Liquid Crystalline Phase Behavior and Sol–Gel Transition in Aqueous

Article pubs.acs.org/Langmuir

Liquid Crystalline Phase Behavior and Sol−Gel Transition in Aqueous Halloysite Nanotube Dispersions Zhiqiang Luo,† Hongzan Song,*,† Xiaorui Feng,† Mingtao Run,† Huanhuan Cui,† Licun Wu,† Jungang Gao,† and Zhigang Wang*,‡ †

College of Chemistry & Environmental Science, Hebei University, Baoding, Hebei Province 071002, China CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui Province 230026, China

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S Supporting Information *

ABSTRACT: The liquid crystalline phase behavior and sol−gel transition in halloysite nanotubes (HNTs) aqueous dispersions have been investigated by applying polarized optical microscopy (POM), macroscopic observation, rheometer, small-angle X-ray scattering, scanning electron microscopy, and transmission electron microscopy. The liquid crystalline phase starts to form at the HNT concentration of 1 wt %, and a full liquid crystalline phase forms at the HNT concentration of 25 wt % as observed by POM and macroscopic observation. Rheological measurements indicate a typical shear flow behavior for the HNT aqueous dispersions with concentrations above 20 wt % and further confirm that the sol−gel transition occurs at the HNT concentration of 37 wt %. Furthermore, the HNT aqueous dispersions exhibit pH-induced gelation with more intense birefringence when hydrochloric acid (HCl) is added. The above findings shed light on the phase behaviors of diversely topological HNTs and lay the foundation for fabrication of the long-range ordered nano-objects.

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such as those observed for carbon nanotubes (CNTs),30−37 graphene oxide (GO),38,39 mineral,40−46 gibbsite platelets,47,48 cellulose, and chitin nanocrystals,49−51 which are recognized as key potential precursors for the fluid phase processing of particles into aligned materials with outstanding properties.52,53 However, most studies neglect the key feature of HNTs, i.e., their anisotropic shape (300−1500 nm in length and 15−50 nm in outer diameter, that is, the aspect ratio typically ranging between 6 and 100). Up to now, to the best of the authors’ knowledge, there are no available data concerning on the liquid crystalline behaviors of HNT aqueous dispersions. In this work, we report that HNT aqueous dispersions show the isotropic−liquid crystalline−liquid crystalline gel phase transitions. For a complete understanding, we have investigated the liquid crystalline phase transition behaviors versus HNT concentration for HNT aqueous dispersions. We also have

INTRODUCTION Halloysite nanotubes (HNTs, Al2Si2O5(OH)4·nH2O), a type of newly emerging clay with a nanotubular structure, are available in abundance in many countries and have recently become the subject of research attention as a new type of material.1−11 More importantly, HNTs possess huge specific surface area, abundant hydroxyl groups, environmental friendliness, and biocompatibility, not only making them have potentials as additives for enhancing the mechanical performances,12−14 thermal stability,15,16 and nucleating agents for crystallization of polymers17,18 but also making them attractive candidates for a variety of potential applications, including controlled release of protective agents,19−22 biomimetic reaction vessels,23 adsorption agents,24,25 corrosion prevention agents,26 and catalysts.27,28 On the other hand, dispersions of nanoparticles are widely investigated because of their uses in many industrial applications (foods, pharmaceuticals, cosmetics, paints, etc.).29 In particular, due to their high anisosymmetric structures, the anisotropic nanoparticles can form liquid crystalline phases, © 2013 American Chemical Society

Received: July 25, 2013 Revised: September 11, 2013 Published: September 26, 2013 12358

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monochromatic Cu Kα radiation (wavelength λ = 0.154 nm). The SAXS patterns were recorded with a two-dimensional gas-filled wire detector (Bruker Hi-Star). The HNT aqueous dispersions were injected into 1 cm diameter disklike copper cell, which was sealed with two Kapton film windows and aligned perpendicular to the X-ray beam. The sample-to-detector distance was 2463 mm. The SAXS intensity profiles were extracted from the isotropic two-dimensional SAXS patterns. The rheological measurements were performed on a stress-controlled rheometer (TA-AR2000EX, TA Instruments) equipped with a cone-and-plate geometry (diameter 40 mm; angle 1°). Before the oscillatory shear measurements, a strain sweep from 0.1 to 100% with a fixed frequency of 6.28 rad/s was performed for each dispersion to determine the linear viscoelastic regime. The chosen strains of 1−10% fell well within the linear viscoelastic regime for the frequency range of 0.1−100 rad/s in the oscillatory shear measurements. The experimental temperature was mainly set at 25 °C. For each sweep measurement, repeat specimens are requested, and the number of repeat specimens is three in order to examine the data reproducibility.

investigated the influence of pH on the formation of liquid crystalline and gelation of HNT aqueous dispersions. Finally, the possible formation mechanism for the liquid crystalline and liquid crystalline gel has been proposed. The current work illustrates that the liquid crystalline and liquid crystalline gel characters of HNT aqueous dispersions can be enhanced with increasing HNT concentration and/or with an addition of acid. Our findings could facilitate the large-scale alignments of HNTs in the fluid phase, open the way to make the long-range ordered structures of HNT-based functional materials, and offer the opportunities to uncover the complex phase transition behaviors for HNT dispersions with particular topologies.

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EXPERIMENTAL SECTION

Materials. Halloysite nanotubes (HNTs) were obtained from GuangZhou Shinshi Metallurgy and Chemical Company Ltd. (Guangzhou, China). Sodium hexametaphosphate [(NaPO3)6], sodium hydroxide (NaOH), and hydrochloric acid (HCl, concentrated 37% v/v) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), which were all analytical grade reagents and were used with no further treatments. Purification of HNTs. About 250 g of halloysite nanotubes powder and 500 mL of deionized water were mixed in the flask and stirred for 2 h. Sodium hexametaphosphate [(NaPO3)6] (1.25 g) was added gradually under continuous stirring, and then 10 wt % NaOH aqueous solution was added to adjust the pH value to between 8 and 9. The mixture was further stirred for 24 h and then left to stand for 6 h. The impurities and large bundled HNTs precipitated at the bottom of the flask, while individual HNTs mainly remained in supernatant. The supernatant was decanted and then centrifuged at 3000 rpm for 5 min. The supernatant was decanted again for removing much long unbundled HNTs, and then a centrifugation at 7000 rpm for 10 min was further performed. Afterward, the obtained precipitates were repeatedly washed with deionized water and centrifuged successively for at least three centrifugation cycles until the decantate became neutral. Finally, the milky solids were obtained. The vacuum freezedrying method was used to further enhance the solubility of HNTs and improve the stability of HNT dispersions. The final sample represented a white solid that could be easily crushed into powder using mortar. Preparation of HNT Aqueous Dispersions. The purified HNTs were added into deionized water under stirring in sample bottles, and the mass concentrations were measured in wt % at 25 °C. The HNT concentration range was from 0.1 to 50 wt %. Characterizations. The measurements of zeta potentials of the HNT dispersions were performed on DelsaNano C Particle Size and Zeta Potential Analyzer (Beckman Coulter, Inc.). The HNT aqueous dispersions were loaded in beakers and quenched in liquid nitrogen. The frozen solids were quickly transferred to the vacuum freeze-dryer and kept at −60 °C until completely dried. The fluffy solids were carefully put on flat substrates precoated with carbonic glues and then were coated with platinum for scanning electron microscopy (SEM) observations in the secondary electron imaging mode by using a JEOL SEM 6700 operating at 5 kV. Transmission electron microscopy (TEM) observations on HNTs were performed by using a JEOL JEM2200 FS with an accelerating voltage of 200 kV. Drops of each HNT dispersion were cast and sandwiched between two glass slides to form the film with a thickness of about 50 μm, and then the polarized optical micrographs were taken by using the Olympus BX51 polarized optical microscope. In order to observe the color changes and distinguish the structural changes during phase transitions, a 530 nm sensitive tint plate (1λ, U-TP530, Olympus, Japan) was used as a test plate compensator, which resulted in a magenta background for the taken optical micrographs. All the observations were conducted under a nitrogen atmosphere. Small-angle X-ray scattering (SAXS) measurements were performed by using an in-house setup with a sealed tube equipped with two parabolic multilayer mirrors (Bruker, Karlsruhe), giving a highly parallel beam (divergence about 0.012°) of

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RESULTS AND DISCUSSION To obtain the liquid crystalline phase for the HNT aqueous dispersions, the first step is to guarantee sufficient solubility/ dispersibility and stability. The oven-dry method was widely used to prepare HNTs. However, the obtained HNT samples by this method are difficult to crush into powder and its aqueous dispersions are poorly dispersed and unstable.54 Large HNT aggregates can be seen in the sample prepared by the oven-dry method (see Figure S1 in the Supporting Information). On contrast, the freeze-drying method has many applications for nanoparticle technology, especially for preventing from particle aggregation and improving solubility and long-term nanoparticle stability.55 Final HNT sample prepared by the freeze-drying method represents a white fluffy solid that can be easily crushed into powder using mortar. Large HNT aggregates are not seen in the sample prepared by the freeze-drying method (Figure S2). Therefore, compared with the common oven-dry method, we applied the vacuum freezedrying method to further enhance the dispersity of HNTs and improve the stability of HNT dispersions. TEM micrographs shown in Figure S3 clearly demonstrate that HNTs prepared by the freeze-drying method have better dispersity in water than that prepared by the oven-dry method. This is because the freeze-drying method can decrease the aggregation of HNT particles and increase the specific surface area, resulting in the increase of solubility and stability of HNTs in water. Figure 1

Figure 1. Photographs of HNT dispersions with HNT concentrations of 0.1, 1, 10, 20, and 40 wt %.

shows photos of the HNT dispersions in aqueous media for 48 h after mixing. (We note here that no sediments appear for these dispersions for 2 weeks after mixing.) Interestingly, the HNT dispersions are homogeneous with no sediments observed, which indicates the sufficient dispersion and stability of HNTs in aqueous media. Noticeably, the 40 wt % aqueous 12359

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HNT dispersion does not flow when turning the sample bottle upside down, indicating it becomes a gel-like sample. It is known that the dispersion stability can be expressed by the electrostatic interactions evidenced by the zeta potential measurements.54 Figure 2 shows the changes of zeta potential

the aggregation of HNT particles to form larger aggregates than the oven-dry method at the same HNT concentration. In other words, the oven-dry method produces larger particles, resulting in the changes of particle movement in the electric field due to the gravitational effect. It is known that the liquid crystalline formation is influenced by the aspect ratio and size distribution of dispersed nanoparticles. Therefore, the sizes of HNTs measured by SEM and TEM are displayed in Figure 3. The results indicate that the HNTs have an average length of 572 nm with a standard deviation (σ) of 196 nm, an average outer diameter of 56 nm (σ = 14 nm), and an average lumen diameter of 21 nm (σ = 7 nm). The corresponding average aspect ratio (mean length/mean diameter) is about 10. Such a sufficient aspect ratio and appropriate solubility/dispersibility and stability in water for HNTs should be prerequisites for the formation of a liquid crystalline phase and lay the foundation for our detailed studies on the liquid crystalline behaviors of HNT dispersions.56 It is well recognized that a direct evidence for appearance of the lyotropic liquid crystalline phase is the evolved birefringence between analyzer and polarizer upon increasing particle concentration. The polarized optical micrographs of HNT aqueous dispersions at 25 °C are displayed in Figure 4. In order to clearly demonstrate the related phase transitions, the micrographs were taken with the sensitive tint plate insertion. (For comparison, the micrographs taken without using the sensitive tint plate are shown in insets.) When the HNT concentration is 0.1 wt % or lower, the dispersion is isotropic (Figure 4a). At the HNT concentration of 1 wt % the emergence of microscopic birefringence and threadlike textures indicates the onset of formation of a liquid crystalline phase (Figure 4b). With increasing HNT concentration, the optical textures become more compact and the birefringence becomes stronger (Figure 4c,d). As HNT concentration increases to 25 wt %, the dispersion shows birefringence with intense colors, which indicates the formation of the anisotropic phase (Figure 4e). A similar phase transition behavior was observed in our

Figure 2. Changes of zeta potential with pH for 10 wt % HNT aqueous dispersions prepared by the oven-dry method and freezedrying method.

with pH for 10 wt % HNT aqueous dispersions prepared by the oven-dry method and freeze-drying method, respectively. Compared with the 10 wt % HNT dispersion prepared by the oven-dry method, which generally features a low zeta potential (−35.6 mV) of HNTs in water, the 10 wt % HNT dispersion prepared by the freeze-drying method shows a higher zeta potential (−56.3 mV). The higher zeta potential for the freeze-drying HNT dispersion makes the aqueous HNT dispersion more stable. Note the differences of zeta potential between the oven-dry method and freeze-drying method are affected by HNT concentration, which increases with increasing HNT concentration. For example, the zeta potential changes from −51.3 to −62.5 mV for the 0.1 wt % HNT dispersions. The reason may be that the freeze-drying method can reduce

Figure 3. (a, b) TEM images of HNTs dispersed in water, (c) SEM image of HNTs, and (d, e, f) the length, outer, and lumen diameter distributions of HNTs measured from (a, b). 12360

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Figure 4. Polarized optical micrographs of HNT dispersions with different HNT concentrations at 25 °C with the sensitive tint plate insertion: (a) 0.1, (b) 1, (c) 10, (d) 20, (e) 25, and (f) 37 wt %. The white scale bar represents 100 μm and is applied to all the micrographs. Insets show corresponding micrographs without the sensitive tint plate insertion. The white scale bar in the top left inset represents 100 μm and is applied to all insets.

previous study on the concentrated microcrystalline cellulose (MCC)/ionic liquid (1-ethyl-3-methylimidazolium acetate, EMIMAc) (MCC/EMIMAc) solutions, in which the texture was ascribed to the cholesteric phase (chiral nematic phase) formation, and the planar textures were interpreted as a nonaligned cholesteric phase associated with the formation of the lyotropic liquid crystalline solution.57,58 When the HNT concentration reaches 37 wt %, the texture looks more random with bright colors, and the rheological characterization at this concentration reveals that the dispersion system eventually becomes a gel (Figure 4f). A similar behavior was observed in aqueous mixtures of carbon nanotubes (CNTs),59 chitin nanocrystal,49 and cellulose nanocrystal.50 These dispersions contain a cholesteric phase (chiral nematic phase), which disappears at the high particle concentrations. On the other hand, for the liquid crystalline HNT dispersions, macroscopic textures can be generally observed if the dispersions are placed between two crossed polarizers. The phase separation occurs with long-time standing or can be speeded up by centrifugation, and the equilibrium between diffusion and sedimentation results in two phases with a distinct interface. These macroscopic results intuitively affirm the liquid crystalline phase transition of the dispersions.38,47,48 We performed the macroscopic texture observation. Figure S4 shows the macroscopic photographs of aqueous HNT dispersions after phase separation taken between two crossed polarizers (Figure S4A) and the plotted relationship between the volume fraction of the anisotropic phase after phase separation and HNT concentration (Figure S4B). The results in Figure S4 reveal an evolution of the three phase states including isotropic, biphasic, and liquid crystalline phases with increasing HNT concentration and confirm that the phase transition concentration from the biphasic phase to a full liquid crystalline phase is 25 wt %, which is consistent with that obtained from the polarized optical microscope observation (Figure 4). Once the liquid crystalline phase of HNT dispersions has been validated, we investigated the liquid crystalline structural information by scanning electron microscopy (SEM) and smallangle X-ray scattering (SAXS). The freeze-dried HNTs derived from the isotropic HNT dispersion (0.5 wt %) show disordered

structure, and no obvious HNT clusters can be observed in Figure S5a,b. For the HNTs derived from the biphasic HNT dispersion (20 wt %), some ordered domains surrounded by irregular HNT tubes can be seen in Figure S5c,d. For the HNTs derived from the fully anisotropic HNT dispersion (35 wt %), various oriented HNT alignments are clearly shown in Figure S5e,f. In addition, it can be seen from these SEM micrographs that the liquid crystalline phase of HNTs has positional ordering at the high HNT concentrations. Note that the HNT dispersions contain a type of cholesteric phase, nonaligned cholesteric phase, and the corresponding texture is a planar kind (Figure 4). The reason for appearance of nonaligned cholesteric phase may lie in the high viscosity or high concentration, which prevents from migration or aggregation of the liquid crystalline phase into more organized domains, resulting in some irregular structures with low orientation. Recall that the HNTs have the average length of 572 nm, average outer diameter of 56 nm, and average lumen diameter of 21 nm. The average aspect ratio is about 10, which is smaller than other common anisotropic nanoparticles.50,59 Therefore, the critical HNT concentration for the formation of liquid crystalline phase is higher than other common anisotropic nanoparticle dispersions, which results in the higher viscosity. For our HNT dispersions, the smectic phase is not observed. However, the common nematic phase can be easily observed under shear or centrifugation conditions (Figure S6).60 The SAXS profiles in Figure 5 further provide the detailed information on the positional ordering as a function of HNT concentration for the HNT dispersions. It can be seen that all the dispersions show one broad weak SAXS peak except for 0.1 wt % HNT dispersion. The reason for the appearance of one broad weak SAXS peak may be that the HNT dispersions contain nonaligned cholesteric liquid crystals, which have some irregular structures with low orientation. Furthermore, the SAXS peak position (remarked by the orange dashed line) for 1−50 wt % HNT dispersions shifts to higher q value with increasing HNT concentration, indicating that the average distance d (d = 2π/q) between HNT particles in planes perpendicular to the director decreases with increasing HNT concentration. 60 Thus, SEM and SAXS results clearly 12361

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ω2 and G″ ∼ ω in the low frequency range. Whereas for the 37 wt % HNT dispersion, G′ and G″ almost superpose with each other within the whole frequency range, and the power law relation is no doubt obeyed, which indicates a transition from the liquidlike to solidlike behaviors, and this transition can be used to define the critical gel point. This result indicates that the HNT aqueous dispersions change from a viscous fluid to an elastic gel as the HNT concentration increases. The dependences of steady shear viscosity, η on shear rate at 25 °C for the HNT dispersions with different HNT concentrations are shown in Figure 7. At the HNT Figure 5. SAXS profiles of HNT dispersions with HNT concentration ranged from 0.1 to 50 wt %. The spectra depict the Lorentz-corrected scattering intensity as functions of scattering vector, q (q = (4π sin θ)/ λ = 2π/d, where 2θ is the scattering angle).

demonstrate that the orientation ordering of HNTs in the dispersions are strongly dependent on the HNT concentration. Rheological methods have been widely used to study the nanoparticle (rods, plates, etc.) dispersions because they can detect the presence of internal microstructures. The microscopic connectivity with three-dimensional networks produced from physical interactions (van der Waals, hydrogen bonds, and electrostatic interactions) can be investigated using the rheological methods.49,61 At a critical concentration of nanoparticles, the viscoelastic response of the dispersion system changes from the liquid-like to solid-like behaviors. The dynamic rheological behaviors of the HNT aqueous dispersions were examined. The changes of storage modulus, G′, and loss modulus, G″, as functions of angular frequency, ω, for the HNT aqueous dispersions at 25 °C are shown in Figure 6. The HNT

Figure 7. Changes of steady shear viscosity with shear rate for HNT dispersions with different HNT concentrations. The measurements were performed at 25 °C.

concentrations lower than 20 wt %, low shear plateaus appear after the initial shear thinning region. For HNT concentrations between 20 and 35 wt %, the HNT dispersions signify a typical shear flow behavior and show decreased viscosity upon shear, likely because of the deformation of the existing gel network. Moreover, for the 40 and 50 wt % HNT dispersions the viscosity shows almost linear decrease with increasing shear rate, which suggests the deformation of the existing networks for gelation. Note that the viscosity at the low shear rate does not change monotonically with increasing HNT concentration; that is to say, it has a steep increase up to the maximum value at the HNT concentration of 15 wt %, a drop to the minimum value at the HNT concentration of 20 wt %, and then a steep increase again at higher HNT concentrations. However, at the high shear rates the viscosity shows a monotonic increase with increasing HNT concentration. A particular characteristic for the lyotropic liquid crystals is that the steady shear viscosity and dynamic complex viscosity are not equal when the shear rate equals the frequency, which indicates that the Cox−Merz rule is not obeyed. Figure S7 shows the evolutions of steady shear viscosity with shear rate and dynamic complex viscosity with frequency for 20 and 50 wt % HNT aqueous dispersions, respectively. For the lower HNT concentration dispersion, the biphasic dispersion shows less obvious deviation from the Cox−Merz rule at the low shear rate/frequency but shows obvious deviation at the high shear rate/frequency (Figure S7a). Whereas for the higher HNT concentration dispersion, the liquid crystalline gel dispersion shows obvious deviation from the Cox−Merz rule throughout the whole shear rate/frequency range (Figure S7b). For a normal polymer solution or colloidal dispersion, the viscosity at the low shear rate increases with concentration. However, for the liquid crystalline dispersions, the viscosity

Figure 6. Changes of storage modulus, G′, and loss modulus, G″, as functions of angular frequency, ω, at 25 °C for HNT aqueous dispersions with different HNT concentrations.

concentrations of 10, 37, and 50 wt % are indicated in the figure. The typical characteristics of these modulus−frequency curves can be figured out directly. Approximately two distinct groups of curves are separated by the concentration value of 37 wt %. For an ideal gel that behaves elastically, the G′ values are expected to be independent of frequency and G′ > G″. As it can be seen from Figure 6, for the 50 wt % HNT dispersion, the G′ values are always higher than G″ in the explored frequency range, showing a strong frequency independence. However, at the low concentration of 10 wt %, the HNT dispersion exhibits a liquid-like behavior because G′ < G″ in the whole frequency range and G′ and G″ values scale approximately with ω by G′ ∼ 12362

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does not change monotonically with increasing concentration and goes through a maximum in the biphasic region. Figure 8

Figure 9. Changes of G′, G″, and |η*| as functions of ω at 25 °C for 15 wt % HNT aqueous dispersions under pH = 5 (acidic) and pH = 7 (neutral) conditions. Insets show the HNT dispersion under neutral condition (the bottom right corner) and the dispersion under acidic condition (the top left corner), corresponding to the typical flow and gel states, respectively.

Figure 8. Changes of steady shear viscosity as functions of HNT concentration for HNT dispersions at the shear rates of 1, 10, and 100 s. The vertical dashed lines separate the HNT dispersions into the different phase regions.

behavior. This result indicates that the associative interactions become dominating in the system, leading to the network formation. The associative interactions can be attributed to the electrostatic screening of HNTs by the H+ addition, which leads to the minimization of the repulsive forces due to the electrostatic charges on the outer surfaces of HNT nanotubes. As shown in the polarized optical micrographs of Figure S8, the H+-free 15 wt % HNT aqueous dispersion exhibits relatively weak birefringence (Figure S8a), whereas the dispersion exhibits strong birefringence when the dispersion approaches the pH value of 5.0 (Figure S8b). It is also worth noting that the sol−LC gel transition is more evident under the acidic condition for the HNT aqueous dispersion. However, these particular LC gels display inhomogeneous structures with some dark nonbirefringent portions, indicating some different structural organization from the LC gel sample with the higher HNT concentration (37 wt % HNTs, with no addition of HCl) shown in Figure 4f, which exhibits the birefringence in the whole volume range. Similar results have been reported in the chitin nanocrystal dispersions at different pH levels.49 On the basis of the above results, the microstructural changes in the HNT aqueous dispersions with increasing HNT concentration and addition of HCl are schematically depicted in Figure 10. HNTs have positively charged inner surfaces and negatively charged outer surfaces in water, resulting in strong repulsive electrostatic forces rather than attractive interaction, and the dilute HNT dispersions show a random distribution of HNTs (Figure 10a), whereas some ordered alignments of HNTs appear in the biphasic region, which can be explained by the Onsager theory for parallel alignments of anisotropic particles on the entropic term (Figure 10b). With further increasing HNT concentration, the completely ordered alignments of HNTs are obtained (Figure 10c), and eventually a percolated liquid crystalline network forms as a result of strong correlation between the adjacent liquid crystalline domains (Figure 10d). Liquid crystalline network also forms when HCl is added in the HNT aqueous dispersion in the biphasic region (Figure 10e). The reason for this particular liquid crystalline network formation is that the repulsive electrostatic forces are reduced, and the HNTs can easily aggregate when adding the acid. As the pH reaches even lower values, i.e. pH 3.0, the HNT dispersions exhibit more obvious liquid crystalline phase and stronger gel-like behavior. On the contrary, as the pH reaches

shows the changes of steady shear viscosity as functions of HNT concentration for HNT dispersions at the shear rates of 1, 10, and 100 s−1. Note that the separation of phase transitions by vertical dashed lines in Figure 8 is based on the polarized optical microscopic observation (Figure 4), macroscopic observation (Figure S4), and rheological measurements (Figures 6 and 7). Clearly, the maxima are observed at the HNT concentration of 15 wt % in the biphasic region at the shear rate of 1 s−1, which is due that the viscosity increases with HNT concentration as long as the dispersions are predominantly isotropic, whereas the increasing fraction of anisotropic domains (liquid crystalline) eventually results in a decreased resistance to shear flow, and thus the viscosity decreases with further increasing HNT concentration. Furthermore, the magnitude of this maximum decreases when the shear rate increases to 10 s−1. Surprisingly, no maximum can be observed at the 15 wt % HNT concentration in the biphasic region at the shear rate of 100 s−1. This result may suggest that the liquid crystalline ordering is disrupted by so high shear rate, and the viscosity only shows a monotonic increase with HNT concentration.57 The shear viscosity−concentration curves for HNT dispersions can be separated into four regions corresponding to the isotropic, isotropic + liquid crystalline, liquid crystalline, and LC gel phases, as remarked by the vertical dashed lines in Figure 8. This observation is similar to some reports about the lyotropic liquid crystalline solutions or dispersions.38,61 For the charged nanoparticle dispersions, the isotropic to anisotropic and the liquid crystalline to gel transitions are sensitive to pH values (acid control), and the phase transitions occur at the lower particle concentrations at the acidic condition compared with the H+-free aqueous dispersions (neutral condition).49,62 In our case, drops of 1 M hydrochloric acid (HCl) were added into the 15 wt % HNT dispersion to obtain the pH value of 5.0. Figure 9 shows the dynamic rheological behaviors of HNT aqueous dispersions under the acidic (pH = 5) and neutral (pH = 7) conditions. As can be seen from Figure 9, the 15 wt % HNT aqueous dispersion with no addition of HCl shows a typical liquidlike behavior. However, when the dispersion added with HCl approaches the pH value of 5.0, the dispersion exhibits a strong gel-like 12363

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Figure 10. Schematic illustration of self-organizations of HNTs and phase transitions with increasing HNT concentration and with addition of HCl for the HNT aqueous dispersions.

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even higher pH value, i.e. pH 9.0, the HNT dispersions show a weaker liquid crystalline phase and weaker gel-like behavior. Thus, the H+ can be considered as cross-linking points between adjacent HNTs, which have the negatively charged outer surfaces. Furthermore, some HNTs can be oriented to form new parallel aggregates, leading to the formation of the liquid crystalline network structure.63,64

Corresponding Authors

*E-mail: [email protected] (H.S.). *E-mail: [email protected] (Z.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Z.W. acknowledges the financial support from the National Basic Research Program of China with Grant No. 2012CB025901 and National Science Foundation of China with Grant No. 51073145. H.S. acknowledges the financial support from National Science Foundation of China with Grant No. 21304029, the Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20121301120004), the Natural Science Foundation of Hebei Province (Grant No. B2013201117), the Plan of Science Technology Research and Development of Hebei Province (Grant No. 12211204), and Hebei University (Grant No. Y2011223). The authors thank Dr. Jun Zhang’s group at Institute of Chemistry, Chinese Academy of Sciences, for the assistance on TEM measurement.

CONCLUSIONS The HNT aqueous dispersions shift from isotropic toward lyotropic liquid crystalline and liquid crystalline gel phases with increasing HNT concentration. HNT aqueous dispersions exhibit the pH-induced gelation and more intense birefringence with addition of HCl. To the authors’ knowledge, this is the first report on the lyotropic liquid crystalline and liquid crystalline gel phases for the HNT aqueous dispersions. These results are essential to expand the understanding on the relations between the liquid crystalline phase and sol−gel transition. It is believed that these liquid crystalline and liquid crystalline gels can have some optical functions for the HNT long-range ordered materials and potentially provide the foundation for their biological applications, especially for fabrication of highly ordered supramolecular complex of HNTs and biopolymers (DNA, amylose, etc.).

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AUTHOR INFORMATION

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REFERENCES

(1) Abdullayev, E.; Joshi, A.; Wei, W. B.; Zhao, Y. F.; Lvov, Y. M. Enlargement of Halloysite Clay Nanotube Lumen by Selective Etching of Aluminum Oxide. ACS Nano 2012, 6, 7216−7226. (2) Ma, Z.; Wang, J. Y.; Gao, X.; Ding, T.; Qin, Y. N. Application of Halloysite Nanotubes. Prog. Chem. 2012, 24, 275−283. (3) Lvov, Y. M.; Shchukin, D. G.; Mohwald, H.; Price, R. R. Halloysite Clay Nanotubes for Controlled Release of Protective Agents. ACS Nano 2008, 2, 814−820. (4) Yah, W. O.; Takahara, A.; Lvov, Y. M. Selective Modification of Halloysite Lumen with Octadecylphosphonic Acid: New Inorganic Tubular Micelle. J. Am. Chem. Soc. 2012, 134, 1853−1859. (5) Joshi, A.; Abdullayev, E.; Vasiliev, A.; Volkova, O.; Lvov, Y. M. Interfacial Modification of Clay Nanotubes for the Sustained Release of Corrosion Inhibitors. Langmuir 2013, 29, 7439−7448.

ASSOCIATED CONTENT

* Supporting Information S

Figures S1−S8 show SEM and TEM micrographs of HNTs prepared by the oven-dry method and the freeze-drying method, macroscopic observation on the phase transition for HNT aqueous dispersions, SEM micrographs of freeze-dried HNT foams, SEM micrograph of dried HNT sample after shear, Cox−Merz rule for HNT aqueous dispersions, and polarized optical micrographs for 15 wt % HNT aqueous dispersions at different pH values. This material is available free of charge via the Internet at http://pubs.acs.org. 12364

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Gonzalez-Montiel, A. Surface Modification of Natural Halloysite Clay Nanotubes with Aminosilanes. Application as Catalyst Supports in the Atom Transfer Radical Polymerization of Methyl Methacrylate. Appl. Catal., A 2011, 406, 22−33. (28) Zatta, L.; Gardolinski, J.; Wypych, F. Raw Halloysite as Reusable Heterogeneous Catalyst for Esterification of Lauric Acid. Appl. Clay Sci. 2011, 51, 165−169. (29) Cavallaro, G.; Lazzara, G.; Milioto, S. Dispersions of Nanoclays of Different Shapes into Aqueous and Solid Biopolymeric Matrices. Extended Physicochemical Study. Langmuir 2011, 27, 1158−1167. (30) Firestone, M. A.; Williams, D. E.; Seifert, S.; Csencsits, R. Nanoparticle Arrays Formed by Spatial Compartmentalization in a Complex Fluid. Nano Lett. 2001, 1, 129−135. (31) Vigolo, B.; Penicaud, A.; Coulon, C.; Sauder, C.; Pailler, R.; Journet, C.; Bernier, P.; Poulin, P. Macroscopic Fibers and Ribbons of Oriented Carbon Nanotubes. Science 2000, 290, 1331−1334. (32) Tardani, F.; Gentile, L.; Ranieri, G. A.; La Mesa, C. Shear Orientation in Nematic Carbon Nanotube Dispersions: A Combined NMR Investigation. J. Phys. Chem. C 2013, 117, 8556−8562. (33) Song, W.; Kinloch, I. A.; Windle, A. H. Nematic Liquid Crystallinity of Multiwall Carbon Nanotubes. Science 2003, 302, 1363. (34) Badaire, S.; Zakri, C.; Maugey, M.; Derre, A.; Barisci, J. N.; Wallace, G.; Poulin, P. Liquid Crystals of DNA-Stabilized Carbon Nanotubes. Adv. Mater. 2005, 17, 1673−1676. (35) Wang, R. R.; Sun, J.; Gao, L. A. Liquid-Crystal Phase Reinforced Carbon Nanotube Fibers. J. Phys. Chem. C 2010, 114, 4923−4928. (36) Moulton, S. E.; Maugey, M.; Poulin, P.; Wallace, G. G. Liquid Crystal Behavior of Single-walled Carbon Nanotubes Dispersed in Biological Hyaluronic Acid Solutions. J. Am. Chem. Soc. 2007, 129, 9452−9457. (37) Tardani, F.; La Mesa, C.; Poulin, P.; Maugey, M. Phase Behavior of DNA-Based Dispersions Containing Carbon Nanotubes: Effects of Added Polymers and Ionic Strength on Excluded Volume. J. Phys. Chem. C 2012, 116, 9888−9894. (38) Xu, Z.; Gao, C. Aqueous Liquid Crystals of Graphene Oxide. ACS Nano 2011, 5, 2908−2915. (39) Xu, Z.; Gao, C. Graphene Chiral Liquid Crystals and Macroscopic Assembled Fibres. Nat. Commun. 2011, 2, 571. (40) Dozov, I.; Paineau, E.; Davidson, P.; Antonova, K.; Baravian, C.; Bihannic, I.; Michot, L. J. Electric-Field-Induced Perfect Anti-Nematic Order in Isotropic Aqueous Suspensions of a Natural Beidellite Clay. J. Phys. Chem. B 2011, 115, 7751−7765. (41) Michot, L. J.; Bihannic, I.; Maddi, S.; Funari, S. S.; Baravian, C.; Levitz, P.; Davidson, P. Liquid-Crystalline Aqueous Clay Suspensions. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 16101−16104. (42) Kajiwara, K.; Donkai, N.; Hiragi, Y.; Inagaki, H. Lyotropic Mesophase of Imogolite, 1. Effect of Polydispersity on Phase Diagram. Makromol. Chem. 1986, 187, 2883−2893. (43) Buining, P. A.; Philipse, A. P.; Lekkerkerker, H. N. W. Phase Behavior of Aqueous Dispersions of Colloidal Boehmite Rods. Langmuir 1994, 10, 2106−2114. (44) Miyamoto, N.; Iijima, H.; Ohkubo, H.; Yamauchi, Y. Liquid Crystal Phases in the Aqueous Colloids of Size-Controlled Fluorinated Layered Clay Mineral Nanosheets. Chem. Commun. 2010, 46, 4166− 4168. (45) Gabriel, J. C. P.; Davidson, P. New Trends in Colloidal Liquid Crystals Based on Mineral Moieties. Adv. Mater. 2000, 12, 9−20. (46) Nakato, T.; Miyamoto, N. Liquid Crystalline Behavior and Related Properties of Colloidal Systems of Inorganic Oxide Nanosheets. Materials 2009, 2, 1734−1761. (47) Mourad, M. C. D.; Belov, D. V.; Petukhov, A. V.; De Winter, D. A. M.; Verkleij, A. J.; Lekkerkerker, H. N. W. Sol-Gel Transitions and Liquid Crystal Phase Transitions in Concentrated Aqueous Suspensions of Colloidal Gibbsite Platelets. J. Phys. Chem. B 2009, 113, 11604−11613. (48) Wijnhoven, J. E. G. J.; Van’t Zand, D. D.; Beek, D. V. D.; Lekkerkerker, H. N. W. Sedimentation and Phase Transitions of Colloidal Gibbsite Platelets. Langmuir 2005, 21, 10422−10427.

(6) Zhao, Y. F.; Abdullayev, E.; Vasiliev, A.; Lvov, Y. M. Halloysite Nanotubule Clay for Efficient Water Purification. J. Colloid Interface Sci. 2013, 406, 121−129. (7) Konnova, S. A.; Sharipova, I. R.; Demina, T. A.; Osin, Y. N.; Yarullina, D. R.; Ilinskaya, O. N.; Lvov, Y. M.; Fakhrullin, R. F. Biomimetic Cell-Mediated Three-Dimensional Assembly of Halloysite Nanotubes. Chem. Commun. 2013, 49, 4208−4210. (8) Fix, D.; Andreeva, D. V.; Shchukin, D. G.; Moehwald, H. Application of Inhibitor-Loaded Halloysite Nanotubes in Active AntiCorrosive Coatings. Adv. Funct. Mater. 2009, 19, 1720−1727. (9) Abdullayev, E.; Price, R.; Shchukin, D. G.; Lvov, Y. M. Halloysite Tubes as Nanocontainers for Anticorrosion Coating with Benzotriazole. ACS Appl. Mater. Interfaces 2009, 1, 1437−1443. (10) Guimaraes, L.; Enyashin, A. N.; Seifert, G.; Duarte, H. A. Structural, Electronic, and Mechanical Properties of Single-Walled Halloysite Nanotube Models. J. Phys. Chem. C 2010, 114, 11358− 11363. (11) Du, M. L.; Guo, B. C.; Jia, D. M. Newly Emerging Applications of Halloysite Nanotubes: a Review. Polym. Int. 2010, 59, 574−582. (12) Liu, M. X.; Guo, B. C.; Du, M. L.; Cai, X. J.; Jia, D. M. Properties of Halloysite Nanotube-epoxy Resin Hybrids and the Interfacial Reactions in the Systems. Nanotechnology 2007, 18, 455703. (13) Lecouvet, B.; Sclavons, M.; Bourbigot, S.; Devaux, J.; Bailly, C. Water-Assisted Extrusion as a Novel Processing Route to Prepare Polypropylene/Halloysite Nanotube Nanocomposites: Structure and Properties. Polymer 2011, 52, 4284−4295. (14) Cavallaro, G.; Gianguzza, A.; Milioto, S.; Lazzara, G.; Piazzese, D. Alginate Gel Beads Flled with Halloysite Nanotubes. Appl. Clay Sci. 2013, 72, 132−137. (15) Cavallaro, G.; Donato, D. I.; Lazzara, G.; Milioto, S. Films of Halloysite Nanotubes Sandwiched between Two Layers of Biopolymer: From the Morphology to the Dielectric, Thermal, Transparency, and Wettability Properties. J. Phys. Chem. C 2011, 115, 20491−20498. (16) Cavallaro, G.; Lisi, R. D.; Lazzara, G.; Milioto, S. Polyethylene Glycol/Clay Nanotubes Composites:Thermal Properties and Structure. J. Therm. Anal. Calorim. 2013, 112, 383−389. (17) Ning, N. Y.; Yin, Q. J.; Luo, F.; Zhang, Q.; Du, R.; Fu, Q. Crystallization Behavior and Mechanical Properties of Polypropylene/ Halloy Site Composites. Polymer 2007, 48, 7374−7384. (18) Liu, M. X.; Guo, B. C.; Du, M. L.; Chen, F.; Jia, D. M. Halloysite Nanotubes as a Novel Beta-nucleating Agent for Isotactic Polypropylene. Polymer 2009, 50, 3022−3030. (19) Vergaro, V.; Abdullayev, E.; Lvov, Y. M.; Zeitoun, A.; Cingolani, R.; Rinaldi, R.; Leporatti, S. Cytocompatibility and Uptake of Halloysite Clay Nanotubes. Biomacromolecules 2010, 11, 820−826. (20) Ariga, K.; Lvov, Y. M.; Kawakami, K.; Ji, Q. M.; Hill, J. P. Layerby-Layer Self-Assembled Shells for Drug Delivery. Adv. Drug Delivery Rev. 2011, 63, 762−771. (21) Veerabadran, N. G.; Mongayt, D.; Torchilin, V.; Price, R. R.; Lvov, Y. M. Organized Shells on Clay Nanotubes for Controlled Release of Macromolecules. Macromol. Rapid Commun. 2009, 30, 99− 103. (22) Abdullayev, E.; Lvov, Y. M. Halloysite Clay Nanotubes for Controlled Release of Protective Agents. J. Nanosci. Nanotechnol. 2011, 11, 10007−10026. (23) Shchukin, D. G.; Sukhorukov, G. B.; Price, R. R.; Lvov, Y. M. Halloysite Nanotubes as Biomimetic Nanoreactors. Small 2005, 1, 510−513. (24) Luo, P.; Zhao, Y. F.; Zhang, B.; Liu, J. D.; Yang, Y.; Liu, J. F. Study on the Adsorption of Neutral Red from Aqueous Solution onto Halloysite Nanotubes. Water Res. 2010, 44, 1489−1497. (25) Liu, L.; Wan, Y. Z.; Xie, Y. D.; Zhai, R.; Zhang, B.; Liu, J. D. The Removal of Dye from Aqueous Solution Using Alginate-halloysite Nanotube Beads. Chem. Eng. J. 2012, 187, 210−216. (26) Shchukin, D. G.; Mohwald, H. Surface-engineered Nanocontainers for Entrapment of Corrosion Inhibitors. Adv. Funct. Mater. 2007, 17, 1451−1458. (27) Barrientos-Ramirez, S.; de Oca-Ramirez, G. M.; RamosFernandez, E. V.; Sepulveda-Escribano, A.; Pastor-Blas, M. M.; 12365

dx.doi.org/10.1021/la402836d | Langmuir 2013, 29, 12358−12366

Langmuir

Article

(49) Tzoumaki, M. V.; Moschakis, T.; Biliaderis, C. G. Metastability of Nematic Gels Made of Aqueous Chitin Nanocrystal Dispersions. Biomacromolecules 2010, 11, 175−181. (50) Habibi, Y.; Lucia, L. A.; Rojas, O. J. Cellulose Nanocrystals: Chemistry, Self-Assembly, and Applications. Chem. Rev. 2010, 110, 3479−3500. (51) Eichhorn, S. J. Cellulose Nanowhiskers: Promising Materials for Advanced Applications. Soft Matter 2011, 7, 303−315. (52) Pan, J. H.; Hamad, W.; Straus, S. K. Parameters Affecting the Chiral Nematic Phase of Nanocrystalline Cellulose Films. Macromolecules 2010, 43, 3851−3858. (53) Dikin, D. A.; Stankovich, S.; Zimney, E. J.; Piner, R. D.; Dommett, G. H. B.; Evmenenko, G.; Nguyen, S. T.; Ruoff, R. S. Preparation and Characterization of Graphene Oxide Paper. Nature 2007, 448, 457−460. (54) Cavallaro, G.; Lazzara, G.; Milioto, S. Exploiting the Colloidal Stability and Solubilization Ability of Clay Nanotubes/Ionic Surfactant Hybrid Nanomaterials. J. Phys. Chem. C 2012, 116, 21932−21938. (55) Abdelwahed, W.; Degobert, G.; Stainmesse, S.; Fessi, H. FreezeDrying of Nanoparticles: Formulation, Process and Storage Considerations. Adv. Drug Delivery Rev. 2006, 58, 1688−1713. (56) Aboutalebi, S. H.; Gudarzi, M. M.; Zheng, Q. B.; Kim, J. K. Spontaneous Formation of Liquid Crystals in Ultralarge Graphene Oxide Dispersions. Adv. Funct. Mater. 2011, 21, 2978−2988. (57) Song, H. Z.; Niu, Y. H.; Wang, Z. G.; Zhang, J. Liquid Crystalline Phase and Gel-Sol Transitions for Concentrated Microcrystalline Cellulose (MCC)/1-Ethyl-3-methylimidazolium Acetate (EMIMAc) Solutions. Biomacromolecules 2011, 12, 1087−1096. (58) Song, H. Z.; Zhang, J.; Niu, Y. H.; Wang, Z. G. Phase Transition and Rheological Behaviors of Concentrated Cellulose/Ionic Liquid Solutions. J. Phys. Chem. B 2010, 114, 6006−6013. (59) Ao, G.; Nepal, D.; Aono, M.; Davis, V. A. Cholesteric and Nematic Liquid Crystalline Phase Behavior of Double-Stranded DNA Stabilized Single-Walled Carbon Nanotube Dispersions. ACS Nano 2011, 5, 1450−1458. (60) Belamie, E.; Davidson, P.; Giraud-Guille, M. M. Structure and Chirality of the Nematic Phase in α-Chitin Suspensions. J. Phys. Chem. B 2004, 108, 14991−15000. (61) Urena-Benavides, E. E.; Ao, G. Y.; Davis, V. A.; Kitchens, C. L. Rheology and Phase Behavior of Lyotropic Cellulose Nanocrystal Suspensions. Macromolecules 2011, 44, 8990−8998. (62) Kim, H. S.; Yoon, S. H.; Kwon, S. M.; Jin, H. J. pH-Sensitive Multiwalled Carbon Nanotube Dispersion with Silk Fibroins. Biomacromolecules 2009, 10, 82−86. (63) Chan, C.; Crawford, G.; Gao, Y. M.; Hurt, R.; Jian, K. Q.; Li, H.; Sheldon, B.; Sousa, M.; Yang, N. Liquid Crystal Engineering of Carbon Nanofibers and Nanotubes. Carbon 2005, 43, 2431−2440. (64) Behabtu, N.; Lomeda, J. R.; Green, M. J.; Higginbotham, A. L.; Sinitskii, A.; Kosynkin, D. V.; Tsentalovich, D.; Parra-Vasquez, A. N. G.; Schmidt, J.; Kesselman, E.; Cohen, Y.; Talmon, Y.; Tour, J. M.; Pasquali, M. Spontaneous High-concentration Dispersions and Liquid Crystals of Graphene. Nat. Nanotechnol. 2010, 5, 406−411.

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