Controlling the Supramolecular Architecture of Molecular Gels with

Center for Soft Condensed Matter Physics and Interdisciplinary Research, Soochow University, Suzhou 215006, P. R. China. Langmuir , 2016, 32 (4), pp 1...
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Controlling the Supramolecular Architecture of Molecular Gels with Surfactants J. Y. Chen,† B. Yuan,‡ Z. Y. Li,† B. Tang,† E. Ankers,† X. G. Wang,† and J. L. Li*,† †

Institute for Frontier Materials, Deakin University, Geelong, Australia Center for Soft Condensed Matter Physics and Interdisciplinary Research, Soochow University, Suzhou 215006, P. R. China

‡

S Supporting Information *

ABSTRACT: Manipulating molecular assembly is significant for achieving materials with desirable performances. In this paper, two nonionic surfactants, Span 20 and Triton X-100, are used to tune the nucleation and fiber growth of a molecular gelator 2,3-di-n-decyloxyanthracene (DDOA). Confocal microscopic images show that Span 20 induces elongation of DDOA spherulites, and promotes fiber side branching. In contrast, Triton X-100 enhances the primary nucleation of DDOA leading to the formation of smaller DDOA spherulites, and promotes fiber tip branching. 1H NMR investigation demonstrates strong interactions between the hydrophobic tails of the surfactants and the alkyl chains of DDOA molecules.The interactions significantly reduce the diffusion of DDOA molecules. The different effects of the two surfactants could be attributable to their different alkyl hydrophobic tails. The hydrophobic tail of Span 20 is similar to the alkyl chain of DDOA, which could promote the adsorption of Span 20 on the fiber side surface rich in alkyl chains of DDOA.While the benzene ring in the hydrophobic tail of Triton X-100 could facilitate the primary nucleation of DDOA and the adsorpion of Triton X-100 on the fiber tip surface rich in aromatic structure of DDOA. The observations of this work will help the development of a convenient approach to tune the fiber network structure of molecular gels.

1. INTRODUCTION Structures formed through supramolecular assembly to provide specific functions are common in nature. This bottom-up approach for materials formation has stimulated the design of functional materials with significant applications in various fields. Molecular gel is a class of supramolecular materials.1−3 Depending on their properties, molecular gels have promising applications in light emitting devices, energy harvesting and transfer, cosmetics and foods, controlled drug release and controlled crystal growth, chemical sensing, and environmental remediation,4−9 to name just a few. The fibers in a gel form a three-dimensional network that effectively traps a solvent. Controlling the fiber structure is significant as it determines the physical properties and applications of a gel.10 Understanding the role of the molecular structure of a gelator and the properties of the solvent in the assembly has led to the design of a number of new families of gelators. However, designing a new gelator that can produce the desired properties remains very challanging due to the strong solvent-dependence of the self-assembly process (Figure 1a).11,12 In this context, to facilitate the applications of this class of material, it is essential to develop convenient ways to engineer their supramolecular structures so that the desired properties can be achieved.13,14 This approach to engineer the supramolecular architecuture (Figure 1b) of materials is becoming a subject of interest. The nucleation and growth mechanism of fiber formation in © 2016 American Chemical Society

Figure 1. Two routes for the development of molecular gels with desirable structure and properties.

molecular gels has led to the development of some convenient approaches to control their fiber structures by manipulating the Received: December 1, 2015 Revised: January 10, 2016 Published: January 11, 2016 1171

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Figure 2. Confocal microscopic images of DDOA fiber networks formed in the absence (a) and presence of Span 20 (b−e) and Triton X-100 (f−h). The concentration of DDOA was fixed at 2 mM. The scale bars represent 100 μm. The circles in (c) and (g) demonstrate fiber side and tip branching, respectively. The scale bar in (a) represents 100 μm. All the images are on the same scale.

thermodynamic driving force,15−17 applying volume confinement,18,19 and using molecular additives.14,15,20 Due to their interfacial adsorption properties, surfactants have been widely used to control the growth of inorganic crystals.21,22 However, they have not been used widely to control fiber crystallization in molecular gels.23 During the fiber crystallizaion in a molecular gel, the fiber-solvent interfaces being created provide a huge interfacial area for adsorption of surfactants. Similar to the inorganic systems, surfactant molecules can potentially affect the nucleation and growth of fibers. It is worthy of investigating on a molecular level how surfactant molecules interact with gelator molecules: do surfactant molecules only adsorb at the interface or are they incorporated into the fibers to affect fiber nucleation and growth? Understanding the interactions of surfactants with molecular gelators is important for the selection of suitable molecular additives to control fiber crystallization in molecular gels. In this work, we studied the fiber formation of 2,3-di-ndecyloxyanthracene (DDOA) in dimethyl sulfoxide (DMSO) in the presence of two nonionic surfactants, sorbitan monolaurate (Span 20) and polyethylene glycol tert-octylphenyl ether (Triton X-100). The molecular structures of DDOA and these two surfactants are shown in the Supporting Information. The fiber networks of DDOA were characterized using confocal fluorescence microscopy, and 1H NMR spectroscopy was used to investigate the interactions between surfactant and DDOA molecules. The surfactants were used at concentrations both below and at/above their critical micelle concentrations (CMCs), which are 20 and 1 mM for Span 20 and Triton X-100, respectively. When the concentration of a surfactant in a solvent is above CMC, surfactant monomers will self-assemble into aggregates, which are in dynamic equilibrium with monomers.

2.2. Methods. Synthesis of DDOA. The synthesis and purification of DDOA followed the procedures reported.24 Gel Formation and Fluorescence Imaging. Thin (300 μm) gel films of DDOA, without and with surfactants, were formed in selfmade glass cells. A linkam heating and cooling stage (THMS600) was used to melt the gels at 90 °C and cooled to 25 °C at a cooling rate of 20 °C/min for gel reformation. In this manner, all the gel samples were formed under the same thermal conditions. GP-1/PG gel was prepared following the same procedures. The DDOA fibers were imaged with a Leca confocal microscope. The excitation wavelength was 405 nm. Fluorescence spectra were obtained with a Hitachi fluorescence spectrometer (F4500). The excitation wavelngth was 300 nm. Proton Nuclear Magnetic Resonance (1H NMR). All 1H NMR experiments were carried out on a Bruker Avance III 500 MHz 5 mm broadband NMR spectrometer, operating at 500.13 MHz at 295 K unless stated otherwise. All spectra were calibrated according to the residual peak of DMSO at 2.50 ppm or CHCl3 at 7.26 ppm. The spectra were processed with Origin. The integration was carried out with ACD/NMR processor academic edition. In all the experiments of variable temperature, unless further specified, gels were heated to 90 °C, cooled to selected temperature, and equilibrated for 20 min before testing. The diffusion measurements were performed with stimulated echo employing bipolar gradients and a longitudinal eddy current delay. The parameters are as follows: Δ = 40 ms; δ = 2 ms; ns =16; Fid = 32 000 × 16; recycle delay = 0.31s. The gradient strength was linearly incremented from 2.24 to 36.79 G/cm. Data acquisition and processing were performed using TOPSPIN (Bruker). Rheological Measurements. The rheological properties were characterized with an Advanced Rheological Expansion System (ARES-2). Briefly, the sol−gel process was performed in situ between two parallel plates with a gap of 0.5 mm. The samples were subjected to sinusoidal oscillation by moving both the upper (with a diameter 40 mm) and the lower plates. The amplitude of the oscillation was controlled to obtain a strain of 0.02% in the sample and the oscillation frequency was set at 0.1 Hz. The temperature ramp rate was 20 °C/ min.

3. RESULTS AND DISCUSSION 3.1. Influence of Surfactants on the Fiber Network Structure. In the absence of any surfactant, DDOA fibers grow spherulitically (Figure 2a). Each spherulitic fiber network orginates from a nucleation center and the fibers sprouting from the nucleation center have limited branches. Figure 2b−e shows the confocal fluorescence images of DDOA fiber networks formed in the presence of Span 20 at 0.25, 2, 10, and 20 mM. In the presence of this surfactant, DDOA fibers

2. MATERIALS AND METHODS 2.1. Materials. Sorbitan monolaurate (Span 20), polyethylene glycol tert-octylphenyl ether (Triton X-100), dimethyl sulfoxide (DMSO), deuterated chloroform (CDCl3), deuterated DMSO, propylene glycol (PG), as well as all the chemicals for the synthesis of DDOA were obtained from Sigma-Aldrich. N-Lauroyl-L-glutamic acid di-n-butylamide (GP-1) was purchased from Kishimoto Sangyo Asia. All the chemicals were used as received. 1172

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Langmuir tend to grow nonspherulitically, which becomes more evident with an increase in the concentration of Span 20. As shown in Figure 2b, in the presence of Span 20 (2 mM), although nucleation centers of DDOA fibers are observable, the spherulites became elongated. This indicates at the initial fiber nucleation and growth stage, the molecules of this surfactant modified the surface of the fiber nuclei (small stacked columns) so that the nucleation and growth of fibers in some directions are hindered. Due to the similarity between the hydrophobic (alkyl) tail/chains of Span 20 and the alkyl chain of DDOA, it is highly possible that molecules of Span 20 adsorb at the side surface of DDOA fibers through van der Waals’ interactions between the alkyl chains of DDOA and Span 20. This led to some short side branches on DDOA fibers (the circled in Figure 2c). When the concentration of Span 20 is 20 mM (its CMC) (Figure 2e), only a few fiber arms originate from the nucleation center, which makes the spherulitic pattern hardly identifiable. Triton X-100 shows quite different effects on DDOA fibers. Figure 2 f-h shows the fiber network of DDOA formed in the presence of Triton X-100 at 0.2 mM, 1 mM, and 2 mM (2 CMC). Increasing the concentration of Triton X-100, the spherulites become smaller but the number density of the spherulites is bigger, indicating the promotion of primary nucleation of DDOA. Triton X-100 has a more rigid hydrophobic tail containing a benzene ring. Once adsorbed on a substrate (dust particle, etc.), it is not easy to dissociate and the benzene ring helps adsorption and nucleation of DDOA on the substrate. In addition, enhanced fiber tip branching is more evident in the presence of this surfactant. The 1H NMR characterization that follows indicate both π−π stacking and van der Waals’ interactions contribute to the formation of DDOA fibers, which indicates the surface of fiber is rich in the alkyl chain of DDOA, while the aromatic part of DDOA is exposed on the fiber tip surface. The strong interactions of the aromatic moiety of Triton X-100 with the aromatic part of DDOA facilitates the adsorption of this surfactant on the fiber tip surface, inducing fiber tip branching. The fiber side surface which is rich in the alkyl chain of DDOA is more preferably adsorbed by Span 20 and hence enhanced fiber side branching was induced by this surfactant. A schematic illustration of the fiber formation and branching mechanism in the presence of the two surfactants is shown in Figure 3 in the next section. 3.2. 1H NMR Spectroscopic Characterization. To understand the effects of surfactants, it is necessary to figure out how the DDOA molecules arrange in a fiber and how the surfactant molecules interact with DDOA. We recorded the 1H NMR spectra of the DDOA in deuterated DMSO with temperatures ranging from 22 to 110 °C (Figure S1, Supporting Information) (the gelation temperature of 2 mM DDOA in DMSO is ∼40 °C). An increase in temperature resulted in sharper peaks in the aromatic region (Figure S1a), indicating the DDOA molecules obtained more freedom of movement. In the alkyl region, the peaks shifted slightly downfield at above 90 °C (Figure S1b), for example, the peak of Db moved from 4.12 ppm (40 °C) to 4.16 ppm (110 °C) implying that the interactions between the alkyl chains are weakened at high temperatures. However, all the peaks in the aromatic region did not show observable shifts at above 90 °C, indicating that the electron environment of the anthracene was not changed (Figure S1a). The diffusion coefficients of DDOA at different temperatures were also measured using 1H NMR

Figure 3. Illustration of mechanism of DDOA fiber growth in the presence of surfactants. DDOA in complex with surfactant molecules diffuse to the surface of columns, where surface (side or tip) integration occurs. Surfactant molecules can interrupt the integration and cause mismatch nucleation and fiber branching. The preferable adsorption of Span 20 and Triton X-100 on the fiber side and tip surface enhances fiber side and tip branching, respectively.

spectroscopy. At 90 °C the coefficient was 5.6 × 10−8 m2/s, which was higher than that at 40 °C (4.9 × 10−10 m2/s). This 2 orders of magnitude increase in diffusion coefficient supports the smaller mass unit of the DDOA aggregates at a higher temperature. To further understand the molecular interactions that contribute to DDOA fiber formation, the solvent was changed to deuterated chloroform (CDCl3), in which DDOA dissolves instead of forming a gel. As compared in DMSO, all peaks in the aromatic region moved upfield, while those in the alkyl region move downfield (Figure S1). This suggests a tighter stacking between the anthracene portion of DDOA and weaker interaction between their alkyl chains in CDCl3. Although the π−π interaction in CDCl3 is stronger, DDOA cannot gel this solvent, which implies π−π interaction alone is not sufficient for gel formation. This further demonstrates the significant role of the van der Waals’ interactions between alkyl chains of DDOA in gel fiber formation. Combining the above information, it is proposed that in the fibers, DDOA molecules stack into columns via π−π interactions between the anthracene moieties. The van der Waals’ interactions between the alkyl chains group individual columns into a fiber (Figure 3).25 π−π interaction is also supported by the enhanced fluorescence emission while a hot solution of DDOA in DMSO is cooled to form a gel (Figure S2a, Supporting Information). A slight blue shift (from 416 to 412 nm) indicates H type (cofacial) aggregates/stacking. In general, H type stacking induces a nonradiative process that reduces fluorescence emission. However, the π−π stacking and van der Waals’ interaction between alkyl chains of DDOA would facilitate the planarization of the aromatic units to enhance radiation, as observed in several H-type gels.26 1 H NMR spectroscopy was utilized to study the molecular interactions between the surfactants and DDOA. DDOA gels containing 2 or 10 mM Span 20 were characterized at 22 °C (Figure 4). In the presence of Span 20, there was no shift of peaks in the aromatic region of DDOA, when compared with the plain DDOA gel (without surfactant), indicating the surfactant did not have an observable effect on π−π stacking of DDOA. As shown in the fluorescence spectra, the presence of this surfactant (and Triton X-100) did not induce an observable shift in the emission peak wavelength of DDOA, which further indicates the π−π stacking of DDOA was not affected (Figure 1173

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Figure 4. 1H NMR spectra of gels of DDOA without surfactant and with 2 mM and 10 mM Span 20, and spectra of a solution of 10 mM Span 20 in deuterated DMSO. Spectra of the aromatic region of DDOA is displayed in (a) while spectra of the alkyl chains of DDOA are depicted in (b).

Figure 5. 1H NMR spectra of gels and solutions in deuterated DMSO, containing 2 mM Triton X-100, and 2 mM DDOA/2 mM Triton X100. Spectra of the aromatic region are displayed in (a) while those of the alkyl tails are shown in (b).

In both the spectra of DDOA-Triton X-100 and Triton X100 solution, all the T1 peaks are integrated according to the solvent residual peak of DMSO at 2.50 ppm. By comparing the two relative integration values (0.54 in Triton X-100 solution, and 0.29 in DDOA-Triton X-100 gel), it can be deduced that almost 50% (1 mM) of Triton X-100 are associated with DDOA fibers. Note that the molar ratio of total Triton X-100 to DDOA is 1:1. The high boiling point of DMSO makes it hard to get xerogels, particular with the presence of surfactants. An SEM image of the DDOA fiber show that the thickness of the xerogel fiber is in the range of 300 to 500 nm (Figure S4, Supporting Information). On the basis of this information, it can be deduced that one DDOA fiber shall be made up of hundreds of stacked DDOA columns. Therefore, the fiber surface only contains a small fraction of the total DDOA to interact with surfactant molecules. This means most of the Triton X-100 molecules shall be associated with DDOA inside fibers. Increasing the temperature to 60 °C (gel was melted to sol), Triton X-100 did not induce shifts in DDOA (Figure 5), which indicates the mode of stacking of DDOA was not affected. However, the integration ratio of T1/ (T4, 5 and 6) increases from 0.15 (at 22 °C) to 0.20 (60 °C), implying the release of Triton X-100 molecules from DDOA fibers. The presence of DDOA leads all the peaks of Triton X-100 to move downfield (zoomed spectra are given in Figure S5, Supporting Information), which suggests that the molecules of DDOA interact with the molecules of surfactant, most likely through van der Waals’ interaction between their alkyl chains. At 60 °C, the diffusion coefficient of DDOA is 8.95 × 10−10 m2/s, which was reduced to 4.21 × 10−10 m2/s when Triton X-100 (2 mM) was present. The retarded diffusion of DDOA by surfactant indicates the interactions between their molecules even in a solution. Although the concentration of this surfactant is above its CMC in DMSO, the existence of micelles in the presence of DDOA is unlikely. The interaction with DDOA significantly reduces the concentration of surfactant monomers for selfaggregation. The molecular weights of DDOA and Triton X100 are 491 and 625, respectively, which are close to each other. Since the diffusion coefficient of DDOA is nearly halved

S2b). In addition, DDOA also did not show observable effects on the characteristic peak (peak S1) of Span 20. This indicates the hydrophobic chain of Span 20 interacts with DDOA in a manner similar to that between the molecules of Span 20 due to the similarity between the alkyl tail of Span 20 and the alkyl chain of DDOA, which also made it impossible to distinguish the other NMR peaks for their alkyl chains. Besides, some peaks of the −OH in the hydrophilic part of Span 20 overlapped the water peak at 3.34 ppm (not shown). The 1H NMR spectra of gel (22 °C) and sol (60 °C) of DDOA in the presence of Span 20 (10 mM) and those of Span 20 at the two temperatures are given in Figure S3 (Supporting Information). Increasing temperature, downfield shifts of peaks (at 1.5, 1.25, and 1.4 ppm, belong to both DDOA and Span 20) were observed, indicating more freedom of motion of the groups at a higher temperature. Similarly, the characteristic S1 peak of Span 20 did not show any shift at 60 °C. The hydrophobic tail of Triton X-100 is rich in −CH3 groups and its hydrophilic head is composed of ethylene oxide. All of these groups give peaks that are easy to distinguish from the signals of DDOA and water, which helps understand the changes of DDOA fibers induced by surfactants. Figure 5 shows the 1H NMR spectra of Triton X-100 solution (without DDOA) in DMSO at 22 °C and DDOA (2 mM) with Triton X-100 (2 mM) at 22 and 60 °C. The interactions between Triton X-100 and DDOA can be deduced from the changes in the relative peak intensity of the hydrophobic and hydrophilic parts of the surfactant. At 22 °C, for the DDOA gel with Triton X-100, the integration ratio of peaks of T1 to the ethylene oxide chain (T4, 5, and 6) is 0.15. While in the spectra of Triton X-100 solution (without DDOA), this ratio is around 0.25, in agreement with the ratio calculated from its molecular formula. In the gel, the integration ratio of T7 in the aromatic ring of Triton X-100 to those in ethylene oxide (T4, 5, and 6) is 0.041, while in solution of Triton X-100, this ratio is 0.053. The reduction in the ratios of both T1 and T7 (in the hydrophobic tail) to those in the hydrophilic head indicates that the mobility of the hydrophobic tail of Triton X-100 in gel is reduced. 1174

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Langmuir by this surfactant, it is thus highly possible that DDOA and surfactant form complexes with a molar ratio of these two compounds close to 1:1. From the crystallization point of view, branching of fibers is caused by structural mismatch between the nucleation phase and existing fiber surface, which is affected by surface integration kinetics and presence of impurities.14 The association between DDOA and surfactant molecules reduces the diffusion of DDOA molecules and their surface integration kinetics, which generally reduces mismatch nucleation and branching of fibers.14 On the other hand, association with surfactant molecules can physically interrupt the surface (tip and side) integration of DDOA molecules which enhances mismatch nucleation and promotes fiber branching. Therefore, the overall effects of a surfactant on fiber growth is a trade-off between these two opposite influences. In the case of the gelator and the surfactants used in this work, enhanced fiber branching resulted due to the strong interactions between the surfactants and gelator. 3.3. Influence of Surfactants on the Rheological Properties of DDOA/DMSO Gel. In general, the fiber network of a gel consists of a collection of individual networks, each originating from a primary nucleation center. The fiber network can be classified into domain and single networks depending on whether clear boundaries between neighboring individual networks can be identified.14 If they are isolated/ unlocked from each other, the network is termed as a domain network. In such a network, clear boundaries between the individual networks can be identified. Gels with such a network is generally weak in elasticity due to the weak/noninteractions between the domains.15,17 If fibers from the individual networks are not compact, the fibers from one individual network can penetrate into the neighboring networks to form an interconnecting/interpenetrating whole network. For a same gelator and solvent, an interconnecting network which although consists of a collection of individual networks, is similar to a single network in terms of rheological properties. Both of these networks will give a gel a higher elasticity compared to a domain network that consists of isolated spherulites (boundaries between neighboring spherulites/domains are clear). A schematic summary of the influence of surfactants on the fiber network structure of DDOA is shown in Figure 6a. Without any surfactant, DDOA crystallizes into spherulitic fiber networks. Due to the low degree of fiber branching, the fibers from spherulites penetrate into the neighboring spherulites, forming an interconnecting/interpenetrating network. When Triton X-100 was present, significant fiber tip branching was induced, making the spherulites more compact. While, clear boundaries between neighboring spherulites are still not clear and the whole network is still an interconnecting network. The presence of Span 20 reduces fiber branching from the primary nucleation centers, leading to the formation of a fiber network that is apparently like a “single” network (Figure 2e), since the individual networks cannot be identified. For clarity, a domain network with clear boundaries between neighboring networks, formed by another molecular gelator, GP-1 in propylene glycol, is shown in Figure 6b. Gels of GP-1 have been well studied.15 Figure 7 shows the evolution of storage modulus G′ of DDOA gels without and with the two surfactants. The surfactants at the concentrations of 2 mM, which is the same as the DDOA concentration, improved the G′ of the gel. In particular, the G′ was improved from 1600 to 2400 Pa, which is

Figure 6. Schematic summary of the influence of surfactants on the fiber network structure of DDOA (a) and a domain network formed by GP-1 (5 wt %) in propylene glycol at 25 °C. Clear boundaries between neighboring domains exist (b).

Figure 7. Evolution of storage modulus of DDOA gel in the absence and presence of surfactants.

an increase of 50%. At high concentrations of Span 20 (10 and 20 mM), the G′ was reduced, from 1600 to 600 and 350 Pa, respectively. As discussed, the interpenetrating networks of DDOA, formed either in the absence and presence of surfactants are similar to single networks. For single networks, fiber branching density has an important impact on the storage modulus.14 A network with more branched fibers has a higher storage modulus. For the single network containing spherulites, branching include both at the centers of the spherulites, that is, number of fiber arms from the centers, and on fibers. In the absence and presence of surfactants at low concentrations, the centers/nodes of the individual networks are highly branched, which provides rigid and more elastic networks. In contrast, at high concentrations of Span 20, the centers are less branched. Although some fiber side branches were brought about by this surfactant, the fibers are too short to give observable positive impact on G′. Although compared to the DDOA fiber network formed without any surfactant, the single network obtained in the 1175

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(2) Lan, Y.; Corradini, M. G.; Weiss, R. G.; Raghavan, S. R.; Rogers, M. A. To Gel or Not to Gel: Correlating Molecular Gelation with Solvent Parameters. Chem. Soc. Rev. 2015, 44, 6035−6058. (3) Terech, P.; Weiss, R. G. Low Molecular Mass Gelators of Organic Liquids and the Properties of Their Gels. Chem. Rev. 1997, 97, 3133− 3159. (4) Foster, J. A.; Piepenbrock, M.-O. M.; Lloyd, G. O.; Clarke, N.; Howard, J. A. K.; Steed, J. W. Anion-Switchable Supramolecular Gels for Controlling Pharmaceutical Crystal Growth. Nat. Chem. 2010, 2, 1037−1043. (5) Praveen, V. K.; Ranjith, C.; Bandini, E.; Ajayaghosh, A.; Armaroli, N. Oligo(Phenylenevinylene) Hybrids and Self-Assemblies: Versatile Materials for Excitation Energy Transfer. Chem. Soc. Rev. 2014, 43, 4222−4242. (6) Lim, P. F. C.; Liu, X. Y.; Kang, L. F.; Ho, P. C. L.; Chan, Y. W.; Chan, S. Y. Limonene Gp1/Pg Organogel as a Vehicle in Transdermal Delivery of Haloperidol. Int. J. Pharm. 2006, 311, 157−164. (7) Rogers, M. A.; Kontogiorgos, V. Temperature Dependence of Relaxation Spectra for Self-Assembled Fibrillar Networks of 12Hydroxystearic Acid in Canola Oil Organogels. Food Biophys. 2012, 7, 132−137. (8) Liu, K.; Liu, T.; Chen, X.; Sun, X.; Fang, Y. Fluorescent Films Based on Molecular-Gel Networks and Their Sensing Performances. ACS Appl. Mater. Interfaces 2013, 5, 9830−9836. (9) Cheng, N.; Hu, Q.; Guo, Y.; Wang, Y.; Yu, L. Efficient and Selective Removal of Dyes Using Imidazolium-Based Supramolecular Gels. ACS Appl. Mater. Interfaces 2015, 7, 10258−10265. (10) Sakakibara, K.; Chithra, P.; Das, B.; Mori, T.; Akada, M.; Labuta, J.; Tsuruoka, T.; Maji, S.; Furumi, S.; Shrestha, L. K.; Hill, J. P.; Acharya, S.; Ariga, K.; Ajayaghosh, A. Aligned 1-D Nanorods of a PiGelator Exhibit Molecular Orientation and Excitation Energy Transport Different from Entangled Fiber Networks. J. Am. Chem. Soc. 2014, 136, 8548−8551. (11) Dasgupta, D.; Srinivasan, S.; Rochas, C.; Ajayaghosh, A.; Guenet, J.-M. Solvent-Mediated Fiber Growth in Organogels. Soft Matter 2011, 7, 9311−9315. (12) Dasgupta, D.; Thierry, A.; Rochas, C.; Ajayaghosh, A.; Guenet, J. M. Key Role of Solvent Type in Organogelation. Soft Matter 2012, 8, 8714−8721. (13) Lloyd, G. O.; Steed, J. W. Anion-Tuning of Supramolecular Gel Properties. Nat. Chem. 2009, 1, 437−442. (14) Li, J. L.; Liu, X. Y. Architecture of Supramolecular Soft Functional Materials: From Understanding to Micro-/Nanoscale Engineering. Adv. Funct. Mater. 2010, 20, 3196−3216. (15) Li, J. L.; Yuan, B.; Liu, X. Y.; Wang, R. Y.; Wang, X. G. Control of Crystallization in Supramolecular Soft Materials Engineering. Soft Matter 2013, 9, 435−442. (16) Rogers, M. A.; Wright, A. J.; Marangoni, A. G. Engineering the Oil Binding Capacity and Crystallinity of Self-Assembled Fibrillar Networks of 12-Hydroxystearic Acid in Edible Oils. Soft Matter 2008, 4, 1483−1490. (17) Huang, X.; Terech, P.; Raghavan, S. R.; Weiss, R. G. Kinetics of 5 Alpha-Cholestan-3 Beta,6-Yl N-(2-Naphthyl)Carbamate/N-Alkane Organogel Formation and Its Influence on the Fibrillar Networks. J. Am. Chem. Soc. 2005, 127, 4336−4344. (18) Yuan, B.; Li, J. L.; Liu, X. Y.; Ma, Y. Q.; Xu, H. Y. Critical Behavior of Confined Supramolecular Soft Materials on a Microscopic Scale. Chem. Commun. 2011, 47, 2793−2795. (19) Liu, Y.; Zhao, W. J.; Li, J. L.; Wang, R. Y. Distinct Kinetics of Molecular Gelation in a Confined Space and Its Relation to the Structure and Property of Thin Gel Films. Phys. Chem. Chem. Phys. 2015, 17, 8258−8265. (20) Adhia, Y. J.; Schloemer, T. H.; Perez, M. T.; McNeil, A. J. Using Polymeric Additives to Enhance Molecular Gelation: Impact of Poly(Acrylic Acid) on Pyridine-Based Gelators. Soft Matter 2012, 8, 430−434. (21) Griffiths, M. B. E.; Koponen, S. E.; Mandia, D. J.; McLeod, J. F.; Coyle, J. P.; Sims, J. J.; Giorgi, J. B.; Sirianni, E. R.; Yap, G. P. A.; Barry,

presence of Span 20 is inferior in terms of elasticity, the structure is more homogeneous, which could be more beneficial to some applications, such as controlled drug release where uniform pore size of a fiber network is more important. In addition, rheological properties can be improved by increasing the fiber mass (i.e., concentration of gelators). Gels with sufficient rheological properties and desirable networks can be achieved at the same time by choosing a suitable gelator concentration and with the aid of suitable surfactants.

4. CONSLUSIONS In summary, this study shows that surfactant molecules interact with DDOA molecules and are embeded in DDOA fibers, which is different from simple surface adsorption of surfactants as in the controlled formation of inorganic crystals. The two surfactants Span 20 and Triton X-100 led to different DDOA fiber networks. It can also be concluded from this study that a preliminary understanding of the molecular organization of the gelator fibers will help the selection of suitable surfactants if a certain type of fiber network is preferred. The findings of this work will contribute to the development of a convenient approach to control the fiber growth in molecular gels. Nevertheless, to get a complete understanding of the surfactant effects, more detailed work needs be carried out to examine how surfactant molecular structures (hydrophobic tail and hydrophilic head) affect the growth of fibers. To this end, surfactants with the same hydrophobic tails but different hydrophilic heads, and those with same hydrophilic heads but different hydrophobic tails will be examined. Gelators of various structures will also need to be included.

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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.5b04384. Fluorescence spectra of DDOA/DMSO sol−gel process, SEM of DDOA fibers, 1H NMR spectra of Triton X-100 solution, DDOA gels without and with Triton X-100 (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 This work was supported by the Australian Research Council (ARC) through a Future Fellowship Grant (FT130100057) and an Industrial Transformation Research Hub in Future Fibers (IH140100018). The ARC is also acknowledged for funding Deakin University’s Magnetic Resonance Facility through a LIEF grant (LE110100141).

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REFERENCES

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DOI: 10.1021/acs.langmuir.5b04384 Langmuir 2016, 32, 1171−1177

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DOI: 10.1021/acs.langmuir.5b04384 Langmuir 2016, 32, 1171−1177