Interplay of Molecular Hydrogelators and SDS Affords Responsive Soft

Jun 27, 2013 - Emily R. Draper , Matthew Wallace , Ralf Schweins , Robert J. Poole , and Dave J. Adams. Langmuir 2017 33 (9), 2387-2395. Abstract | Fu...
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Interplay of Molecular Hydrogelators and SDS Affords Responsive Soft Matter Systems with Tunable Properties Vicent J. Nebot,†,§ Beatriu Escuder,† Juan F. Miravet,*,† Johan Smets,‡ and Susana Fernández-Prieto‡ †

Departament de Química Inorgànica i Orgànica, Universitat Jaume I, 12071 Castelló, Spain Procter & Gamble, Teemselaan 100, 1853 Strombeek-Bever, Belgium

‡

S Supporting Information *

ABSTRACT: The gelation efficiency of low molecular weight bolaamphiphilic hydrogelators 1 and 2 is influenced by the presence of SDS micelles. Similarly, the critical micellar concentration value of SDS is reduced in the presence of the studied molecular hydrogelators. Rheological measurements indicate that the strength of the hydrogels can be modulated with SDS, the gels becoming weaker in the presence of micelles. This behavior has been rationalized with the help of NMR studies using diffusion measurements and NOE correlations. The results obtained clearly point to the formation of mixed micelles composed of SDS and the hydrogelators. In the case of 1, the gelator:SDS ratio in the mixed micelles has been estimated from solubility studies to be ca. 1:2.5. Electron microscopy reveals that when SDS is present, the morphology of the xerogels is modified in its appearance at the micrometer scale but fibers with diameter in the nanometer range are observed in all the cases. The interplay between the surfactant and the gelators provides with new possibilities for the modulation of both gel and micelle formation. Examples are shown to highlight the potential usefulness of this type of interconnected system. In one case the release of a gel entrapped dye is modulated by the presence of SDS and sodium chloride. In another example, an intricate system that responds to a temperature excursion by irreversible micelle disassembly is shown.

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components may assemble orthogonally,12−14 self-sorting into separated phases, or mutually interact leading to interconnected complex systems. The interplay between molecular hydrogelators and surfactants has been scarcely addressed. In one case SDS and a zwitterionic amphiphile were shown to form a two-component gel which was heat- and pH-responsive.15 Within the study of orthogonal self-assembly of surfactants and hydrogelators, it was reported that the critical micellar concentration (cmc) values of SDS and other surfactants were not significantly altered in the presence of hydrogels formed from 1,3,5-triamide-cis,cis-cyclohexane-based hydrogelators.12−14 On the contrary, in the same work an important process of gelator solubilization was observed in the presence of cationic micelles. Recently, a system composed of an enzymesensitive hydrogelator and a liposome containing the appropriate enzyme was used for controlled release.16 Additionally, it has been shown that the addition of ionic surfactants to a tripodal urea is key for the formation of hydrogels,17 and a supramolecular gelator and SDS have been used in protein separation by electrophoresis.18

INTRODUCTION The interest in molecular (hydro)gels has grown exponentially in recent years, and applications have been developed in a wide range of areas such as regenerative medicine, controlled drug release, optoelectronic materials, or catalysis among others.1−5 The formation of molecular gels represents an intriguing case of self-assembly of low molecular weight species. Commonly the aggregation produces nano(micro)fibrillar networks that percolate the solvent and transform it into a viscoelastic material. These materials are formed by weak noncovalent interactions that may be conveniently reversed by external physicochemical stimuli leading to environment responsive systems.6 Besides, the study of complex supramolecular systems is gaining increased attention in the past years after the blossoming of the field of systems chemistrythe chemistry of mixtures of interrelated components.7−11 In this context, the development of complex and multiresponsive gels is of great interest considering that many of the real-world applications of those materials will face with complex dynamic mixtures, i.e., biological fluids, cells, contaminated water, etc., where spatiotemporal combination of stimuli may be determinant for the reliability of the response. One of such complex systems is obtained when mixing two different kinds of soft matter building blocks: molecular gelators and surfactants. These © 2013 American Chemical Society

Received: May 2, 2013 Revised: June 21, 2013 Published: June 27, 2013 9544

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cuvette (optical path 1 cm) a hot aqueous solution of methylene blue (0.2 mM) and gelator (21 mM). Then, 2.5 mL of an aqueous solution containing SDS at the desired concentration was deposited in the cuvette on the top of the gel. For the studies with NaCl solutions the same procedure was followed. For the studies involving the polymorphic transition of compound 2, a hydrogel of 0.5 mL containing 5 mM SDS and 4 mM 2 was prepared into a 5 mm NMR tube as reported above. Then the gel was heated in thermostatic bath at 70 °C for 30 min and later cooled to room temperature. For the electron microscopy studies a vial containing the hydrogel was immersed in liquid N2 at −196 °C for 1 min and connected to a Telstar Lyoquest freeze-drying system at −85 °C and 0.005 mbar. All the samples were freeze-dried for at least 3 days. The resulting solid was used as obtained for the required experiments. Field-emission scanning electron micrographs were taken on a JEOL 7001F microscope equipped with a digital camera. The corresponding freeze-dried gels were placed on top of an aluminum specimen mount stub and sputtered with Pt.

Here we report on a fundamental study and potential applications of the complex systems which emerge from the interplay of bolaamphiphilic hydrogelators 1 and 2 and SDS.

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EXPERIMENTAL SECTION

Compounds 1 and 2 were prepared as described previously.19,20 Their hydrogels have also been reported by our group.20 Sodium dodecyl sulfate (99%), Sudan I (98%), and pyrene (98%) were purchased from ACROS. Milli-Q water was used for the gel preparation. To prepare the gels formed by compounds 1 or 2, weighed amounts of these solidsand SDS if requiredwere dissolved in water by heating with caution in a screw-capped vial and then rested for 30 min at room temperature (vial dimensions: 2.5 cm width, 6.5 cm height or, alternatively, 1.5 cm width and 4.5 cm height; Teflon-lined caps were used). Typical quantities of gelator for gelation of 1 mL of water were 10 mg of 1 and 2 mg in the case of 2. Rheological measurements were carried out with oscillatory experiments in a TA AR-G2 rheometer equipped with a Peltier temperature control accessory, using a steel parallel plate−plate geometry (40 mm diameter). The gap distance was fixed at 1 μm. The gels were aged for 24 h before measurement. A homogeneous layer of gel was placed between the two plates, and the sample was rested for 10 min before the measurement. Frequency and stress sweeps were performed at 20 °C. To compare the rheological profiles in the presence and absence of SDS (Figure 2), a frequency sweep at a constant oscillatory stress (0.2 Pa) and a oscillatory stress sweep at a constant frequency (6.28 rad s−1) were used, conditions which fall within the linear viscoelastic regime for both systems (see Supporting Information). 1 H and NOE NMR measurements were carried out in a Varian Inova 500 MHz spectrometer at 30 °C. NOE experiments where recorded with a mixing time of 100 ms. Diffusion experiments were carried out with the bipolar pulse pair stimulated echo sequence at 293 K. Diffusion coefficients were calculated using the values of the intensity of the observed signal for 15 different gradient strengths in the Stejskal−Tanner equation (ln(I/I0) = −γg2δ2(Δ − δ/3)D),4 where I and I0 are the signal intensities in the presence and absence of the pulsed-field gradients, respectively, γ is the gyromagnetic ratio (rad s−1 G−1), g is the strength of the diffusion gradients (G m−1), D is the diffusion coefficient of the observed spins (m2 s−1), δ is the length of the diffusion gradients (s), and Δ is the time separation between the leading edges of the two diffusion pulsed gradients (s). To prepare the gels for NMR studies, a hot solution of the gelator in D2O (99.8%) was transferred from a vial into a 5 mm NMR tube. The samples were aged for 24 h at room temperature before the measurements. A concentric capillary tube (1 mm width) containing a D2O solution of 4,4-dimethyl-4-silapentane-1-sulfonic acid (DSS; 2 mg/mL) was used as external standard for NMR solubility measurements. The relative integral value of the pyridine protons to DSS was measured in the gel samples. To obtain samples with all the gelator in solution, the gel in the NMR tube was disassembled by addition of 30−60 μL of trifluoracetic acid. The results were reproduced successfully at least twice. The minimum concentration required for gelation was assayed with the vial inversion methodology.21 For the experiments regarding the solubilization of Sudan I, 5 mg of Sudan I was suspended in 1 mL of either a 5 mM aqueous solution of SDS or an aqueous solution containing 5 mM SDS and 4 mM compound 1. The suspensions were sonicated for 5 min in a vial and then filtered (0.45 μm mesh, nylon HPLC filter). For the experiments regarding fluorescence quenching of pyrene, 1 mg of pyrene was suspended in either a 5 mM aqueous solution of SDS or an aqueous solution containing 5 mM SDS and 4 mM compound 1. The suspensions were sonicated for 5 min in a vial and then filtered (0.45 μm mesh, nylon HPLC filter). The picture of the vials was taken under 254 nm UV light. Methylene blue release studies were followed by absorbance measurements with a UV−vis spectrophotometer at 660 nm. A gel of 0.5 mL was prepared by cooling to room temperature in a UV-

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RESULTS AND DISCUSSION We reported previously that compound 1 (Scheme 1) forms hydrogels at a minimum concentration of 18 mM by cooling its Scheme 1. Chemical Structure of SDS and Compounds 1−3

hot aqueous solutions to 25 °C.20 Hydrophobic effect and Hbonding were shown to be driving forces for gelation. In the context of the results reported below, it is important to recall here that the solid-like fibrillar network of molecular gels, which is NMR silent and coexists with free, nonaggregated, soluble species. For example, the hydrogel formed by 1 presents a solubility of 9 mM at 25 °C. In the present work while studying the possible interaction between 1 and SDS we found that these species mutually interfere in both the gelation and micelle formation processes. The interplay was manifested clearly when the solubility of the gelator 1 was studied in the presence of different amounts of SDS. For this study the solubility was assessed using 1H NMR integration of the soluble gelator. It can be seen in Figure 1 that for [SDS] < 4 mM the solubility of 1 slightly decreases upon addition of SDS, most likely as a result of the increasing ionic strength. It has to be noted that in 9545

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volume of micelles when compared to free surfactant provokes a diminution of the diffusion coefficient. For example, in agreement with previous reports,23 we observed that free SDS, at a concentration below the critical micelle concentration, presented a diffusion coefficient at 30 °C in D2O of 5.1 × 10−10 m2 s−1 while in the presence of micelles ([SDS] = 20 mM) the value was 2.3 × 10−10 m2 s−1, roughly half of the previous one (entries 1 and 2, Table 1). It has to be noted that the change in Table 1. Self-Diffusion Coefficients at 30 °C of Aqueous Systems Containing Hydrogelators 1 or 2 and SDS at Different Concentration Valuesa

Figure 1. Plot of the solubility at 30 °C of 1 in its hydrogels vs the concentration of SDS present in the medium.

this initial concentration range SDS is not forming micelles. A sharp tendency change in the solubility of 1 is detected for [SDS] > 4 mM, resulting in a linear increase. We have ascribed this behavior to the incorporation of free, soluble, hydrogelator molecules into the SDS micelles in such a way that the equilibria involved move the system toward gel disassembly. As a consequence, the concentration required for gelation of 1 is clearly affected by SDS. For example, at [SDS] = 20 mM, the minimum gelator concentration value for hydrogelator 1 changes from 19 to 26 mM. Very conveniently, the composition of the micelles containing SDS and compound 1 can be estimated considering that the slope of the graph in Figure 1 is equal to the molar ratio 1:SDS in the mixed micelles, which is calculated in this way to be of 1:2.5. The reduced gelation efficiency of the gelator in the presence of SDS micelles correlates with the change observed in the rheological properties. The data represented in Figure 2 show

entry

sample composition

1 2 3 4 5 6

SDS (4 mM) SDS (20 mM) 1 (2 mM) + SDS (4 mM) 1 (2 mM) + SDS (20 mM) 2 (1 mM) + SDS (1 mM) 2 (1 mM) + SDS (14 mM)

a

Dgelator/10−10 m2 s−1

DSDS/ 10−10 m2 s−1

2.6 1.7 3.3 1.6

5.1 2.3 3.9 1.7 4.5 1.9

Estimated error is 5%.

the diffusion coefficient is not proportional to the change in the mass of the species involved but to the hydrodynamic radius, which is related to the mass by a cubic factor. Therefore, although SDS micelles are known to consist of ca. 60 molecules, the diffusion coefficient reported above is only 2.4 times smaller than that of the free molecules. Additional factors to be taken care of in this context are the changes in the solvation shell associated with aggregation and the fact that the diffusion coefficient reflects the average diffusion of free and micellar species, and therefore its value depends on the total concentration. A marked decrease of the diffusion coefficient of 1 from 2.6 × 10−10 to 1.7 × 10−10 m2 s−1 was measured upon increasing the concentration of SDS from 4 to 20 mM (entries 3 and 4 in Table 1; the diffusion coefficient of 1 in the absence of SDS is 3.4 × 10−10 m2 s−1; see Supporting Information). This result clearly confirms the incorporation of the free hydrogelator into the SDS micelles as hinted in the previous solubility experiments. Compound 2, which contains an L-isoleucine moiety instead of a L-valine unit, was similarly studied. The introduction of the L-isoleucine residue makes 2 a quite more efficient hydrogelator than 1, being the reported solubility in water 1 mM and the concentration required for gelation 4 mM, values which are significantly lower than in the case of 1.20 Hydrogelator 2, similarly to 1, is able to form mixed micelles with SDS as demonstrated by diffusion measurements. As revealed in Table 1 (entries 5 and 6), the diffusion coefficient of 2 was significantly reduced from 3.3 × 10−10 to 1.6 × 10−10 m2 s−1 upon addition of 14 mM micellar SDS, confirming its incorporation into the micelles. Gelation efficiency of 2 is also reduced in the presence of SDS. For example, in the presence of 20 mM SDS the minimum concentration required for gelation increases dramatically from 4 to 20 mM. Considering that the cmc of SDS in the presence of 2 is 5 mM (vide infra) and that the measured solubility increase is 16 mM, it can be concluded the presence of approximately equimolar quantities of 2 and SDS in the mixed micelles. Additionally, diffusion coefficient measurements also permitted to calculate the variation of SDS cmc value in SDS− hydrogelator systems. When plotting the diffusion coefficient of

Figure 2. Oscillatory rheological measurements recorded at a constant frequency of 6.28 rad s−1 for hydrogels composed of 1 (21 mM) and 1 (21 mM) + SDS (20 mM). G′ = storage modulus; G″ = loss modulus.

how the rheological moduli G′ and G″ of the hydrogel formed by 1 are reduced several orders of magnitude in the presence of micellar SDS. In the conditions of the rheological measurement, the hydrogel of 1 was stable at oscillating stress values higher than 100 Pa (G′ > G″), but in the presence of SDS micelles the gel was disassembled at stress values higher than 1 Pa (G″ > G′). On the other hand, in order to evaluate the influence of hydrogelator 1 in SDS micelle formation and to confirm the formation of mixed micelles, NMR diffusion measurements were carried out. As reported in the literature, the magnitude of the diffusion coefficient of surfactants experiences changes associated with micelle formation.22 The large hydrodynamic 9546

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In additional experiments, naked-eye detection of mixed micelles was achieved using colored and fluorescent probes. First, an assay on the solubilization of the water insoluble dye Sudan I was carried out. This dye could not be dissolved neither in the presence of SDS at premicellar concentrations nor in the presence of 4 mM 1. However, upon addition of compound 1 (4 mM) to the SDS solution dye solubilization took place, indicating the formation of micelles in this system (see Supporting Information). Second, taking advantage of the fact that the fluorescence of pyrene is quenched in the presence of pyridine derivatives by an electron transfer mechanism,25 a solution of pyrene in water containing SDS micelles was prepared. In this system pyrene, which otherwise is insoluble in water, is incorporated into the SDS micelles (pyrene is not soluble in the presence of compound 1 without addition of SDS). When the pyrene−SDS solution was observed under the UV light (254 nm), fluorescence from pyrene was detected all over the sample. Upon addition of compound 1 to this solution the fluorescence of pyrene was quenched, indicating the incorporation of the gelator in the SDS micelles (see Supporting Information). It could be hypothesized that the incorporation of hydrogelators in the mixed micelles occurs by protonation of the pyridine units, yielding pyridinium species that would exchange with the sodium counterions of the negatively charged micelles. Indeed, ion-pairing of amphihiles at the head groups of aqueous micelles has been reported affording lamellar fiber formation.26,27 However, in this case NMR studies show that the signal of pyridine protons is not shifted in the presence of SDS micelles, as would be the case if protonation would occur in the pyridine ring. This result indicates the incorporation of neutral hydrogelator in the mixed micelles (see Supporting Information). The morphology found in a xerogel of 1 using electron microscopy (SEM) revealed a fibrillar network as commonly observed in supramolecular gels. Fiber diameters were in the nanometer range in both the presence and absence of SDS micelles. However, evident differences can be seen in the micrometer size scale. When the gels were prepared in the presence of SDS micelles, layered, honeycomb-like structures were found (Figure 5). This is a common artifact as a result of the formation of ice crystals instead of vitrified ice during the freeze-drying process,28 which in this case seems to be favored with the presence of SDS in the samples. In order to further

SDS recorded at different total concentrations of SDS, a clear discontinuity is observed upon reaching the micelle formation concentration (Figure 3). We found that with this methodology

Figure 3. Self-diffusion coefficient of SDS vs the concentration of SDS present in the medium at 30 °C for systems containing only SDS (full circles) and for systems containing gelator 1 (4 mM) and SDS (empty circles).

the cmc value obtained for SDS was 8 mM, in accordance with previous literature reports.24 Noticeably, micelle formation took place at significantly lower cmc values when compound 1 or 2 was present. For example, when a 4 mM solution of 1 was present, the cmc was reduced from 8 to 5 mM (Figure 4), and in the presence of 1 mM 2 the cmc was shifted also to 5 mM (see Supporting Information). Remarkably, the bolaamphiphilic structure of the hydrogelators is key both for the formation of hydrogels20 and for the formation of mixed micelles. Indeed, we found that compound 3 (Scheme 1) which when compared to compound 1 lacks an isonicotinoylvalinyl moiety did not affect the cmc value of SDS (see Supporting Information). NMR NOE experiments also supported incorporation of the hydrogelators in the micelles. For example, it was observed that while the molecules of compound 1 in solution provide positive intramolecular NOE signals, as expected for molecules with molecular weight below ca. 1000 Da, in the presence of SDS micelles negative intramolecular NOE correlations were detected which are commonly associated with high molecular weight species (see Supporting Information). Furthermore, intermolecular NOE correlations were detected between the methyl groups of gelator 1 and the aliphatic chain of SDS, revealing the spatial proximity of the two molecules in the mixed micelles (Figure 4).

Figure 4. 2D-NOESY spectrum of a sample containing micellar SDS (20 mM) and gelator 1 (2 mM); arrows point to the correlations mentioned in the text. 9547

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resulting from SDS-stimulated gel disassembly, was monitored by UV−vis spectroscopy analysis of the aqueous SDS solution deposited over the gel. No significant release was detected in the absence of SDS in the time scale of the experiment. However, in the presence of SDS micelles, the gel was progressively disassembled and, consequently, the dye released (see Supporting Information). For a concentration of SDS of 0.1 M ca. 50% of the dye was liberated after 30 min. It has to be stressed here that the release process comes from the incorporation of the gelator into the SDS micelles and the consequent gel disassembly, being the equilibrium depicted in Scheme 2 shifted toward soluble species. Noticeably, the release rate can be easily regulated, and for example, when 0.2 M SDS micelles were present, full gel disassembly and dye release were attained after 30 min. Further regulation of the controlled release system described above can be achieved by modulation of micelle formation with ionic strength. A system was devised that would release the entrapped dye in the presence of NaCl aqueous solutions, similar to biological media, but not in the absence of salts. For this purpose systems were prepared containing the gel entrapped dye and a solution with premicellar SDS (5 mM) deposited over the gel. In one case the system was prepared in distilled water, and in the other one the solvent was aqueous sodium chloride (40 mM). It was expected than the system prepared in aqueous sodium chloride would result in SDS micelle formation due to the increased ionic strength30 and, therefore, in the incorporation of gelator 1 into the SDS micelles with the consequent dye release. Indeed, as shown in Figure S9, UV−vis measurements revealed that methylene blue was not released when the gel was prepared in distilled water and a premicellar SDS solution was deposited on it. Also, if the gel was prepared in 40 mM sodium chloride and the same saline solution deposited over the gel, without SDS, no release took place. However, a steady release of the dye was monitored when the solvent used in the gel preparation was 40 mM NaCl solution and a 5 mM solution of SDS in aqueous NaCl was deposited on top of it (see Supporting Information). Finally, in order to explore another potential application of the complex systems resulting from the interplay of hydrogelators and SDS, we aimed to link the temperature sensitivity of the supramolecular gels with the formation of mixed micelles. In this case a rather intricate system was designed taking advantage of our previous knowledge of the hydrogelation properties of 2 (Scheme 3). This compound forms hydrogels which undergo a polymorphic transition upon heating above 70 °C.20 The transition is irreversible, and when cooling back to room temperature, the new polymorph is conserved. For the purposes of this paper, a very relevant point is that the solubility of the gelator changes 1 order of magnitude with the polymorphic transition from 1 mM for polymorph I to less than 0.1 mM for polymorph II. We hypothesized that the formation of micelles could be regulated with the mentioned polymorphic transition. A system containing the gel formed by 2 from a solution of premicellar SDS (5 mM) was prepared. It has to be recalled here that this gel contains, aside from the solid-like fibrillar network of 2, the soluble gelator (1 mM). Although SDS is at premicellar concentration in this system, the presence of 2 permits the formation of mixed micelles, as demonstrated clearly with NMR diffusion measurements. The diffusion coefficient measured for the SDS molecules in this gel was 2.6 × 10−10 m2 s−1well below the value of free SDS and in

Figure 5. SEM images of freeze-dried hydrogels: (A) compound 1 (19 mM); (B−D) compound 1 (32 mM) and SDS (30 mM).

characterize the xerogels, X-ray diffraction studies were carried out, affording quite broad diffractograms which did not provide useful information (see Supporting Information). The results presented up to this point may allow concluding that the interplay between SDS and 1 connects the formation of two reversible supramolecular systems, hydrogel fibers and micelles, in such a way that SDS represents a stimulus that influences gelation and the presence of 1 represents a stimulus that influences micelle formation (see depiction in Scheme 2). Scheme 2. Equilibria Involving Gel Fibers, SDS Micelles, and Mixed Micelles

It could be said that this interplay creates a soft matter system which might allow for a fine-tuning by means of multiple variables such as gelator or SDS concentration and, especially interesting, stimuli that regulate either gelation (for example, temperature) or SDS micelle formation (for example, ionic strength). To test the feasibility of using this type of system in different applications, in first place the modulation of the release rate of gel-entrapped species was assayed. The controlled release from molecular gels is becoming increasingly of interest, especially in the context smart drug delivery systems.29 A system consisting of methylene blue entrapped into a gel formed by 1 and an aqueous solution of SDS deposited over the gel was devised. The gel loaded with methylene blue was prepared by gelation of a dye solution. Then the release of the entrapped dye, 9548

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Scheme 3. Pictorial Representation of Micelle Disassembly within a Hydrogel Formed by Compound 2

Overall, as a general conclusion, it could be stated that here we have shown that the interplay of two equilibria that involve different soft materials results in systems with interesting emergent properties. To some point, this work could be regarded as a proof of principle for envisaged complex, smart systems that could be constructed following this approach. For example, the development of multipurpose smart molecular hydrogel systems which compete with their polymeric counterparts31 seems to be very appealing, taking into account applications such as controlled drug release. Finally, it should be mentioned that the work is somewhat related to the complex chemistry of life which is characterized by an intricate interplay of multiple equilibria related to both chemical reactions and aggregation processes.

agreement with formation of mixed micelles (Figure S10). As expected, when the system was heated to 70 °C and cooled back to room temperature, the polymorph II was formed and the solubility of 2 decreased below 0.1 M. At this stage, the absence of free compound 2 in solution precludes the formation of mixed micelles with SDS, and the final result is an irreversible micelle disassembly, as revealed by the significant increase in the diffusion coefficient of SDS from 2.6 × 10−10 to 4.4 × 10−10 m2 s−1. In summary, as depicted in Scheme 3, the devised system responds to a temperature excursion above 70 °C with irreversible micellar disassembly.

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CONCLUSIONS Here we have reported that hydrogelators 1 or 2 and SDS interplay in the micelle and gel forming processes. The key for this behavior is the formation of mixed micelles with an estimated SDS:gelator composition of 2.5:1 for the case of 1 and almost equimolar in the case of 2. The bolaamphiphilic characteristics of the gelators, which contain relatively polar isonicotinoyl terminal groups and nonpolar moieties derived from L-valine or L-isoleucine, could explain this tendency to interact with the surfactant. The gelation capabilities of 1 and 2 are significantly reduced in the presence of SDS due to the solubilization of the solid-like fibrillar network. On the other side, the cmc value of SDS is clearly diminished in the presence of hydrogelators 1 or 2 due to their role as comicellar species. In this paper, as an example, a tuning of the release rate of gelentrapped methylene blue is described. The presence of SDS micelles provokes dye release due to gel disassembly. The fact the amount of released dye after 30 min is approximately doubled when the concentration of SDS is changed from 0.1 to 0.2 M reveals that a fine-tuning release rate could be achieved in this type of system playing with the concentration of surfactant. This approach has been further elaborated with the use of NaCl to induce micelle formation. Gels loaded with the dye did not release their content when a premicellar solution of SDS in distilled water was used as medium. However, when the medium was aqueous sodium chloride, the release proceeded steadily. One could think of the use of this approach, with the pertinent modifications, for the preparation of smart gels that when inserted in biological media would release their content. Finally, another point of complexity and versatility to these systems has been introduced when the gels formed by 2 are considered. The polymorphic transition experimented by this gel allows for a temperature-regulated micelle disassembly process which could be of potential interest, for example, in the signaling of a temperature excursion.

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S11. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.F.M.). Present Address

́ V.J.N.: Laboratorio de Polimeros Terapéuticos, Centro de Investigación Principe Felipe, 46012 Valencia, Spain. §

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Spanish Ministry of Science and Innovation (Grants CTQ2009-13961 and CTQ2012-37735) and Universitat Jaume I (Grant P1.1B2012-25) for financial support.

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REFERENCES

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dx.doi.org/10.1021/la401653b | Langmuir 2013, 29, 9544−9550