Gelled Lyotropic Liquid Crystals - Langmuir (ACS Publications)

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Gelled Lyotropic Liquid Crystals Yang Xu,† Michaela Laupheimer,† Natalie Preisig,† Thomas Sottmann,† Claudia Schmidt,‡ and Cosima Stubenrauch*,† †

Institute of Physical Chemistry, University of Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany Department of Chemistry, University of Paderborn, Warburger Str. 100, 33098 Paderborn, Germany

‡

S Supporting Information *

ABSTRACT: In our previous work we were able to prove that gelled bicontinuous microemulsions are a novel type of orthogonal selfassembled system. The study at hand aims at complementing our previous work by answering the question of whether gelled lyotropic liquid crystals are also orthogonal self-assembled systems. For this purpose we studied the same system, namely, water−n-decane/12hydroxyoctadecanoic acid (12-HOA)−n-decyl tetraoxyethylene glycol ether (C10E4). The phase boundaries of the nongelled and the gelled lyotropic liquid crystals were determined visually and with 2H NMR spectroscopy. Oscillating shear measurements revealed that the absolute values of the storage and loss moduli of the gelled liquid crystalline (LC) phases do not differ very much from those of the binary organogel. While both the phase behavior and the rheological properties of the LC phases support the hypothesis that gelled lyotropic liquid crystals are orthogonal self-assembled systems, freeze−fracture electron microscopy (FFEM) seems to indicate an influence of the gel network on the structure of the Lα phase and vice versa. microemulsions.12−16 In order to prove unambiguously that gelled bicontinuous microemulsions are orthogonal selfassembled systems we carried out an extensive study on the properties and microstructure of gelled bicontinuous microemulsions and compared the results with those of the base systems, namely, the binary organogel17 and the nongelled microemulsion.18 The system of choice consisted of water−ndecane/12-hydroxyoctadecanoic acid (12-HOA)−n-decyl tetraoxyethylene glycol ether (C10E4). On the basis of the results obtained from phase studies, rheometry, self-diffusion NMR, SANS, and freeze-fracture electron microscopy (FFEM) we concluded that the gelator molecules indeed form a network in the presence of a bicontinuous microemulsion. Moreover, the two microstructures do not significantly influence each other, such that gelled bicontinuous microemulsions were clearly identified as one more example of an orthogonal self-assembled system.14−16 A topic we came across in our previous studies of the gelled systems was the presence of lyotropic liquid crystalline (LC) phases, which we did not study in detail. Thus, the question of whether gelled lyotropic liquid crystals are also orthogonal selfassembled systems is still to be answered. Note that gelled lamellar lyotropic liquid crystals have already been reported.19,20 These so-called lamellar hydrogels consist of self-assembled surfactant bilayers which are decorated with short poly(ethylene glycol)-based amphiphilic block copolymers (PEG-lipids with

1. INTRODUCTION The phenomenon that two self-assembled structures form independently but simultaneously within a single system is called orthogonal self-assembly. This term was introduced in 1989 by Laibinis et al. for monolayers of alkanethiols and alkane carboxylic acids forming in an ordered manner in differently treated regions of a surface exposed to a common solution of both adsorbates.1 The observed behavior was explained by the fact that different coordination chemistry is involved in the two self-assembly processes.1 On the basis of this concept, orthogonal self-assembly is obviously by no means restricted to surface chemistry. In nature as well as in science there are numerous bulk systems consisting of different structures which self-assemble due to selective and noninterfering, noncovalent interactions.2−7 One example is a mammal cell which is confined by a phospholipid bilayer that coexists with a wide variety of other self-assembled architectures, such as the cytoskeleton. Other orthogonal self-assembled systems can be obtained by adding a gelator to an aqueous surfactant solution.8−11 In these systems, surfactant assemblies, i.e., micelles, wormlike micelles, or vesicles, coexist with a three-dimensional network made up of a low-molecular-weight hydrogelator. This is particularly interesting if one wants to mimic cells by combining surfactant vesicles and gelator networks10 or if one wants to study synergistic effects such as those seen in the rheological properties of gelled solutions of wormlike micelles.11 In recent years we also studied gelled systems in the context of orthogonal self-assembly. However, instead of gelling binary water−surfactant systems we gelled thermodynamically stable and nanostructured ternary water−oil−-surfactant systems, i.e., © XXXX American Chemical Society

Received: June 3, 2015 Revised: July 16, 2015

A

DOI: 10.1021/acs.langmuir.5b01992 Langmuir XXXX, XXX, XXX−XXX

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Figure 1. T−γ phase diagrams of the nongelled system D2O−n-decane−C10E4 (ϕ = 0.50, diamonds, left) and of the gelled system D2O−n-decane/12HOA−C10E4 (ϕ = 0.50, η = 0.015, circles, right). The phase-transition temperatures are determined visually, and the dark-gray stars indicate the sol−gel transition. The region denoted with Lα includes both the single Lα phase as well as the two-phase region 1 + Lα. The same holds true for the region denoted with H1 (Figure 4).

Mn ≤ 5000 g mol−1). The structure of the gel phase is described as a “highly defected microstructure comprised of a network of connected membrane bilayers with PEG-lipid segregated to the high curvature region”.19 Thus, the gel properties are not based on a three-dimensional network but on the random orientation of lamellar domains: domains that have their layer normals along the flow direction resist shear. Since even concentrated (>50 wt %) binary water−PEG mixtures do not gel while as little as 0.5 wt % PEG is sufficient to gel the lamellar phase, lamellar hydrogels cannot be classified as orthogonal self-assembled systems. To find out whether the gelled lyotropic liquid crystals observed in our system are orthogonal self-assembled systems we completed the previous phase studies of the system water−ndecane/12-HOA−C10E4 by focusing on the lyotropic LC phases. In order to identify both the extension and the nature of the lyotropic LC phases we used the visual method in combination with 2H NMR spectroscopy.21,22 2H NMR spectroscopy has proven to be an excellent tool to investigate the phase equilibria and the microstructure of lyotropic LC phases even in the presence of an isotropic phase without the need for macroscopic phase separation.18,23−35 Note that it was necessary to replace protonated by deuterated water for the 2H NMR measurements. For the sake of consistency we thus carried out all studies with the systems D2O−n-decane−C10E4 and D2O−n-decane/12HOA−C10E4, respectively. We first determined the phase diagrams of the nongelled and the gelled systems visually as well as with 2H NMR. The aim of the 2H NMR study was to distinguish coexisting phases and to clarify the microstructure of the lyotropic mesophases. We complemented the phase studies with the investigation of the rheological behavior of both nongelled and gelled lyotropic LC phases and compared the results with those obtained for the binary organogel. Finally, we visualized the gel structures of the gelled lamellar phase with transmission electron microscopy.

ϕ=

Vn ‐ decane = 0.50 VD2O + Vn ‐ decane

(1)

which corresponds to an oil mass fraction in the water−oil mixture of α = 0.40. The surfactant mass fraction in the systems is defined as mC10E4 γ= m D2O + mn ‐ decane + mC10E4 + m12 ‐ HOA (2) and the gelator mass fraction is defined as m12 ‐ HOA η= m D2O + mn ‐ decane + mC10E4 + m12 ‐ HOA

(3)

For all gelator-containing samples we used η = 0.015. We weighed all compounds in suitable glass tubes and sealed these tubes. For homogeneous mixing, the gelator-containing samples first had to be heated in a water bath to ∼70 °C to melt the solid 12-HOA. Subsequently, the samples were gelled by putting them into an ice bath for about 2 min or by letting them cool to room temperature for at least 1 h. Note that slow cooling was never a problem with binary organogel ndecane/12-HOA. However, for the quaternary system D2O−n-decane/ 12-HOA−C10E4 slow cooling yielded homogeneously gelled samples only for high surfactant mass fractions (γ > 0.24) where the sol-gel boundary lies below the upper microemulsion phase boundary (Figure 1, right). Here the sample can be homogenized in the nongelled onephase region and then can be gelled at a slightly lower temperature (usually in the Lα phase). For lower γ values this procedure is not possible because the sample is already gelled within the one-phase region. We thus homogenized the two-phase system at high temperatures in the sol state before we gelled the sample very quickly in an ice bath to prevent macroscopic demixing. 2.2. Visual Phase Studies. As already mentioned in the Introduction we needed to replace H2O by D2O for the envisaged 2H NMR measurements. We thus carried out all studies with the system D2O−n-decane−C10E4 and D2O−n-decane/12-HOA−C10E4. How the exchange of H2O by D2O influences the phase behavior is shown in the Supporting Information (Figure S1). The visual phase studies were carried out in a water basin equipped with a thermostat (Thermo Scientific DC30) and a homemade sample holder. A lamp behind the basin helped to identify the number of phases. Crossed polarizers behind and in front of the basin were used to identify the presence of an anisotropic lyotropic liquid crystal. In the composition range under question, the ternary and quaternary systems D2O−n-decane−C10E4 and D2O−n-decane/12-HOA−C10E4 form either one phase or two phases, depending on the temperature. Thus, we visually inspected a sample of a given composition at different temperatures and took the sample’s turbidity as a measure of its homogeneity. Accordingly, one-

2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. For this work we purchased and used without further purification D2O (99.8 atom % D) from AcroSeal, n-decane (≥99%) from Sigma-Aldrich, n-decyl tetraoxyethylene glycol ether (C10E4, ≥98.1%) from Bachem, and 12hydroxyoctadecanoic acid (12-HOA, 99%) from Sigma-Aldrich. All ternary and quaternary systems were prepared with equal volumes of water and oil B


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Langmuir phase (clear) and two-phase (turbid) regions were identified. The solgel transitions were determined by monitoring the sudden drop in viscosity. 2.3. 2H NMR Measurements. The 2H NMR measurements were carried out with a Bruker Avance III 400 MHz WB spectrometer equipped with a 30−75 MHz filter. The probe was self-built and equipped with a heating unit and a thermometer. Furthermore, a cooling unit (Bruker, BCU II −80/60) was connected to adjust the measuring temperature. The samples were placed into 5 mm NMR tubes (Norell, ST500-8) which were cut to a 3 cm length and sealed with a Teflon plug and epoxy glue. The NMR tube was then placed in the probe where the tube axis was oriented perpendicular to the external magnetic field. To study the phase behavior with 2H NMR we measured the spectra of each sample as a function of temperature. The temperature was changed in steps of 0.5−1 °C in temperature ranges where we expected phase transitions. Otherwise the temperature was changed in steps of 1.5−2 °C depending on the temperature range of the respective one-phase and two-phase regions. Before inserting a sample into the probe, the sample and the probe were prethermostated separately. For each new temperature setting the sample was kept at the set temperature for at least 30 min to reach equilibrium. The spectra were recorded with the help of TopSpin 3.0 software. A quadrupole echo pulse sequence was used, and 32 scans were accumulated. Subsequently, the recorded data were processed using MATLAB routines. 2.4. Rheometry. The rheometry measurements were carried out with a Physica MCR 501 rheometer from Anton Paar. (A StressTech rheometer from Rheologica Instruments was used for the binary organogel.) A plate-plate assembly with an upper (moving) plate of 2.5 cm diameter was used. The temperature of the lower (stationary) plate was set to the respective temperature via a Peltier element with a precision of ±0.1 K (Table 1).

specimen were prepared using the EM BAF060 freeze-fracture and etching system from Leica. The preparation procedure of the binary organogel is described in detail in one of our previous publications.17 For the other two systems, i.e., the nongelled and the gelled lamellar phases, the samples were prepared at a surfactant mass fraction of γ = 0.29 as described in section 2.1. Two copper plates (4.5 mm × 3.0 mm) and two copper grids were assembled into a so-called sandwich which was immersed in the nongelled lamellar phase at a temperature of T = 29 °C for at least 30 min. (The temperature range of the lamellar phase extends from 23.5 to 30.1 °C for γ = 0.29.) For the gelled lamellar phase the temperature was first set to T = 30 °C, at which the sample is a sol and thus can easily penetrate the sandwich. After 30 min the sample was slowly gelled at T = 22 °C (below the sol-gel transition temperature Tsol-gel and within the range of the lamellar phase) for at least 1 h to ensure the formation of the gelled lamellar phase within the sandwich. The sample with 1.5 wt % 12-HOA is a nongelled one-phase microemulsion in a temperature range from 28 to 32 °C, while it is a gelled lamellar phase between 20.7 and 24.7 °C. The specimen sandwiches were quickly frozen in liquid ethane, fractured, and subsequently shadowed with platinum-carbon (∼2 nm) at 45° and covered by a layer of pure carbon (∼20 nm) at 90° in the vacuum chamber of the BAF060, whose specimen stage was cooled to −150 °C (details in Laupheimer et al.17). The replicas were cleaned with acetone and ethanol (for the samples with 12-HOA), dried, and inspected either with the Tecnai G2 Sphera transmission electron microscope from FEI (binary gel) or with the EM10 from Zeiss (gelled and nongelled lamellar phases).

3. VISUAL AND 2H NMR PHASE STUDIES The focus of this section is on the unambiguous determination of phase boundaries and two-phase regions at high surfactant concentrations, i.e., at surfactant concentrations where lyotropic liquid crystalline phases form. In a first approach T−γ sections through the phase prism were determined visually at a volume fraction of oil of ϕ = 0.50 in the mixture of water and oil. It turned out that the visual determination of the phase diagrams at high surfactant mass fractions is time-consuming since the samples are very viscous. In Figure 1 the visually recorded T−γ phase diagrams of the nongelled system D 2O−n-decane−C10E4 (diamonds, left) and of the gelled system D2O−n-decane/12HOA−C10E4 (η = 0.015, circles, right) are shown. The sol-gel transition is indicated as a dashed dark-gray line and stars in Figure 1 (right). As mentioned above, the phase studies are performed at surfactant mass fractions of γ > γ̃, where γ̃ is the minimum surfactant mass fraction needed to obtain a one-phase microemulsion at the respective oil-to-water ratio. At lower values of γ and at temperatures around the phase inversion temperature T̃ , an extended three-phase region (3) is formed, where the microemulsion phase coexists with water and oil excess phases. As discussed elsewhere14−16 both the nongelled system and the gelled system show the typical phase behavior of microemulsions formulated with a nonionic surfactant. At low temperatures the coexistence of an oil-in-water (o/w)-microemulsion with an oil excess phase is found (2), while at high temperatures a water-in-oil microemulsion coexists with a water excess phase (2̅). The region in which the one-phase microemulsion is formed (1) lies between these two-phase regions and gets broader with increasing γ until the formation of the anisotropic Lα phase can be observed at large surfactant mass fractions. In comparing the phase diagrams, one sees that the phase boundaries of the gelled system are shifted to lower temperatures. As we could show via an extensive SANS study,16 a large fraction of the 12-HOA (gelator) molecules are not involved in the formation of the gel network but instead act both as a

Table 1. Parameters of Rheometry Measurementsa average standard deviation of sample binary gel nongelled Lα gelled Lα nongelled H1 gelled H1

σ/Pa

system

T/°C

n-decane/12-HOA D2O−n-decane−C10E4

25.0 26.7

10 0.1

G′/% G″/% 9 19

7 19

D2O−n-decane/12HOA−C10E4 D2O−n-decane−C10E4

22.7

0.5 and 1

22

23

14.5

2

7

7

D2O−n-decane/12HOA−C10E4

13.5

2

27

14

Temperature T, shear stress σ in the oscillation frequency sweeps, average standard deviations of G′ and G″. a

Then the samples (∼600−800 μL/0.75 g) were transferred to the plate with an Eppendorf pipet in a one-phase liquid state or with a spatula in the gelled state (particularly the binary organogel). Subsequently, the upper plate was lowered to the measuring position at a gap width of 1 mm. Initially, oscillation stress sweeps were performed with a constant oscillation frequency of 1 Hz in order to identify the linear viscoelastic (LVE) ranges of the different samples. Then stress values within these LVE ranges were chosen for measuring the oscillation frequency sweeps (Table 1). In these rheometry measurements the storage modulus G′ and the loss modulus G″ were determined. We repeated all measurements with good reproducibility and calculated the average values of four to five oscillation frequency sweep measurements. The respective standard deviations averaged over these four to five measurements are listed in Table 1. All C10E4containing samples were measured at γ = 0.29 and ϕ = 0.50 at the temperatures given in Table 1. 2.5. Freeze-Fracture Electron Microscopy (FFEM). In order to visualize the gelled and nongelled lamellar phase and the binary organogel with a transmission electron microscope, replicas of the C


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Langmuir hydrophobic cosurfactant and as a cosolvent making the oil more hydrophilic. Both effects have the same impact on the phase diagram, namely, a shift in the phase boundaries to lower temperatures. With regard to the part of the phase diagrams where the anisotropic lamellar phase (Lα) occurs, only the phase transition between the isotropic microemulsion phase (1) and the coexistence of the microemulsion and lamellar phases (1 + Lα) could be determined visually for the nongelled system. The phase boundary between two-phase region 1 + Lα and the pure Lα phase, however, could not be determined accurately by visual inspection. The same holds true for the anisotropic hexagonal phase (H1). In the case of the gelled samples the visual determination of all phase boundaries is very difficult due to a slight turbidity caused by the presence of the gel network. We thus decided to complement the visual studies with a very versatile technique, namely, 2H NMR of D2O. The quadrupolar 2H nucleus (spin I = 1) shows a spectral splitting in anisotropic phases. The splitting depends on the residual orientational order of the rapidly moving D2O molecules and on the orientation of the phase axis (director) with respect to the magnetic field.25,36 In an isotropic phase the molecules have no preferred orientation; their motion is isotropic and the splitting vanishes. In the anisotropic lamellar and hexagonal LC phases, however, the interaction with the aligned surfactant molecules results in a weak alignment of the D2O molecules and quadrupole splittings are observed. The size of the splitting decreases both with increasing water concentration and with increasing temperature as the orientational order becomes lower. If an LC phase is not macroscopically aligned and domains with all possible director orientations exist, then the spectrum has a characteristic line shape known as a Pake pattern.37 Samples with a uniform director alignment at an angle θ with respect to the magnetic field yield spectra consisting of two peaks with a splitting proportional to (3 cos2θ − 1). Hence, a sample shows the maximum splitting when the director is uniformly aligned at θ = 0° (parallel) and only half of the maximum splitting when it is aligned at θ = 90° (perpendicular). Since the negative anisotropy of the magnetic susceptibility of ethylene oxide surfactants favors a perpendicular orientation of the molecules in a magnetic field a (partial) alignment of the sample is often observed, in particular, when the anisotropic phase is formed slowly from an isotropic one. This enables us to distinguish between lamellar and hexagonal phases because the director aligns in the perpendicular and parallel orientations, respectively. As already mentioned, the 2H NMR measurements in the study at hand serve to complement the visual phase studies. We carried out 2H NMR measurements at γ = 0.22, 0.23, and 0.30 for the nongelled system and at γ = 0.24 and 0.28 for the gelled system as a function of temperature. For the sake of clarity we picked out one surfactant mass fraction and telling temperatures of each system for the following discussion. We will first show and discuss the corresponding 2H NMR spectra (Figures 2 and 3) before we add the phase-transition temperatures determined via the 2H NMR spectra to the phase diagrams (Figure 4). Figure 2 shows six exemplary spectra of the system D2O−ndecane−C10E4 measured at a surfactant mass fraction of γ = 0.30 at different temperatures. The phase-transition temperatures determined via the temperature scans at all three γ values are summarized in Table 2. Since we investigated this system with 2H NMR in one of our previous studies18 we could easily assign the splitting of the top two spectra in Figure 2 to the lamellar phase. The fact that a doublet of two narrow peaks is observed tells us that the sample is perfectly aligned by the magnetic field. The

Figure 2. 2H NMR spectra of the system D2O−n-decane−C10E4 (ϕ = 0.50) recorded at γ = 0.30 for six different temperatures T.

single isotropic peak at somewhat lower temperatures (spectrum at 18 °C in Figure 2) is indicative of the one-phase microemulsion. When the temperature is decreased further, additional peaks occur at 16.8 and 16.0 °C which can be attributed to the formation of a different anisotropic phase, namely, the hexagonal phase. At 13 °C the spectrum of the pure hexagonal phase is observed, showing four peaks. The outer doublet corresponds to hexagonal domains aligned parallel to the magnetic field, while the inner two peaks with half the splitting of the outer doublet are the maxima of a Pake spectrum, which results from the unaligned fraction of the sample. Such a partial alignment in the magnetic field is very common for hexagonal phases, which, due to their high viscosity, are aligned by a magnetic field less readily than lamellar phases.24,26 Note that the inner splitting of the hexagonal phase and the splitting of the Lα phase, which both correspond to the same director orientation (θ = 90°), differ, as expected, by roughly a factor of 2 (when the overall temperature dependence of the splitting is ignored). The position of the hexagonal phase at temperatures below the lamellar one allows us to identify it as an H1 phase, as was the case in our previous work.18 At temperatures above the Lα phase, i.e., between 34 and 38 °C, we found no experimental evidence for the occurrence of an H2 phase. The gelled system D2O−n-decane/12-HOA−C10E4 was studied at two different γ values, namely, at 0.24 and 0.28, as a function of temperature. As an example, nine spectra are shown in Figure 3 for γ = 0.28. In the Supporting Information five more D


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(partially aligned parallel to the magnetic field as can be seen from the high shoulders), respectively. In the spectra at 26 and 26.5 °C an additional feature, namely, the coexistence of two doublets, is clearly seen. In addition to the doublet with broad peaks one sees a sharper doublet with a larger splitting and an increased intensity at the baseline that extends to approximately ±500 Hz. This subspectrum results from a partial demixing of the sample at the sol−gel transition and the formation of a second lamellar phase from the sol. Note that the existence of two lamellar phases with different surfactant mass fractions is an experimental artifact and not a thermodynamically stable state. Accordingly the respective phase-transition temperatures are neither listed in Table 3 nor plotted in Figure 4 (right). The peaks from this subspectrum can still be recognized as shoulders at 23 °C. The spectrum at 18.5 °C is assigned to the biphasic 1 + Lα regime because the line width is still much larger than at 18 °C. In a nutshell, the 2H NMR study confirms that the gelled system shows the same phase sequence as the nongelled one, just at lower temperatures. Again, we found no experimental evidence for the formation of an H2 phase at temperatures above the Lα phase, i.e., between 27 and 31 °C. This finding, however, does not rule out the existence of an H2 phase in a very narrow temperature range. All phase-transition temperatures determined visually (black and gray symbols) and via 2H NMR measurements (open symbols, see Tables 2 and 3) are plotted in Figure 4. As can be seen, the results are in line and complement each other. The 2H NMR measurements provide the location of the phase boundaries between the one-phase microemulsion (1) and two-phase region 1 + Lα as well as between 1 + Lα and the pure Lα phase. For the nongelled system it is possible to directly compare the temperatures of the phase boundaries between 1 and 1 + Lα determined with the two methods. A close look at Figure 4 reveals that the upper phase boundary between 1 + Lα and 1 determined via 2H NMR is observed at slightly higher temperatures. Plausible explanations are the fact that the sample cannot be stirred, i.e., homogenized, and that the temperature is not really constant but slightly graded within the NMR tube, which is why small remaining fractions of the Lα phase may melt only at higher measured temperatures. In addition, the phase boundary of the two-phase region where the hexagonal phase and the microemulsion coexist (1 + H1) could be distinguished from that of the pure H1 phase with the 2H NMR spectra. Summarizing the results so far, one can say that we identified two more examples of orthogonal self-assembled systems, namely, a gelled lamellar liquid crystal and a gelled hexagonal liquid crystal. This conclusion is based on the fact that the phase sequence of the nongelled system is not affected by the gelatoronly a slight shift to lower temperatures is observed, which we discussed intensively in one of our previous studies.14 On the other hand, the gelator still forms a gel network, i.e., gelator 12-HOA “accepts” the two lyotropic liquid crystals as solvent. In the following discussion these findings will be double-checked by rheological measurements and transmission electron microscopy pictures.

Figure 3. 2H NMR spectra of the system D2O−n-decane/12-HOA− C10E4 (ϕ = 0.50) recorded at η = 0.015 and γ = 0.28 for nine different temperatures T.

spectra are shown for γ = 0.24 (Figure S2). The phase-transition temperatures determined via the temperature scans at both γ values are summarized in Table 3. As was the case for the nongelled system, the spectra of the gelled system can be assigned to two different anisotropic phases (Figure 3). In comparison to the spectra of the nongelled system, the doublets are less sharp. This is reminiscent of the D2O spectra of lyotropic liquid crystalline polymers and elastomers.38,39 The most probable explanation is a smaller domain size, which results from a higher defect density. Such a defect structure may allow for a slow exchange (on the time scale of the NMR experiment) of the D2O molecules between the domains, which in turn would lead to the observed line broadening (onset of motional narrowing). Again, the spectra at 23 and 13 °C can be assigned to a perpendicularly aligned lamellar phase and a hexagonal phase

4. RHEOLOGY AND TRANSMISSION ELECTRON MICROSCOPY After having carried out the phase studies we investigated the rheological properties of the gelled lyotropic LC phases and took electron microscopy pictures in order to provide more evidence for the orthogonal self-assembly of the gelled systems. Oscillating E


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Figure 4. Phase boundaries determined visually (black and gray symbols) complemented by phase-transition temperatures determined via 2H NMR measurements (open symbols, see Tables 2 and 3) for both the nongelled system D2O−n-decane−C10E4 (ϕ = 0.50, diamonds, left) and the gelled system D2O−n-decane/12-HOA−C10E4 (ϕ = 0.50, η = 0.015 circles, right). Note that the phase transition between the Lα phase and the two-phase region 1 + Lα as well as the phase boundaries between the H1 phase and the two-phase region 1 + H1 could only be determined with 2H NMR.

decane−C10E4 at ϕ = 0.50, γ = 0.29, and T = 26.7 °C (Figure 5, right), and (c) the nongelled H1 phase of the system D2O−ndecane−C10E4 at ϕ = 0.50, γ = 0.29, and T = 14.5 °C (Figure 6, right). The measurements yield the storage modulus G′ and the loss modulus G″, which are measures of the solidlike and the liquidlike properties of a viscoelastic material. In order not to destroy the self-assembled structures of the samples it was important to first carry out stress sweep measurements and to identify the limits of the linear viscoelastic (LVE) range in the different systems. In the case of the binary gel the LVE range stretches up to 18 Pa,15,16 while for the nongelled Lα phase linear viscoelasticity is lost already at about 0.2 Pa. These values directly reflect the viscosity of the phases, i.e., the low viscosity of the nongelled Lα phase and the high viscosity of the binary gel. Accordingly, adding the gelator 12-HOA to the nongelled Lα phase expands the LVE range of the Lα phase up to about 2 Pa. For the H1 phase, which is highly viscous even without gelator, the LVE range stretches up to about 4 Pa both with and without 12-HOA. On the basis of this information suitable stress values within the respective LVE ranges were chosen for the subsequent frequency sweep measurements (Table 1). The results of the frequency sweep measurements for the gelled Lα phase and its base systems are shown in Figure 5, while those for the gelled H1 phase and its base systems are shown in Figure 6. One can see that for all systems containing gelator 12HOA, i.e., for the binary gel and for the gelled lyotropic LC phases as well as for the nongelled Lα phase, the storage modulus G′ is considerably higher than the loss modulus G″. This situation, i.e., G′ ≫ G″, shows a solidlike character of the systems.40 It is furthermore characteristic for strong gels or soft solids that G′ and G″ only slightly depend on the frequency.41 Note that the rheology of the binary system oil−12-HOA has been studied extensively in the past.42−44 Thus, one finds a rather small slope of G′ and G″ for these systems. For the nongelled H1 phase the situation is different. Here one finds a rather high viscosity which originates from densely packed cylindrical surfactant aggregates and leads to storage and loss moduli G′ and G″ of the same order of magnitude. In addition, one can see a crossover point of G′ and G″ in the frequency sweep of the H1 phase at about 0.1 Hz, where the viscoelastic behavior changes from viscous, liquidlike (G″ > G′) to elastic, solidlike (G′ > G″)

Table 2. Phase-Transition Temperatures Ttrans Determined from 2H NMR Measurements of the System D2O−n-Decane− C10E4 (ϕ = 0.50)a γ

phase transition

Ttrans/°C

0.30 0.30 0.30 0.30 0.30 0.30 0.23 0.23 0.22 0.22

isotropic → isotropic + Lα isotropic + Lα → Lα Lα → isotropic + Lα isotropic + Lα → isotropic isotropic → isotropic + H1 isotropic + H1 → H1 isotropic → isotropic + Lα isotropic + Lα → isotropic isotropic → isotropic + Lα isotropic + Lα → isotropic

36.0 ± 0.5 30.5 ± 0.5 23.2 ± 0.8 18.7 ± 0.8 17.1 ± 0.4 14.5 ± 0.5 31.0 ± 0.5 23.5 ± 0.5 29.8 ± 0.3 25.3 ± 0.3

a

The data are plotted in Figure 4 (left) as open diamonds. Note that the transitions are listed for decreasing temperatures.

Table 3. Phase-Transition Temperatures Ttrans Determined from 2H NMR Measurements of the System D2O−n-Decane/ 12-HOA−C10E4 (ϕ = 0.50, η = 0.015)a γ

phase transition

Ttrans/°C

0.24 0.24 0.24 0.24 0.28 0.28 0.28 0.28 0.28 0.28

isotropic → isotropic + Lα isotropic + Lα → Lα Lα → isotropic + Lα isotropic + Lα → isotropic isotropic → isotropic + Lα isotropic + Lα → Lα Lα → isotropic + Lα isotropic + Lα → isotropic isotropic → isotropic + H1 isotropic + H1 → H1

25.5 ± 0.5 23.8 ± 1.3 21.8 ± 0.8 20.5 ± 0.5 27.3 ± 0.3 24.5 ± 1.5 20.8 ± 2.3 18.2 ± 0.3 14.8 ± 0.3 13.7 ± 0.8

a The data are plotted in Figure 4 (right) as open circles. Note that the transitions are listed for decreasing temperatures.

shear rheometry measurements were carried out for the gelled Lα phase in the system D2O−n-decane/12-HOA−C10E4 (ϕ = 0.50, η = 0.015) at γ = 0.290 and 22.7 °C (Figure 5, middle) as well as for the gelled H1 phase at 13.5 °C (Figure 6, middle). For comparison we also studied the base systems, namely, (a) the binary gel n-decane/12-HOA at η = 0.015 and 25.0 °C (Figures 5 and 6, left), (b) the nongelled Lα phase of the system D2O−nF


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Figure 5. Storage modulus G′ (filled symbols) and loss modulus G″ (open symbols) of the binary gel n-decane/12-HOA (η = 0.015) at 25.0 °C (squares, left), of the gelled Lα phase in the system D2O−n-decane/12-HOA−C10E4 (ϕ = 0.50, η = 0.015) at γ = 0.290 and 22.7 °C (circles, middle), and of the nongelled Lα phase in the system D2O−n-decane−C10E4 (ϕ = 0.50) at γ = 0.290 and 26.7 °C (diamonds, right).

Figure 6. Storage modulus G′ (filled symbols) and loss modulus G″ (open symbols) of the binary gel n-decane/12-HOA (η = 0.015) at 25.0 °C (squares, left), of the gelled H1 phase in the system D2O−n-decane/12-HOA−C10E4 (ϕ = 0.50, η = 0.015) at γ = 0.290 and 13.5 °C (circles, middle), and of the nongelled H1 phase in the system D2O−n-decane−C10E4 (ϕ = 0.50) at γ = 0.290 and 14.5 °C (diamonds, right).

behavior. This crossover is in line with previous studies on hexagonal phases where the crossover is reported in the same frequency range.45 In all other systems, the solidlike behavior dominates over the whole frequency range studied. Looking at the absolute values of the storage and loss moduli of the gelled systems one sees that the values of the gelled LC phases do not differ very much from those of the binary organogel. This is exactly what we observed for the gelled microemulsions,14 which is why we can draw the same conclusion: replacing decane of the binary organogel by a lyotropic liquid crystal (or by a bicontinuous microemulsion) does not significantly affect the gel properties. The gel still forms, and its strength is nearly the same, which means that the gelator 12-HOA “accepts” lyotropic liquid crystals (or bicontinuous microemulsions) as solvent. This finding is in total agreement with the idea of dealing with an orthogonal self-assembled system since the gel network forms irrespective of the solvent’s structure. In other words, the rheological behavior of gelled lyotropic LC phases is dominated by the rheological behavior of the 12-HOA gel network, as was the case for the gelled bicontinuous microemulsions. In conclusion one can say that the rheometry results further support the idea that gelled lyotropic liquid crystals are orthogonal self-assembled systems. On the one hand, the phase behavior of the lyotropic liquid crystals is not strongly

affected by the presence of the gelator. On the other hand, the gel formation is not strongly affected by the presence of a lyotropic liquid crystal as solvent. Finally, we used transmission electron microscopy to visualize the structure of the gelled Lα phase and therewith to provide further evidence for the orthogonal self-assembly of the gelled system. As a starting point the structures of both base systems (binary gel and gelator-free Lα phase) are visualized. FFEM pictures of representative structures found throughout the specimen are shown in Figure 7a,b. Looking at Figure 7a one sees the twisted 12-HOA fibers of the binary gel as reported in previous studies.17,46−48 The width of the fibers varies from 12 to 60 nm. Both thicker and thinner fibers as well as areas with less or even no fibers were also observed in other parts of the specimen. Note that with the FFEM technique only tiny sections of a sample are imaged. Regarding the distribution of gelator fibers, these sections are not necessarily representative of the entire sample volume, and thus the density of fibers in a replica is not necessarily related to the total concentration of the gelator. See our previous work for more details.17 Three examples of pictures taken on the same specimen at different areas are shown in the Supporting Information. On the FFEM picture of the gelatorfree Lα phase (Figure 7b) one mainly sees regions of lamellar stacks. In addition, we observed a few spots where the structure G


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Figure 7. FFEM pictures of (a) the binary gel n-decane/12-HOA at η = 0.015 and T = room temperature, (b) the nongelled Lα phase in the system D2O−n-decane−C10E4 at ϕ = 0.50, γ = 0.29, and T = 29 °C, and (c, d) the gelled Lα phase in the system D2O−n-decane/12-HOA−C10E4 at ϕ = 0.50, η = 0.015, γ = 0.29, and T = 22 °C.

work, γ = 0.29 in the present work) and thus the different “solvent” in which the gel network forms. It is well known that the solvent has an enormous impact on the structure of the gel network in general and on the structure of the fibers in particular.40 To clarify this point one needs to study the structure of the gel fibers as a function of the surfactant concentration, which we are planning to do in the future. Finally, we comment on Figure 7d. In some areas of the specimen we found lamellar regions which were much better ordered compared to the gelator-free Lα phase. The higher order is reflected in the fact that the size of the stacks is significantly larger in the gelled Lα phase. However, the repeat distance of the lamellar layers is also d ≈ 4 nm and thus about the same as the distance found for the gelator-free Lα phase. We have two possible explanations for the higher order: (1) The gelator has a certain surface activity and can thus adsorb at the water/oil interface which renders the lamellar layers more rigid, thus promoting a higher order. (2) The simultaneous formation of the gel network may have a directing effect on the formation of the lamellar layers. To conclude, one can say that the simultaneous formation of the gel network and of the Lα phase seems to influence each other. The presence of huge amounts of surfactant leads to nontwisted fibers, while the presence of the gelator

resembles the one known from multilamellar vesicles found in binary water/surfactant mixtures49−52 (denoted as MLV-like). The repeat distance d of the lamellar layers amounts to about 4 nm, which is in almost quantitative agreement with the distance calculated from the composition in a first-order approximation (neglecting the effect of undulations). The distance can be calculated via d ≈ δ/ϕC,i, where δ ≈ 1.1 nm corresponds to the thickness of the surfactant monolayer and ϕC,i ≈ 0.3 corresponds to the volume fraction of surfactant in the lamellar layers.53 Two FFEM pictures of the gelled system D2O−n-decane/12HOA−C10E4 are shown in Figure 7c,d. Both structures, namely, gelator fibers and lamellar layers, can be found on the replica (Figure 7c), while in other areas of the same specimen only lamellar layers were observed (Figure 7d). Let us have a closer look at the structure of both the gel fibers and the lamellar layers. Looking at Figure 7c, one sees that the fibers are not twisted, in contrast to our previous work where 12-HOA fibers in a gelled bicontinuous microemulsion of the same system were found to be twisted.15 In addition, the width of the fibers in the present work is between 35 and 70 nm and thus is a bit thicker than the fibers of the binary gel and of our previously studied gelled microemulsions. A possible explanation for this observation are the different surfactant mass fraction (γ = 0.17 in the previous H


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templating process, while the second case would result in a truly orthogonal self-assembled system.

increases the order of the Lα phase. This calls into question the idea that gelled lyotropic liquid crystals are truly orthogonal selfassembled systems and thus requires further investigations.

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5. CONCLUSIONS AND OUTLOOK We showed in the study at hand that it is possible to obtain gelled lyotropic liquid crystals by adding an appropriate low-molecularweight gelator (LMG) to a lyotropic liquid crystal. The system in question was water−n-decane/12-hydroxyoctadecanoic acid (12-HOA)−n-decyl tetraoxyethylene glycol ether (C10E4) because the properties of the respective gelled bicontinuous microemulsion were extensively investigated in previous studies. Here we studied the properties of lyotropic liquid crystals (LC) which were observed at higher surfactant concentrations in this very system. We determined the phase boundaries of the nongelled and the gelled lyotropic LCs visually and with 2H NMR spectroscopy, and we found that the general pattern of the phase diagram is not altered if one adds the gelator. More specifically, we observed the same phase boundaries but at slightly lower temperatures, which is in line with the observations we made for the gelled microemulsions. Note that the type and the exact location of the pure Lα and H1 phases as well as their coexistence with the microemulsion in the phase diagram could be determined only with 2H NMR spectroscopy. Further experimental evidence for the idea of gelled lyotropic LC phases being orthogonal self-assembled systems was provided by the rheological properties. Oscillating shear measurements on the gelled lyotropic LC phases showed that the absolute values of the storage and loss moduli are only slightly (if at all) influenced when an Lα phase or an H1 phase is used as a solvent (instead of pure decane). Finally, TEM pictures show that the two structures, namely, the gel network and a lyotropic LC, selfassemble simultaneously. However, the two self-assembled systems seem to influence one another: the presence of the surfactant changes the structure of the gelator fibers from being twisted to not being twisted, and the presence of the gelator leads to a higher order of the lamellar phase. In other words, this observation calls into question the idea that gelled lyotropic liquid crystals are truly orthogonal self-assembled systems, which is why it requires further investigations. We conclude this study with an outlook. What we did not look at yet is the time scale of formation or the relation between the sol-gel transition temperature and the clearing point of the LC phase. Kato et al. gelled thermotropic liquid crystals with an LMG and used the fact that liquid crystals are anisotropic and well-ordered to manipulate the structure of the resulting gelled LC phases.54 They gelled systems for which the formation of the LC occurs at temperatures above the sol-gel transition, and they found that the resulting fibers are well-aligned when growing in the LC “template”. However, if the gel network is formed first, then the fibers are randomly distributed and coexist with LC polydomains. In these systems two structures coexist whose structural details (such as the alignment of the fibers) are influenced by the way the final system is formed. Thus, a gelled thermotropic LC phase is a borderline case between orthogonal self-assembly and soft templating, and it may well be that the formation of a gelled lyotropic LC phase can be manipulated in the same way. To answer this question we will follow Kato’s strategy: we will need to first identify and then study (a) a system in which the sol-gel transition is below the melting point of the LC phases and (b) another system where the sol-gel transition is above the melting point of the LC phases. Speculative as it may be, we expect that in the first case one should rather talk about a

ASSOCIATED CONTENT

S Supporting Information *

How the exchange of H2O by D2O influences the phase behavior. Additional 2H NMR spectra of the gelled quaternary system D2O−n-decane/12-HOA−C10E4. Additional FFEM pictures of the binary gel. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.langmuir.5b01992.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +49711-685 64470/64451. Notes

The authors declare no competing financial interest.

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REFERENCES

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