Internally Self-Assembled Submicrometer Emulsions Stabilized with a

Article pubs.acs.org/Langmuir

Internally Self-Assembled Submicrometer Emulsions Stabilized with a Charged Polymer or with Silica Particles Martin Dulle and Otto Glatter* Department of Chemistry, Karl Franzens University, Heinrichstrasse 28, A-8010 Graz, Austria

ABSTRACT: Internally self-assembled submicrometer emulsions were stabilized by F127, by the charged diblock copolymer K151, by L300 particles, and by sodium dodecyl sulfate (SDS). The stabilization of all investigated internal phases and the impact of the stabilizer on them are discussed. The use of charged stabilizers results in a highly negative zeta potential of the emulsion droplets, which can be exploited as a means to control their adsorption onto charged surfaces. Small-angle X-ray scattering and dynamic light scattering were used to determine the internal structure and size of the emulsion droplets, respectively.

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INTRODUCTION In recent years, monoglyceride-based internally self-assembled particles (isasomes) have drawn a lot of attention as vehicles for drug transport and in the field of plant protection.1−4 These particles are not merely dispersed oil in water droplets but possess an internally self-assembled structure that can be tuned by temperature and oil content. A more detailed description of the various liquid crystalline phases5 found within these particles6 can be found in the literature. Because of the ability of isasomes to solubilize hydrophobic, amphiphilic, and hydrophilic substances in the liquid-crystalline internal phase 7−9 and their subsequent dispersion in aqueous medium,10−14 these systems represent an environmentally friendly solution, in that they reduce the need for harmful organic solvents to solubilize hydrophobic substances. Besides their use in transporting hydrophobic substances through aqueous media,15 these particles could also be used, for instance, as an anchor or “sponge” for hydrophobic pollutants in aqueous media16 or as carriers of fungicides onto leafs. One way to increase functionality of the particles is to charge the surface. This would allow many different kinds of surfaces to be coated with the particles and have them adhere to the surface. Also, the surface charge would allow incorporation of a trigger for the release of the particle’s contents once they are at the target site. Traditionally, stabilization of isasomes is achieved by the addition of a nonionic triblock poly(ethylene oxide)−poly(propylene oxide)−poly(ethylene oxide) copolymer (F127) as secondary stabilizer, and recently it has been found that of the many commercially available block copolymers only one was slightly better than F127.17 The stabilization of these dispersed nanostructured phases with materials other than triblock copolymers has received increasing interest,18 as has the © 2011 American Chemical Society

fabrication of nanostructured water-in-oil (w/o) emulsions without surfactants.19 Particle stabilization without the use of surfactant molecules has received particular attention,20−24 while the layer-by-layer technique introduced by Decher et al.25 is still one of the fastest growing in terms of the number of publications reporting its use. The main concept of this relatively simple technique, which works from a few nanometers up to several micrometers, is the subsequent adsorption of oppositely charged molecular species onto a smooth and charged surface. In many cases, polyelectrolytes26−29 are used to build up these multilayered systems. Such systems can, for instance, change the optical properties30 of the surface or change its hydrophobicity31−33 or topography in a very controlled way. 34 They can be used to incorporate particles,35−38 cells,39,40 or other soft materials.41 It has been shown by Drieveret al.42 that cubosomes stabilized by F127 can be incorporated into a polymeric layer-by-layer system while maintaining their internal structure. Our aim is to enable these internally nanostructured emulsion droplets to be used in a simple layer-by-layer approach. This would allow many different kinds of surfaces to be coated with the particles. We present in this paper a variety of stabilization methods that give rise to highly negatively charged internally self-assembled particles with various internal structures.

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EXPERIMENTAL SECTION

Materials. The internal lyotropic phases of all investigated samples were composed of phytantriol (DSM, Germany), tetradecane (Fluka), and Milli-Q water. To disperse the lyotropic phases, we used four Received: October 3, 2011 Revised: December 9, 2011 Published: December 12, 2011 1136

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Scheme 1. Reaction Pathway for the Block Copolymer K151

Table 1. Characteristics of the Block Copolymer name

ma

nb

X mc

Xm(NMR)d

Mcalce [g/mol]

Mn(GPC) [g/mol]

Mw(GPC) [g/mol]

PDI

K151

50

150

0.3

0.31

54774

54900

58700

1.07

a Theoretical number of lyophobic units (monomer A). bTheoretical number of lyophilic units (monomer B). cTheoretical ratio of monomer A to monomer B. dRatio of A to B determined from peak integration of the 1H NMR spectrum. eCalculated molecular weight.

Zürich, Switzerland), an ALV/SO-SIPD/DUAL photomultiplier with pseudo-cross-correlation, and an ALV 5000/E correlator with fast expansion (ALV, Langen, Germany). Measurements were carried out at a scattering angle of 90°. Correlation functions were collected for 60 s ten times and then averaged. From these functions, the average diffusion coefficient D was obtained by cumulant analysis.43 In this method, the correlation function is expanded in a power series, where the expansion coefficients (cumulants) correspond to the moments of the intensity distribution. The first cumulant is the mean of the distribution and the second its variance. The first cumulant gives the zaverage of the diffusion coefficient D, which is related to the hydrodynamic radius RH through the Stokes−Einstein equation:

different stabilizers. As a standard surfactant with negative charge we used sodium dodecyl sulfate (SDS; 99.8% Sigma-Aldrich). In most studies of these internally self-assembled particles, F127 (BASF) is used as the stabilizing agent; thus, we use it here for comparison. A specially synthesized charged block copolymer, K151, was applied as stabilizer; details of its synthesis and purification can be found below. To produce emulsion droplets without surfactant, we used suspended silica particles (L300), a gift from the company H.C. Starck, as the stabilizer. These silica particles have a mean diameter of 18 nm. All materials were used without further purification. Small-Angle X-ray Scattering (SAXS). SAXS is well suited for investigating the internal nanostructure of emulsion droplets. Our SAXS equipment comprises a SAXSess camera (Anton-Paar, Graz, Austria), with high flux and low background, connected to an X-ray generator (Philips, PW1730/10) operating at 40 kV and 50 mA with a sealed-tube Cu anode. A Göbel mirror was used to convert the divergent polychromatic X-ray beam into a focused line-shaped beam of Cu Kα radiation (wavelength λ = 0.154 nm). The 2D scattering pattern was recorded by a PI-SCX fused fiber-optic taper CCD camera from Princeton Instruments, a division of Roper Scientific (Trenton, NJ), and integrated into the 1D scattering function I(q), where q is the length of the scattering vector, defined by q = (4π/λ) sin(θ/2), λ being the wavelength and θ being the scattering angle. The CCD detector features a 2084 × 2084 array with 24 μm × 24 μm pixel size (chip size: 50 mm × 50 mm). The CCD was operated at −30 °C with 10 °C water-assisted cooling to reduce the thermally generated charge. Cosmic-ray correction and background subtraction were performed on the 2D image before further data processing. The temperature of the capillary and the metallic sample holder were controlled by a Peltier element. Samples were equilibrated at 25 °C for 10 min before being exposed to X-rays for 5 min to yield an image; three such images were used for averaging. Hexagonal and cubic space groups were determined by the relative positions of the peaks in the scattering curves, which correspond to reflections from families of planes with particular Miller (hkl) indices. The interplanar distance between two parallel planes of one family is given by dhkl = 2π/qhkl, from which we can determine the mean lattice parameter. For this purpose we used indexing software developed in our group. Concerning the L2 phase, emulsified in our particles, we can deduce the characteristic distance d = 2π/q from the maximum of the broad correlation peak. Dynamic Light Scattering (DLS). The size of the emulsion droplets was measured by DLS. All samples were measured at 5 × 10−4 wt % in 1.5 mL glass vials at 25 °C. The DLS equipment is composed of a goniometer and a diode laser (Coherent Verdi V5, λ = 532 nm, Pmax = 5 W) with single-mode fiber-detection optics (OZ from GMP,

RH =

kBT 6πηD

(1)

where kB is the Boltzmann constant, T the absolute temperature, and η the viscosity of the solvent. The hydrodynamic radius is the radius of an equivalent compact sphere with diffusion coefficient D. Another source of information, especially concerning polydisperse systems, comes from carrying out a Laplace inversion with the program ORT.44 The size distributions were calculated without further assumptions and are therefore weighted with intensity.

Zeta Potential. The electrophoretic mobility was measured using a Malvern Zetasizer Nano-ZS. The samples were diluted by a factor of 1000, 1.5 μL of isasomes in 1.5 mL of ultrapure water, prior to measurement and placed in a disposable measurement cell. The electrophoretic mobility was measured at least three times and averaged. From this data the zeta potential was calculated using the Smoluchowski approximation under the assumption of spherical particles. Synthesis of the Block Copolymer. The polymer we chose as emulsifier for our isasomes is K151 (see Scheme 1), which has block lengths of m = 50 and n = 150. The synthesis was carried out as described by Stubenrauch et al.45 endo,exo[2.2.1]Bicyclo-2-ene-5,6dicarboxylic acid dimethyl ester (monomerA) was purchased from Orgentis Chemicals, and endo,exo[2.2.1]bicyclo-2-ene-5,6-dicarboxylic acid di-tert-butyl ester (monomer B) was supplied by Stubenrauch et al. Under an inert atmosphere monomer A (210 mg 1.0 mmol) and the “first generation Grubbs” initiator RuCl2(PCy3)2(CHPh) (Cy = cyclohexyl) (17.8 mg 0.023 mmol) were dissolved in 5 mL of CH2Cl2 each, and monomer A was added to the initiator solution. The reaction mixture was stirred for 1 h, and the absence of monomer was checked by TLC. Monomer B (884 mg, 3.0 mmol) was dissolved in 5 mL of CH2Cl2 and added to the reaction mixture. It was stirred for 1 h, and the absence of monomer was checked by TLC. To stop the reaction and cleave off the catalyst 3 mL of ethyl vinyl ether (EVE) was added, 1137

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and the mixture stirred for half an hour. The solvent was reduced to half the volume, and the polymer was precipitated in cold methanol. The methanol was decanted off and the polymer dried under vacuum To confirm that the polymerization had worked as planned, gel permeation chromatography was carried out as described by Stubenrauch et al.45 before the tert-butyl protecting group was cleaved off with CF3COOH to give the charged diblock copolymer. This was precipitated from heptane and dried under vacuum. 1H and 13C NMR measurements were made to confirm the structure. Preparation of the Emulsions. To form the lyotropic phases, we used phytantriol, with tetradecane as the oil. All aqueous dispersions were prepared by weighing and mixing all the components (stabilizer, phythantriol, oil, and water) at 25 °C in a sealable glass vial. In the case of the charged copolymer, NaOH solution (5 mM) was used instead of water in order to achieve a pH of 7 after preparation. All the dispersions were 5 wt % dispersed oil/monoglyceride in water. The raw mixture was ultrasonicated for 20 min by using a vibra cell ultrasonic processor from SY-LAB (Pukersdorf, Austria) at 30% of the maximum power in pulse mode (0.5 s pulses interrupted by 1.5 s breaks), which led to a milky dispersion. The weighted oil/phytantriol ratio δ (see eq 2) was varied within the range of 50−100, which corresponds to the percentage of phytantriol added to a normal oil-inwater (O/W) emulsion.

⎡ ⎤ mass of phythantriol δ=⎢ ⎥ × 100 ⎣ mass of phythantriol + mass of oil ⎦

examined. In each case the internal structure could be easily identified from the Bragg peaks. The corresponding space groups and lattice constants are Pn3m and a = 7.1 nm for the cubosomes and H2 and a = 5.1 nm for the hexosomes, respectively. As the EME is not a crystalline structure but a microemulsion phase, the maximum of the correlation peak d at q1 is used to determine the characteristic distance between the water droplets in the internal phase, EME/d = 5.4 nm. We also tested the stabilization of the emulsion under low-temperature (4 °C) conditions because F127 is not a good stabilizer at low temperature. The poly(propylene oxide) chain of the triblock is less hydrophobic at lower temperature. Thus, we split all samples made with F127 and K151 into two fractions. One fraction was stored at 4 °C and the other at room temperature. The samples were then investigated 1, 4, 11, and 31 days after preparation. At room temperature, the two samples behaved in the same manner and showed no change in internal structure. The mean size of the droplets was investigated using DLS; only minor changes were observed. The radius of the cubosomes decreased from about 140 to 130 nm when the charged polymer was used as stabilizer. When F127 was used, the radius was constant at around 128 nm. The radius of the hexosomes and EME droplets increased with both stabilizers from 115 to 145 nm. The width of the size distributions was in the range of 10−20% for all samples over the observed period. The hexosomes and EME stored at 4 °C behaved the same way with both stabilizers; the internal structure and mean size did not change relative to those at room temperature. The cubosomes, on the other hand, behaved differently at the two temperatures. In both cases the peaks shifted slightly to lower q values, which correspond to a widening of the internal structure and a change in the lattice constant from 7.1 nm at room temperature to 7.5 nm at 4 °C. But in the case of K151 the widening stops after 11 days, and the lattice constant remains unchanged thereafter (Figure 2). This is not the case for F127; the lattice constant changes over the whole period. Furthermore, the sample with F127 undergoes a temporary change in structure at day 4, as can be seen from the peak next to the main reflection. This is likely due to the simultaneous presence of two liquid-crystalline structures in the sample (Figure 3). The reason for this may be rooted in the change of solubility of F127 in water with temperature, but we did not investigate this phenomenon further. Isasomes Stabilized by SDS. As an example of a conventional charged surfactant, SDS was selected to stabilize the liquid-crystalline structures. SDS is highly charged, readily available, easy to handle, and known to stabilize emulsions very well,46 but in the case of isasomes a huge oil−water interface is present inside the droplets which enables the SDS molecules to be incorporated into the internal structure and disrupt it. The concentration we used for SDS (β = 10) was very low (0.9 mmol/L)well below its cmc (8 mmol/L).47 With SDS we could stabilize all of the liquid-crystalline structures over a long time at room temperature as well as at 4 °C. But, due to its small molecular weight and size, it does seem to get incorporated into the internal structure, which is clearest in the case of the cubosomes, where the lattice parameter is increased from 7.1 to 7.3 and the number of visible peaks is reduced to two, which are not as high as in case of the cubosomes stabilized with F127; i.e., the structural order is decreased. From these findings we can conclude that SDS is incorporated into the liquid-crystalline structure, which results

(2)

The five different stabilizers described in the Materials section were used in various amounts (as sparingly as possible), according to the β value (eq 3).

⎡ ⎤ mass of stabilizing agent β=⎢ ⎥ × 100 ⎣ mass of phythantriol + mass of oil ⎦

(3)

The sample names and compositions are given in Table 2.

Table 2. List of Samples and Their Compositions

a

name

stabilizer

δ [wt %]

β [wt %]

internal phase

K151_100 K151_85 K151_70 F127_100 F127_85 F127_70 SDS_100 SDS_85 SDS_70 L300_100 L300_85 L300_70

K151 K151 K151 F127 F127 F127 SDS SDS SDS L300 L300 L300

100 85 70 100 85 70 100 85 70 100 85 70

4 2 2 8 8 8 10 10 10 8 8 8

cubic hexagonal EMEa cubic hexagonal EME cubic hexagonal EME cubic hexagonal EME

EME stands for emulsified microemulsion.

The emulsion droplets were stable for at least a month after preparation, with no change in size or internal structure, as confirmed by DLS and SAXS, respectively.

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RESULTS AND DISCUSSION Isasomes with Polymeric Stabilizer. Our synthesized charged block copolymer stabilized the isasomes with all three different internal phases used in this study. As can be seen in Figure 1, the internal structure was retained in all cases and was stable over the whole period studied, which lead us to the assumption that incorporation of the stabilizer into the internal nanostructure was negligible. To evaluate the performance of the charged block copolymer, we compared it directly to samples prepared with F127 and found that there was no difference in the internal phases of any of the samples 1138

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Figure 1. SAXS curves of (a) cubosomes, (b) hexosomes, and (c) emulsified microemulsion stabilized by the charged diblock copolymer K151 (curves vertically shifted for better visibility). (d) Peak indexes for Pn3m and H2 and maximum position (q1) for the EME.

Figure 2. SAXS curves of cubosomes stabilized by K151 at 4 °C (curves vertically shifted for better visibility). The vertical line is a guide to the eye and sits on the first peak of the day 1 curve.

Figure 3. SAXS curves of cubosomes stabilized by F127 at 4 °C (curves vertically shifted for better visibility). The vertical line is a guide to the eye and sits on the first peak of the day 1 curve.

in a widening of the water channels due to the charged headgroup of SDS and to the deterioration of the liquidcrystalline structure as a whole (Figure 4). Isasomes Stabilized by L300 Silica Particles. To stabilize these internally structured emulsions, not only polymers and surfactants can be used but also a variety of small particles, such as Laponite (clay based)20 or silica

particles.23 The resulting so-called Pickering emulsions are very stable due to the big energy gain on attachment of the particles to the water/oil interface. The nature of the particles is responsible for the high stability, as they can form a dense layer at the interface.23 In our case all three internal phases could be stabilized with the L300 particles with no phase changes or peak shifts, which demonstrates that the internal structure is 1139

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Figure 6. Zeta potential values of cubosomes (a), hexosomes (b), and EME (c) with the different stabilizers used.

Figure 4. Cubosomes stabilized by SDS compared to F127 (curves vertically shifted for better visibility).

stabilization mechanism. This was also proposed by Salonen et al.23 from SAXS results, where the amount of silica particles on the surface was determined relative to the total amount of silica particles present. But concerning this question, our results are not conclusive enough as the differences in zeta potential for the cubic phases are within the margins of error of the obtained data. The specially designed polymer K151 also offers a handle to influence the surface charge of the droplets. By changing the pH of the surrounding solution we were able to reach zeta potential values from −80 up to −20 mV (Figure 7). This

not influenced by the particles. The only drawback associated with these particles is that, upon storage at 4 °C, the cubosomes and the hexosomes become unstable after 3−5 days. This can be explained by the fact that the viscosity of the liquid crystalline phases in case of inverse cubic and hexagonal symmetry is very high and the L300 particles have problems to adhere on the interface in the same way they do in case of the isasomes with EME as internal phase.

Figure 5. Cubosomes stabilized by L300 compared to F127 (curves vertically shifted for better visibility). The internal structure is the same; the upturn for L300 at low q-values originates from the silica particles.

Figure 7. pH dependence of the zeta potential for cubosomes with K151 as stabilizer.

change in surface charge enables tuning of the interaction of the droplets with other charged materials when used in a layer-bylayer system.

Zeta Potential. All samples investigated showed a negative zeta potential. In case of F127, which is a nonionic copolymer, the charge found by the zeta-potential measurements probably comes from a layer of polarized water around the droplets.48 The zeta potential for these samples is relatively low; all other samples gave highly negative values (Figure 6). Note also that all samples with a cubic internal structure display a different zeta potential from the other two phases; i.e., the internal structure influences the surface layer. In the cases of F127, SDS, and L300, the zeta potentials of the cubic phase are 37% (F127), 8% (SDS), and 19.6% (L300) lower than the average of the other two phases. For K151 it is 15% higher in the cubic phase than for the other two. This difference in zeta potential for the cubic phases (relative to the other liquid crystalline phases with the same stabilizer) probably arises from a different

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CONCLUSIONS We have shown that the surface charge of the dispersions of the liquid-crystalline phases formed by phytantriol in water is increased substantially by using charged stabilizers. When F127 was used as stabilizer, a small charge could be deduced from zeta potential measurements, but this charge is not sufficiently high to form a dense layer of isasomes on an oppositely charged surface. Of the charged stabilizers used, the specially synthesized polymer K151 showed the best overall performance. It gave the highest increase in surface charge alongside SDS, but without changing the internal structure. The 1140

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(23) Salonen, A.; Muller, F.; Glatter, O. Langmuir 2010, 26, 7981. (24) Muller, F.; Salonen, A.; Dulle, M.; Glatter, O. In Trends in Colloid and Interface Science XXIV; Starov, V., Procházka, K., Eds.; Springer: Berlin, 2011; Vol. 138, p 27. (25) Decher, G. Science 1997, 277, 1232. (26) Decher, G. In Templating, Self-Assembly and Self-Organization; Sauvage, J.-P. a. H., M. W., Ed.; Pergamon Press: Oxford, 1996; Vol. 9, p 507. (27) Klitzing, R.; Moehwald, H. Langmuir 1995, 11, 3554. (28) Schlenoff, J. B.; Dubas, S. T. Macromolecules 2001, 34, 592. (29) Dubas, S. T.; Schlenoff, J. B. Langmuir 2001, 17, 7725. (30) Hiller, J. a.; Mendelsohn, J. D.; Rubner, M. F. Nature Mater. 2002, 1, 59. (31) Kolasinska, M.; Warsynski, P. Bioelectrochemistry 2005, 66, 65. (32) Wang, B.; Feng, J.; Gao, C. Colloids Surf., A 2005, 259, 1. (33) Lingström, R.; Wagenberg, L.; Larsson, P. T. J. Colloid Interface Sci. 2006, 296, 396. (34) Lu, C.; Mohwald, H.; Fery, A. Soft Matter 2007, 3, 1530. (35) Decher, G.; Lehr, B.; Lowack, K.; Lvov, Y.; Schmitt, J. Biosens. Bioelectron. 1994, 9, 677. (36) Van Duffel, B.; Schoonheydt, R. A.; Grim, C. P. M.; De Schryver, F. C. Langmuir 1999, 15, 7520. (37) Mamedov, A. A.; Kotov, N. A. Langmuir 2000, 16, 5530. (38) Koktysh, D. S.; Liang, X. R.; Yun, B. G.; Pastoriza-Santos, I.; Matts, R. L.; Giersig, M.; Serra-Rodriguez, C.; Liz-Marzan, L. M.; Kotov, N. A. Adv. Funct. Mater. 2002, 12, 255. (39) Jan, E.; Kotov, N. A. Nano Lett. 2007, 7, 1123. (40) Wang, Y.; Angelatos, A. S.; Caruso, F. Chem. Mater. 2007, 20, 848. (41) Michel, M.; Vautier, D.; Voegel, J.-C.; Schaaf, P.; Ball, V. Langmuir 2004, 20, 4835. (42) Driever, C. D.; Mulet, X.; Johnston, A. P. R.; Waddington, L. J.; Thissen, H.; Caruso, F.; Drummond, C. J. Soft Matter 2011, 7, 4257. (43) Koppel, D. E. J. Chem. Phys. 1972, 57, 4814. (44) Schnablegger, H.; Glatter, O. Appl. Opt. 1991, 30, 4889. (45) Stubenrauch, K.; Fritz-Popovski, G.; Ingolic, E.; Grogger, W.; Glatter, O.; Stelzer, F.; Trimmel, G. Macromolecules 2007, 40, 4592. (46) Vold, R. D.; Groot, R. C. J. Phys. Chem. B 1962, 66, 1969. (47) Turro, N. J.; Yekta, A. J. Am. Chem. Soc. 1978, 100, 5951. (48) Tjipto, E.; Cadwell, K. D.; Quinn, J. F.; Johnston, A. P. R.; Abbott, N. L.; Caruso, F. Nano Lett. 2006, 6, 2243. (49) Ahualli, S.; Iglesias, G. R.; Wachter, W.; Dulle, M.; Minami, D.; Glatter, O. Langmuir 2011, 27, 9182. (50) Ravera, F.; Ferrari, M.; Liggieri, L.; Loglio, G.; Santini, E.; Zanobini, A. Colloids Surf., A 2008, 323, 99. (51) Ravera, F.; Santini, E.; Loglio, G.; Ferrari, M.; Liggieri, L. J. Phys. Chem. B 2006, 110, 19543.

surfactant-free approachL300 (silica particles)did work very well (except at low temperature), which was surprising because L300 particles are very hydrophilic and not able to stabilize a classical oil-in-water emulsion except when modified with low-molecular-weight surfactants.49−51 The reason for this difference is the relatively low interfacial tension of the internal phase. We propose that the added charge and high stability of L300 particles should allow the emulsion droplets to be absorbed onto charged surfaces while retaining their internal structure. This is part of an ongoing research project in our group.

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AUTHOR INFORMATION

Corresponding Author

*Fax +433163809850, e-mail [email protected].

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ACKNOWLEDGMENTS We thank Gregor Trimmel from TU Graz for providing the chemicals and the laboratory time needed to synthesize the charged block copolymer. We also thank Silva Ahualli and Guillermo Iglesias Salto for performing the zeta potential measurements at the University of Granada, Spain.

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REFERENCES

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