Diverse Ordered 3D Nanostructured Amphiphile Self-Assembly

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Diverse Ordered 3D Nanostructured Amphiphile Self-Assembly Materials Found in Protic Ionic Liquids Xavier Mulet,†,‡,# Danielle F. Kennedy,†,# Tamar L. Greaves,†,# Lynne J. Waddington,§ Adrian Hawley,^ Nigel Kirby,^ and Calum J. Drummond*,† †

CSIRO Materials Science and Engineering, Bag 10, Clayton South MDC, VIC 3169, Australia, ‡Drug Delivery, Disposition and Dynamics, Monash Institute of Pharmaceutical Sciences, Monash University (Parkville Campus), 381 Royal Parade, Parkville, Victoria 3052, Australia, §CSIRO Materials Science and Engineering, 343 Royal Parade, Parkville, VIC 3052, Australia, and ^ Australian Synchrotron, 800 Blackburn Road, Clayton, VIC 3169, Australia

ABSTRACT Protic ionic liquids (PILs) have been shown to provide media for small amphiphilic molecule self-assembly into periodic 3D geometries. The ability of phytantriol and monoolein to self-assemble into ordered 3D nanostructures analogous to structures observed in aqueous solvents has been demonstrated. Monoolein in ethanolammonium formate (EOAF) formed the double diamond lyotropic liquid crystalline cubic mesophase, and phytantriol formed the gyroid cubic mesophase at 25 C. The monoolein cubic mesophase was successfully dispersed in excess PIL, forming nanostructured nanoparticles, and imaged by cryo-transmission electron microscopy. The self-assembled materials reported herein can be considered for soft-templating of material structures during synthesis, as encapsulation and controlled release media, as carrier particles in transport phenomena, and as supports for catalysts where the process conditions require a nonaqueous environment. SECTION Macromolecules, Soft Matter

nitrate (EAN) was first reported by Evans et al. in 1983.11 More recently, there have been several reports of the formation of lyotropic liquid crystals in PILs.10 Cross-polarized optical microscopy has been used to observe the presence of hexagonal and lamellar phases and to infer cubic phases.12-15 Cubic phases have been assigned using small-angle X-ray diffraction (SAXS) for the cationic surfactant C16mimCl in EAN and with a triblock copolymer poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (Pluronic 123), also in EAN.16,17 The self-assembly of phytantriol and monoolein, Figure 1, in EOAF was screened at a 50:50 ratio w/w using synchrotron SAXS at 25 C. Detailed information regarding the experimental procedures employed in this Letter can be found in the Supporting Information. The EOAF-lipid systems have shown a high propensity for the formation of curved lyotropic liquid crystalline systems with two cubic phases observed, the double diamond (Pn3m) and gyroid (Ia3d) phases. Samples were homogenized through centrifugation/extrusion processes, a necessary procedure due to the high viscosity of the PIL. Sample homogenization is assumed to be complete as the diffraction rings observed are typically homogeneous with no large domains or significant granularity present.

F

our decades ago, Luzzati et al. began describing the polymorphism of lyotropic liquid crystals in water, with new phases still being discovered as recently as 2009.1,2 Here, we demonstrate the existence of different bicontinuous cubic mesophases in a protic ionic liquid (PIL), ethanolammonium formate (EOAF). The ability to produce advanced nanomaterials with a range of ordered 3D architectures in ionic liquid solvent systems vastly expands the range of applications of ionic liquids. The formation of two bicontinuous cubic liquid crystal phases in PILs is reported, along with the successful formation of nanoparticles with 3D internal order in the PIL EOAF. The understanding of factors that govern the formation and structure of amphiphile-based lyotropic liquid crystalline materials has grown significantly over recent years.3-5 The bicontinuous cubic mesophases, of particular interest here, consist of an amphiphile bilayer draped over a triply periodic minimal surface separating two unconnected water labyrinths. Applications for such materials range from drug delivery to material templating.6,7 PILs are a subclass of ionic liquids synthesized via proton transfer from a Brønsted acid to a Brønsted base. PILs have tunable solvation properties, accessible through control of their hydrophobic/hydrophilic balance, which has seen a marked increase in their development and application.8 Their water-like solvation properties mean that many PILs are capable of supporting the self-assembly of amphiphiles and polymers.9,10 Micelle formation in the PIL ethylammonium

r 2010 American Chemical Society

Received Date: July 17, 2010 Accepted Date: August 20, 2010 Published on Web Date: August 26, 2010

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DOI: 10.1021/jz1009746 |J. Phys. Chem. Lett. 2010, 1, 2651–2654

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Figure 1. Chemical structure of EOAF, monoolein, and phytantriol.

Figure 3. Powder diffraction scattering intensity profiles showing the intensities of the Bragg peaks for the Ia3d symmetry system formed by the phytantriol (51.2 wt %)/EOAF system at 25 C. Inset: model of the structure of the Ia3d phase.

Several PILs, including ethyl ammonium nitrate, have been found to have intermediate range ordering, consisting of polar and nonpolar domains analogous to those observed in amphiphilic self-assembly processes. Greaves et al. have reported an absence of such structure in EOAF and have correlated this lack of ordering of the ions as an influential factor in smallmolecule amphiphile self-assembly.21 In contrast, highly structured PILs have an increased propensity toward amphiphile solubilization. For select amphiphiles, this may lead to higher critical micelle concentrations (CMCs), and therefore, higher concentrations of amphiphile are required to form self-assembled structures. The PIL in which the cubic phases was observed, EOAF, has limited nanostructure, a lower ability to solubilize amphiphiles, and is able to support inverse cubic phases at very low amphiphile concentrations. Sonication was used to disperse monoolein in EOAF (10:90 w/w ratio of amphiphile/PIL), forming submicrometer particles. The resultant nanoparticles possessed double diamond symmetry with a 150 Å ((1 Å) lattice parameter (Bragg reflections in Figure S2, Supporting Information). Cryo-TEM measurements show the sample with polydisperse amphiphile particles ranging in size from 200 nm to 1 μm (Figure S3, Supporting Information) along with their internal nanostructure as shown in Figure 4. Figure S4 (Supporting Information) highlights the ordering observed by cryo-TEM of the monoolein-EOAF compared to neat EOAF, which displays no longrange order. It should be noted here that the TEM contrast that can be obtained in a PIL is low due to its high electron density reducing the number of electrons transmitted and the increased sensitivity of the samples to damage by the electron beam, limiting exposure times. The PIL's high viscosity relative to that of water hinders the formation of a sufficiently thin film during the vitrification process. The self-assembled bulk phases and particles formed in EOAF all had larger lattice parameters than those observed in water. This difference may be attributed to an interaction of the PIL at the bilayer/solvent interface or a change in the amphiphile structural packing parameters. This increase in

Figure 2. Powder diffraction scattering intensity profiles showing the intensities of the Bragg peaks for the double diamond (Pn3m) symmetry system formed by the monoolein (50.8 wt %)/EOAF system at 25 C. Inset: model of the structure of the Pn3m phase.

The powder diffraction patterns for the systems with Pn3m and Ia3d symmetry are shown in Figures 2 and 3, respectively. The indexed peaks are presented in Table 1, and the diffraction patterns are shown in Figure S1, Supporting Information. The lattice parameters corresponding to the nominally 50:50 mixtures in EOAF were 117 Å ((1 Å) for the monoolein (50.8 wt %) in the double diamond phase and 156 Å ((2 Å) for the phytantriol (51.2 wt %) in the gyroid phase. For comparison, in aqueous systems at similar concentrations, monoolein forms a double diamond phase with a lattice parameter of less than 100 Å, and phytantriol (54 wt %) also forms a double diamond phase with a lattice parameter of 68.2 Å.18,19 Phytantriol forms different cubic phases in EOAF than those in aqueous systems, and the lattice parameters of the cubic phases are significantly larger in EOAF compared to those in water-based lyotropic liquid crystals. Studies of ionic liquid structure, such as those described by Triolo et al., have also been performed on PILs.20 It has recently been shown that neat PILs exhibit a diversity of nanostructures and that this intermediate length scale structure influences their ability to serve as amphiphile self-assembly solvents.21 Small-angle neutron scattering, SAXS, viscosity, and mass spectrometry have all been used to analyze the nanostructure and aggregate formation in PILs.15,21-23


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Table 1. Indexing of the Powder Diffraction Data of the 3D Inverse Bicontinuous Cubic Phases with Calculated q Values Monoolein Pn3m phase -1

Phytantriol Ia3d phase -1

index (hkl)

experimental q (Å-1)

calculated q (Å-1)

0.0930

211

0.1002

0.0987

0.1074 0.1315

220 321

0.1159 0.1535

0.1139 0.1507

index (hkl)

experimental q (Å )

calculated q (Å )

110

0.0763

0.0759

111

0.0933

200 211

0.1077 0.1315

220

0.1520

0.1519

400

0.1642

0.1611

221

0.1610

0.1611

420

0.1836

0.1801

310

0.1698

0.1698

332

0.1924

0.1889

311

0.1861

0.1781

422

0.2012

0.1973

222

0.2012

0.2009

431

0.2093

0.2054

321

0.2225

0.2278

Author Contributions: #

These authors contributed equally to the work presented here.

ACKNOWLEDGMENT C.J.D. is the recipient of an Australian Research Council (ARC) Federation Fellowship. This work was also partly supported by an ARC Discovery Project Grant DP0666961. This research was undertaken in part on the SAXS beamline at the Australian Synchrotron, Victoria, Australia.

REFERENCES (1)

Luzzati, V.; Tardieu, A.; Gulik-Krzywicki, T. Polymorphism of Lipids. Nature 1968, 217, 1028–1030. (2) Shearman, G. C.; Tyler, A. I.; Brooks, N. J.; Templer, R. H.; Ces, O.; Law, R. V.; Seddon, J. M. A 3-D Hexagonal Inverse Micellar Lyotropic Phase. J. Am. Chem. Soc. 2009, 131, 1678–9. (3) Kaasgaard, T.; Drummond, C. J. Ordered 2-D and 3-D Nanostructured Amphiphile Self-Assembly Materials Stable in Excess Solvent. Phys. Chem. Chem. Phys. 2006, 8, 4957–4975. (4) Mulet, X.; Gong, X.; Waddington, L. J.; Drummond, C. J. Observing Self-Assembled Lipid Nanoparticles Building Order and Complexity through Low-Energy Transformation Processes. ACS Nano 2009, 3, 2789–2797. (5) Seddon, J. M.; Squires, A. M.; Conn, C. E.; Ces, O.; Heron, A. J.; Mulet, X.; Shearman, G. C.; Templer, R. H. Pressure-Jump X-ray Studies of Liquid Crystal Transitions in Lipids. Philos. Trans. R. Soc. London, Ser. A 2006, 364, 2635–2655. (6) Mulet, X.; Kennedy, D. F.; Conn, C. E.; Hawley, A.; Drummond, C. J. High Throughput Preparation and Characterisation of Amphiphilic Nanostructured Nanoparticulate Drug Delivery Vehicles. Int. J. Pharm. 2010, 395, 290–297. (7) Braun, P. V.; Stupp, S. I. CdS Mineralization of Hexagonal, Lamellar, and Cubic Lyotropic Liquid Crystals. Mater. Res. Bull. 1999, 34, 463–469. (8) Greaves, T. L.; Drummond, C. J. Protic Ionic Liquids: Properties and Applications. Chem. Rev. 2008, 108, 206–237. (9) Lu, J. M.; Yan, F.; Texter, J. Advanced Applications of Ionic Liquids in Polymer Science. Prog. Polym. Sci. 2009, 34, 431–448. (10) Greaves, T. L.; Drummond, C. J. Ionic Liquids as Amphiphile Self-Assembly Media. Chem. Soc. Rev. 2008, 37, 1709–1726. (11) Evans, D. F.; Yamauchi, A.; Wei, G. J.; Bloomfield, V. A. Micelle Size in Ethylammonium Nitrate as Determined by Classical and Quasi-Elastic Light-Scattering. J. Phys. Chem. 1983, 87, 3537–3541. (12) Greaves, T. L.; Weerawardena, A.; Fong, C.; Krodkiewska, I.; Drummond, C. J. Protic Ionic Liquids: Solvents with Tunable

Figure 4. (A) Cryo-TEM image of monoolein dispersed in EOAF. Arrows indicate the direction of ordering. The resolution is limited due to the limited vitrification of EOAF and its relatively high electron density. This contrast cannot be improved through solvent replacement as this will clearly alter the self-assembled system formed by the amphiphile. (B) Fast Fourier transform (FFT)of the ordered region demonstrates the presence of ordering (postulated [110] reflection).

lattice size leads to a decrease in the system's interfacial curvature when compared to that of water. To our knowledge, this is the first definitive demonstration of nonaqueous solvents (PILs) serving as media which support (i) the self-assembly of small-molecule amphiphiles into different types of inverse bicontinuous cubic phases (Ia3d and Pn3m symmetries) and (ii) the creation of nanostructured nanoparticles of self-assembled amphiphiles with long-range 3D periodicity. The self-assembled materials reported herein may be considered for soft-templating of material structures during synthesis, as encapsulation and controlled release media, as carrier particles in transport phenomena, and as supports for catalysts, where the process conditions require a nonaqueous environment.

SUPPORTING INFORMATION AVAILABLE Materials, methods, and diffraction patterns for other screened ionic liquids. This material is available free of charge via the Internet at http:// pubs.acs.org.

AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail: calum. [email protected]. Tel: þ61 3 9545 2050. Fax: þ61 3 9545 2059.


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Diverse Ordered 3D Nanostructured Amphiphile Self-Assembly

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