Internally Self-Assembled Submicrometer Emulsions Stabilized by

Published on Web 03/23/2010 pubs.acs.org/Langmuir .... measured to be 8.85 atT= 25°C. For convenience, these particles will be noted as TM in the res...
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Internally Self-Assembled Submicrometer Emulsions Stabilized by Spherical Nanocolloids: Finding the Free Nanoparticles in the Aqueous Continuous Phase Anniina Salonen,† Franc-ois Muller,‡ and Otto Glatter* Department of Chemistry, Karl-Franzens-University, Heinrichstrasse 28, A-8010 Graz, Austria. Present address: Laboratoire de Physique des Solides, Universit e Paris Sud, CNRS UMR 8502, 91405 Orsay Cedex, France. ‡Present address: Laboratoire L eon Brillouin, CEA Saclay, 91191 Gif sur Yvette Cedex, France †

Received December 23, 2009. Revised Manuscript Received February 1, 2010 This article reports on the behavior of colloids during the stabilization of internally liquid-crystalline droplets. The ability and limitations of spherical silica colloids to act as stabilizers of liquid-crystalline bulk phases have been demonstrated for monoglyceride/tetradecane/water and phytantriol/tetradecane/water systems using small-angle X-ray scattering and dynamic light scattering as probes. It has been demonstrated that these nanoparticles are very good stabilizers of phytantriol-based phases. In particular, these data showed that they are nondisruptive stabilizers for these bulk phases at ambient temperature. Interestingly, it was shown that the spherical nanoparticles were not able to stabilize the monoglyceride-based bicontinuous cubic phase (Pn3m symmetry group), in contrast with their phytantriol-based counterparts. They were, however, good stabilizers of the monoglyceride-based emulsified microemulsions and inverse micellar cubic phases (Fd3m symmetry group). We then further examined the influence of the concentration of stabilizer on the phytantriol-based dispersed particles. We showed that the scattering signals of the liquid crystal and the nanoparticles are uncorrelated, whatever the identity of the liquid-crystalline phase. A careful analysis method coupled with dynamic light scattering measurements allowed us to determine and discriminate qualitatively the cases with or without free nanoparticles in the continuous phase. The results indicate for the first time fundamental differences in the stabilization by solidlike nanoparticles of emulsified microemulsions and of bicontinuous cubic phases.

I. Introduction 1,2

Ramsden or Pickering emulsions are an integral component in the formulation of smart materials in many fields of application as well as in fundamental science. These stable emulsion materials are based on the ability of colloids (solid particles) to adsorb onto a liquid-liquid interface (e.g., a water-oil interface); thus, oilin-water or water-in-oil domains can be obtained with very good long-term stability. Since the recent experimental work of Velev and co-workers3,4 as well as that of Weitz et al.5 demonstrating that particles (latex colloids, for instance) adsorbed at a liquidliquid interface of emulsion droplets self-assembled into supracolloidal structures, various shapes and sizes such as spherical silica colloids,6-10 polymeric rods,11 and clay platelets12-14 have been used to investigate the emulsion properties as a function of the system parameters and to fabricate novel, smart, stable Pickering emulsion materials with controlled properties. It was only recently that the principle of Pickering emulsions was used to disperse and stabilize liquid-crystalline (LC) phases.15 *Corresponding author. E-mail: [email protected]. (1) Ramsden, W. Proc. R. Soc. London 1903, 72, 156–164. (2) Pickering, S. U. J. Chem. Soc. 1907, 91, 2001–2021. (3) Velev, O. D.; Furusawa, K.; Nagayama, K. Langmuir 1996, 12, 2374–2384. (4) Velev, O. D.; Furusawa, K.; Nagayama, K. Langmuir 1996, 12, 2385–2391. (5) Dinsmore, A. D.; Hsu, M. F.; Nikolaides, M. G.; Marquez, M.; Baush, A. R.; Weitz, D. A. Science 2003, 298, 1006–1009. (6) Binks, B. P.; Lumsdon, S. O. Phys. Chem. Chem. Phys. 1999, 1, 3007–3016. (7) Binks, B. P.; Lumsdon, S. O. Langmuir 2000, 16, 2539–2547. (8) Binks, B. P.; Lumsdon, S. O. Langmuir 2000, 16, 3748–3756. (9) Binks, B. P.; Lumsdon, S. O. Langmuir 2000, 16, 8622–8631. (10) Binks, B. P.; Lumsdon, S. O. Phys. Chem. Chem. Phys. 2000, 2, 2959–2967. (11) Noble, P. F.; Cayre, O. J.; Alargova, R. G.; Velev, O. D.; Paunov, V. N. J. Am. Chem. Soc. 2004, 126, 8092–8093. (12) Ashby, N. P.; Binks, B. P. Phys. Chem. Chem. Phys. 2000, 2, 5640–5646. (13) Cauvin, S.; Colver, P. J.; Bon, S. A. F. Macromolecules 2005, 38, 7887–7889. (14) Bon, S. A. F.; Colver, P. J. Langmuir 2007, 23, 8316–8322. (15) Salonen, A.; Muller, F.; Glatter, O. Langmuir 2008, 24, 5306–5314.

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Indeed, since Larsson and co-workers introduced, around 10 years ago, the idea of mechanically breaking up LC phases in the presence of a hydrophilic stabilizer to form aqueous dispersions,16-19 these smart materials, which are now used in many applications ranging from cosmetics to food products,20,21 have been exclusively stabilized using organic macromolecules such as surfactants, diblock and triblock polymers, and proteins.22-27 In our initial study,15 it was demonstrated that colloidal clay platelets (Laponite XLG, a synthetic hectorite, R ≈ 12-13 nm, h ≈ 1 nm) were physically able to stabilize monoglyceride/water and monoglyceride/tetradecane/ water systems with internal structures of L2 (isotropic inverse micellar phase), Fd3m (micellar cubic phase), H2 (inverse hexagonal), and Pn3m (bicontinuous cubic phase) despite evidence of chemical instability in time due to hydrolysis. (16) Pilman, E.; Larsson, K.; Tornberg, E. J. Dispersion Sci. Technol. 1980, 1, 267–281. (17) Gustafsson, J.; Ljusberg-Wahren, H.; Almgren, M.; Larsson, K. Langmuir 1996, 12, 4611–4613. (18) Gustafsson, J.; Ljusberg-Wahren, H.; Almgren, M.; Larsson, K. Langmuir 1997, 13, 6964–6971. (19) Larsson, K. J. Dispersion Sci. Technol. 1999, 20, 27–34. (20) Mezzenga, R.; Schurtenberger, P.; Burbridge, A.; Michel, M. Nat. Mater. 2005, 4, 729–740. (21) Yaghmur, A; Glatter, O. Adv. Colloid Interface Sci. 2009, 147-148, 333– 342. (22) de Campo, L.; Yaghmur, A.; Sagalowicz, L.; Leser, M. E.; Watzke, H. J.; Glatter, O. Langmuir 2004, 20, 5254–5261. (23) Guillot, S.; Moitzi, C.; Salentinig, S.; Sagalowicz, L.; Leser, M:E.; Glatter, O. Colloids Surf., A 2006, 291, 78–84. (24) Yaghmur, A.; de Campo, L.; Sagalowicz, L.; Leser, M. E.; Glatter, O. Langmuir 2005, 21, 569–577. (25) Yaghmur, A.; de Campo, L.; Salentinig, S.; Sagalowicz, L.; Leser, M. E.; Glatter, O. Langmuir 2006, 22, 517–521. (26) Yaghmur, A.; de Campo, L.; Sagalowicz, L.; Leser, M. E.; Glatter, O. Langmuir 2006, 22, 9919–9927. (27) Dong, Y. D.; Larson, I.; Hanley, T.; Boyd, B. Langmuir 2006, 22, 9512– 9518.

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DOI: 10.1021/la9048538

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We further demonstrated that the system’s chemical instability can be overcome by using phytantriol (PT; 3,7,11,15tetramethylhexadecane-1,2,3-triol), a common cosmetic ingredient that does not hydrolyze. The particular influence of the colloidal nature of Laponite was investigated for the Pn3m phase by separating the effects of pH and temperature for both monoglyceride/water and phytantriol/water systems.28,29 In the meantime, an experimental study was reported investigating the specific case of the inverse hexagonal phase (H2) made of monoglyceride/triglyceride/R(þ)-limonene with mixtures stabilized by different concentrations of two kinds of clay platelets:30 laponite and montmorillonite. The microscopy results unambiguously showed that, up to a clay concentration of 2 wt %, the H2 bulk phase is well dispersed and stabilized by both types of clays into elongated particles. However, X-ray diffraction patterns showed peaks revealing a lamellar structure that was not hexagonal, even with increased oil concentration. The authors claimed that this arises from the elongated shape of the stabilized particles, which deformed the hexagonal structure to a lamellar structure in their internal periphery. Surprisingly, it was excluded from the discussion that the lamellar structure could arise from stacks of clays on the stabilized particle’s surface or in the free water volume, a structure that is found rather commonly in clay systems.31-33 In fact, little is known about the behavior of the colloids during the stabilization of different LC phases, both at the interface and in the bulk. New insights are needed in order to understand clearly and control the properties of the systems, especially because the presence of free colloids in solution can strongly change the rheological properties of the solutions. The determination of the free colloids in solution is the purpose of this article. Because dispersions of clay platelets have particularly complex (and timedependent) phase behavior and the appearance of stacks or ordered phases of clays cannot be excluded even at low concentrations,31 we used an equivalent isotropic colloidal shape as the inorganic stabilizer. Spherical silica nanoparticles were chosen as stabilizers, and their ability and limitations in stabilizing LC bulk phases have been demonstrated. Monoglyceride/tetradecane/water and phytantriol/tetradecane/water systems were studied using SAXS and DLS as probes. The specific cases of two very different internal phases of L2 (isotropic inverse micellar phase) and Pn3m (bicontinuous cubic phase) were examined, and the influence of the colloidal concentration was investigated.

II. Experimental Section II.A. Materials. Dimodan U/J (DU) was supplied by DANISCO A/S (Braband, Denmark). It contained 96% distilled monoglycerides, of which 62% were linoleate. Phytantriol (purity >95%) was purchased from DSM Nutritional Products Ltd. (Basel, Switzerland). Tetradecane (TC), a linear alkane chain of composition C14H30, was purchased from Sigma Chemical Co. (St. Louis, MO). The water utilized in the preparations was doubly distilled. The colloidal silica spheres had an average diameter of 25 nm (Ludox TM-50, DuPont). The particles are (28) Muller, F.; Salonen, A.; Glatter, O. J. Colloid Interface Sci. 2010, 342, 392– 398. (29) Muller, F.; Salonen, A.; Glatter, O. Colloids Surf., A 2010, 358, 50–56. (30) Guillot, S.; Bergaya, F.; de Azevedo, C.; Warmont, F.; Tranchant, J.-F. J. Colloid Interface Sci. 2009, 333, 563–569. (31) Mourchid, A.; Delville, A.; Lambard, J.; Lecolier, E.; Levitz, P. Langmuir 1995, 11, 1942–1950. (32) Levitz, P.; Lecolier, E.; Mouchid, A.; Delville, A.; Lyonnard, S. Europhys. Lett. 2000, 49, 672–677. (33) Shalkevich, A.; Stradner, A.; Bhat, S. K.; Muller, F.; Schurtenberger, P. Langmuir 2007, 23, 3570-3580 and references therein.

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negatively charged with a specific surface area of 140 m2/g and are stabilized with sodium counterions. The surface is then composed of SiOH and SiO-Naþ (ionized as SiO- or with condensed Naþ) groups. The pH of the stock silica solution (50 wt % silica) was measured to be 8.85 at T = 25 °C. For convenience, these particles will be noted as TM in the rest of the text. II.B. Sample Preparation. Dispersed samples with three DU/TC ratios (i.e. 100:0, 70:30 and 50:50) were prepared in excess water in order to form Pn3m, Fd3m, and L2 symmetry groups, respectively. Dispersed PT/TC samples were prepared in excess water at ratios of 100:0 and 50:50. The preparation was carried out by weighing the DU/TC (PT/TC, respectively) mixture, the stabilizer (Ludox TM), and the water into vials. The raw mixture was ultrasonicated (SY-Lab 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 (PT/TC, respectively) mixture was kept constant at 0.05 g 3 g-1 (5 wt %), and that of the stabilizer varied from 0.0050 g 3 g-1 (0.50 wt %) and 0.0175 g 3 g-1 (1.75 wt %). It is noteworthy that the DU-based samples can evolve in time; namely, the hydrolysis of monoglyceride chains occurs because of the relatively high pH value, and the internal transition of phases can happen. pH-induced hydrolysis is not the focus of this article, and all data shown are from fresh samples (i.e., taken less than 2 days after preparation). II.C. Small-Angle X-ray Scattering. The SAXS equipment consisted of a slit-geometry camera with high flux and low background (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 1D 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  5 min for all dispersions (because of the strong scattering by TM particles). 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 dispersions. We are aware that the data were smeared with the beam profile (slit profile). II.D. 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 single-mode fiber detection optics (OZ from GMP, Switzerland), an ALV/SO-SIPD/DUAL photomultiplier with pseudo-cross-correlation, and an ALV 5000/E correlator with fast expansion (ALV, Germany). The measurements were carried out at a scattering angle of θ = 90°, and the temperature was fixed at 25 °C. All prepared samples were turbid, thus excluding direct measurements with standard DLS. However, because the dispersed Pluronic F127-based particles have been previously reported to be kinetically stable, we assumed that the droplets stabilized by TM would also be stable, at least on intermediate timescales. All samples were diluted using doubly distilled water 4000 times immediately before the measurements. The data were collected in repeated measurements of 10  30 s. The intensities of the pseudo-cross-correlation functions were averaged, and the average diffusion coefficient D was obtained from these functions by means of second-order cumulant analysis.34 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 (34) Koppel, D. E. J. Chem. Phys. 1972, 57, 771.

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Figure 1. Macroscopic picture of the prepared DU-based Pn3m cubic bulk phase in the presence of 0.5 wt % TM. The bulk phase is stabilized to some extent (lower part of the picture), but there still remains a large part of the unstabilized bulk phase in the vials (as shown by the arrow), unlike the situation for DU-based microemulsion bulk phases.

Figure 2. Solvent-subtracted SAXS intensities (log-log representation) obtained for dispersed PT-based droplets stabilized by 0.5 wt % TM. Each curve is labeled with its PT/TC ratio together with the TM concentration. The black curve shows the scattering intensity from a dispersion of TM particles only.

to equivalent spheres. In the following text, no comparison is carried out regarding the sizes of the dispersed particles with different internal structures.

III. Experimental Results III.A. Using TM Nanoparticles to Stabilize Different LC Phases. III.A.1. Macroscopic Behavior. Good stabilization of LC dispersions is macroscopically characterized by a white emulsion with no immediate coalescence or phase separation. For PT-based materials, such emulsions were found using TM as a stabilizer for both the cubic phase and the microemulsion as the internal phase (i.e., with and without added oil). For DUbased materials, the situation was different. The dispersion of the phases with highly negative curvature in the presence of oil (L2 and Fd3m phases) resulted in stable emulsions, showing no visible creaming or phase separation within the first few weeks. However, with the same processing it was not possible to prepare DU-based cubosomes (Pn3m phase). It is clear from an optical inspection of the samples that a large part of the bulk phase was not dispersed and thus not stabilized, as shown in the typical picture in Figure 1. Therefore, despite a part of the cubic phase being stabilized, we did not perform further experiments on these samples because the spherical nanoparticles are not good stabilizers of the monoglyceride-based cubic phase. The possible reasons behind the differences in DU and PT stabilization will be discussed below. III.A.2. Scattering of TM-Stabilized PT Parent Bulk Phases. Figure 2 shows the SAXS scattering of mixtures containing PT/TC bulk phases: 100:0 and 50:50 at a concentration of 5 wt % with 0.5 wt % TM as a stabilizer. We observe that the intensities are Langmuir 2010, 26(11), 7981–7987

Figure 3. q3I(q) vs q representation of solvent-subtracted SAXS intensities (lin-lin representation) obtained for dispersed PTbased drops stabilized by 0.5 wt % TM as shown in Figure 2. Each curve is labeled with its PT/TC ratio together with the TM concentration. The graphs have been shifted vertically for the sake of clarity. For the 100:0:0.5 sample, the visible peaks are indexed following the Miller indices as {hkl} = {110}, {111}, and {200} diffraction planes corresponding to Bragg peaks with relative ratios of 21/2, 31/2, and 41/2 of Pn3m symmetry. For 50:50:0.5, the inverse micellar L2 phase observed is indicated by the position of the maximum of the position broad peak q0.

dominated by the scattering of TM for q < 0.5 nm-1. Above this q value, scattering by the LC structure is found, regardless of the mixture. The diffraction peaks need to be indexed carefully because they can be due to different effects, namely, from (i) the internal crystalline phases, (ii) the aggregation of TM in the free volume, (iii) the regular organization of the TM on the surface of dispersed particles, or (iv) any combination of these previous effects. Therefore, to enhance these structural effects, we choose to use the q3I(q) versus q representation as shown in Figure 3 (i.e., similar to a Porod representation when taking into account the smearing of the TM signal by the instrumental beam profile). It is clear that this representation renders possible the discussion of the specificities of the scattering curves. For the 100:0:0.5 mixture, we observed peaks with relative ratios of 21/2, 31/2, and 41/2. These peaks can be unambiguously indexed following Miller’s indices as {hkl} = {110}, {111}, and {200} diffraction planes of the Pn3m structure. For the 50:50:0.5 mixture, we observed a broad peak at q0, typical of the mean center-to-center distance between the micelles, d = 2π/q0, of an inverse micellar L2 phase. This means that the bulk structure (Pn3m or L2) is not modified by the presence of TM particles as stabilizers. From Figure 3, we can deduce that the lattice parameters are 7.1 and 4.8 nm in the Pn3m and L2 phases, respectively, as expected at the given pH value.28 Therefore, this demonstrates that TM particles are nondisruptive stabilizers for PT-based EME and cubosomes under the experimental conditions studied. The hydrodynamic radii obtained by DLS on diluted samples are summarized in Table 1. The Table shows that, within the experimental error, TM-stabilized EME and cubosomes have a similar mean radius of around 140 nm with a rather broad size distribution. Nevertheless, at this point, this size similarity should be viewed only as a coincidence. This will be discussed in the last section of this article when investigating the effects of the TM concentration. III.A.3. Scattering of TM-Stabilized DU Parent Bulk Phases. Figure 4 shows the scattering of the macroscopically stable emulsions containing DU/TC bulk phases: 70:30 and 50:50 at a concentration of 5 wt % with 0.5 wt % TM as the stabilizer. To enhance the structural effects as in the previous Figure, we use the q3I(q) versus q representation as shown in DOI: 10.1021/la9048538

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Table 1. DLS Results for the Studied Mixtures, PT/TC 100:0 and 50:50 and DU/TC 100:0, 90:10, 70:30, and 50:50, Stabilized with TM Nanoparticlesa PT/TC/TM

RH (nm)

width (%)

100:0:0.5 50:50:0.5

140 140

58 40

DU/TC/TM

RH (nm)

width (%)

100:0:0.5 70:30:0.5 98 50:50:0.5 101 a All samples contain 0.5 wt % TM as a stabilizer.

35 29

Figure 5. Solvent-subtracted SAXS intensities (log-log representation) obtained for the dispersed PT-based inverse micellar L2 phase stabilized with 0.5, 1, and 1.75 wt % TM. Each curve is labeled with its PT/TC ratio and the TM concentration. The dashed line corresponds to the scattering intensity at the maximum by the crystalline structure and is independent of TM concentration. (See the text.) The arrow displays the position of q0.

Figure 4. q3I(q) vs q representation of solvent-subtracted SAXS intensities (lin-lin representation) obtained for dispersed DUbased particles stabilized by 0.5 wt % TM. Each curve is labeled with its DU/TC ratio together with the TM concentration. The graphs have been shifted vertically for the sake of clarity. For the 70:30:0.5 sample, the visible peaks are indexed following the Miller indices as {hkl} = {311}, {331}, {333} þ {511}, and {440} diffraction planes with Fd3m symmetry. For the 50:50:0.5 sample, the inverse micellar L2 phase observed is indicated by the position of the maximum of the position broad peak q0.

Figure 4. (Experimental scattering data I(q) vs q are shown in Supporting Information in Figure 1S). For the 50:50:0.5 mixture, we observed an inverse micellar L2 phase with a corresponding average micelle-to-micelle distance of around 8 nm. For the 70:30 mixture, several peaks are observed, which cannot correspond on their own to any known structures in the monoglyceride system except for the Fd3m micellar cubic phase. However, because of the large lattice parameter and the domination of the TM colloids at low q values, the first two Bragg peaks are not visible, namely, the {111} and {220} reflections, and even at higher q values not all of the expected peaks are clearly visible. The scattering from the bulk is shown for comparison in the Supporting Information (Figure 2S). As a consequence, the peaks are indexed as follows in Figure 4: {311}, {331}, {333 þ 511}, and {440} reflections of the Fd3m phase.15,25 This leads to a lattice parameter of 22.7 nm, as expected at the elevated pH used.25 No internal structural changes due to TM for DU-based EME and micellar cubic phases under the experimental conditions studied were observed. The hydrodynamic radii obtained using DLS with diluted samples are summarized in Table 1, and the dispersed DU-based particles are smaller than the PT-based particles. Remarkably, the polydispersity is also smaller for all of these DU dispersions, as the size distribution widths indicate. Having shown the stabilization of different LC phases with TM as the stabilizer, we turn our attention to the location of the TM in the samples via a study at different stabilizer concentrations. 7984 DOI: 10.1021/la9048538

III.B. Influence of TM Concentration on the Dispersion Structure. III.B.1. Observations on the Scattering-Intensity Curves. The experimental scattering data from EME (50:50 PT/TC), samples containing 0.5, 1, and 1.75 wt % TM, are shown in Figure 5. At low q values, the signals for all samples are dominated by the form factor of the TM nanoparticles without additional peaks corresponding to the organization of TM in the samples. This suggests that in this concentration range the TM nanoparticles are not well organized on a regular lattice on the surfaces of the stabilized particles. The internal phase has not changed and is an inverse isotropic micellar phase in all cases. Very interestingly, the height of the L2 phase peak is shown to be TM-concentration-independent, as shown by the dashed line in Figure 5. This is an important result and indeed shows that the contributions of TM nanoparticles and the L2 phase are independent, the scattering signals are uncorrelated, and the cross-term contribution can be neglected. We also studied the case of the stabilized bicontinuous cubic phase (Pn3m cubosomes), which corresponds to a PT/TC ratio of 100:0 (i.e., without oil). These cubosomes should be considered to be the LC phase that is the most different from an L2 phase from the point of view of internal stiffness. We investigated samples containing 0.5 and 1 wt % TM, and again good stabilization was obtained over the entire experiment. The corresponding SAXS intensities (given as Supporting Information, Figure 3S) exhibited characteristics similar to those of EME: (i) no additional peaks corresponding to TM organization in the samples, (ii) an internal Pn3m phase in all cases, and (iii) the crystalline structure of the Pn3m phase being independent of the TM concentration (and the signals being uncorrelated). Thus, the overall signal measured in the experimental q range can be described by the sum of different contributions as eff ðq, CTM Þ Iðq, CTM Þ - Iw ðqÞ ¼ Idrop ðqÞ þ ILC ðqÞ þ ITM

ð1Þ

where Idrop(q) is the contribution of the stabilized particles (spheres or cubes with a radius of ∼100 nm), meaning that this first term contains information on the overall droplet size (beyond the resolution limit of the experiment), ILC(q) is the intensity due to the internal organization of the LC phase considered, and Ieff TM(q) is the effective contribution of the TM nanoparticles (i.e., the contribution in the modified environment with the dispersed LC domains). These various contributions are detailed in the Supporting Information. Langmuir 2010, 26(11), 7981–7987

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Because of the number of parameters as well as the smearing of the signals, which strongly influences some of the parameters, herein we do not perform fits to the experimental data. This would result in large error bars on all parameters and would not give convincing quantitative results. Thus, we performed simulations assuming smearing showing the very good qualitative agreement between the model and the experimental data with increasing TM. Details of the model and the simulations are given in the Supporting Information in section SI.1 and Figures 4S and 5S. III.B.2. Data Analysis and Discussion. To extract qualitative information about the influence of the TM concentration, the simplest way is to subtract from the total intensity scattered by the dispersions (eq 1) the intensity from a sample of free TM particles at the same concentration. Indeed, in this way we obtain Ieff ðq, CTM Þ ¼ ½Iðq, CTM Þ - Iw ðqÞ - ½ITM ðq, CTM Þ - Iw ðqÞ ¼ ILC ðqÞ þ Idrop ðq > 0:1 nm - 1 Þ þ ΔITM ðq, CTM Þ

ð2Þ

where the last term ΔITM(q, CTM) contains the relevant information of interest here and can be written as

Figure 6. Intensities Ieff(q) obtained after the subtraction of ITM(q, CTM) for dispersed PT-based EME shown in Figure 5. (See the text.) Each curve is labeled with its PT/TC ratio and the TM concentration. The dotted line corresponds to the contribution of the TM form factor multiplied by an arbitrary numerical factor k. The solid lines correspond to the adjustments. (See the text.)

eff ΔITM ðq, CTM Þ ¼ D½ðφeff TM  STM ðq, CTM ÞÞ

- ðφTM  STM ðq, CTM ÞÞ

ð3Þ

where all terms follow the previous convention and D = (ZTM)2  PTM(q)  VTM is a constant in the current system. The scattering profiles of TM at the concentrations studied as given in the Supporting Information (Figure 6S) are concentration-independent. Thus, we can assume that STM(q > 0.1 nm-1, CTM) ≈ 1. Note that small differences can be expected at lower q values (below 0.1 nm-1) because of the existence of structure factors, in particular, for 1.75 wt %. Furthermore, as shown in Figure 5, no additional correlation peaks are observed in LC dispersions. This means that in the samples studied and for the -1 q range investigated it can be assumed that Seff TM(q > 0.1 nm , CTM) ≈ 1. Therefore, in our approach ΔITM(q, CTM) is simply given by ΔITM ðq, CTM Þ ¼ D  ðφeff TM - φTM Þ

ð4Þ

Thus, the following conditions arise from our scattering treatment, allowing the determination of cases where some TM particles are present in excess: ΔITM ðq, CTM Þ ¼ 0 ðno TM in the water continuous volumeÞ

ΔITM ðq, CTM Þ ¼ 6 0 ðexcess of TM in the water continuous volumeÞ

ð5Þ Because these conditions can appear to some extent to be counterintuitive, we explain them in more detail in the Supporting Information (section SI.2). We applied eq 1 to the experimental data, as shown in Figures 6 and 7 for EME and cubosomes, respectively. For EME at CTM e 1 wt % (Figure 6), we observe an increase in scattering at small q values, which is fully compatible with a crossover between the diffraction signal of IL2(q) and the asymptotic law of Idrop(q > 0.1 nm-1). Because we are interested in the signal at low q values (typically for q < 0.5-0.6 nm-1) and because we saw that IL2(q) is an invariant of the system, we assume IL2(q) ≈ B (with B being a constant) for all TM concentrations, namely, meaning that it is not necessary to describe IL2(q) fully. We then adjust the signals by Kq-3 þ B with K = 2.2  10-3 and Langmuir 2010, 26(11), 7981–7987

Figure 7. Intensities Ieff(q) obtained after the subtraction of ITM(q, CTM) for the dispersed PT-based bulk phase composed of a 5 wt % 100:0 mixture with 0.5 and 1 wt % TM. (See the text.) Each curve is labeled with its PT/TC ratio and the TM concentration. The dotted lines correspond to the TM form factors multiplied by an arbitrary numerical factor (k or k0 ). The solid lines correspond to the adjustments. (See the text.)

3  10-3 for 0.5 and 1 wt %, respectively, and B = 0.22. The variation of K reflects that the drops have changed size between concentrations. Such adjustments are in very good agreement with the experimental curves for q < 0.5 nm-1 as expected. This means that for CTM e 1 wt % we have ΔITM(q, CTM) = 0. For CTM = 1.75 wt %, the signal has changed radically and can no longer be described by Kq-3 þ B. In our understanding, this 6 0 and Ieff(q, 1.75) have shapes that are means that ΔITM(q, CTM) ¼ similar to scattering from a collection of TM in water, more specifically, as I(q, 1.75) - Iw(q). We now qualitatively adjust the signal by kPTM(q) þ Kq-3 þ B with k = (ZTM)2  VTM  a where a mimics the ratio φeff TM/φTM, B = 0.22 as above, and K = 1.4  10-3. This adjustment is displayed in Figure 6, showing good agreement with the experimental scattering curve for q < 0.5 nm-1. Therefore, in our understanding, these data indicate that at CTM = 1.75 wt % some of the TM nanoparticles are not used in the stabilization process and thus are dispersed in the continuous water volume whereas between 0.5 and 1 wt % all nanoparticles are used for stabilization, meaning that all TMs are located on the surfaces of the L2 drops. At this point, it is interesting to compare the overall particle sizes. As shown in Table 2, the particle sizes are strongly TMconcentration-dependent. We showed that the particle radius approximately doubles for a change in TM concentration from DOI: 10.1021/la9048538

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Table 2. DLS Results for 100:0 and 50:50 Mixtures of PT/TC as a Function of TM Nanoparticle Concentration PT/TC

CTM (wt %)

LC phase

RH (nm)

width (%)

50:50 50:50 50:50 100:0 100:0

0.5 1.0 1.75 0.5 1.0

L2 L2 L2 Pn3m Pn3m

140 286 203 140 142

40 46 55 58 54

0.5 to 1 wt %: R1/R0.5 ≈ 2.05 within experimental error. This means that the surface of the particles has been divided by 4 and the stabilizer concentration has been multiplied by 2. Thus, because we found by SAXS that all TM nanoparticles are used in the stabilization process, the TM surface density is much higher at 1 wt % than at 0.5 wt %. This suggests that the maximum TM concentration on the particle surface has not been reached and that the L2 phase domains have some ability to reorganize into larger particles, adsorbing more TM on their surfaces. Of course, this would lead to changing TM-TM interactions at the drop surfaces and thus two different stabilization regimes and strongly indicates the help of PT chains in the stabilization. At CTM = 1.75 wt %, the particles have been found to have a smaller radius than at 1 wt % with a radius ratio of R1.75/R1 ≈ 1.40 within experimental error. Furthermore, it should be noted that the width of the size distribution also strongly increased to 54% from 45 and 40% at 1 and 0.5 wt %, respectively. Such an increase can be associated with the presence of excess stabilizer during dispersion and is compatible with the existence of a maximum surface concentration between 1 and 1.75 wt %, which is further corroborated with finding excess free TM nanoparticles in the volume of water at CTM = 1.75 wt % (Figure 6). Indeed, above this concentration, it is expected that the surface coverage should be constant, thus the stabilized particle radii will decrease with increasing TM concentration. Of course, this excludes the possibility of changing TM-TM interactions at higher concentrations and thus the packing at the surfaces of the drops. We have demonstrated that a PT-based L2 phase stabilized by TM nanospheres exhibits properties very close to those of regular Pickering emulsions. A stable regime has been seen at very low TM concentrations, similar to what has been observed in a few cases of Pickering emulsions.35,36 In this regime, the surface coverage on the droplets is assumed to be small and very much below the maximum coverage that could be obtained. However, in our case the presence of lipid chains at the interface aiding in the stabilization cannot be excluded and in fact is very probable. These new insights should be considered to be general features of L2 phase stabilization. The analyzed data obtained for the stabilized cubosomes are shown in Figure 7. Remarkably, for both concentrations the observed Ieff(q, CTM) data are clearly inconsistent with a simple 6 0 IPn3m(q) þ Kq-3contribution, suggesting that ΔITM(q, CTM) ¼ in all cases. Therefore, some TM nanoparticles are not used for stabilization, surprisingly, even at a concentration as low as 0.5 wt %. Furthermore, we can observe that the intensities at low q values are different between 0.5 and 1 wt %, meaning that the free TM concentration in the water continuous phase is different in both cases. Namely, at 0.5 wt %, it appears that there are fewer eff nanoparticles present in excess: φeff 1 /φ1 > φ0.5/φ0.5. To verify such assumptions, we thus qualitatively adjusted the signals by kPTM(q) þ B and k0 PTM(q) þ B, where k and k0 are arbitrary constants (35) Vignati, E.; Piazza, R.; Lockhart, T. P. Langmuir 2003, 19, 6650–6656. (36) Gautier, F.; Destribats, M.; Perrier-Corner, R.; Dechezelles, J.-F.; Giermanska, J.; Heroguez, V.; Ravaine, S.; Leal-Calderon, F.; Schmitt, V. Phys. Chem. Chem. Phys. 2007, 9, 6455–6462.

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(k > k0 ) for 1 and 0.5 wt % TM, respectively, and B = 0.13 (Figure 7). In this case, we are ignoring the contribution of the whole drop. This is reasonable for a qualitative adjustment of the signal due to the shape of the form factor of the TM particles as well as the q range under investigation. This approach is in very good agreement with the experimental curves in both cases for q values below 0.5 to 0.6 nm-1, confirming the existence of excess TM in solution. Parameters k and k0 have been found to be equivalent to TM concentrations of 0.138 and 0.014 wt %, respectively. Although the values are not absolute, the differences between the two TM concentrations are evident and we can be sure that at 1 wt % TM the amount of free TM in solution is much higher than at 0.5 wt %. Interestingly, for both concentrations, the drop sizes have been found to be similar (Table 2). This, coupled with the presence of TM in excess in the water continuous phase, leads to two possible scenarios: either the Pn3m drops are able to reorganize as a function of TM concentration, like an L2 internal phase, or the drops are not able to reorganize and the size is defined by the input energy. In both cases, the existence of a TM concentration (below 0.5 wt %) is expected to correspond to the threshold for a good stabilization of the drops. Below this threshold, in the first case there would be a second region of stability with lower surface cover. In the second case, below the threshold the drops would not be well stabilized, and immediately above, increasing amounts of TM would be found in the continuous water phase with increasing concentration. The exact differentiation between the two scenarios would require a full set of experiments at lower TM concentrations. Nevertheless, for our purpose here, this indicates that the stabilization process of the Pn3m phase is rather different from that of the inverse micellar L2 phase and that the amounts of stabilizer required for the two phases are very different. It can be assumed that for oil-loaded microemulsion phases the stabilization process occurs via the trapping of the nanocolloids at the oil/water interfaces as for regular Pickering emulsions12,37 (i.e., through a process where particle wetting is a fundamental parameter and can be changed by the presence of the lipid as a second emulsifier). This explains the similarity with Pickering emulsions. The situation is different in the case of bicontinuous cubosomes because of (i) their higher internal stiffness38 and (ii) their cubic shape exhibiting patchy flat interfaces. Indeed, their stabilization is different. These physical stability criteria are combined with the chemical compatibility between the lipid and the stabilizer, which is probably why the DU-based cubosomes were not stabilized by TM. In this article, we have used spherical colloids, and already differences have been noted in comparison with previous work on disklike laponite particles as stabilizers.15 This means that the geometry of the colloidal stabilizer is another important parameter in the system because the surface chemistry of laponite is similar to that of TM and further work is surely required to separate and understand all of these effects.

IV. Conclusions We have demonstrated the ability of spherical silica nanoparticles to stabilize lipid-based liquid-crystalline bulk phases into submicrometer particles. Two types of lipid-based systems have been explored. The nanoparticles have been seen to be suitable stabilizers for all PT-based LC phases with no effect of TM on the internal structure; they were not good stabilizers for DU-based (37) Aveyard, R.; Binks, B. P.; Clint, J. H. Adv. Colloid Interface Sci. 2003, 100, 503–546. (38) Pouzot, M.; Mezzenga, R.; Leser, M.; Sagalowicz, L.; Guillot, S.; Glatter, O. Langmuir 2007, 23, 9618–9628.

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cubosomes, even if good stabilization was obtained for emulsified microemulsions. This raises the question of colloidal adsorption onto patterned interfaces and the interfacial chemistry, which is a particularly interesting area of research and requires more detailed experiments and modeling. The role and the influence of the TM concentration have been studied as a function of the LC internal phase. Thanks to the development of an overall scattering model, which allows for the determination of free and surface-bound colloidal stabilizers as a result of a normalization approach, we qualitatively demonstrated for the first time the fundamental difference in the stabilization between EME and bicontinuous cubic phases. For a similar final size of droplets, much less TM is required to stabilize cubic phases than L2. The TM-stabilized EME particle size is changing when the TM concentration is modified, but TM-stabilized cubosomes remained the same size with many free TM nanoparticles in the water volume at comparable concentrations, again highlighting the importance of the surface and the structure of the particles. The approach developed gave

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considerable insights into these complex systems by allowing us to study, for the first time, the point of view of the nanocolloidal stabilizer. Acknowledgment. We are indebted to G. Scherf for his support with the scattering device. Baxter (A.S.) and Zukunftsfond Steiermark are acknowledged for partially funding this work. Supporting Information Available: SAXS intensities obtained for an Fd3m bulk phase at pH 6 for dispersed DU-based particles stabilized by 0.5 wt % TM with different DU/TC ratios and for the dispersed PT-based Pn3m phase stabilized at different TM concentrations. The scattering model used as an overall signal is developed, followed by simulations. SAXS intensities obtained for dispersed TM particles in pure water and their concentration-normalized scattering. Conditions used for data analysis (eq 5 in the main text). This material is available free of charge via the Internet at http://pubs.acs.org.

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