Inverse Pickering Emulsions with Droplet Sizes below 500 nm

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Inverse Pickering emulsions with droplet sizes below 500 nm Susanne Sihler, Anika Schrade, Zhihai Cao, and Ulrich Ziener Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b02735 • Publication Date (Web): 08 Sep 2015 Downloaded from http://pubs.acs.org on September 12, 2015

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Inverse Pickering emulsions with droplet sizes below 500 nm Susanne Sihler, Anika Schrade, Zhihai Cao† and Ulrich Ziener* Institute of Organic Chemistry III-Macromolecular Chemistry and Organic Materials, University of Ulm, Albert-Einstein-Allee 11, 89081 Ulm, Germany ABSTRACT: Inverse Pickering emulsions with droplet diameters between 180 and 450 nm, a narrow droplet size distribution and an outstanding stability were prepared using miniemulsion technique. Commercially available hydrophilic silica nanoparticles were used to stabilize the emulsions. They were hydrophobized in-situ by the adsorption of various neutral polymeric surfactants. The influence of different parameters such as kind and amount of surfactant as hydrophobizing agent, size and charge of the silica particles, amount of water in the dispersed phase as well as the kind of osmotic agent (sodium chloride and phosphate buffered saline (PBS)) on the emulsion characteristics was investigated. The systems were characterized by dynamic light scattering (DLS), transmission electron microscopy (TEM), cryo scanning electron microscopy (Cryo-SEM), thermogravimetric analysis (TGA), and semi-quantitative attenuated total reflection infrared spectroscopy (ATR-IR). Cryo-SEM shows that some silica particles are obviously rendered hydrophilic and form a three-dimensional network inside the droplets.

INTRODUCTION

ACS Paragon Plus Environment

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Apart from surfactants, solid particles can be used to stabilize emulsions.1,2 For the stability of these so-called Pickering emulsions many different parameters have to be taken into account. Beside the properties of particles such as size,3 shape,4–6 and material,2,6–16 the nature of the oil,11,17 temperature18 and composition of both the aqueous and the oil phase17,19,20 are rather important. Nevertheless, one of the most essential parameters is the particle wettability. Only if the particles are wetted partially by both liquids they are able to stabilize Pickering emulsions. In terms of the contact angle θ this means that θ must be neither 0° (fully hydrophilic) nor 180° (fully hydrophobic).7,10,15,21–23 Rather hydrophilic particles with a contact angle of less than 90° are able to stabilize oil-in-water (o/w, direct) emulsions, while rather hydrophobic particles will stabilize water-in-oil (w/o, inverse) Pickering emulsions.21,24 From a thermodynamic point of view the adsorption energy of the particle at the interface is an important parameter for the stability of Pickering emulsions and reaches a maximum for θ = 90°. On the other hand the capillary pressure between the particles must be taken into account, which leads to an optimum contact angle slightly smaller or higher than 90°.25–28 To adjust the contact angle of native (too hydrophilic) particles reactive groups on the surface of the particles can be derivatized, for example the silanol groups on silica particles can be converted into hydrophobic groups via a reaction with, e.g., alkyl silanes.10,14,18,29–31 On the other hand, (macro)molecules such as potassium hydrogen phthalate,32 cetyltrimethylammonium bromide,33,34 double chain cationic surfactants,34,35 lecithin,36 oleylamine,36 stearic acid and stearyl amine,37 switchable surfactants38 as well as different non-ionic surfactants such as poly(ethylene imine),39 Tween, Brij, and synperonic®40 can adsorb to the hydrophilic surface and consequently change the wetting behavior. While the chemical derivatization exhibits the advantage that the surface is irreversibly modified, the adsorption process is reversible. This in


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turn offers the possibility of an in-situ hydrophobization of the particles during the preparation of the emulsion and therefore the saving of one reaction step. Most of the publications dealing with Pickering emulsions report on droplet sizes in the µm range. With regard to for example bio-applications it is interesting to produce stable emulsions with droplet diameters in the range of several hundred nanometers.41 One possibility to obtain emulsions with droplets smaller than one µm is to apply the miniemulsion technique delivering fairly monodisperse emulsion droplets with diameters in the range of 30–500 nm, which is achieved by the application of very high shear forces, for example by high-pressure homogenization or ultrasonication, and the addition of an osmotic agent to suppress Ostwald ripening.42–46 So far there are only two groups, which have already reported on Pickering emulsions with droplet sizes below 1 µm.36,37 While the former reported on a method to prepare direct Pickering emulsions with droplet sizes in the submicron range stabilized by a combination of hydrophilic silica particles and lecithin or oleylamine, the latter could show that colloidosomes with droplet sizes below 600 nm but with a trimodal particle distribution can be prepared by means of inverse Pickering emulsions stabilized by silica and aluminium oxide nanoparticles, respectively, together with charged surfactants. Here, we report on the preparation of inverse Pickering emulsions by miniemulsion technique to obtain stable emulsions with droplet sizes below 500 nm and a monomodal droplet size distribution (see Scheme 1 and Chart 1). The employed particles are commercially available, plain hydrophilic silica particles with a diameter of 22 and 12 nm, respectively. They are in-situ hydrophobized by the adsorption of different oil soluble surfactants onto the silica surface. Hence, the particles are originally dispersed in the aqueous phase, which becomes the dispersed


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phase in the emulsion. We focus on poly(ethylene-co-butylene)-block-poly(ethylene oxide) with an HLB value of 8.3 and a molecular weight of about 7,200 g·mol-1 as hydrophobizing agent. Additionally, we demonstrate the flexibility of the approach by using a variety of uncharged and charged surfactants, which are also able to hydrophobize the silica particles and stabilize the emulsions.


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Scheme 1. Preparation of inverse Pickering emulsions with droplet sizes below 500 nm stabilized by in-situ hydrophobized silica particles.


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Chart 1. Employed surfactants to partially hydrophobize the silica particles.

EXPERIMENTAL SECTION Materials. The silica sols LUDOX®CL (kindly donated by Grace Davison, coated with an aluminium compound, positively charged, ζ-potential: 24.0 ± 2.7 mV, counterion: chloride, 12 nm, 28 wt% in water), LUDOX®CL-P (kindly donated by Grace Davison, coated with an aluminium compound, positively charged, ζ-potential: 38.3 ± 3.3 mV, counterion: chloride, 22


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nm, 34 wt% in water), LUDOX®HSA (kindly donated by Grace Davison, negatively charged, ζpotential: -19.9 ± 5.4 mV counterion: sodium, 12 nm, 31 wt% in water) and LUDOX®TMA (Aldrich, negatively charged, ζ-potential: -20.5 ± 4.0 mV, counterion: sodium, 22 nm, 36 wt% in water), the solvents IsoparM (CALDIC Deutschland), n-dodecane (99+%, Merck), n-tetradecane (99+%, Aldrich) and acetone (99+%, Merck), the surfactants poly(ethylene-co-butylene)-blockpoly(ethylene oxide) (P(E/B)-PEO_1, Mw = 7,200 g·mol-1 (NMR), HLB = 8.3, P(E/B)-PEO_2, Mw = 8,500 g·mol-1 (NMR), HLB = 10 and P(E/B)-PEO_3, Mw = 10,500 g·mol-1 (NMR), HLB = 12, synthesized via anionic polymerization

as described elsewhere),47 polyglycerol

polyricinoleate (PGPR 90, Grinsted), lecithin (≥ 97 %, Roth), polyisobutylene succinimide pentamine (Lubrizol®U, Lubrizol, France) and sodium chloride (99.5 %, Merck) were used as received. Demineralized water with Milli-Q grade (resistivity: 18 MΩ) was used for all experiments. Preparation of emulsions. The aqueous phase (disperse phase, DP) and oil phase (continuous phase, CP) were prepared separately. If not stated otherwise, the DP was prepared as follows. 10 mg of sodium chloride were dissolved in 600 µL of water. Thereafter, 400 mg of the respective silica sol was added. The oil phase consists of a certain amount of surfactant dissolved in 16.5 mL of IsoparM, which is a mixture of several linear and branched hydrocarbons mainly in the range of C12 to C15. Both phases were mixed in a 30 mL screw cap jar and stirred magnetically for 15 min (1000 rpm, cylindric stirring bar (6 x 15 mm)). The dispersion was ultrasonicated with a Branson W450 digital sonifier by using a 1/2" titanium horn under ice cooling (3 min, 90% amplitude, 0 °C).


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The emulsions used to investigate the structure of the particle aggregates were prepared as described above, but instead of IsoparM, a mixture of each 8.25 mL of n-dodecane and ntetradecane was used. Adsorption of P(E/B)-PEO_1 to silica particles. A certain amount of P(E/B)-PEO_1 was dissolved in 16.5 mL of acetone. Afterwards, 144 mg of dry silica particles (received by drying some LUDOX®TMA silica sol and crushing the obtained solid in a mortar) were added and dispersed in the surfactant solution by treatment in an ultrasonic bath for 30 min. The dispersion was then stirred at room temperature with 950 rpm for 4 d and finally centrifuged. The residue was dried and analyzed. Characterization. Dynamic light scattering (DLS). Both the size and the size distribution (polydispersity index (PDI)) of the emulsion droplets were measured by DLS on a NanoZetasizer (Malvern Instruments) at 20 ºС under a scattering angle of 173º at λ = 633 nm. Droplet sizes and PDIs are given as the average of five measurements. 20 µL of the emulsion were diluted with 1.5 mL of IsoparM in a polystyrene cuvette before the measurement. Zeta potential measurements. The zeta potential of the samples was measured on a NanoZetasizer (Malvern Instruments) at 25 °C under the mode of zeta potential. A total of 2 µL of the dispersion was diluted with 1 mL of aqueous solution of KCl (10-3 M). Zeta potentials are given as the average of three measurements. Transmission electron microscopy (TEM). TEM measurements were performed on a Zeiss EM 10 microscope with an acceleration voltage of 80 kV. A 300 square mesh copper grid was dipped into the emulsion. The sample was allowed to dry at room temperature. Cryo-scanning electron microscopy (Cryo-SEM). For the analysis of the emulsions by CryoSEM, the following process was applied, which differs slightly from the method described in


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literature.48 A 300 square mesh copper grid was dipped into the emulsion and placed between two aluminium discs. This “sandwich” was then dipped into liquid propane to freeze the sample and stored in liquid nitrogen. Afterwards, the samples were fractured at -170 °C, tempered to 100 °C for 15 minutes and vapor coated with platinum (1800 V, ca. 70 mA) and carbon (2400 V, ca. 90 mA) in a Balzer BAF 300 freeze-etching unit. After transfer of the sample into a Gatan Cryo holder, the measurements were performed at -100 °C in a Hitachi S-5200 (10 kV, 10 µA). Thermogravimetric analysis (TGA). To determine the amount of surfactant adsorbed on the silica particles, TGA measurements were performed. At least 2 mL of the emulsion or dispersion was centrifuged (13,000 rpm, 5 min). After the removal of the supernatant, the residue was dried in an oven (100 °C) for three days. About 10 mg of the dried sample was placed in a specimen holder (aluminium oxide, 70 µL) and analyzed with a TGA/SDTA851e (Mettler Toledo) using a heating rate of 10 °C min-1 under oxygen supply (50 mL min-1) in a temperature range between 25 °C and 800 °C. As the surfactant combusts in a temperature range of 180 °C to 600 °C, these values were taken as borders for the analysis of all measurements. Attenuated total reflection infrared (ATR-IR) spectroscopy. ATR-IR spectroscopy was used as a very fast and easy alternative to TGA measurements. The samples were prepared as described above for the TGA measurements. A small quantity of the dried substance was put on the diamond of a Tensor 27 (Bruker) and pressed on the diamond very slightly with a metal finger. Afterwards, the spectrum was recorded in a wave number range of 400 cm-1 – 4000 cm-1 with a resolution of 1 cm-1. For further analysis of the spectrum, the “silica band” (650 cm-1 – 1330 cm1

, Si–O bending and stretching) as well as the band between 2800 cm-1 – 3000 cm-1 (C–H

stretching) were integrated and standardized to the value of the “silica band”.


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Surface tension measurements. All surface tension measurements were performed on a tensiometer (DCAT21, DataPhysics) using the Du Noüy ring method. The Pt-Ir-ring (wire radius: 0.185 mm, ring radius: 9.4425 mm, height: 25 mm) was annealed prior to every measurement. The measurement itself was performed in the push–pull mode with each 10 push and pull operations per measurement. Surface tensions are given as the average of at least four measurements.

RESULTS AND DISCUSSION Formation of inverse Pickering emulsions with LUDOX®TMA and various amounts of P(E/B)-PEO_1. In a first series of experiments the influence of the amount of the polymeric surfactant P(E/B)PEO_1 on the emulsion stability is investigated. For the emulsions prepared in run 1–11 (Table 1), the silica sol LUDOX®TMA was used. The amounts of IsoparM (16.5 mL), water (600 µL), silica sol (400 mg), and sodium chloride (10 mg) as osmotic agent were kept constant. Both the mass of P(E/B)-PEO_1 used to prepare the emulsions and the data received from the analyses by DLS are listed in Table 1.

Table 1. Recipes and characteristics of the resulting emulsions.

run 1

hydrophobizing agent (amount / mg) P(E/B)-PEO_1 (7.5)

silica dispersion (amount / mg) LUDOX®TMA (400) ®

droplet size after 4 weeksd

water / µL

droplet size right after preparationd Z-average / PDI nm

600

475 (65)

0.14 (0.09)

435 (13)

0.15 (0.13)

Z-average / nm

PDI

2

P(E/B)-PEO_1 (15)

LUDOX TMA (400)

600

391 (16)

0.06 (0.05)

420 (10)

0.17 (0.10)

3

P(E/B)-PEO_1 (30)

LUDOX®TMA (400)

600

289 (8)

0.08 (0.06)

308 (3)

0.15 (0.03)

4

P(E/B)-PEO_1 (60)

LUDOX®TMA (400)

600

231 (5)

0.07 (0.03)

237 (3)

0.14 (0.01)

5

P(E/B)-PEO_1 (93.5)

LUDOX®TMA (400)

600

212 (1)

0.05 (0.02)

216 (2)

0.13 (0.01)

6

P(E/B)-PEO_1 (120)

LUDOX®TMA (400)

600

204 (3)

0.04 (0.04)

202 (3)

0.09 (0.01)

7

P(E/B)-PEO_1 (240)

LUDOX®TMA (400)

600

183 (2)

0.08 (0.03)

181 (3)

0.15 (0.01)


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P(E/B)-PEO_1 (500)

LUDOX®TMA (400)

600

165 (3)

0.08 (0.02)

188 (1)

0.15 (0.05)

9

P(E/B)-PEO_1 (800)

LUDOX®TMA (400)

600

160 (1)

0.10 (0.02)

167 (1)

0.13 (0.03)

a

®

10

P(E/B)-PEO_1 (30)

LUDOX TMA (400)

-

334 (9)

0.11 (0.07)

316 (2)

0.13 (0.07)

11b

P(E/B)-PEO_1 (30)

LUDOX®TMA (400)

600

312 (13)

0.06 (0.05)

338 (3)

0.25 (0.02)

12

P(E/B)-PEO_1 (7.5)

-

1000

867 (238)

0.29 (0.14)

1021 (50)

0.63 (0.10)

13

P(E/B)-PEO_1 (15)

-

1000

610 (108)

0.16 (0.12)

543 (13)

0.49 (0.04)

14

P(E/B)-PEO_1 (30)

-

1000

383 (3)

0.27 (0.04)

389 (15)

0.25 (0.08)

15

P(E/B)-PEO_1 (7.5)

-

866

845 (171)

0.31 (0.18)

1099 (22)

0.65 (0.11)

16

P(E/B)-PEO_1 (15)

-

866

425 (22)

0.20 (0.04)

475 (14)

0.23 (0.16)

17

P(E/B)-PEO_1 (30)

866

314 (6)

0.24 (0.02)

333 (4)

0.28 (0.05)

18

P(E/B)-PEO_1 (30)

LUDOX®HSA (250)

750

365 (17)

0.13 (0.06)

430 (7)

0.19 (0.03)

19

P(E/B)-PEO_1 (30)

LUDOX®CL-P (400)

600

290 (6)

0.20 (0.04)

308 (5)

0.18 (0.04)

-

®

20

P(E/B)-PEO_1 (30)

LUDOX CL (250)

750

297 (3)

0.17 (0.05)

302 (2)

0.16 (0.04)

21

P(E/B)-PEO_1 (30)

LUDOX®TMA (400)

400

262 (3)

0.10 (0.05)

307 (3)

0.21 (0.02)

22

P(E/B)-PEO_1 (30)

LUDOX®TMA (400)

200

244 (5)

0.06 (0.04)

254 (2)

0.08 (0.02)

23

P(E/B)-PEO_1 (30)

LUDOX®TMA (400)

100

236 (4)

0.06 (0.04)

238 (4)

0.09 (0.02)

24

P(E/B)-PEO_1 (30)

LUDOX®TMA (400)

-

-

-

-

-

24ac

P(E/B)-PEO_1 (60)

LUDOX®TMA (800)

-

256 (4)

0.05 (0.04)

245 (7)

0.06 (0.02)

25

P(E/B)-PEO_2 (30)

LUDOX®TMA (400)

600

304 (7)

0.21 (0.03)

332 (6)

0.14 (0.07)

26

P(E/B)-PEO_3 (30)

LUDOX®TMA (400)

600

330 (7)

0.18 (0.05)

354 (16)

0.13 (0.07)

27

Lubrizol®U (15)

LUDOX®TMA (400)

600

363 (7)

0.28 (0.05)

241 (5)

0.07 (0.02)

28

Lubrizol®U (30)

LUDOX®TMA (400)

600

242 (5)

0.17 (0.01)

223 (2)

0.06 (0.05)

29

Lubrizol®U (45)

LUDOX®TMA (400)

600

214 (1)

0.15 (0.03)

197 (3)

0.07 (0.03)

®

®

30

Lubrizol U (100)

LUDOX TMA (400)

600

216 (13)

0.47 (0.05)

263 (5)

0.35 (0.04)

31

lecithin (30)

LUDOX®TMA (400)

600

350 (5)

0.20 (0.04)

360 (8)

0.19 (0.02)

32

lecithin (60)

LUDOX®TMA (400)

600

227 (3)

0.18 (0.03)

228 (2)

0.19 (0.01)

33

lecithin (120)

LUDOX®TMA (400)

600

190 (3)

0.19 (0.02)

191 (3)

0.17 (0.01)

34

PGPR (50)

LUDOX®TMA (400)

600

604 (81)

0.18 (0.15)

1290 (253)

0.59 (0.41)

35

PGPR (80)

LUDOX®TMA (400)

600

270 (14)

0.26 (0.07)

424 (44)

0.22 (0.12)

36

PGPR (100)

LUDOX®TMA (400)

600

215 (6)

0.32 (0.03)

424 (4)

0.23 (0.20)

a

600 µL of PBS instead of 10 mg of NaCl dissolved in 600 µL of water; b 8.25 mL of ndodecane and 8.25 mL of n-tetradecane instead of 16.5 mL of IsoparM; c 20 mg of NaCl, 33 mL of IsoparM, prepared in a higher screw-cap jar; d measured by DLS, standard deviation in brackets.

With surfactant amounts below 7.5 mg, no stable emulsion can be obtained as determined by DLS (not shown). The employed hydrophilic silica particles and even those in combination with a small amount of P(E/B)-PEO_1 are obviously too hydrophilic to stabilize Pickering emulsions with the given compounds and can be characterized by very low contact angles. On the contrary, emulsions prepared with a combination of hydrophilic silica particles and 7.5 – 240 mg P(E/B)PEO_1 (runs 1 – 7) are stable both directly after preparation and several weeks later.


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Presumably, the adsorption of the polymeric surfactant to the silica surface, which goes along with a decrease in hydrophilicity and increase of contact angle of the particles is responsible for this finding (see last section). The droplet sizes are in the sub-micrometer range (between 200 and 450 nm) with very small standard deviations and outstanding PDIs (< 0.1). The TEM images confirm an excellent size distribution of particle aggregates (see Figure 1). As the diameters of the emulsion droplets determined via DLS are only slightly bigger than the diameters of the particle aggregates seen in the TEM images, it is quite likely that one particle aggregate contains the particles that contributed to the stabilization of one emulsion droplet. The hybrid surface layers of the droplets may have enough stability to avoid contraction with the evaporation of water. The diameter of the droplets can be fine-tuned via the employed amount of P(E/B)-PEO_1; it decreases with an increasing surfactant concentration (see Figure 2). As a smaller mean diameter of the emulsion droplets is accompanied by a larger interface if the volume fraction between oiland water phase is not changed, this observation indicates that not all silica particles are partially hydrophobized and contribute to the stabilization of the emulsion (see also Cryo-SEM images below in Figure 3). At least at lower surfactant concentrations, some particles must be rendered hydrophilic. This observation will be discussed in detail later.


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Figure 1. TEM images with different magnifications of the emulsions from run 3 (a and c) and run 4 (b and d).

Figure 2. Correlation between the employed amount of P(E/B)-PEO_1 and the average droplet diameter (blue) and the PDI (red) determined by DLS; some selected size distributions (inset). A further increase in the P(E/B)-PEO_1 amount to 500 and 800 mg, respectively (run 8 and 9), results in stable emulsions according to DLS. In the TEM images however no spherical particle


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aggregates can be seen any longer; instead some rod shaped or oval objects with very high electron density are observed, which could be attributed to the presence of NaCl or micelle formation by excess P(E/B)-PEO_1. The latter is supported by foam formation upon shaking the emulsions. Thus, there is a significant amount of P(E/B)-PEO_1 that is not adsorbed to the silica particles. Surface tension measurements of P(E/B)-PEO_1 in IsoparM reveal a cmc of ca. 0.26 mmol·L-1 (Supporting Information, Figure S1) and DLS measurements show that the average diameter of P(E/B)-PEO_1 micelles in IsoparM is about 55 nm. These findings are in accordance with the surface tension of the emulsions from run 3 (30 mg P(E/B)-PEO_1, below cmc, σ high) and 9 (800 mg P(E/B)-PEO_1, above cmc, σ low). To accommodate the emulsions for potential applications in biological environments a phosphate buffered system (PBS: 137 mM NaCl, 2.7 mM KCl, 10 mM Na2HPO4, 1.8 mM KH2PO4) was tested as aqueous phase (run 10). Both the analysis via DLS (see Table 1) and the TEM image show similar results as for run 3 with NaCl as sole osmotic agent confirming a stable system. Hence, sodium chloride can be replaced by PBS to produce stable emulsions with small droplets and a narrow size distribution. If the assumption made above that not all silica nanoparticles are (partially) hydrophobized is true, a special kind of morphology built by the silica particles, which participate in the stabilization of one emulsion droplet, is expected. The exchange of IsoparM by a mixture of ndodecane and n-tetradecane as the continuous phase led to stable emulsions (run 11, see Table 1) and allowed investigations of the (frozen) emulsions by Cryo-SEM. The corresponding images in Figure 3 reveal that the emulsion droplets stabilized by the partially hydrophobized silica nanoparticles are hollow objects with a shell composed of particles. On the other hand it becomes clear that not all silica particles are situated at the oil-water interface of the droplets;


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some particles stay inside the droplets and form a kind of three-dimensional network (Figure 3b). Presumably due to kinetic reasons the polymeric surfactant does not adsorb to all particles (see above). Hence, some nanoparticles are left mainly hydrophilic and stay in the aqueous phase. The formation of the three-dimensional network inside the droplets as a drying-artifact is rather unlikely as the freezing-process is very fast. Destabilization and cross-linking of the negatively charged particles (zeta potential of -20.5 mV at a pH 6.5) by the presence of NaCl (osmotic agent) can be excluded because of the overall stability of the (inverse) emulsions. The formation of the particle network could be caused by the high energy input during ultrasonication, which promotes the evaporation of water from the inner phase. This in turn is responsible for an increase in the electrolyte concentration in the aqueous phase, which can lead to the flocculation of the particles. Three-dimensional networks of particles inside the droplets cannot be observed in all stable emulsions described within this work (Supporting Information Figure S5), which indicates that they are not essential for the stabilization of the emulsions.

Figure 3. Cryo-SEM images with different magnifications of the emulsion from run 11.

Adsorption of P(E/B)-PEO_1 to silica particles.


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To further investigate the interaction between the polymeric surfactant and the hydrophilic silica particles and to verify the assumption that P(E/B)-PEO_1 adsorbs to the silica particles, thermogravimetric analysis and semi-quantitative ATR-IR spectroscopy were applied. On the one hand the adsorption of the surfactant to the particles in acetone was investigated (A1 – A5, compositions and results see Supporting Information, Table S1). IsoparM as dispersion medium for SiO2 could not be used because of its low polarity. On the other hand, some selected emulsions were centrifuged, the residue was dried and also analyzed via TGA and ATR-IR spectroscopy. To enable a comparison between the different samples with respect to the amount of surfactant, which was adsorbed to the surface of the silica particles, the “silica band” (650 cm1

– 1330 cm-1) as well as the band of the C-H stretching vibration (2800 cm-1 – 3000 cm-1) were

integrated and standardized to the value of the “silica band”. Figure S2 shows exemplarily the integrated spectrum of run 5. If the ATR-IR spectra of A1 – A5 are compared with each other, it can be found that the intensity of the C-H stretching band increases with an increase in the amount of surfactant used for the experiment, while the intensity of the “silica bands” stays mainly unchanged. The same is true for the intensity of the C-H-bending band at about 1460 cm1

(Supporting Information, Figure S3, Table S1). Hence, in the investigated range of

concentration the adsorption of P(E/B)-PEO_1 in acetone to the silica particles increases with an increase of the mass ratio between polymeric surfactant and silica that is applied initially. The samples prepared by centrifuging and drying of the emulsions from runs 1-3 and 5-9 (7.5 mg, 15 mg, 30 mg, 93.5 mg, 120 mg, 240 mg, 500 mg and 800 mg of P(E/B)-PEO_1) show the same trend. If the standardized values of the integrals of the C-H-stretching are plotted against the employed mass ratio between P(E/B)-PEO_1 and silica particles (Figure 4a) it becomes obvious that P(E/B)-PEO_1 to silica ratios higher than 3.4 (corresponding to a mass of the


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polymeric surfactant of 500 mg) do not lead to a significant further increase in the standardized integral and a plateau is visible in the diagram.

a)

b) content of organic material / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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30 25 20 15 10 5 0

in emulsion in acetone

0

1

2

3

4

5

6

mP(E/B)-PEO / mSilica

c)


17

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

adsorbed P(E/B)-PEO / applied P(E/B)-PEO / %

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80 70 60 50 40 30 20 10 0 0

2 4 mP(E/B)-PEO / mSilica

6

Figure 4. a) Plot of the standardized values of the integrals of the C-H-stretching against the mass ratio between P(E/B)-PEO_1 and silica particles used to prepare the respective emulsions. b) Adsorption isotherm of P(E/B)-PEO_1 to silica particles in emulsion determined by TGA. For comparison the value of A2 (see Table S1) is given. c) Same data as in b) but as percentage of adsorbed P(E/B)-PEO_1 with respect to employed amount.

Those results from the semi-quantitative analysis of the samples via ATR-IR spectroscopy are affirmed by quantitative thermogravimetric analysis (TGA) showing a clear plateau (Figure 4b). It is conspicuous that the percentage of P(E/B)-PEO_1, that is adsorbed to the particles is much smaller than the initially applied ratio between surfactant and silica might assume (Figure 4c). This means that especially in the region of higher masses of surfactant significant amounts of polymer stay in the organic phase, its cmc is exceeded and micelles of P(E/B)-PEO_1 are formed (see above). As the particles present at the interface can be assumed closely packed only ca. 10 % of the oil/water interface will not be covered by particles. Hence, we cannot exclude the presence of surfactant at the accessible oil/water interface but the corresponding amount is rather negligible. This assumption is further supported by the following semi-quantitative considerations. Taking into account that (i) the surface area of the silica particles is 130 m² g-1


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(determined by BET, see Table 2), (ii) polyethylene glycol derivatives adsorbed to colloidal silica from nonpolar solvents have a space requirement of 15 Å² per ethylene oxide group,49 (iii) P(E/B)-PEO_1 contains 68 ethylene oxide groups and (iv) presumably about 50 % of the particles’ surface is accessible for the polymeric surfactant - assuming that the contact angle is ca. 90° - it is possible to estimate roughly the surface coverage of P(E/B)-PEO_1 on the silica particles from the values obtained by TGA. It turns out that for run 1 and 2 (7.5 and 15 mg of polymer) sub-monolayer adsorption occurs, while 30 mg of P(E/B)-PEO_1 (run 3) result in monolayer adsorption. For run 4–9 (60–800 mg of polymer) the amount of adsorbed surfactant exceeds a monolayer corresponding to almost 5 layers for runs 8 and 9. As for these two emulsions no particle aggregates can be seen in TEM (see above) it is questionable if the assumptions above are fully applicable. For these samples it is more likely that almost all particles are totally hydrophobized. Presumably the diameters measured by DLS do not belong to droplets of a Pickering emulsion but to small water droplets stabilized by the surfactant alone as well as hydrophobic silica particles in oil and micelles of P(E/B)-PEO_1 (see above). Comparing by TGA the amount of adsorbed P(E/B)-PEO_1 from acetone or in emulsion, the latter one is much lower (13.6 % in acetone vs. 6.5 % in emulsion) if a comparable mass ratio between surfactant and particles is applied (in both cases 0.21). This can be explained by (i) the effect of the solvent,49 (ii) the limited accessibility of particles’ surface in emulsion at the oil/water interface or even completely inside the water droplets (see Figure 3). Assuming that thermodynamic and kinetic solvent effects are minor a ratio of adsorbed polymer of roughly 2:1 in acetone and emulsion, respectively, means that in emulsion the accessible silica surface is half as large as in the acetone dispersion corresponding to a contact angle close to 90°. Keeping in mind that a minor part of particles is still located inside the droplets indicates that even more


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than half of the particles’ surface area of the particles at the interface is wetted by IsoparM, i.e. the contact angle is larger than 90°, which would be in total accordance to the theory (see Introduction).

Influence of the adsorption of P(E/B)-PEO_1 to silica particles on the emulsion stability. To elucidate the role of the P(E/B)-PEO_1 with regard to emulsion stability, emulsions stabilized by the surfactant alone were prepared (run 12–17) and compared to emulsions containing silica particles and the same amount of P(E/B)-PEO_1 (run 1–5, see Table 1). The emulsions from run 12–14 are stable directly after preparation, which shows that the applied P(E/B)-PEO_1 with an HLB value of 8.3 is able to stabilize inverse emulsions sterically in accordance to the literature50 but they destabilize by forming a white precipitate within four weeks and DLS displays big aggregates. To exclude any effects of the slightly lower amount of water in the emulsions with particles due to the mass of the silica, samples with exactly the same amount of water (0.866 g) as in the emulsions without particles were prepared (run 15–17) delivering the same behavior as in run 12–14. The presence of particles leads to smaller droplet diameters and narrower size distributions (Figure 5a and c). Furthermore, the combination of silica particles and P(E/B)-PEO_1 exhibits a much higher long-term stability (see Figure 5b and d). These observations confirm the synergy of silica particles and surfactant with regard to the stabilization of the investigated emulsions. To gain further insight into the synergy between silica particles and polymeric surfactant, two more emulsions with the composition of run 3 and 17 were prepared and treated with ultrasonication pulses à 3 minutes several times. As mentioned above, after 3 minutes of ultrasonication not all of the initially employed P(E/B)-PEO_1 is adsorbed to the silica particles;


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on the other hand, some particles are still rendered hydrophilic. Therefore it can be assumed that further ultrasonication leads to an increase of partially hydrophobized particles and thus a decrease in droplet diameter. This assumption is verified by the experiment (Figure S4, Supporting Information). After a total ultrasonication time of 21 minutes, the droplet diameter does not decrease further indicating that the equilibrium is reached. TGA confirms that now almost all of the employed P(E/B)-PEO_1 is adsorbed to the particles. The emulsion stabilized only by the polymeric surfactant shows a corresponding behavior upon sonication time which means that in run 17 not all P(E/B)-PEO molecules contribute to the stabilization of the water droplets. Although the droplet diameter can be further decreased by the application of up to seven ultrasonication pulses, it does not approximate or even fall below the droplet diameter of the emulsion stabilized by both polymer and silica particles. This indicates that P(E/B)-PEO_1 does not desorb from the particles to adsorb to the oil-water interface and stabilize the emulsion droplets itself proving the synergy between silica particles and polymeric surfactant with respect to the stabilization of inverse emulsions.


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Figure 5. Surfactant amount vs. droplet diameter (a and b) and PDI (c and d), respectively, directly after preparation of the emulsion (a and c) and 4 weeks later (b and d). The legend in b is


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true for all diagrams. Especially at low surfactant concentrations the emulsions prepared with the combination of surfactant and particles turn out to be much more stable than the one with the same amount of surfactant but without silica particles (0.866 mL of water). The data from runs 12–14 are not illustrated for clarity.

Variation of size and charge of the silica particles. To check whether the in-situ hydrophobization can be also performed with other than the specific silica particles emulsions with silica particles of different size and charge were prepared. The aqueous phase contained - besides sodium chloride - 400 mg of LUDOX®TMA or LUDOX®CL-P, respectively, diluted with 600 µL of water. For the smaller particles LUDOX®HSA and LUDOX®CL, only 250 mg of the silica sol were applied and diluted with 750 µL of water (see Table 1). The different volumes guarantee that the required space of all particles at the interface for runs 3 and 18-20 is approximately the same. The characteristics of the employed silica dispersions are summarized in Table 2.

Table 2: Characteristics of the different silica dispersions. name

particle size / nm

content of SiO2 in dispersion / wt%

ζ-potential / mV

counterion

surface area (BET) / m2/g

diameter (DLS) / nm

LUDOX®TMA

22

36

-20.5 ± 4.0

sodium

130

20

®

12

31

-19.9 ± 5.4

sodium

255

13

®

LUDOX CL-P

22

34

38.3 ± 3.3

chloride

93

26

LUDOX®CL

12

28

24.0 ± 2.7

chloride

134

21

LUDOX HSA

Emulsions with smaller (run 18 and 20) and with positively charged silica particles (run 19 and 20) are certainly as stable as the reference emulsion (run 3). They all display droplet diameters


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below 400 nm and PDIs below 0.2. This global information is confirmed by TEM (Figure 6). Only run 20 (Figure 6d) seems to show massive objects instead of capsules and a rather broad size distribution. Nevertheless, Cryo-SEM confirms the formation of capsules (Supporting Information, Figure S5). Furthermore, this emulsion can be classified as stable because one month after preparation the droplet size and PDI did hardly change. These findings show that the applied surfactant is not only able to adsorb to negatively charged particles but it can also hydrophobize positively charged silica particles. This independence on charge can be attributed to the fact, that P(E/B)-PEO_1 is an uncharged surfactant and the main interactions with the surface of the particles are van der Waals interactions and hydrogen bonding. As the positively charged particles bear a coating with an aluminium compound, these results indicate that not only silica particles can be used for the presented approach but also nanoparticles made of other materials. In addition, the system is quite flexible towards changes in the size of the applied particles.


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Figure 6. TEM images of samples prepared from the emulsions with different kinds of silica particles (a: negatively charged silica particles, d = 22 nm; b: negatively charged silica particles, d = 12 nm; c: positively charged silica particles, d = 22 nm; d: positively charged silica particles d = 12 nm).

Variation of the volume of the dispersed phase. In the next step, the influence of the volume of the dispersed phase is studied. The aqueous phase was diluted with 600 µL (run 3), 400 µL (run 21), 200 µL (run 22), 100 µL (run 23) and 0 µL (run 24) of water, respectively. Without any additional water (run 24) no stable emulsion can be obtained but only a two-phase system. This is a matter of volume effect. When the amount of all compounds is doubled and a higher screw cap jar with the same diameter is used (run 24a), a stable emulsion is obtained. All other emulsions turned out to be stable and show emulsion droplets in the range of about 250 nm with small standard deviations as well as rather good PDIs (see Table 1). The good visual appearance of the emulsions resulting from runs 21 – 23 is confirmed by TEM (Supporting Information, Figure S6). The average diameter of the droplets decreases with a decrease in the volume of the aqueous phase expectedly if the amount of surfactant is kept constant and subsequently the ratio between P(E/B)-PEO_1 and dispersed phase increases. The finding that the emulsions are also flexible with respect to the volume of the dispersed phase and that this offers again a possibility to fine-tune the droplet size highlights another advantage of the investigated system.

Variation of the hydrophobizing agent.


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In the last series of experiments variable amounts of several different surfactants were applied. Two other poly(ethylene-co-butylene)-block-poly(ethylene oxide)s (P(E/B)-PEO_2, P(E/B)PEO_3) with larger PEO blocks and, thus, higher HLB values (10 and 12, respectively) than P(E/B)-PEO_1 (HLB 8.3) were investigated. The higher hydrophilicity of P(E/B)-PEO_2 und P(E/B)-PEO_3 does not reduce the stability of the emulsions, but the droplet sizes increase slightly with an increase of the molecular weight of the surfactant (see Table 1). This is attributed to the lower number of molecules for the same employed mass of surfactant. Nevertheless, all emulsions prepared with any of the three block copolymers are stable for at least 1 month. This behavior is remarkable as surfactants with an HLB value of 10 or 12 tend to stabilize direct emulsions rather than inverse ones. However this is not important for the system investigated here as both surfactants are obviously able to adsorb to the silica particles and render them sufficiently hydrophobic. If Lubrizol®U is used as surfactant quite stable emulsions can be obtained (run 27-30, Table 1). In contrast to P(E/B)-PEO, the hydrophilic part of the molecule is a pentamine which is most likely protonated and, thus, charged at a pH of about 5 of the silica dispersion. Lubrizol®U seems to be able to adsorb to the surface of the silica particles presumably because of electrostatic interaction. Although the diameters of the droplets are almost the same for runs 29 and 30 (45 and 100 mg of surfactant, respectively), the TEM images in Figure 7 show clearly that the size of the particle aggregates decreases with an increase of the surfactant amount.


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Figure 7. TEM images of the emulsions from run 28 (a) and 29 (b).

In a final step, the biocompatible surfactants lecithin and PGPR were used to prepare inverse emulsions with droplet diameters below 500 nm. In both cases, an amount of 15 mg of the surfactant is not sufficient to stabilize the Pickering emulsions. However, stable emulsions can be obtained by using at least 30 mg of lecithin or 80 mg of PGPR. The different amounts of hydrophobizing agent required to stabilize the emulsions might be caused by different molecular weights and structures of the surfactants. While for example P(E/B)-PEO and Lubrizol®U are linear molecules, lecithin and especially PGPR are branched. Another difference between PGPR and the other hydrophobizing agents is that it does not display a defined ratio between the hydrophilic and the hydrophobic part. This may result in a totally different morphology of an adsorbed PGPR molecule compared to the other surfactants and could explain the lower stabilizing efficiency. The finding that both PGPR and lecithin can be used to hydrophobize silica particles is rather important with respect to any possible application, where biocompatibility is required. Thus, the system is also flexible with respect to the choice of surfactant.


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Proposed mechanism for the formation of inverse emulsions stabilized by the in-situ synergistic interaction of an amphiphilic block copolymer surfactant and silica particles. In the following we propose a general scheme for the in-situ stabilization mechanism exemplarily for the surfactants P(E/B)-PEO. On the one hand, P(E/B)-PEO is able to stabilize inverse emulsions but the stability can be enhanced by the addition of silica particles. On the other hand, not all particles contribute to the stabilization of the emulsion at the interface but stay non-hydrophobized inside the droplets. From these findings we propose the following mechanism for the formation of the emulsion: As the particles are initially totally hydrophilic and reside in the aqueous dispersed phase it can be assumed that in the start-up period the emulsion droplets are stabilized solely by P(E/B)-PEO via steric stabilization (see Figure 8a). Presumably, the hydrophilic PEO blocks loom into the aqueous phase and interact with the hydrophilic silica particles via, e.g., van der Waals interactions and hydrogen bonding between silanol groups on the particles’ surface and the ether groups of PEO leading to the adsorption of the polymer (see Figure 8b). Those partially hydrophobized particles are in turn able to adsorb to the oil-water interface (see Figure 8c). As the surface of the particles is negatively charged, the emulsion is not only stable because of steric but also by electrostatic stabilization which could explain the enhanced stability of those emulsions in comparison with emulsions stabilized only by P(E/B)-PEO. This mechanism is in accordance with the observations that there are (i) “naked” silica particles inside the droplets without significant amounts of adsorbed surfactant and (ii) some free surfactant molecules in the oil phase (e.g., almost 20 mg in run 3). As soon as a layer of partially hydrophobized particles has formed at the oil-water interface, the remaining hydrophilic particles in the aqueous phase cannot reach the interface any more. Moreover, P(E/B)-PEO is virtually insoluble in water (distribution coefficient of P(E/B)-PEO_1 between


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IsoparM and water at rt: 4650) and thus the excess surfactant in the continuous phase is kinetically hindered and not able to diffuse into the emulsion droplets and interact with the residual hydrophilic silica particles. Consequently, the particle-surfactant-composites shield the free particles in the aqueous phase from the free surfactant in the oil phase.

Figure 8. Proposed mechanism for the formation of emulsions stabilized by in-situ hydrophobized silica particles. Initially, the polymeric surfactant P(E/B)-PEO stabilizes the emulsion (a), afterwards the PEO block adsorbs to the hydrophilic particles (b) and the particlesurfactant-composites adsorb to the interface and stabilize the droplets via both electrostatic repulsion and steric hindrance (c).

CONCLUSION Inverse Pickering emulsions of an outstanding stability with droplet diameters between 180 and 450 nm as well as excellent size distributions could be prepared by particle stabilized emulsions produced via miniemulsion technique. In the key step plain silica particles were hydrophobized in-situ by the adsorption of a surfactant. The system investigated herein turned out to be very flexible with respect to the size and charge of the silica particles, the osmotic agent, the various surfactants with different HLB-values, and the volume of the dispersed phase. Cryo-SEM images show that the silica particles do not only form the shell of the emulsion


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droplets but some particles are obviously rendered hydrophilic and form a three-dimensional network in the inner of the droplets. Structures like these could act as a stabilizing framework and might be used to prepare stable colloidosomes without any kind of locking the particles together. The high flexibility of the presented systems opens up the possibility for biomedical applications like capsule cell interactions for drug delivery or diagnostics.

ASSOCIATED CONTENT Supporting Information. Plot of surface tension vs. concentration of (P/E)-PEO_1 in IsoparM; composition and results of the integration of the ATR-IR spectra of A1-A5; ATR-IR spectra; plot of droplet size vs. emulsification time; Cryo-SEM image of an emulsion stabilized by LUDOX®CL silica dispersion and P(E/B)-PEO_1; TEM image of the emulsion from run 22. This material is available free of charge via the Internet at http://pubs.acs.org.” AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Present Addresses † Key Laboratory of Advanced Textile Materials and Manufacturing Technology and Engineering Research Center for Eco-Dyeing & Finishing of Textiles, Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, China Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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ACKNOWLEDGMENT The authors greatly thank G. Weber for the synthesis of P(E/B)-PEOs and Dr. C. HoffmannRichter for the thermogravimetric analyses. They also thank Grace Davison for the kind donation of the silica dispersions LUDOX®CL and LUDOX®CL-P. Prof. Dr. M. Lindén from the Institute of Inorganic Chemistry II, University of Ulm, is thanked for providing the ATR-IR spectrometer. Prof. Dr. P. Walther from the Central Facility for Electron Microscopy, University of Ulm, is greatly thanked for the preparation of the samples for Cryo-SEM, for his help with the CryoSEM measurements and for helpful discussion. Z. C. is grateful for financial support by the Open Foundation of State Key Laboratory of Chemical Engineering of Zhejiang University (SKLChE-15D02). REFERENCES

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Inverse Pickering Emulsions with Droplet Sizes below 500 nm

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