Dispersions of Internally Liquid Crystalline Systems Stabilized by

(Mount Olive, NJ). .... At a DU:TC ratio of 100:0, the scattering pattern exhibited at least seven Bragg peaks with ... (19) Figure 1b displays the sc...
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Langmuir 2008, 24, 5306-5314

Dispersions of Internally Liquid Crystalline Systems Stabilized by Charged Disklike Particles as Pickering Emulsions: Basic Properties and Time-Resolved Behavior Anniina Salonen, Franc¸ois Muller, and Otto Glatter* Institute of Chemistry, Karl Franzens UniVersity, Heinrichstrasse 28, A-8010 Graz, Austria ReceiVed January 21, 2008. ReVised Manuscript ReceiVed February 27, 2008 The present paper reports on dispersions of internally liquid crystalline particles, formed from monoglyceride and oil mixtures, stabilized with discrete disklike particles of Laponite clay. Small-angle X-ray scattering (SAXS) was used to probe the presence of dispersed particles as well as their internal liquid crystalline structure. The data were compared to scattering results of reference systems, namely, from the bulk as well as from well-defined particles formed with a polymer as the emulsifier. The submicrometer sizes of the various particles could be derived using dynamic light scattering (DLS). The possible mechanisms involved in the stabilization of each of the different phases by the Laponite platelets, including the role of the residual salt, are discussed. Time-resolved experiments were performed over 60 days in order to follow the evolution of both the internal structure and size of the particles. In particular, we discuss the peculiar behavior of the sample without added oil, where the cubosomes transform into hexosomes over time. The effect of the high pH induced by the Laponite platelets in water, which could result in a hydrolysis of the monoglycerides, was shown to be responsible for the observed cubosome-to-hexasome transition, as well as for the decrease in the lattice parameters.

Introduction Dispersed liquid crystalline phases are of central interest as a result of their many potential applications in a range of fields from cosmetics to the food industry.1–4 In particular, these dispersed particles are highly relevant as further carriers of active molecules due to their low viscosity, their high interfacial area, and the presence of both hydrophilic and hydrophobic regions.5 Such materials consist of stabilized dispersions of a fully hydrated crystalline bulk phase, typically monoglyceride-based, in an aqueous medium. Larsson and co-workers early demonstrated in model systems that microemulsions and hexagonal and cubic phases can be dispersed in such a way.6–9 Since then, several studies have been carried out using a variety of internal components in order to extend the fundamental knowledge of these systems.10–20 Such investigations include exploring the thermodynamic equilibrium of particles’ internal structure, tuning * Corresponding author. E-mail: [email protected]. (1) Chang, C. M.; Bodmeier, R. J. Pharm. Sci. 1997, 86, 747–752. (2) Chang, C. M.; Bodmeier, R. J. Controlled Release 1997, 46, 215–222. (3) Esposito, E.; Menegatti, E.; Cortesi, R. J. Appl. Cosmetol. 2005, 23, 105– 116. (4) Mezzenga, R.; Schurtenberger, P.; Burbridge, A.; Michel, M. Nat. Mater. 2005, 4, 729–740. (5) Chung, H.; Jeong, S. Y.; Kwon, I. C. In Bicontinuous Liquid Crystals; Lynch, M. L., Spicer, P. T., Eds.; CRC Press: Boca Raton, FL, 2005; Chapter 13. (6) Pilman, E.; Larsson, K.; Tornberg, E. J. Dispersion Sci. Technol. 1980, 1(3), 267–281. (7) Gustafsson, J.; Ljusberg-Wahren, H.; Almgren, M.; Larsson, K. Langmuir 1996, 12, 4611–4613. (8) Gustafsson, J.; Ljusberg-Wahren, H.; Almgren, M.; Larsson, K. Langmuir 1997, 13, 6964–6971. (9) Larsson, K. J. Dispersion Sci. Technol. 1999, 20, 27–34. (10) Spicer, P. T.; Hayden, K. L. Langmuir 2001, 17, 5748–5756. (11) Nakano, M.; Teshigawara, T.; Sugita, A.; Leesajakul, W.; Taniguchi, A.; Kamo, T.; Matsuoka, H.; Handa, T. Langmuir 2002, 18, 9283–9288. (12) Kamo, T.; Nakano, M.; Leesajakul, W.; Sugita, A.; Matsuoka, H.; Handa, T. Langmuir 2003, 19, 9191–9195. (13) de Campo, L.; Yaghmur, A.; Sagalowicz, L.; Leser, M. E.; Watzke, H. J.; Glatter, O. Langmuir 2004, 20, 5254–5261. (14) Yaghmur, A.; de Campo, L.; Sagalowicz, L.; Leser, M. E.; Glatter, O. Langmuir 2005, 21, 569–577. (15) Mezzenga, R.; Meyer, C.; Servais, C.; Romoscanu, A. I.; Sagalowicz, L.; Hayward, R. C. Langmuir 2005, 21, 3322–3333.

the internal structure with temperature or with additives without destabilization of the particles, as well as designing useful materials for use as industrial products. These studies have led to a new class of dispersed materials, nowadays commonly called hexosomes (internal hexagonal phase), cubosomes (internal cubic phases), and emulsified microemulsions (EMEs). To date, most of the experimental work has been carried out using surfactant molecules or polymers (diblock or triblock) that include proteins as stabilizers. However, since the pioneering work of Ramsden and Pickering one century ago,21,22 it is known that solid colloidal particles can be used to stabilize emulsions and miniemulsions, and solid molecules should thus be seen as ideal candidates for the stabilization of liquid crystalline bulk phases into well-defined particles. During the last decades, materials that are nowadays commonly referred to as Pickering emulsions have gone through a renaissance. Indeed, some years ago, Velev and co-workers demonstrated that latex particles adsorbed at the liquid-liquid interface of emulsion droplets self-assembled into supracolloidal structures.23,24 Later on, the fabrication of similar permeable structures was reported by Weitz and co-workers, and these were given the generic name of colloidosomes.25 Recent advances (16) Yaghmur, A.; de Campo, L.; Salentinig, S.; Sagalowicz, L.; Leser, M. E.; Glatter, O. Langmuir 2006, 22, 517–521. (17) Dong, Y. D.; Larson, I.; Hanley, T.; Boyd, B. Langmuir 2006, 22, 9512– 9518. (18) Yaghmur, A.; de Campo, L.; Sagalowicz, L.; Leser, M. E.; Glatter, O. Langmuir 2006, 22, 9919–9927. (19) Guillot, S.; Moitzi, C.; Salentinig, S.; Sagalowicz, L.; Leser, M. E.; Glatter, O. Colloids Surf. A. 2006, 291, 78–84. (20) Popescu, G.; Barauskas, J.; Nylander, T.; Tiberg, F. Langmuir 2007, 23, 496–503. (21) Ramsden, W. Proc. R. Soc. London 1903, 72, 156–164. (22) Pickering, S. U. J. Chem. Soc. 1907, 91, 2001–2021. (23) Velev, O. D.; Furusawa, K.; Nagayama, K. Langmuir 1996, 12, 2374– 2384. (24) Velev, O. D.; Furusawa, K.; Nagayama, K. Langmuir 1996, 12, 2385– 2391. (25) Dinsmore, A. D.; Hsu, M. F.; Nikolaides, M. G.; Marquez, M.; Baush, A. R.; Weitz, D. A. Science 2003, 298, 1006–1009.

10.1021/la800199x CCC: $40.75  2008 American Chemical Society Published on Web 04/19/2008

Dispersions of Liquid Crystalline Systems

have enabled the fabrication of colloidosomes by using magnetic26 or pH-driven27,28 particles as emulsifiers, as well as particles of various shapes and sizes, such as spherical silica colloids,29–33 polymeric rods,34 and clay pellets.35–37 This has led to the creation of novel, smart, and stable emulsion-based materials. Our interest lies in using clay particle emulsifiers for the stabilization of dispersed hexosomes, cubosomes, and EME particles of submicrometer sizes. This clay-based system, which can be considered as a nanocomposite (i.e., a mixture of selfassembled organic and inorganic materials of nanosizes), is of great interest both for fundamental material physics as well as for numerous applications. Indeed, such inorganic fillers are known to enhance properties of organic materials, e.g. the mechanical or thermal properties, even when added at rather low concentrations (typically below 5 wt %). Furthermore, clay colloids (natural or synthetic) are already present as additives in a wide range of industrial products, including printing, inks, paints, toothpastes, cosmetics, and pharmaceutical formulations, just to name a few. Therefore, combining clay pellets and dispersed liquid crystalline phases should be a new and interesting way of obtaining useful and versatile particles. Ashby and Binks were the first to focus on the specific effects of such clay nanoparticles as stabilizers in Pickering emulsions,35 although early studies had always associated these particles in combination with surfactants or other solid materials.38–42 The authors succeeded in preparing toluene-in-water (o/w) colloidosomes that were stable to creaming and coalescence for at least 6 months by using Laponite clay particles. However, they were only able to find stable toluene domains under restrictive conditions, namely, when the clay particles were flocculated due to the addition of external salt. This induced a slight colloidal instability through a reduction of the electrostatic repulsion and thereby increased the capacity of the clay particles to allow partitioning to an oil-water interface. Recently, Bon and coworkers studied a similar clay-stabilized system,36,37 where they prepared clay-based colloidosomes as armored vessels of submicrometer size for further polymerization of latexes. In particular, they showed that their data were a direct result of the reversibility of the adhesion process of the Laponite clay disk under the emulsification conditions used. Laponite clay particles are commercially available and synthetically fabricated. They are made up of disklike particles with a thickness of 1 nm and a mean diameter of approximately 30 nm. Laponite is composed of a central octahedral coordinated magnesium-oxygen-hydroxide sheet that is sandwiched between two tetrahedral coordinated silica-oxygen sheets. Iso(26) Melle, S.; Lask, M.; Fuller, G. G. Langmuir 2005, 21, 2158–2162. (27) Ngai, T.; Behrens, S. H.; Auweter, H. Chem. Commun. 2005, 3, 331–333. (28) Fujii, S.; Read, E. S.; Binks, B. P.; Armes, S. P. AdV. Mater. 2005, 17, 1014–1018. (29) Binks, B. P.; Lumsdon, S. O. Phys. Chem. Chem. Phys. 1999, 1, 3007– 3016. (30) Binks, B. P.; Lumsdon, S. O. Langmuir 2000, 16, 2539–2547. (31) Binks, B. P.; Lumsdon, S. O. Langmuir 2000, 16, 3748–3756. (32) Binks, B. P.; Lumsdon, S. O. Langmuir 2000, 16, 8622–8631. (33) Binks, B. P.; Lumsdon, S. O. Phys. Chem. Chem. Phys. 2000, 2, 2959– 2967. (34) Noble, P. F.; Cayre, O. J.; Alargova, R. G.; Velev, O. D.; Paunov, V. N. J. Am. Chem. Soc. 2004, 126, 8092–8093. (35) Ashby, N. P.; Binks, B. P. Phys. Chem. Chem. Phys. 2000, 2, 5640–5646. (36) Cauvin, S.; Colver, P. J.; Bon, S. A. F. Macromolecules 2005, 38, 7887– 7889. (37) Bon, S. A. F.; Colver, P. J. Langmuir 2007, 23, 8316–8322. (38) Tsugita, A.; Takemoto, S.; Mori, K.; Yoneya, T.; Otani, Y. J. Colloid Interface Sci. 1983, 95, 551–560. (39) Abend, S.; Bonnke, N.; Gutschner, U.; Lagaly, G. Colloid Polym. Sci. 1998, 276, 730–737. (40) Lagaly, G.; Reese, M.; Abend, S. Appl. Clay Sci. 1999, 14, 83–103. (41) Lagaly, G.; Reese, M.; Abend, S. Appl. Clay Sci. 1999, 14, 273–298. (42) Yan, N.; Masliyah, J. H. J. Colloid Interface Sci. 1996, 181, 20–27.

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morphic substitutions of magnesium atoms by lithium atoms lead to the formation of negative charges within the lattice, and these negative charges are balanced by counterions, typically sodium ions, located on the surface. Due to their anisotropic shape, the phase behavior of the clay particles in an aqueous system is expected to be richer than for isotropic particles, especially since they can form orientated phases. Several experimental groups have extensively studied phase diagrams in water and in brine of Laponite as well as other natural clays.43–58 Equilibrium phases, in addition to a wide variety of nonequilibrium states that undergo either phase separation or gel/glass formation, have been observed. However, the status of this phase diagram is still under considerable debate, in particular, the structure at the local scale in the nonequilibrium states. The concentrations used herein are restrained to the liquid phases, specifically to clear dispersions of discrete disklike particles in water. Complications arising from aggregated Laponite structures can thus be avoided. The aim of the present contribution is to demonstrate that nanometer-sized Laponite clay particles (i.e., with radii of around 12 nm) can be used as building blocks in order to stabilize cubosomes, hexosomes, and EMEs in water. For this purpose, we used small-angle X-ray scattering (SAXS) to probe the presence of internally self-assembled particles that were stabilized by the Laponite nanoparticles. The obtained scattering curves were compared with reference samples from the bulk phases, as well as from well-defined dispersed particles formed using the triblock copolymer Pluronic F127 as stabilizer. The hydrodynamic radii for diluted dispersions of the various samples were derived by using dynamic light scattering (DLS). In order to verify the time evolution of the particles and their properties, SAXS and DLS were carried out over a period of 60 days.

Experimental Section Materials. Dimodan U/J (DU) was supplied by DANISCO A/S (Braband, Denmark). It contained 96% distilled monoglycerides, of which 62% were linoleate and 25% oleate. Tetradecane (TC), a linear alkane chain of composition C14H30, was purchased from Sigma Chemical Co. (St. Louis, MO). Clay particles of Laponite XLG (Southern Clay Products, Inc., Gonzales, TX), which is a form of Laponite RD further characterized for utilization in the cosmetics industry, were used. These Laponite XLG particles were disklike with a mean radius of RXLG) 12.5 nm and a thickness of h ) 1 nm. Their surface area was 370 m2 g-1 and the pH of aqueous suspensions (43) Dijkstra, M.; Hansen, J. P.; Madden, P. A. Phys. ReV. Lett. 1995, 75, 2236–2239. (44) Mourchid, A.; Delville, A.; Lambard, J.; Le Colier, E.; Levitz, P. Langmuir 1995, 11, 1942–1950. (45) Kroon, M.; Wegdam, G. H.; Sprik, R. Phys. ReV. E. 1996, 54, 6541–6550. (46) Bonn, D.; Tanaka, H.; Wegdam, G.; Kellay, H. J. M. Europhys. Lett. 1999, 45, 52–57. (47) Bonn, D.; Kellay, H.; Tanaka, H.; Wegdam, G.; Meunier, J. Langmuir 1999, 15, 7534–7536. (48) Levitz, P.; Lecolier, E.; Mouchid, A.; Delville, A.; Lyonnard, S. Europhys. Lett. 2000, 49, 672–677. (49) Knaebel, A.; Bellour, M.; Munch, J.-P.; Viasnoff, V.; Lequeux, F.; Harden, J. L. Europhys. Lett. 2000, 52, 73–79. (50) Trizac, E.; Bocquet, L.; Agra, R.; Weis, J. J.; Aubouy, M. J. Phys.: Condens. Matter 2002, 14, 9339–9352. (51) Tomba´cz, E.; Szekeres, M. Appl. Clay Sci. 2004, 27, 75–94. (52) Ruzicka, B.; Zulian, L.; Ruocco, G. J. Phys.: Condens. Matter 2004, 16, S4993–S5002. (53) Tanaka, H.; Meunier, J.; Bonn, D. Phys. ReV. E 2004, 69, 031404. (54) Michot, L:J.; Bihannic, I.; Porsh, K.; Maddi, S.; Baravian, C.; Mougel, J.; Levitz, P. Langmuir 2004, 20, 10829–10837. (55) Mongondry, P.; Tassin, J. F.; Nicolai, T. J. Colloid Interface Sci. 2005, 283, 397–405. (56) Tanaka, H.; Jabbari-Farouji, S.; Meunier, J.; Bonn, D. Phys. ReV. E 2005, 71, 021402. (57) Ruzicka, B.; Zulian, L.; Ruocco, G. Langmuir 2006, 22, 1106–1111. (58) Shalkevich, A.; Stradner, A.; Bhat, S. K.; Muller, F.; Schurtenberger, P. Langmuir 2007, 23, 3570–3580.

5308 Langmuir, Vol. 24, No. 10, 2008 at concentrations below 2 wt % was around 9-10, thus indicating a repulsive potential between the particles. The Laponite XLG contained, in majority, 59.5% SiO2 and 27.5% MgO with traces of LiO2 and Na2O of about 0.8% and 2.8%, respectively. The triblock copolymer Pluronic F127, which consisted of PEO99-PPO67-PEO99, where the subscripts displayed the number of monomers of each species, was generously donated by the BASF Corp. (Mount Olive, NJ). All materials were used without further purification. The water utilized in the preparations was double distilled. The effect of pH on the samples was verified using a 100 mM Tris buffer adjusted with HCl to pH 10. Sample Preparation. Dispersed samples with four DU:TC ratios, i.e. 100:0, 90:10, 70:30, and 50:50, were prepared in excess water in order to form Pn3m, H2, Fd3m, and L2 symmetry groups, respectively. The preparation was carried out by weighing the DU: TC mixture, the stabilizer (Laponite XLG or Pluronic F127), and the water into vials. The raw mixture was ultrasonicated (SYLaboratory GmbH, Pukersdorf, Austria), without external cooling, for 20 min at 30% of the maximum power in pulse mode (0.5 s on and 1.5 s off), and the samples were then sealed and left to equilibrate at room temperature. The weight fraction of the DU:TC mixture was kept constant at 0.05 g g-1 (5 wt %) and that of the stabilizer at 0.005 g g-1 (0.5 wt %), thus giving rise to a constant stabilizer to DU:TC weight ratio of 0.1. The bulk samples were prepared by adding water to a molten DU:TC mixture (T ∼ 50 °C), in order to obtain samples with approximately 40 wt % water and 60 wt % DU:TC, i.e., enough water to be beyond the hydration limit for all the mixtures. The samples were sealed in Pyrex tubes and heated using a Bunsen burner (up to 100 °C for a few seconds at a time) with intermittent vigorous mixing with a vortex to obtain a thorough homogenization. Subsequently, they were left to cool for 2 h at room temperature before being frozen (T ) -4 °C) for 1 h, after which they underwent a re-equilibration at room temperature. Small-Angle X-ray Scattering. The SAXS equipment consisted of a slit-geometry camera (SAXSess, Anton-Paar, Austria) connected to an X-ray generator (Philips, PW1730/10) operating at 40 kV and 50 mA with a sealed-tube Cu anode (λ ) 0.154 nm). The 2D scattering patterns were recorded with a CCD camera from Princeton instruments, which is a division of Roper Scientifics (Trenton, NJ). The images were then integrated into the one-dimensional scattering function I(q). The temperature of the capillary in the metallic sample holder was controlled by a Peltier element. The temperature was fixed at 25 °C and a thermal equilibration time of 30 min was used prior to each SAXS measurement. The measuring times were 3 × 15 min for all dispersions and 3 × 3 min for the bulk samples. This allowed for the proper subtraction of cosmic rays that were registered when using a CCD camera. The scattering of the water solvent, Iw(q), was measured for equivalent times and further subtracted from I(q), thus giving the scattering intensity from the particles. We are aware that the data were smeared with the beam profile (slit profile); however, the influence of this effect was minimized by the various normalizations used for the data interpretation and since the study concerned liquid crystalline structures. Dynamic Light Scattering. The DLS instrument used was a laboratory-built goniometer equipped with a diode laser (Coherent Verdi V5, λ ) 532 nm, maximum power 5 W, average power used 50 mW) with a single mode fiber detection optic (OZ from GMP), an ALV/SO-SIPD/DUAL photomultiplier with a pseudocrosscorrelation, and an ALV 5000/E correlator with fast expansion (ALV). The measurements were carried out at a scattering angle of θ ) 90° The temperature was fixed at 25 °C. All prepared samples were turbid, thus excluding direct measurements with standard DLS. However, since the dispersed Pluronic F127-based particles have been previously reported to be kinetically stable, we assumed that the particles formed using Laponite pellets as building blocks would follow a similar behavior, at least at intermediate time-scales. As a consequence, all samples were diluted using double-distilled water 4000 times immediately before the measurements. The data were collected in repeated measurements of 10 × 30 s. The intensities

Salonen et al.

Figure 1. (a) SAXS intensities (lin-log representation) of liquid crystalline bulk phases of the bicontinuous cubic Pn3m space group (DU:TC ) 100:0), the hexagonal phase H2 (90:10), the micellar cubic Fd3m space group (70:30), and the micellar phase L2 (50:50). The visible reflections are indexed for each phase. The curves have been vertically shifted for the sake of clarity. (b) Solvent-subtracted scattering intensities (lin-log representation) obtained for dispersed particles stabilized by 0.5 wt % Pluronic F127. The initial liquid crystalline phases are presented in the same order as in part a. The curves have been shifted for the sake of clarity.

of the pseudo-cross-correlation functions were averaged, and the average diffusion coefficient (D) was obtained from these functions by means of a second-order cumulant analysis.59 The hydrodynamic radii, RH, were derived from D using the well-known Stokes-Einstein relation, RH ) (kT)/(6πηD), where η is the viscosity of the water solvent at the experimental temperature. Thus, the hydrodynamic radii of the particles corresponded to equivalent spheres. In the following, no comparison is carried out regarding the sizes of the dispersed particles with different internal structures. Each sample was measured from at least three independent preparations. Thus, we present in the following tables the average values found from these measurements.

Results and Discussions Scattering of Reference Systems. We start by presenting the SAXS scattering patterns observed for our two reference systems, i.e., the bulk and dispersed particles stabilized by the triblock copolymer Pluronic F127. The SAXS intensity of the four bulk phases of interest is displayed in Figure 1a. At a DU:TC ratio of 100:0, the scattering pattern exhibited at least seven Bragg peaks with the relative ratios of 2, 3, 4, 6, 8, 9, and 10. Such peaks are typical of a crystalline cubic lattice with a Pn3m symmetry and can be indexed as hkl ) 110, 111, 200, 211, 220, 221, and 310 reflections. At a DU:TC ratio of 90:10, we observed a scattering pattern with five Bragg peaks with relative positions in the ratios 1, 3, 4, 7, and 9. These (59) Dynamic Light Scattering; Pecora, R., Ed.; Plenum: New York, 1985.

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Table 1. Average DLS Results on Different Preparations for the Four Studied Mixtures, DU:TC 100:0, 90:10, 70:30, and 50:50, Stabilized with Pluronic F127a sample DU:TC/F127

(m2 s-1)

〈R〉 (nm)

〈width〉 (%)

100:0/0.5 90:10/0.5 70:30/0.5 50:50/0.5

2.1 × 10 2.2 × 10-12 2.2 × 10-12 2.5 × 10-12

116 114 110 99

39.8 43.9 46.7 38.1

a

-12

All samples contain 0.5 wt % of Pluronic F127 as stabilizer.

peaks identify the symmetry as a 2D hexagonal phase H2. Further addition of TC (to a 70:30 ratio) resulted in a scattering pattern with at least nine visible Bragg peaks in the ratios of 3, 8, 11, 12, 16, 19, 24, 27, and 32, which were indexed as hkl ) 111, 220, 311, 222, 400, 331, 422, 333 + 511, and 440 reflections of an inverse micellar cubic phase of Fd3m symmetry. Finally, at a DU:TC ratio of 50:50, we measured a broad peak characteristic of an L2 phase, where the characteristic distance d ) 2π/q0 could be deduced from the maximum of this correlation peak seen at q0. The oscillation seen at around q ≈ 1.5 nm-1 (arrow in Figure 1a) corresponds to the form factor signal of the micelles composing the phase and not to a harmonic Bragg peak. The generic phase diagram (temperature versus DU:TC ratio) of dispersions stabilized by Pluronic F127 has been investigated by Guillot et al.19 Figure 1b displays the scattering patterns of the dispersions with a concentration of 5 wt % after subtraction of the solvent scattering, obtained with a stabilizer weight fraction of 0.5 wt %. All curves were characterized at intermediate q values by the scattering of the internal liquid crystalline phase, while an increase in the scattering could be observed at small q-values, in contrast to the bulk scattering. Instead, we observed a crossover to another scattering regime whatever the internal phase was. This is a sign that the liquid crystalline phases were stabilized into defined particles by the triblock copolymer. The average size of these particles could not be determined by the presented SAXS experiments, as there was no sign of a crossover to a Guinier regime in the accessible q range of the instrument. Therefore, dynamic light scattering experiments on much more diluted samples were used to derive the particle sizes. The diffusion coefficients and the hydrodynamic radii obtained are summarized in Table 1. This indicates that the particles stabilized by the Pluronic F127 were of submicrometer size with radii on the order of 100 nm in all cases. Another noteworthy effect that can be observed in the SAXS curves is that the dispersed particles referred to as cubosome 1 in Figure 1b did not exhibit an internal phase corresponding to a Pn3m symmetry group (bulk phase used) but rather to a mixture of Pn3m and Im3m structures. Im3m is another reverse bicontinuous phase with Bragg peaks in the ratios of 2, 4, 6, 8, 10, 12, and 14, just to name some of the reflections. The presence of the Pluronic F127 significantly disturbed the internal phase and a phase transition occurred. This effect of the stabilizer on the reverse bicontinuous cubic phases has been described in the literature,8,60 and recent experiments have demonstrated effects on the internal structure of the other phases, however, at considerably higher stabilizer contents.61 Laponite XLG as Stabilizer as Opposed to Pluronic F127. This section describes the investigation of samples comprised of mixtures of DU:TC bulk phases and Laponite XLG. Figure 2 displays solvent-subtracted SAXS patterns for dispersions of the four bulk phases of interest at concentrations of 5 wt % with 0.5 wt % Laponite XLG as stabilizer. The pattern (60) Nakano, M.; Sugita, A.; Matsuoka, H.; Handa, T. Langmuir 2001, 17, 3917–3922. (61) Salentining, S.; Guillot, S.; Glatter, O. Unpublished results.

Figure 2. Solvent-subtracted SAXS intensities (lin-log representation) obtained for dispersed particles stabilized by 0.5 wt % Laponite XLG. Each curve is labeled with its DU:TC ratio together with the Laponite concentration. The inset shows the concentration-normalized scattering of Laponite XLG (log-log representation) at 0.25 wt % (full triangles), 0.5 wt % (full circles), and 1 wt % (cross). Every 20th point is displayed.

for a pure Laponite dispersion at a concentration of 0.5 wt %, prepared in the same way, is shown for comparison. The figure demonstrates that the scattering of the Laponite particles strongly contributes to the total scattering of the mixtures. Therefore, it is necessary to start by discussing this contribution. We clearly observe that the scattering curve exhibited by Laponite particles at a concentration of 0.5 wt % in pure water does not follow a simple scaling law in the q range investigated here but rather shows a downward bending at high q values. This is typical for the thickness contribution of disklike particles. Furthermore, no interaction peaks are observed and the bending is small, indicating, respectively, that no stacks exist in the dispersion and that the mean thickness of the particles is small, as would be expected for a full exfoliation of the Laponite particles into single layers (thickness h ≈ 1 nm). These observations are confirmed when comparing the scattering function with that obtained at concentrations of 0.25 and 1 wt % (see the inset in Figure 2). It is evident that the scattering curves of the Laponite XLG particles in this concentration range are similar to those within the investigated q range. We can therefore assume that the contribution of these particles is due only to well-dispersed single layers, which is a prerequisite for our purpose, and can be described by the form factor P(q) of disklike particles (of radius R and thickness h) integrated over all spatial orientations:62

P(q) )

[( )(

J1(qR) 2 12 qR (qR)

) ][

sin(qh/2) qh/2

]

(1)

Here, J1(χ) is the Bessel function of first order. The first term of eq 1 represents the contribution from the radius of the particles, while the second term corresponds to the thickness. Nevertheless, we assume that, in particular, an aggregation of the platelets resulting from a possible change in the interaction potential between the particles could occur when the Laponite (62) Pedersen, J. S. In Neutrons, X-rays and Light: Scattering Methods Applied to Soft Condensed Matter; Linder, P., Zemb, T., Eds.; Elsevier: North-Holland, 2002; Chapter 16.

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Salonen et al. Table 2. Average DLS Results on Different Preparations for the Four Studied Mixtures, DU:TC 100:0, 90:10, 70:30, and 50:50, Stabilized with Laponite XLGa sample DU:TC/Laponite b

100:0/0.5 90:10/0.5 70:30/0.5 50:50/0.5 a

Figure 3. Excess intensity, IEXC(q), as described in the text (lin-log representation), for all mixtures shown in Figure 2. Each curve is labeled with its DU:TC ratio together with the Laponite concentration: 100: 0/0.5 (open circles), 90:10/0.5 (open squares), 70:30/0.5 (up triangles), and 50:50/0.5 (down triangles). The dashed line corresponds to IEXC(q) ) 1 (see the text).

particles were mixed with the bulk phases. Therefore, we now discuss the SAXS experimental scattering functions of all mixtures (Figure 2) in more detail. It can be observed that the scattering functions, at high q values (q g 2.5 nm-1), were similar and quite compatible, within the experimental errors, with the scattering signal of the clay particle thickness. At small q values, the scattering intensity was found to increase with the TC content, while at intermediate q values, interaction peaks, whose positions depend on the composition of the mixtures, were visible. The peak intensities were relatively small, mostly due to the strong contribution of the platelet form factor. However, their positions needed to be carefully indexed. Indeed, they could originate from various phenomena, all of which might give rise to peaks in this q range, namely, (i) the bulk phases or internal liquid crystalline phases, (ii) the clay particles in the stack, (iii) a regular organization of the Laponite on the surface of the dispersed particles, and (iv) any combination of these previous effects. We found that the best way to emphasize these diverse contributions was to express the excess intensity as compared to pure Laponite at 0.5 wt %, since the latter contribution corresponded only to the form factor of the particles. IEXC(q) is thus defined as

IEXC(q) )

IDU:TC⁄Lapo(q) - Iw(q) IDU:TC⁄Lapo(q) - Iw(q) ) ILapo(q) - Iw(q) kPLapo(q) (2)

where k is an arbitrary mathematical factor, related in particular to the concentration and the electronic density of the Laponite particles. The excess intensities IEXC(q) obtained for the scattering curves shown in Figure 2 are represented in Figure 3. The IEXC(q) observed clearly enhanced the structural effects of the DU:TC/ Laponite mixtures, regardless of the mixture, and rendered it possible to discuss their specificities in comparison with the reference systems. First, the increase in scattering that was observed at small q values, regardless of the mixture composition, as well as its TC-dependence, were compatible neither with a contribution of the bulk phases (Figure 1a) nor with an aggregation of the pellets in clusters. Indeed, in the latter case, scaling laws such as I(q) ≈ q-χ should be observed. The most probable explanation is that what we observed was a crossover to another scattering regime as previously seen when using Pluronic F127 as stabilizer (Figure 1b). Therefore, this is a first clear sign that the liquid crystalline phases were stabilized by the Laponite

〈D〉 (m2 s-1)

〈R〉 (nm)

〈width〉 (%)

2.7 × 2.6 × 10-12 2.7 × 10-12 3.0 × 10-12

91 93 92 83

46.1 48.3 37.0 31.9

10-12

All samples contain 0.5 wt % of Laponite XLG as stabilizer. b Cubosomes.

particles whatever the initial bulk phase was. Moreover, the peak sequences observed for all the mixtures could be fully indexed by the liquid crystalline bulk phases used, i.e., the L2, Fd3m, H2, and Pn3m space groups (Figure 1a). This means that the initial bulk phases defined the internal structures of the dispersed particles in the first few days. These observations were similar to those obtained with Pluronic F127 as stabilizer (Figure 1b). We can thus assume that we were able to form EMEs, hexosomes, and cubosomes by using Laponite XLG at a concentration of 0.5 wt % as stabilizer, similarly to when Pluronic F127 was employed under equivalent concentrations. Furthermore, in contrast to the 100:0/0.5 mixtures prepared with Pluronic F127, only one cubic phase of Pn3m was observed when Laponite pellets were used. This indicated a smaller influence of the stabilizer during the initial process of stabilization. However, at this stabilizer concentration, we observed a slight shift of the peak positions in comparison with those of the bulk phases, as was also seen with Pluronic F127-stabilized particles. Finally, Figure 3 displayed no additional Bragg peaks for any of the mixtures. This suggests that there was no periodic organization of the pellets at the interface between the solvent and the internal liquid crystalline phases. As a consequence, we assume in the following a random ordering of the Laponite particles at the surface of the dispersed internally self-assembled particles. In order to derive the dispersed particle sizes, we performed dynamic light scattering with diluted samples (with a dilution factor on the order of 4000). The diffusion coefficients and the hydrodynamic radii obtained are summarized in Table 2. The table shows that, with the same internal phase, the Laponitestabilized particles were smaller than their Pluronic F127stabilized counterparts, thus indicating that the effective surface covered per gram of Laponite XLG was higher than for Pluronic F127. We have demonstrated that Laponite XLG particles at a concentration of 0.5 wt % were able to stabilize 5 wt % internally self-assembled particles ranging from EMEs to cubosomes, within the experimental conditions studied. However, the stabilization mechanism for these phases has not yet been discussed. In comparison with the fundamental work of Ashby and Binks concerning Pickering emulsions using clay pellets,35 we were able to stabilize all types of internally self-assembled particles without addition of salt, i.e., without screening the repulsive interactions between the clay pellets. The effect of residual salt that may exist in the Laponite powder was also verified. For this purpose, an attempt was made to prepare a normal o/w emulsion with TC according to the same procedure as for the particles (i.e., 5 wt % TC and 0.5 wt % Laponite XLG). This TC emulsion was found to be unstablescreaming was followed by phase separation within hourssthus leading us to believe that the residual salt was insufficient and not the main parameters to allow for stabilization in this case. It is therefore necessary to discuss the differences between internally self-assembled particles and ordinary emulsions, where the stability and properties of

Dispersions of Liquid Crystalline Systems

internally structured emulsions have already been found to differ from emulsions, especially in terms of increased stability. There are three main differences between the internally structured particles and ordinary emulsions: (i) the additional component, i.e. the surfactant monoglyceride, strongly reduces the surface tension, although it is not capable of stabilizing o/w emulsions on its own; (ii) the shape of the particles themselves is not always spherical; cryo-TEM of hexosomes and cubosomes has demonstrated hexagonal or cubic shaped particles;63 and (iii) bicontinuous cubic structures can present open water channels at the surface, thus rendering a patchy surface (complex interface) to the Laponite particles, especially as the diameter of the channels was found to be clearly smaller than the diameter of the Laponite particles. The diameter of the water channels for DU without oil were between 2 and 4 nm for Pn3m.15 In the case of H2 hexosomes, two morphologies have been proposed:64 hexosomes with no curved striation, corresponding to a hexagonal platelike particle shape,65 and hexosomes with curved longitudinal axis, corresponding to a spherical particle shape.13 However, in both cases, there are no open water channels between the inside and outside of the particles. Therefore, since also the preparation of particles with L2 and Fd3m internal structures was possible, a general feature, considering that the particles normally provide a continuous hydrophobic surface, seemed to be that the role of DU as a secondary stabilizer at the surface was important when using Laponite XLG particles. This suggests an adsorption of the DU onto the pellets. Let us now turn our attention to the effect of the different shapes of the dispersed particles themselves. It was possible to prepare hexosomes (DU:TC 90:10); however, a visual inspection of the samples showed some solid material attached to the vial walls at the top of the sample, even after a few days. The preparation of the other phases (L2, Fd3m, and Pn3m cubosomes), on the other hand, posed no problems, as they were all found to be stable, showing no visible creaming or phase separation within the first weeks. It should be noted that, with the exception of the top of the Laponite hexosome sample, the rest seemed to be homogeneous with no visible aggregates. Moreover, SAXS and DLS experiments could be carried out. It did however become clear when studying the width of the size distribution (Table 2) that the Laponite hexosomes displayed a considerably higher polydispersity as compared to the other particles (around of 54% as opposed to around 40%). This suggests that the particle morphology played a significant role. Indeed, cryo-TEM images demonstrated that most of the MLO:TC hexosomes stabilized with Pluronic F127 exhibited shapes from spherical to hexagonal; thus, most of the particles had some flat facets.63 The EMEs and Fd3m cubosomes displayed a spherical form but with a fluid interior and a discontinuous micellar cubic phase, respectively, possibly allowing for deformations at the surface. The Pn3m cubosomes, on the other hand, were cubic, always allowing for a flat contact between the Laponite XLG and the particle. This suggests that, in the case of hexosomes, the Laponite pellets preferentially adsorbed onto the flat facets and the observed heterogeneity originated from the difficulty in binding onto a stiff spherical surface. Despite this difficulty, dispersed particles were found in all cases, and the next step is to follow their behavior and stability with time. (63) Sagalowicz, L.; Adiran, M.; Michel, M.; Frossard, P.; Rouvet, M.; Watzke, H. J.; Yaghmur, A.; de Campo, L.; Glatter, O.; Leser, M. E. J. Microsc. 2006, 221, 110–121. (64) Sagalowicz, L.; Mezzenga, R.; Leser, M. Curr. Opin. Colloid Interface Sci. 2006, 11, 224–229. (65) Johnsson, M.; Barauskas, J.; Yam, L.; Tiberg, F. Langmuir 2005, 21, 5159–5165.

Langmuir, Vol. 24, No. 10, 2008 5311

Figure 4. (a) The time evolution of the normalized lattice parameter observed at day 1 for DU:TC 90:10 hexosomes (squares), 70:30 micellar cubosomes (up triangles), and 50:50 EMEs (down triangles). (b) The time-evolution of the lattice parameter of the DU:TC 100:0 sample stabilized with Laponite XLG for the Pn3m lattice parameters (circles) and the H2 lattice parameters (open diamonds). The inset shows the evolution of the Pn3m structure in more detail. The transition from a Pn3m to a Pn3m-H2 coexistence occurs at around 10 days, and after 18 days, only H2 is observed.

Time Evolution of Laponite-Stabilized Dispersed Particles and Their Characteristics. A prerequisite to use the previous particles is their stability with time. Therefore, their structural characteristics should be followed to gain further insights about the rule and the behavior of the Laponite particles. The form factor of the Laponite particles (Figure 2) is used as a normalization function for all investigated times. We thus study IEXC(q), which, in turn, can be used to determine the lattice parameters. We always found stabilized internally structured particles with no significant decrease of the scattering. This indicates that all types of internally structured particles are wellstabilized by Laponite colloids; thus, they can be referred to as Laponite-stabilized internally structured particles. However, the data showed a decrease in the lattice parameter with time, as can be seen in Figure 4. Starting with the TC-loaded particles (Figure 4a), the decrease in the lattice parameter was slightly lower for the hexosomes as compared to the micellar cubosomes and EMEs. This decrease could be a consequence of a lower internal DU concentration, i.e., a larger number of DU chains located at the surface of the dispersed particles. However, all the lattice parameters displayed decrease by as much as 10% over 60 days (Figure 4), and typically a lowered DU concentration of the internal phase should lead to only a slight decrease in the lattice parameters.19 Furthermore, in the literature, phase transitions from H2 to Fd3m or Fd3m to L2 were observed, and they were believed to correspond to a shift in the phase diagram due to the change in the DU:TC ratio. In Laponite-stabilized particles, no

5312 Langmuir, Vol. 24, No. 10, 2008

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Figure 6. The chemical hydrolysis process in which monolinolein (left) breaks up into linoleic acid and glycerol (right). The linoleic acid can be deprotonated so as to have an O- group at the end and thus acts as an acid by reducing the pH of the solution.

Figure 5. The time evolution of IEXC(q) for the DU:TC 100:0 sample (lin-lin representation). Initially, the Pn3m structure can be seen (filled circles); at intermediate times, a coexistence of Pn3m and H2 is evidenced (filled diamonds), and at longer times, only the hexagonal structure is found (empty diamonds). The corresponding lattice parameters are shown in Figure 4b. Every 10th point is displayed.

phase transitions were seen, suggesting that the results could not be attributed merely to a decrease in the internal DU concentration. Furthermore, the DU:TC ) 100:0 sample, which in the last section was shown to start off in the Pn3m phase, interestingly transformed into an H2 phase via a coexistence region. The excess intensity, IEXC(q), for selected times is shown in Figure 5. At short times, a Pn3m structure can be seen, whereas only the three peaks indicating a hexagonal phase can be indexed at longer times. The hexagonal peaks were visible from 10 days onward, while after 18 days the Pn3m structure was no longer apparent. When looking in closer detail at the evolution of the Pn3m lattice parameter (inset in Figure 4a), one can see that, after an initial decay over the first 5 days, the lattice parameter remained almost constant until the onset of the two-phase region during which it continued to decay until the disappearance of the Pn3m structure. It should also be noted that the lattice parameters observed for the H2 phase after the transition were clearly smaller than those found for the hexosomes prepared with the addition of TC. Therefore, it is central to discuss in more detail the reasons behind the structural change of the DU:TC 100:0 sample, as well as the decrease of the lattice parameters observed for all internal phases. Recently, Pouzot and co-workers reported that Fd3m inverse micellar cubic phase formed by a DU/limonene oil/water system with Pluronic F127 as stabilizer admits a very slow kinetics of formation and, thus, an evolution of the internal structure factor in time.66 This physical mechanism was attributed to the partitioning of water into two populations of micelles of different sizes. Here, we suspect that the transition from Pn3m cubosomes to H2 hexosomes (Figure 5), as well as the decrease of all lattice parameters for all internal phases (from L2 to Pn3m, Figure 4), arises from a chemical mechanism rather than a physical mechanism. Indeed, Laponite pellets in water (9) coupled with DU. It should also be noted that diluting with a buffer enabled a higher pH to be maintained over time than when simply using Laponite, which led to smaller lattice parameters after 19 days than those found without added buffer. This reinforces the view of more MLO and MO chains having been hydrolyzed with time when the pH is kept at a higher value for longer. Furthermore, (67) Shui, L.-L.; Wang, Z.-N.; Zheng, L. Q. Chin. J. Chem. 2005, 23, 245– 250. (68) Wadsten-Hindrichsen, P.; Bender, J.; Unga, J.; Engstro¨m, S. J. Colloid Interface Sci 2007, 315, 701–713.

Dispersions of Liquid Crystalline Systems

Langmuir, Vol. 24, No. 10, 2008 5313 Table 3. Average DLS Results on Different Preparations 3 weeks after Preparation for the Laponite XLG Stabilized Particles of the Four Studied Mixtures, DU:TC 100:0, 90:10, 70:30, and 50:50a sample DU:TC/Laponite b

100:0/0.5 90:10/0.5 70:30/0.5 50:50/0.5 a

Figure 8. (a) IEXC(q) for the DU:TC 100:0 sample stabilized with Laponite pellets and mixed with a buffer at pH 10. After 1 day, the particles display a Pn3m internal structure (red circles) that transforms into H2 by 19 days (black diamonds). (b) I(q) - Iw(q) for the DU:TC 100:0 sample stabilized with Pluronic F127 and mixed with a buffer at pH 10. After 1 day, the particles display an internal cubic structure (red circles), but after 19 days this is transformed into H2 (black diamonds).

it is interesting and fundamental to report that the direct preparation of Laponite-stabilized cubosomes at pH 8.5 (which is close to the saturation value of the pH observed at long times) gave rise to completely and immediately unstable solutions. Such a behavior does not take place when the pH evolves to similar values with time, i.e., the Laponite particles become less destabilized by a lower pH once they are adsorbed onto the surface of the dispersion droplets. This points out that, when using Laponite particles as stabilizers, a high enough initial pH (>9) needs to be ensured to obtain stable internally structured particles, whatever the further pH evolution. These results demonstrate that the structural evolution observed is intrinsic to the system used (DU:TC) due to the alkaline conditions needed to use Laponite as stabilizer. A further natural extension to this first attempt to understand the stabilization of liquid crystalline nanoparticles by Laponite particles will be the use of a pH-independent system, such as, for instance, Phytantriol. However, despite the fact that the degradation of MO and MLO is unfortunate from a chemical stability perspective, this provides the clear advantage to allow the formation of in situ internally phases, here for instance H2 internal phase, without destabilization of the particles and at constant temperature. Therefore, whereas a quantitative evaluation of the ratio between MLO/MO and LA/OA with time is beyond the purpose of the present study, a discussion is needed for all types of Laponite-stabilized particles (namely from L2 phase to Pn3m cubosomes), describing qualitatively how the amount of acid molecules in the system could influence their internal structure. Due to this system’s intrinsic hydrolysis, we now have a system with MLO, MO, LA, OA, glycerol, and TC instead of an (MLO/

〈D〉 (m2 s-1)

〈R〉 (nm)

〈width〉 (%)

2.8 × 2.8 × 10-12 2.8 × 10-12 3.0 × 10-12

88 89 88 81

44.6 50.2 39.7 36.3

10-12

All samples contain 0.5 wt % Laponite XLG as stabilizer. b Hexosomes.

MO):TC system. The phase behavior of mixtures of MO and OA has been previously studied, and as the generic phase behavior of MLO and MO are comparable, we will assume here that the time-dependent structural behavior of our particles can be qualitatively described in terms of the MO-OA system. With increasing concentrations of oleic acid in the MO-OA system, a transformation from Pn3m to H2 is observed.69 The structural changes can be explained in terms of changing curvature. With the removal of the glycerol headgroup, the effective surface area of the headgroup (a0) is diminished. However, if the acid is in contact with water, it can be deprotonated, which leads to a slight increase in a0, again due to the repulsion between the headgroups. However, an H2 structure is still formed. Similarly, mixing with a lipase, which effectively hydrolyzes the monoolein, results in a phase behavior that can be directly correlated with the OA and MO concentrations. The effect of high pH values should be similar in this case; i.e., a high pH in our system causes the hydrolysis of MO and MLO, and once enough acid molecules are present in the system, a phase transformation into a hexagonal phase occurs. The change of internal structure is probably seen only for the DU:TC 100:0 sample, since the phase boundary with H2 is very close. Indeed, the phase behavior of MO with OA and sodium oleate (NaO) has been studied70 since NaO resembles the deprotonated form of OA. Therefore, from this phase behavior (again assuming similarity between MO and MLO, and OA and LA), we can infer that the maximum amount of MLO and MO that was hydrolyzed was approximately 25%, even after more than 100 days. This was not enough to allow for a further transition to yet another internal structure with the DU:TC compositions used, with the exception of DU:TC 100:0 cubosomes. However, we assume the hydrolysis that took place in all phases was the underlying explanation for the decrease in lattice parameters as well as for the very small final values found; thus, all initial DU:TC compositions near an internal phase transition, whatever the internal phase, should change phase with time. We also emphasize that the H2 internal phase sample resulting from the pH-induced transition was always visually more homogeneous than the direct preparation of an H2 phase, even though sample aging could not be excluded. This was reinforced by the DLS experiments that are discussed in the next paragraph. We therefore believe that such a pH-induced transition could be used as a fruitful parameter in the preparation process. DLS measurements showed that there was no significant change in the size of the particles of any of the samples during the studied time period (Table 3). In view of previously investigated systems, this was not surprising, since the internal structure of the particles has been found to exert little influence on the particle size. However, the variations might be masked by the change in the shape of the particles, which would alter the diffusion characteristics. We note that the average width of the size (69) Borne´, J.; Nylander, T.; Khan, A. J. Colloid Interface Sci. 2003, 257, 310–320. (70) Borne´, J.; Nylander, T.; Khan, A. Langmuir 2001, 17, 7742–7751.

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distribution found after 3 weeks, as a result of the pH-induced transition, was smaller for the H2 internal phase (∼45%), as opposed to for the H2 phase that was directly prepared and verified after 3 weeks (∼50%). Another interesting consequence of the hydrolysis of the DU chains was that, although dispersed particles from L2 to cubosomes with Coulombic repulsions between them could be formed when using Laponite pellets as stabilizers, the formation of LA and OA inside the particles suggested that the internal liquid crystalline phases could also be charged. This is of central interest for incorporation and release of active molecules.

Conclusions In summary, we demonstrated that disklike nanosized Laponite XLG particles could be used to stabilize dispersions of internally liquid crystalline particles of well-defined submicrometer sizes, whatever the initial liquid crystalline bulk phase. The stabilization of the internally structured particles was found to differ from that of o/w Pickering emulsions, where stabilization was achieved

Salonen et al.

only with addition of salt. However, DU seemed to have a role as a cosurfactant in the stabilization process, acting either as a preferential binding site for the pellets or aiding in surface coverage between the pellets. The internal structure and size of the particles were followed with time, and the internal structures were found to gradually evolve. A phase transition from Pn3m to H2 was shown to occur in the samples without added oil. The high pH induced by the Laponite pellets played a central part in the time evolution of the particles, due to hydrolysis of the DU chains. We believe that these results open a new and interesting way to obtain versatile and smart nanosized particles. Moreover, the results particularly give rise to fundamental questions due to the system’s complexity as well as its incredible potential and richness. Acknowledgment. Many thanks are expressed to G. Scherf for his support with the scattering device. Baxter (A.S.) is gratefully acknowledged for partly financing this work. LA800199X