Controlled Assembly of Nanocellulose-Stabilized Emulsions with

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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

Controlled Assembly of Nanocellulose-Stabilized Emulsions with Periodic Liquid Crystal-in-Liquid Crystal Organization Guang Chu, Gleb Vasilyev, Rita Vilensky, Mor Boaz, Rui-Yan Zhang, Patrick Martin, Nitsan Dahan, Shengwei Deng, and Eyal Zussman Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02163 • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 17, 2018

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Controlled Assembly of Nanocellulose-Stabilized Emulsions with Periodic Liquid Crystal-in-Liquid Crystal Organization Guang Chu1*, Gleb Vasilyev1, Rita Vilensky1, Mor Boaz1, Ruiyan Zhang1, Patrick Martin1, Nitsan Dahan2, Shengwei Deng3 and Eyal Zussman1* 1NanoEngineering

Group, Faculty of Mechanical Engineering, Technion-Israel Institute of

Technology, Haifa 3200003, Israel 2Lorry

I. Lokey Interdisciplinary Center for Life Sciences and Engineering, Technion-Israel

Institute of Technology, Haifa 3200003, Israel 3College

of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, China

ABSTRACT: Colloidal particles combined with polymer can be used to stabilize oil-water interface forming stable emulsions. Here, we described a novel liquid crystal (LC)-in-LC emulsion composed of nematic oil phase and cholesteric or nematic aqueous cellulose nanocrystal (CNC) continuous phase. The guest oil droplets were stabilized and suspended in liquid-crystalline CNCs, inducing distortions and topological defects inside the host LC phase. These emulsions exhibited anisotropic interactions between the two LCs that depended on the diameter-to-pitch ratio of

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suspended guest droplets and host CNC cholesteric phase. When the ratio was high, oil droplets were embedded into a cholesteric shell with a concentric packing of CNCs layers, took on a radial orientation of the helical axis. Otherwise, discrete surface trapped LC droplets assemblies with long-range ordering were obtained, mimicking the fingerprints configuration of the cholesteric phase. Thus, the LC-in-LC emulsions presented here define a new class of ordered soft matter in which both nematic and cholesteric LC ordering can be well-manipulated.

KEYWORDS: Cellulose Nanocrystal, Liquid Crystal Emulsion, Controlled Assembly, Periodic Organization

INTRODUCTION Emulsions are a special type of mixture that consist of two or more immiscible liquids, where one liquid contains a droplet dispersion of the other. Classic emulsions are usually prepared by emulsifying surface-active materials to decrease the interfacial tension at the oil-water interface.1, 2

Liquid crystal (LC) emulsions have been widely explored as a new type of soft matter, where

LCs are confined either directly within the suspended droplet or in the inverted continuous phase.35

Both systems exhibit complex long-range ordered structures, i.e., periodic arrangement of LC

molecules over a long distance, typically on the order of several hundred nanometers, which are driven by the anisotropic interactions within the fluid LC medium. However, reports of LC-in-LC emulsions with hierarchically ordered structures remain relatively rare.6 It’s well known that some anisotropic particles in water can form lyotropic colloidal LCs at high concentrations, combining the features of colloids and long-range structure of LCs.7 When a colloidal LC emulsion (in which the continuous phase is composed of liquid crystalline particles) is formed, the suspended droplets can enforce an interfacial particle orientation realignment (planar or homeotropic anchoring) that


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leads to elastic distortions of the surrounding LC field, generating topological defects in the bulk phase and anisotropic colloidal interactions between the guest droplets.8 Cellulose nanocrystals (CNCs) are rigid, twisted, rod-like nanoparticles with negatively charged surface that arising from controlled sulfuric acid hydrolysis.9 Above a critical concentration, CNCs can spontaneously self-organize into a cholesteric phase, in which the CNC rods rotate in a helical manner to form lamellae of equally spaced planes with a common particle orientation.10 Importantly, the length of the helical pitch in CNC cholesteric phase can be tuned by changing the liquid conditions, e.g., applying ultrasonication or adding salt.11, 12 Unlike other LCs which sustain their ordering in liquid state, the cholesteric order of liquid crystalline CNCs can be retained in dry films after evaporation, yielding a robust matrix for material templating and nanoparticle assembly.13-21 Beyond these studies, no attempt has been made to design and fabricate CNC-based LC emulsions. Capron and Cranston et al.22, 23 recently created a family of CNCstabilized emulsions with high oil volume fractions, but none showed any liquid crystalline properties. On the other hand, some works have studied colloidal oil-in-water emulsions with the LC ordering existing either in continuous phase or suspended droplet.24, 25 Therefore, the LC-inLC emulsions that derive from aqueous CNC continuous phase and oil anisotropic phase still require further investigation. In this contribution, we show the design and assembly of micrometer-sized nematic LC (4Cyano-4'-pentylbiphenyl, 5CB) droplets dispersed in continuous cholesteric or nematic phases of CNCs, generating LC-in-LC emulsions with periodic assembly. After emulsification, CNC served as both stabilizing agents and host LC fluid medium, generating a strong planar anchored, layered structure at the oil-water interface and complex elastic distortions in surrounding bulk phase. By varying the ratio between droplet diameter and CNC cholesteric pitch, we show that the


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confinement-induced self-assembly mechanism can trigger and control the structure transitions from disorder to order. When confined to a thin film, the LC-in-LC emulsions offer fascinating distance-dependent deformations in the fingerprint textures around the suspended droplets as well as collective long-range structures of LC droplet assemblies that derive from the array of disclination line defects in cholesteric CNC phase. Moreover, the structural diversities of these emulsions can be manipulated by tuning the inner and outer configuration of the two coupled immiscible LC phase. This work not only reveals the LC-in-LC emulsion that can be defined as a new periodic structured soft matter, but also deepens our understandings of how the interplay in chirality, size, confinement and defect affects the assembly and internal interactions in LC colloids. EXPERIMENTAL SECTION Materials and Apparatus All chemicals were used as received without further purification. 4-Cyano-4'-pentylbiphenyl (5CB, 98%), Pluronic F127, 3-(N, N-dimethylmyristylammonio) propanesulfonate (DMAPS, ≥ 99%), N, N0-methylenebisacrylamide (99%, AR), 2, 2-diethoxyacetophenone (>95%, AR), silicone oil (Lot # MKBT3510V) and polyvinyl alcohol (PVA, Mw=31000, AR) were purchased from Sigma Aldrich. Acrylamide (≥99%) was obtained from Merck KGaA. CNC powder was obtained from the U.S. Forest Products Laboratory at University of Maine. The CNCs were prepared by controlled sulfuric acid hydrolysis and freeze-drying into solid powders. Polarized optical microscopy (POM) image was conducted on Olympus BX51-P microscope with images taken by polarizers in a perpendicular arrangement to verify to the anisotropy of the composite samples. A high-resolution DP71 camera (Olympus, resolution of 5760×3600 pixels) was used to record the images of the emulsion sample. The POM images were


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analysed using ImageJ. Surface morphologies were characterized using a Zeiss Ultra Plus highresolution scanning electron microscope (HR-SEM) at an accelerating voltage of 3 kV. Transmission electron microscopy (TEM) was conducted on a FEI Tecnai G2S-Twin with a field emission gun operating at 200 kV. The zeta potential of CNC suspension before and after postmodification was measured using a Malvern Zetasizer Nano-ZS90. The droplet size distribution of the emulsion was determined using a Malvern Mastersizer 3000. During the measurement, the instrument reservoir was first filled with water and then mixed with 1 mL of the emulsion phase. The droplet size distribution was calculated on a number average basis. Surface and interfacial tension measurements were conducted on Ramé-hart Model 200 through the pendant drop method.26 To measure the interfacial tension, a transparent container was filled with polymer suspension containing 5CB droplet. Confocal laser scanning microscopy (CLSM) was performed with LSM 510 META microscope (Zeiss). A 514 nm argon laser (30 mW) was used as the light source combined with a 40×oil immersion objective to image the sample. Polarized CLSM imaging was conducted on the same microscope without fluorescent labelling. The optical birefringence was collected in the transmission mode using the polarized laser and a linear polarized filter oriented perpendicular to the polarization plane of the incident light. Preparation of cholesteric CNC phase. CNC powder (1.0 g) was dispersed in 13.3 g H2O and stirred overnight, generating a homogeneous suspension with concentration of 7.0 wt%. Then the CNC suspension was sonicated for 30 sec to 2 min in an ice bath and sealed in a 20 ml bottle. After standing for 2 days, the resulting suspension was separated into two phases with the upper isotropic and bottom cholesteric phase. The helical pitch of the CNC phase ranged from 4 to 12 µm, depending on the sonication time.


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Preparation of Isotropic CNC stabilized LC Emulsion. Typically, 5 g isotropic CNC phase (the upper phase) was mixed with amphiphilic Pluronic F127 or hydrophilic PVA (0.05 g) and stirred for 30 min to obtain a homogeneous polymer-CNC suspension. Then 20 µL 5CB were added into the suspension and emulsified through a homogenizer at high speed for 30 s. After emulsification, the resulting emulsion was drop-casted onto a glass slide to track the drying process. For comparison, isotropic CNC alone without polymer was also emulsified with 5 CB. Preparation of polymer stabilized LC Emulsion without CNC. Typically, 5 g water was mixed with amphiphilic Pluronic F127 or hydrophilic PVA (0.05 g) and stirred for 30 min to obtain a homogeneous polymer suspension. Then, 20 µL 5CB were added into the suspension and emulsified through a homogenizer at high speed for 30 s. After emulsification, the resulting emulsion was injected into a cell for optical characterization. Preparation of LC-in-LC Emulsion. Typically, 5 g cholesteric CNCs phase (with short or long pitch, respectively) was mixed with 0.05 g Pluronic F127 or PVA and stirred for 30 min. Then 30 µL 5CB was added into the suspension and emulsified through a homogenizer at high speed for 30 s. The resulting emulsions exhibited a milky white color with the 5CB droplet sizes ranged from 1 to 100 µm. After emulsification, this oil-in-water emulsion was standing overnight to get equilibrium and generating a CNCs stabilized nematic-in-cholesteric LC emulsion. A nematic (radial configuration)-in-cholesteric emulsion was prepared in a similar method with the addition of a small amount of DMAPS (0.01 g) into the cholesteric CNCs-polymer system


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(5g CNCs/ 0.05 g Pluronic F127 or PVA). After the addition of 5CB, the mixture was emulsified for 30 s to generate the CNCs stabilized oil droplets. The addition of DMAPS can trigger an ordering transition of 5CB droplet from bipolar to radial configuration. The nematic-in-nematic emulsion was prepared using a different procedure. Typically, a freshly prepared CNCs suspension (5 g, 5.0 wt%) without sonication was mixed with Pluronic F127 or PVA (0.05 g) and stirred for 30 min. After that, a small amount of NaCl solution was added into the mixture with the final concentration of 1 mM and stirred for another 30 min. Then, 30 µL 5CB were emulsified in this mixture. After emulsification, the resulting emulsion was dropcasted onto a glass slide without the coverslip to enable tracking of the evolution of LC emulsion during the drying process. Preparation of LC-in-LC Emulsion hydrogel. Typically, 0.5 g acrylamide (monomer), 100 mg N, N0-methylenebisacrylamide (cross linker) and 50 µL 2,2-diethoxyacetophenone (photoinitiator) were mixed with 5g CNC/Pluronic F127 suspension (7.0 wt%). After stirring for 1 h, this homogeneous mixture was emulsified with 5CB (30 µL) resulting LC-in-LC emulsions. After equilibrated for 2h, the resulting emulsion was poured into a 30 mm polystyrene Petri dish. Photo-polymerization was carried out for 1 h using illumination from an ultraviolet light source (20 W, 300 nm), yielding polyacrylamide composite hydrogels with 5CB droplets inside. Then the hydrogel was freeze-dried for SEM analysis and the 5CB was removed by vacuum volatilization. Preparation of CNC-stabilized cholesteric bubbles and silicone oil Emulsion. The CNC-stabilized cholesteric bubbles were prepared as previously described.27 Briefly, the cholesteric ordered CNC phase (5.0 g) was mixed with 0.05 g PVA under tenderly stirring (30


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min) at room temperature. Then, air bubbles were injected into the liquid crystal-filled bottle via a capillary (650 µm) in conjunction with a pneumatic pump. The resulting bubble emulsion was introduced in a quartz cell (500 µm), and sealed with epoxy glue. Before optical characterization, the bubble emulsion was equilibrated for 12 h. The CNC-stabilized cholesteric silicone oil emulsion was prepared similar to the LC-inLC emulsion process. Typically, 5.0 g cholesteric CNC phase was mixed with 0.05 g polymer (Pluronic F127 or PVA) and stirred for 30 min to obtain a homogenous suspension. Then, 0.05 g silicone oil was added into the CNC-polymer suspension and emulsified through a homogenizer at high speed for 30 s. After emulsification, this oil-in-water emulsion was allowed to stand overnight to reach equilibrium and generate a CNC stabilized isotropic oil-in-cholesteric LC emulsion. RESULTS AND DISCUSSION The CNC particles had an average length of 114 nm and diameter of 10 nm, exhibited a uniform rod-like morphology (Figure S1). Due to the surface sulfate ester groups following the hydrolytic process, the pristine CNCs exhibited strong electrostatic repulsions and highly negative charges. Addition of salt or polymer can screen such repulsions and decrease the surface tension, thereby enabling CNCs to act as stabilizers in oil-in-water emulsions.23,

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An aqueous 7.0 wt% CNC

suspension was prepared by stirring and sonication, generating phase separation: upper isotropic phase and bottom anisotropic phase (Figure 1). Sonication can effectively increase the helical pitch by ejecting ions from electrical double layer into the bulk suspension, resulting in a larger electrical double layer and weaker chiral interactions between CNC particles.29 Then the two phases were separated by a pipette and mixed with a small amount of polymer (Pluronic F127 or PVA),


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respectively. Pluronic F127 is a non-ionic amphiphilic block copolymer and PVA is a hydrophilic polymer, both of which can bind to the CNC surface through hydrogen bonding and maintain the cholesteric liquid crystalline ordering. After modification, the zeta potentials of the as-prepared CNCs declined from -58.5 to -37.1 (Pluronic F127) and -40.3 mV (PVA), due to the masking effect of polymer. In addition, the corresponding surface tension of the CNC-polymer suspension decreased from 70.1 to 47.3 and 49 mN·m-1 (Pluronic F127 and PVA), respectively (Table S1). Similarly, the interfacial tension of 5CB between CNCs-polymer mixtures was much lower than CNCs alone, which indicated that the polymer-modified CNCs can be used to stabilize the oilwater interface.

Figure 1. Photograph of the CNCs suspension with crossed polarizers, showing upper isotropic phase and bottom cholesteric phase. We first inspected the ordering transition of 5CB droplets suspended in isotropic CNC phase. A freshly prepared CNC-5CB polymer-free emulsion was drop-casted onto a glass slide and evaporated under ambition condition. Initially, the 5CB droplet was in a bipolar configuration inside the isotropic CNC host phase (Figure 2a-c). However, as evaporation continued, the


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ordering of the droplet shifted from bipolar to radial configuration (Figure 2d and Figure S2), suggesting a perpendicular surface anchoring of the 5CB molecules inside the droplets and implying that the 5CB droplets were sensitive to the pristine CNCs. Besides, we have also sealed the fresh prepared 5CB-CNC emulsion into a liquid crystal cell without evaporation. After equilibrium overnight, we found that some of the droplets were transformed into a radial configuration (Figure S3), which indicated that the drying process is dispensable in triggering the transition of 5CB droplet configuration. It should be noted that sometimes the configuration of 5CB droplets do depend on size, however, in our case the 5CB droplets exhibited both bipolar and radial configurations with varying diameters which implied that the size-dependent droplet configuration was not significant. As expected, CNCs were highly negatively charged due to the sulfate ester groups on their surface, which could serve as stabilizer around oil droplet. As evaporation progressed, the charge density of CNC suspension increased and triggered the bipolarto-radial configuration transition. This phenomenon may be the result of the highly charged hydrophobic face of the CNC extending into the 5CB droplets and inducing the radial configuration.30 Thus, we speculated that the surface charge of CNCs played a possible role in triggering the configuration transition and host CNC phase was strongly coupled with the guest 5CB droplet.


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Figure 2. POM images of the 5CB droplet (bipolar configuration) suspended in pure CNC with crossed polarizers (a), one polarizer (b) and without polarizer (c). (d) POM image of the 5CB droplet with radial configuration following evaporation for 5 min. All the images were taken in reflection mode. On the other hand, 5CB droplets with bipolar configuration were stable in an isotropic polymer modified CNC suspension, showing no configuration transition during the evaporation process (Figure 3a, b and Figure S4). Of note, this phenomenon was very similar to the pure polymer stabilized 5CB emulsions (Figure S5). This might be due to the partial screening of surface charges on CNC nanorods by the modified polymer (Pluronic F127 and PVA), as suggested by the increasing of zeta potentials after polymer addition. However, when the sample was kept still for some time and water evaporation continued, spherical-shaped cholesteric tactoids were observed inside the polymer-modified isotropic CNC suspension (Figure 3c, Pluronic F127 modified CNC for example), indicating the phase transition from isotropic to cholesteric in CNC host matrix. Further drying of the samples led to growth of the isolated cholesteric tactoids that eventually merged into a large cholesteric structures (Figure 3d), which then fused to form a


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polydomain area with the helical axes oriented along a different direction in each domain. It should be noted that the pitches in each domain were slightly different, due to the different cholesteric structures in domains emerging from different tactoids. We also found that the cholesteric fingerprint textures were tended to align around the 5CB droplet during the tactoids merging process (Figure 3e, f), implying the coupling effect between polymer-modified CNCs and 5CB droplet.

Figure 3. (a), (b) POM images of bipolar 5CB droplet suspended in Pluronic F127 and PVA modified isotropic CNC suspension. Each 5CB droplet shows two surface point defects boojums at the poles (marked by red arrow). (c) POM image of the cholesteric CNC tactoids formation during evaporation. (d) POM image of the merging of the cholesteric tactoids. Figure (a)-(d) were taken under reflection mode. POM image of the growing of a fingerprint texture around a 5CB droplet which were taken under reflection mode (e) and transmission mode (f), showing the differences of the same structure between two modes.


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The LC-in-LC emulsion was prepared by emulsifying 5CB in the cholesteric CNC-polymer mixtures (Pluronic F127 for example). The CNC-polymer stabilized 5CB droplets exhibited a broad size distribution, ranging from 1-100 µm, with three distinct populations of 5CB droplets at around 4, 12 and 86 µm (Figure 4a). POM images showed that the 5CB droplets exhibited a bipolar configuration, while the surrounding medium was in a nematic-cholesteric transition state, with the fingerprint texture growing along the curve of the 5CB droplet (Figure 4b). This implied that some of the CNC particles were located at the surface of the droplet, acted as nucleation sites for the cholesteric ordering in continuous phase. Then, the emulsion was sealed and stored undisturbed for 3 days, during which the emulsion re-separated into two co-existing phases with the 5CB droplets gathering in the bottom phase with milky white appearance, while the upper phase was more transparent (Figure 4c). The upper phase proved fully isotropic and the bottom phase was in LC-in-LC state (Figure S6). The separation process may largely be the result of forces of gravity, acting on 5CB and lyotropic CNC, which have distinct density differences (1.38 g/cm3 and 1.08 g/cm3, respectively). This also led us to infer the droplet-selective exclusion effect of the cholesteric phase, in which suspended droplets preferred to enter into the lattice of LC rather than remain in the isotropic phase.31 In order to visualize the CNC arrangements around the 5CB droplets, we solidified the LC-in-LC emulsion into a polyacrylamide hydrogel matrix. After equilibrium, the mixtures were subjected to UV irradiation to trigger the cross-linking of the acrylamide, resulting in a LC-in-LC hydrogel (Figure S7, S8). Low-magnification scanning electron microscopy (SEM) images of the macroporous solid monolith clearly demonstrated preservation of spherical shape and smooth surface at the inner and outer droplet-CNC interface (Figure 4d, e), implying a stabilization effect of CNCs. At higher magnification, the CNCs around the droplet cavity were arranged in a periodic layered structure, similar to the morphologies of


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pure cholesteric CNC film with left-handed twisting, indicating the cholesteric structure of CNCs in the surrounding phase (Figure 4f).

Figure 4. (a) Size distribution of CNC-stabilized 5CB droplets. (b) POM image of the CNC/Pluronic F127-stabilized 5CB droplet emulsion, showing typical fingerprint textures around the droplet. Image were taken in transmission mode. (c) Photograph of the LC-in-LC emulsion with an upper isotropic phase and bottom droplet-rich anisotropic phase. Low-magnification SEM images of the emulsion hydrogel, showing the outer (d) and inner (e) surface of the droplet curve. (f) High-magnification SEM image of the hydrogel, showing the periodic helical ordering of the CNC arrangement. The assembly behaviors of guest particles in LC matrixes greatly depend on the particle diameter D and the size of the LC molecule d. If D ≤ d, entropic effects will play a crucial role in particle assembly, i.e., the particles can either act as LC molecular impurities, or be confined into sub-region for LC phase. However, when D >> d, the suspended particles will distort the spatial ordering of the LC matrix, resulting in long-range particle-particle and particle-LC interactions. In


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our LC-in-LC emulsion system, there are two complementary parameters which guide the assembly process, one is the diameter D of the droplet and the other is the helical pitch p of the CNC continuous phase. Both are size-tunable, with the size of the 5CB droplets being much larger than the size of CNC nanorods. Consequently, the behavior of such system can be rather complex with the formation of new types of self-assembly mechanisms. In order to describe the distortions in the cholesteric phase, we used the diameter-to-pitch ratio, 𝛼 = 𝐷 𝑝, which highly impacts the assembly relationship between the guest droplet and host matrix. This assessment demonstrated a strong α-dependent distortion, with a strongly twisted cholesteric emulsion at α>>1 and a weakly twisted cholesteric emulsion at α

Controlled Assembly of Nanocellulose-Stabilized Emulsions with

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