal energy. Within picoseconds, the temperature (i.e., kinetic energy) decreases to a value of 300 K; the excess energy has dissipated into the chloroform bath. The RMs remain essentially intact during the isomerization process. The water pool size, demonstrated here in terms of the SES volume (VSES), remains the same after isomerization occurs. The MD simulations cannot access the time scales needed for the description of experimentally observed water shedding, yet the obtained molecular picture of photoswitching still yields valuable information. For instance, we can conclude that the excess energy deposited into the system does not lead to an immediate disintegration of RMs but is very quickly dissipated into the environment, and RMs are stable. The partial release of water molecules observed experimentally is not triggered by the excess energy released during photoisomerization but is a thermodynamically driven process, taking place on a longer time scale than that of the MD simulations. We performed additional simulations for cases where only either four or one amphiphilic 1 molecule undergoes isomerization. Also in this case, no dramatic changes in micellar size or properties take place. The end-to-end distances for these simulations are presented in the Supporting Information in Figure S3 and S4. In addition, we performed simulations of the E-1 and Z-1 RMs at 233 K. The first question was whether the water pool can be considered to be frozen, as has been suggested by NMR experiments. The simulations suggest that the water dynamics are much slower at 233 K than at 300 K, yet the diffusion is still observed. This is in agreement with previously studied CTAB RMs.11 The description of freezing the water pool is limited by the possibilities of the employed model; complete freezing most probably requires a temperature below 233 K. The structural characteristics of the micelles at a lower temperature are very similar to those found at room temperature, and thus we can expect that even amphiphile photoisomerization will not lead to the destruction of the micelle in this case. There are two experimental findings that would need a further detailed rationalization: (i) water shedding in RMs and (ii) a lower solubility of the E form. Unfortunately, such processes cannot be described within the nanosecond time scale of our simulations.

only a negligible amount, 3 and