Creation of well-defined particle stabilized oil-in-water nanoemulsions

Colloids and Surfaces A: Physicochem. Eng. Aspects 459 (2014) 48–57

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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

Creation of well-defined particle stabilized oil-in-water nanoemulsions Karin H. Persson ∗ , Irena A. Blute, Isabel C. Mira, Jonas Gustafsson 1 SP Technical Research Institute of Sweden, Box 5607, SE-114 86 Stockholm, Sweden

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• Particle-stabilized o/w nanoemul• • •

•

sions with a stability of at least 3 years were produced. A commercially available silica sol was used to stabilize nanoemulsions. Pickering nanoemulsions were produced using a high shear, commercially available homogenizer. The effect of different types of oils, amount of oil, oil viscosity, particle/oil ratios and processing conditions on the droplet size was examined. The stability of the emulsion is discussed in terms of oil solubility, Ostwald ripening and particle packing density at the interface.

a r t i c l e

i n f o

Article history: Received 8 April 2014 Received in revised form 17 June 2014 Accepted 21 June 2014 Available online 2 July 2014 Keywords: Pickering emulsions Nanoemulsions Alkanes Squalene Silicon oil Nanoparticles

a b s t r a c t The preparation of oil-in-water (o/w) nanoemulsions stabilized with silica nanoparticle sols has been investigated. The emulsification was performed using a high shear homogenizer (Microfluidizer TM processor, Microfluidics, USA). The effect of different processing conditions on the droplet size distribution and stability was investigated in emulsions prepared using different types of oils, oil concentration and particle/oil ratios. It was the ability of the particles to attach to, and stabilize the newly created interface, rather than their ability to lower the interfacial tension, what proved important for the drop size of the resulting emulsions. Changes in drop size distribution with time, attributed to Ostwald ripening effects, were observed for the more soluble oils, while stable nanoemulsions with droplet size of ∼100–200 nm could be produced using a virtually water-insoluble oil such as squalene. © 2014 Elsevier B.V. All rights reserved.

1. Introduction

∗ Corresponding author. Tel.: +46 10516 6072; fax: +46 8 208998. E-mail address: [email protected] (K.H. Persson). 1 Present address: GE Healthcare, Life Science R&D, Björkgatan 30, SE-751 84, Uppsala, Sweden. http://dx.doi.org/10.1016/j.colsurfa.2014.06.034 0927-7757/© 2014 Elsevier B.V. All rights reserved.

The stabilization of emulsions with particles, to produce socalled Pickering emulsions, has been receiving substantial attention by the scientific community for the last decade. Early studies were focused on establishing the influence of the wettability of the particle by the two immiscible liquids on the type of emulsion formed. It was established that, in certain analogy to surfactants, particles

K.H. Persson et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 459 (2014) 48–57

preferentially wetted by the water phase generally form watercontinuous emulsions, while particles preferentially wetted by the oil phase (or non-polar solvent) form oil continuous emulsions [1,2]. Many types of particles, either inorganic or organic, fulfill the partial wetting condition for most common oils [3]. In contrast to most surfactant-stabilized emulsions the initial dispersion of the particles in either oil or water may influence the resulting emulsion type. For such particle/oil/water systems the phase in which particles are first dispersed becomes the continuous phase [4]. Inversion of emulsion type from w/o to either o/w or multiple w/o/w can also be affected by changing the initial location of the particle, by varying the oil/water ratio and by continuous agitation [5]. Adding more of the dispersed phase may lead to either phase inversion [4,6] or emulsion failure [7]. By using catastrophic inversion both water and oil continuous emulsions can be produced without altering the particle wettability [8]. Spontaneous emulsification leading to the formation of monodisperse emulsions, which were claimed to be thermodynamically-stable, has been reported for systems consisting of specific combinations of solid particles and methacrylate oils [9–11]. In general, however, emulsions are thermodynamically unstable and the method of preparation is expected to affect the properties of the resulting emulsions. Pickering emulsions are no exception. The emulsion droplet size depends on several factors such as solid/dispersed phase ratio, particle size and the energy of mixing. A systematic study using a homogenizer for the preparation of o/w Pickering emulsions showed that the oil-to-silica particle ratio had a significant effect on the emulsion droplet size and stability [7]. At low particle/oil ratio the droplets are unstable. For emulsions stabilized by fumed silica particles this has been described in terms of a combination of flocculation and permeation of the oil, an aromatic hydrocarbon, across the particle layers [12]. On the other hand, the use of higher particle/oil ratios leads to emulsions which are very stable against coalescence and where the resulting droplet size is controlled by the particle/oil ratio. Under such conditions and for a fixed particle/oil ratios, the droplet size can be tuned by the emulsification process [7]. The mean droplet size of Pickering emulsions reported in the literature can extend up to several mm for emulsions produced by a low energy method such as shaking by hand [2,13]. More typically the drops are in the micron range, and can extend down to ∼0.6 ␮m when the emulsification is done using homogenizers [8]. High shear fluid processors such as the Microfluidizer are often used to produce nanoemulsions stabilized by proteins and surfactants. A nanoemulsion is defined as an emulsion with droplets with diameters within the 20–200 nm range. Nanoemulsions have several advantages such as the Brownian motion being sufficient to overcome gravity [14]. However, nanoemulsions are particularly prone to growth in particle size due to Ostwald ripening, where the larger droplets grow at the expense of smaller droplets because of the molecular diffusion of oil between the droplets. This process is driven by the difference in Laplace pressure between droplets of different size. The Ostwald ripening can be reduced by using oils with low solubility in the continuous phase, adding a poorly soluble oil to a more soluble one [15] as well as reducing the droplet size distribution. Nanoemulsions have various applications in the industrial field, such as personal care and cosmetics as well as health care, food and agrochemicals. Very limited numbers of submicron sized particle-stabilized emulsions have been mentioned in the open literature [16–18], where the pharmaceutic and cosmetic sectors are the fields within which most attention is being paid to nanoemulsions [19]. In this paper we explore the possibility of producing particlestabilized emulsions by means of a Microfluidizer and using a commercially available silica sol with a mean particle size of ∼7 nm. The effect of different processing conditions on the droplet size

49

distribution and stability was investigated for emulsions prepared using different types of oil, oil concentration and particle/oil ratio. Further insight into relevant surface chemical phenomena was sought by determining the interfacial tension between the aqueous and the oil phases used. Changes in the droplet size distribution over time (mainly attributed to Ostwald ripening) were followed as a means to assess the stability of the produced nanoemulsions

2. Experimental 2.1. Materials Commercial silica colloidal dispersions from Eka Chemicals AB (AkzoNobel, Sweden) were used as supplied. The hydrosols were kindly provided by the manufactures as commercial samples. Several of the commercially available sols from this supplier have previously been tested in foamability studies [20]. Out of the different dispersions tested, Bindzil CC30 (CC30) proved to be the most efficient due to its reduced hydrophilicity. The preparation of aqueous silane modified silica sols has been previously described elsewhere [21]. As CC30 was discontinued during the course of this work and replaced by Bindzil CC301 (CC301), both sols have been investigated. A SiO2 -sol is normally considered to have 8 ␮mol SiOH/m2 , of which CC301 has approx. 2.3 ␮mol glycerolpropylsilyl/m2 . Table 1 describes some properties of the two hydrosols. No significant differences between the two sols were observed in terms of their emulsification ability, see Tables 2 and 4. Some of the physical properties of the test oils included in this study are specified in Table 3. High purity water (resistivity of 18.2 m cm) was used, obtained by a MilliQ PLUS unit (Millipore Corp., Bedford, MA). In order to elucidate the effect of contaminants in hexadecane on the o/w interfacial tension ( ow ), the oil was purified with water to remove water soluble surface active substances. Extraction with

Table 1 Description of the silica sols. Information from the supplier unless otherwise stated. Silica sols

Bindzil CC30

Bindzil CC301

Particle diameter from supplier [nm] Z average particle size [nm] Glycerolpropylsilane Silane per surface area [/nm2 ] Concentration Alcohol type and content pHa Point of zero charge at 10 wt% solids in 0.01 M NaCl Anions

7

7

a

9.1 Methoxy 1.4

Ethoxy 1.4

30% (w/w) 1.8–1.9% methanol

30% (w/w) 2.5–2.6% ethanol

7.5 <1.5 [20]

7.1

50–100 ppm sulfates and 300–400 ppm chlorides

A sodium free sol of CC30 would have a pH of approximately 2.5.

Table 2 Comparison of droplet size for a 10% decalin-in-water emulsion after emulsification of 20 ml emulsion with Ultra-Turrax. Emulsification time 3 min, cooling with ice, dispersion tool S25 N-18G. Silica sol

10% smallest volume percentile d (0.1) [␮m]

Median particle size by volume d (0.5) [␮m]

90% smallest volume percentile d (0.9) [␮m]

CC30 (9%) CC301 (9%)

1.5 1.3

2.2 1.9

3.4 2.8

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Table 3 Quality, manufacturer and viscosity of the oil used. Oil phase

Quality

Supplier

Viscosity at 25 ◦ C[mPa s]

Density at 22 ◦ C [g/ml]

Isooctane (2,2,4-trimethylpentane) Decane Dodecane Tetradecane Hexadecane Decahydronaphtalene (decalin) Squalene Silicone oil PH-1555

p.a. p.s. 99%, min. 99% min. 99% 99% p.s.

Merck Merck Schuchardt Alfa Aesar Sigma Sigma Merck Merck Dow corning

0.50 (20 ◦ C)a 0.8 [22] 1.36 [22] 2.0 [22] 3.1 [15] 2.99a 17.6 (3.0) 175a

0.69 0.730 0.749

a

0.773 0.833–0.879 0.855 1.124

From supplier.

water was repeated, until the surface tension of the water phase was the same as for water (72.5 mN/m). 2.2. Emulsification Before microfluidization the systems were all preemulsified. To prepare the pre-emulsion the original silica sol was diluted with MillieQ water to obtain the desired particle concentration (both particle and oil concentration will be consistently reported in %, v/v). The required volume of oil was then added. Typically a 200 ml batch of emulsion was produced in this way. The pre-emulsion was prepared using an Ultra-Turrax homogenizer equipped with a S25N-5F dispersing tool, the RPM was increased from 8000 to 20 000 rpm for 3.5 min and the emulsion was cooled with ice during the emulsification. The emulsion was kept on magnetic stirring for no more than 2 h prior to microfluidization. The pre-emulsions were further processed using a M-110Y Microfluidizer processor (MF) (Microfluidics, USA). Once inside the channels of the of the MF chambers, the emulsion is exposed to consistent and intense impact and shear forces and is then immediately cooled by an open coiled-type cooling jacket placed in line just after the interaction chamber, see Fig. 1. Unless otherwise specified, microfluidization was performed at 600 bar and using a F2OY 75 ␮m interaction chamber (Y type) in combination with a H30 Z 200 ␮m auxiliary chamber (Z type) placed inline (Fig. 2). 2.3. Particle size distribution determination The drop size distribution (DSD) of the emulsions was determined by means of laser diffraction (Mastersizer 2000, Malvern Instruments, UK). The results were verified by dynamic light scattering (Nanosizer, Malvern Instrument, UK) for some emulsions with smaller droplet size. If separation (creaming) took place during

the storage time of the emulsions, the emulsions were re-dispersed by gentle turning the sample vials. The samples were diluted with MilliQ water (RI = 1.33) to an obscuration between 10 and 20%. The refractive index used for the dispersed phase (oil and silica), 1.9, was obtained by a fitting procedure to minimize the weighted residual. The weighted residual was less than 4% for all measurements. The results shown are an average of three measurements. 2.4. Cryo-TEM imaging Part of the electron microscopy investigations were performed with a Zeiss 902 A instrument, operating at 80 kV. A thin film of the sample solution was prepared by a blotting procedure, performed in a chamber with controlled temperature and humidity. A drop of the solution was placed onto an EM-grid coated with a perforated polymer film [23]. Excess solution was removed with a filter paper, leaving a film of the solution to span the holes of the EM-grid. Vitrification of the thin film was achieved by plunging the grid into liquid ethane held at its freezing point. The vitrified specimens were then transferred to the microscope and investigated in transmission at about 100 K. Further electron microscopy analysis, courtesy of Vironova AB, was performed either using negative stain (nsTEM) or cryoelectron microscopy (cryoEM). In nsTEM analysis, the specimens were embedded in a thin film of amorphous uranyl acetate by a series of blotting steps. EM images were acquired using a FEI Tecnai 10 electron microscope operated at 100 keV. CryoEM grids were prepared using a Vitrobot under temperature and humidity controlled conditions, and treated as described above. The vitrified specimen was then transferred and imaged under cryo conditions using a JEOL JEM-2100F field emission electron microscope operated at 200 keV. 2.5. Interfacial and surface tension The interfacial tension measurements of oils against water were determined with the du Noüy balance (Sigma 70, KSV Instruments

Fig. 1. Schematic representation of the MicrofluidizerTM processor. Used with permission. Copyright Microfluidics 2013.

Fig. 2. The schematic construction of the Y and Z chambers respectively. Used with permission. Copyright Microfluidics 2013.

K.H. Persson et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 459 (2014) 48–57

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Table 4 Droplet size and method of emulsification decalin and squalene emulsions. The samples were let to rest for 60 min before sizing took place. Processing

Sol and oil

10% smallest volume percentile d (0.1) [␮m]

Median particle size by volume d (0.5) [␮m]

90% smallest volume percentile d (0.9) [␮m]

Ultra- Turrax, 5 min MF, 13 min MF, 30 min MF, 60 min MF, 15 min MF, 5 min

8.3% CC301/20% decalin 3.8% CC301/10% decalin 3.8% CC301/10% decalin 3.8% CC301/10% decalin 3.8% CC30/10% decalin 3.8% CC30/10% squalene

1.2 0.2 0.5 0.5 0.4 0.08

1.7 0.8 0.8 0.7 0.6 0.13

2.5 4.0 2.8 1.1 0.8 0.22

Ltd., Finland) using a platinum/iridium ring. The interfacial tension oil (solvent)/water was measured by using two different methods. In the first method, analogous to surface tension measurements of liquid in air, the ring is first immersed in the oil (light phase). The second method used, is useful when the du Noüy ring can be contaminated by impurities in the oil phase. In this method the ring is first immersed in water (heavy phase) with a starting depth of 4–5 mm. Then the light phase (oil or solvent) is carefully poured on to the surface of the heavy phase and the ring is lifted as close as possible, but not in contact with, the phase boundary. The interfacial tensions were measured 30–90 s after equilibration of the liquid/liquid interface. The surface tension of MilliQ water was 71.8–72.7 mN/m. The ring was cleaned between the measurements in bichromate sulfuric acid, rinsed with MilliQ water, 99.5% ethanol and heated in gas burner flame. The measurements were carried out at ambient temperature (21–23 ◦ C).

Fig. 3. Effect of number of passes in the MF for 11.3% CC301 and 10% purified hexadecane.

2.6. Rheological properties The viscosity of the oils at ambient temperature was determined using a rotational rheometer (Kinexus, Malvern instruments) operated under controlled shear rate mode and using a cup and bob geometry. The values reported are the mean values of viscosities in the shear rate interval 0.1–100 s−1 . 3. Results 3.1. Effect of emulsification time and emulsification protocol on DSD 3.1.1. Decalin and squalene systems Using an Ultra-Turrax homogenizer to emulsify a 8.3% CC301/20% decalin system, an emulsion with a median droplet size of a 1.7 ␮m was obtained. After 13 min processing in the MF at 600 bar the median droplet size was reduced to 0.8 ␮m. Continued processing in the MF for up to an hour reduced the width of the size distribution, see Table 4. Replacing decalin by squalene, an unsaturated hydrocarbon well-known by the pharmaceutical community for its ability to facilitate formation of small drops [24], resulted in an emulsion with even smaller median drop size. The 8.3% CC30/10% squalene emulsion exhibited a monomodal narrow DSD with a median droplet size of 0.13 ␮m. 3.1.2. Hexadecane systems Emulsions containing purified hexadecane were prepared with both 3.8 and 11.3% CC301. Increasing the number of passes through the MF chambers reduced the median droplet size in both emulsions. When using 11.3% particles, emulsions with a bimodal distribution were obtained where the fraction of droplets with smaller size increased at the expense of the larger ones as the emulsion was passed through the MF, see Fig. 3. For the 3.8% particle system a similar observation was made although the bimodal distribution was less distinct, see Fig. 4. No significant further reductions in the median size or changes in mono/bimodality

Fig. 4. Effect of number of passes in the MF for 3.8% CC301 and 10% purified hexadecane at 600 bar.

occurred after 3 passes nor after continuous processing for 10 min (see 6 passes + 10 min in Fig. 4). Interestingly enought, removing the interaction chamber and thus using only the auxillary chamber in continuous mode for 5 min, for 1.3% CC301 and 10% hexadecane, also resulted in an emulsion with a monomodal DSD with a DSD similar to those observed using the interaction chamber (volume mean D [4,3] = 118 nm) (data not shown). Continued MF processing did not alter the droplet size (not shown). 3.1.3. Silicone oil systems Pre-emulsions (emulsified with an Ultra-Turrax) of systems containing 10% or 5% silicone oil and two different particle concentrations (3.8 and 11.3% CC301) exhibited similar median drop size distributions. When processed in the MF, increasing the processing reduces the average droplet size. However, for this high viscosity oil, it seems that pressures higher than 600 bar would be required to obtain droplet size distributions as narrow as the ones attained for lower viscosity oils (see previous sections), see Figs. 5 and 6. Continued microfluidization could possibly have narrowed the size distribution even more. It is worth noting that this emulsion showed no sign of coalescence after 1.5 months. 3.2. Effect of oil type on the drop size and stability The influence of the type of oil on the drop size and stability of the emulsions was investigated by preparing a series of o/w emulsions containing 3.8% of CC30 particles and 10% of different alkanes (iso-octane (C8), decane (C10), dodecane (C12), tetradecane (C14) and hexadecane (C16)). The DSD of these emulsions after different aging times is shown in Fig. 7.

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Fig. 5. Effect of processing conditions for 3.8% CC301 and 10% silicone oil as a function of increasing pressure and increasing number of passes in the MF.

Fig. 8. Surface tension at natural pH for CC30 (empty squares, pH = 7.14) and CC301 (solid diamonds, pH = 7.49) at 24 ◦ C.

Fig. 6. Effect of processing conditions for 11% CC301 and 5% silicone oil as a function of pressure and increasing number of passes in the MF.

Except for emulsions prepared with the iso-octane all emulsions have very similar or even identical DSD directly after emulsification. A small population of large drops (ca. 1 ␮m) was present in some of these emulsions. However, this fraction of large drops could not be consistently found during subsequent measurements, which suggests that the appearance of this fraction in the DSD has something to do with the sampling procedure. Immediately after emulsification, the emulsion containing iso-octane, was monodisperse but with a significantly larger mean drop size (ca. 350 nm vs. ca. 120 nm). This is likely due to the extremely rapid Ostwald ripening observed for octane emulsions [15].

During the course of 2 weeks, the emulsions experienced coarsening at different rates depending on the chain length of the oil. Coarsening occurred very rapidly for the iso-octane emulsion where already 1 day after preparation the DSD had shifted to much larger drop sizes (from ca. 350 nm to ca. 8 ␮m). Out of all the linear alkanes tested, only the one prepared with hexadecane remained unchanged after two weeks. After 1.5 months also this emulsion showed signs of Ostwald ripening as the volume mean size had grown from 130 to 164 nm. 3.3. Interfacial/surface tension studies 3.3.1. Surface tension of silica sols The surface tension at natural pH is high and similar for CC30 and CC301 respectively, see Fig. 8. These sols contain methanol (CC30) and ethanol (CC301), which both can reduce surface tension [25].

Fig. 7. DSD of o/w emulsions containing 10% alkane and 3.8% CC30 after different aging times. From top to bottom: freshly prepared; 30 min old, 1 day old and 2 weeks old.

K.H. Persson et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 459 (2014) 48–57 Table 5 Oil/water interfacial tension at 21–23 ◦ C. Oil

Interfacial tension [mN/m]

Decalin Decane Dodecane Tetradecane Hexadecane Hexadecane, purified Squalene Silicone oil PH-1555

48.6 51.6 47.3 51.8 36.7 39.0 18.9 26.8

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and 1% (v/v) CC30 and 1% (v/v) squalene, 20 mM CaCl was added and the pH was adjusted to 3, 7 and 10. All the emulsions were stable and showed similar DSD for least 2 weeks at ambient temperature.

4. Discussion 4.1. The role of interfacial tension

Table 6 Surface tension of squalene and CC30 stabilized emulsions at 21–23 ◦ C. System

Surface tension [mN/m]

Specific surface area [m2 /g]

Squalene 1.3% CC30/10% squalene emulsion 1.9% CC30/10% squalene emulsion 3.8% CC30/10% squalene emulsion

31 41.5 43 60

26.2 43.6 52.5

A previous study of the surface tension of supernatants of CC30 obtained by ultracentrifugation for between 1 and 20 h, showed no systematic influence of the interfacial tension on the particle concentration in the supernatant. For a concentration range of 1.4–16%, w/w the surface tension was in the range 65.9–67.2 mN/m [26]. On the contrary, the interfacial tension of CC30 sols diluted with MilliQ water to the same solid range is 70–72 mN/m. The 5–6 mN/m surface tension reduction observed for the supernatant is expected also for 2% methanol in water [27]. It is also similar to the observed reduction of the undiluted CC30 sol. The discrepancy between the findings for supernatants and diluted sols is thus due to the methanol content which is constant in the supernatants but reduced for the diluted sols. 3.3.2. Interfacial tension at the oil–water interface The oils, used as supplied, were investigated with respect to their interfacial tension with water. The results of these analysis (summarized in Table 5) were reproducible and in very good agreement with literature data for alkanes and decalin [28–31]. The only exception is the hexadecane/water system (36.7 mN/m) which has a reference value of 55.2 mN/m [29], which indicates the presence of surface active impurities in the oil used in this study. The interfacial tension of purified hexadecane against water was 39.0 mN/m, which is still substantially lower than literature data. This demonstrates that the oil contains surface active compounds that are not water soluble at ambient conditions. Generally, the interfacial tensions obtained using Method 2 for interfacial tension determination (see experimental section), were in good agreement with the values obtained by Method 1. A significant difference was observed for decalin (50%) and squalene (11.8%), likely due to adsorption of impurities for Method 1. Thus, results for Method 2 are shown for these two oils. The surface tension of the air (vapor)/liquid interface of squalene-in-water emulsions show that it increases with increasing specific surface area of emulsion droplets, see Table 6. This could be a manifestation of water soluble surface active impurities from the squalene preferentially adsorbing to the emulsion droplet interface rather than at the air/liquid interface. 3.4. pH studies The pH of 10% oil emulsions in MillieQ water was in the range 6.9–7.9 for all the oils used in this study. Checks were carried out in order to investigate the influence of pH on the stability on a squalene/water emulsion. An emulsion was diluted to 1.3% CC30

The droplet size of a homogenized emulsion is determined by the balance between two opposing processes: droplet break-up and re-coalescence [32]. According to a prediction by Taylor, valid for low oil volume fraction and negligible continuous phase viscosity (C ), the droplet radius is a function of the oil/aqueous interfacial ˙ as given by the expression: tension ( ow ) and the shear rate () R∼ow /(C ∗ ). ˙ Surfactants are well known to reduce the interfacial tension and their adsorption kinetics influences their ability to prevent droplet re-coalescence [32]. It has been shown that for a high pressure homogenizer, the droplet size of a surfactant stabilized emulsion depends mostly on the ability of the surfactants to stabilize the drops against coalescence rather than their ability to reduce the interfacial tension [33]. During the MF production of the Pickering emulsions presented in this study, the smallest droplet attained was similar for emulsions containing silicone, C10–C16 alkanes and squalene oils. This is, large micron sized droplets of the pre-emulsions are sheared to droplets with DSD centered around 100–200 nm. It should be noted that these oils have interfacial tensions against water in the range of 19–55 mN/m. Concentrated sols reduce the  ow with up to 2 mN/m for squalene and 10 mN/m for hexadecane (not shown). The extent of this reduction is of course expected to be less when more diluted silica sols are used. Taking into account the contribution of the sols, the range of interfacial tension of the oil against the aqueous phase is 17–45 mN/m. Our results show that, similarly to surfactant stabilized emulsions processed in the MF [34], the  ow has limited effect on the minimum droplet size of particle stabilized oil-in-water emulsions prepared with the MF.

4.2. The influence of pH on emulsification and emulsion stability Adjusting pH after emulsification for a squalene emulsion showed that the stability was not altered for the investigated time, 2 weeks. Silica sols are known to be sensitive to the presence of electrolytes as well as pH. However, the used type of silylated silica sols exhibits good stability toward aggregation by salts. The reason for the stability increase is probably a combination of steric stabilization of the sol and a reduced number of reactive groups [21]. The present work has not investigated the effect of pH during the emulsification. However, the Zpotential of CC30 at 10 wt% solids in 0.01 M NaCl is −16 to −26 mV for the pH range used in our work (6.9–7.9) [20]. It has been shown that the Z potential of a 0.001% hexadecane emulsion decreased from ∼0 just below pH 3 to ∼100 at pH 7 in a linear fashion [35]. Thus, for the hexadecane emulsions there should be an electrostatic repulsion between the particles and the oil interphase. The same should be observed for other alkanes, as that the z potential of n-alkane droplets, from n-nonane to nhexadecane, depends only slightly on the length of the hydrocarbon chain [36]. In the pH range 0–5 the Z potential of the CC30 sol was between 0 and −5 mV, while above pH 8 it decrease even further. It would be particularly interesting to investigate the properties of hexadecane emulsions emulsified at pH below 3 where there should be no electrostatic repulsion between the particles and the oil/aqueous interphase, but that is outside the scope of the present work.

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Table 7 Theoretical surface coverage ratio for 10% oil-in-water emulsions stabilized with different amounts of silica. Dispersed phase

Particles

Surface area [m2 /g]

Volume weighted mean D [4,3] [nm]

Surface weighed mean D [3,2] [nm]

Preparation

Surface coverage ratio

Hexadecane

3.8% CC301

51.3

172 bimodal

117

1.58

Hexadecane

3.8% CC301

57.9

118

104

Hexadecane Squalene Squalene Squalene Oil Oil

1.3% CC301 3.8% CC30 1.3% CC30 0.8% CC30 2% 10%

20.3 55.3 26.2 13.0 13 67

470 bimodal 121 264 832 bimodal 450 45

295 108 229 462

6 passes + 10 min MF at 600 bara 30 min MF at 600 bar in H30 Z 200 ␮m auxiliary chamberb 10 pass 600 bar 15 min MF at 600 bar 15 min MF at 600 bar 30 min at 600 barc Theoretical Theoretical

a b c

1.40

1.39 1.46 1.08 1.25 1 1

No DSD reduction after 4 passes. No DSD reduction after 5 min. No DSD reduction after 4 min.

4.3. The influence of particle concentration on emulsification A narrowing of the size distribution was observed when increasing the number of passes in the MF from 1 to 2 at 600 bar for the 11% CC301/10% hexadecane emulsion (see Fig. 3). This is analogous to the observations that have been reported for surfactant stabilized emulsion produced with 10% silicone oil (50 mPa s) in water with 3% Tween 20 at 150 MPa [34]. In agreement with results reported for that system, we also observe that the height of the “shoulder” of larger droplets in the DSD curve is reduced as the number of passes is increased. The MF produces turbulent break-up in the chamber followed by elongational flow at the exit [34]. It has been shown that the elongational flow allows sufficient time for the newly formed interface to be stabilized by emulsifier adsorption for 10% oil using 3% emulsifiers [34]. For particle stabilized emulsions the adsorption kinetics of the nanoparticles onto the oil/water interface will depend on the diffusion length to the newly created surface. A higher particle concentration favors faster adsorption kinetics, which has been manifested in our work. Comparison of the results in Figs. 3 and 4, show that when increasing the particle/oil ratio, the number of passes required to reduce the amount of large emulsion droplets is reduced. This indicates that the adsorption process of the particles and subsequent stabilization of the oil/aqueous interface is more efficient when using higher particle concentrations for a given oil content. Analogous to surfactant stabilized emulsions, if there is an excess of particles the rate of droplet coalescence during the MF processing is negligible, and the droplet size of the emulsion

is predominately determined by the rate of droplet breakup. However, if the particle/oil ratios used are too low, the resulting emulsions are characterized by bimodal size distributions which remain unchanged upon increasing MF processing time. A couple of examples are the systems containing 1.3% CC301/10% hexadecane and 0.8% CC30/10% squalene (see Table 7). Calculations of the theoretical surface coverage using the surface weighed mean droplet size of the emulsions [2], assuming all particles are adsorbed at the droplet interface, are shown in Table 7. For a hexagonally-closed packed monodisperse particle, the surface coverage ratio would be 0.9 [37]. Thus, for all systems investigated here, there is an excess of particles in the system. This finding is also supported by cryo-TEM images of squalene-in-water emulsions containing 1.3% particles (see Figs. 9 and 10). The images also demonstrate that the droplets are stabilized by densely packed particles. Unlike the case for surfactant stabilized emulsions, particles of sufficient size adsorbed at the oil/aqueous interface are expected to be irreversibly adsorbed at the interface. For a spherical particle with a radius of rp with a contact angle  with the oil/aqueous interface with interfacial tension  ow , the energy of adsorption Eads is [4]: Eads = rp2 ow {1 ± cos }

2

(1)  > 90◦

and positive where the sign in the brackets is negative for for  < 90◦ . For CC30 the degree of modification of the Si OH groups has been determined to ∼25% [38] which in turn provides a contact angle of 40◦ [39]. The results for CC301 are expected to be similar. The larger particles in the size distribution of the silica sols used in

Fig. 9. Cryo-TEM images of a 1.5-month-old squalene in water emulsions containing 1.3% of particles. The scale bars are 200 nm.

K.H. Persson et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 459 (2014) 48–57

55

Fig. 10. Images of a 3-year-old squalene-in-water emulsions containing 1.3% of particles. Left, ns-TEM and right cryo-TEM. The scale bars are 200 and 500 nm, respectively. The ns- and cryo-TEM imaging of the emulsion samples was a courtesy of Vironova AB.

this study will have Eads of an order of magnitude larger than kT, while for the smaller ones the Eads is similar to kT. Here it is useful to consider results from foaming studies that have been reported for CC30 supernatants at different particle concentrations. There was a steep decrease in foam stability when only the smallest 25–30% (w/w) particles were left in the supernatant [26]. This is similar to the concentration present in the aqueous phase of our emulsions. It is plausible that the smallest particles are too small to be irreversibly attached to the oil/aqueous and aqueous/air interface and are thus not able to stabilize neither foams nor emulsions. 4.4. The effect of oil viscosity on the DSD For turbulent shear the droplet disruption is most efficient when the viscosity ratio of the dispersed and the continuous phase, D /C is between 0.1 and 5 [40]. Both the silicone and squalene exceed this ratio, with ratios of 175 and 17.6 at 25 ◦ C, respectively. In the MF, the liquid passes a cooling coil in line with the MF chamber (Fig. 1), so the temperature of the liquid should not increase much during the very fast pass through the chamber. However, the exact temperature of the oil and thus D /C in the MF is not known. This will depend on the MF processing protocol and the viscosity of the oil used, which in turn may vary during the emulsification process. Furthermore, batchwise MF processing will provide more time for cooling of the chamber and emulsion than continuous MF. For the silicone, the oil with the highest viscosity, continuous MF was needed to reduce the droplet size (see Figs. 5 and 6). Droplet size has previously been shown to be independent of the viscosity ratio (0.1–80) when using a MF M-110S for surfactant stabilized 10% silicone emulsions [34]. The viscosity ratio was varied from 0.1 to 100. This was done by changing the aqueous phase viscosity through addition of up to 50% glycerol and by selecting silicone oils with viscosities of 10, 30, 50, 75 and 100 mPa s. The droplet size obtained in the MF was reported to be independent of the viscosity ratio investigated, after both 1 and 5 passes. The device creates sufficient shear to break all droplets within this viscosity range after the first pass. Contrary to this finding, for the high viscosity silicone oil used in the present work, with a viscosity ratio of 175, some heat generation may be needed to obtain maximum shear and droplet elongation and minimum droplet size.

4.5. The effect of the amount of oil on the emulsification Reducing the amount of oil from 10 to 5% for the silicone oil while keeping the silica content at 10%, showed that emulsification can be as efficient as for 10% hexadecane with the same particle concentration. Increasing the oil phase fraction increases the coalescence rate, because the increase in collision frequency [33]. However, we propose that for the silicone oil the viscosity is too high for creating efficient shear for higher amounts of oil than 5%. Although not the subject of this study, the same observation was made with rapeseed oil with a viscosity of 72 mPa s (data not shown). 4.6. Emulsion stability The extent to which the emulsions prepared with different alkanes became coarser with time is attributed to Ostwald ripening, which in o/w emulsions is known to occur to a greater extent the higher the solubility of the oil in the aqueous phase [15,41,42] Ostwald ripening is a process whereby the oil from the droplets dissolve in the continuous aqueous phase followed by diffusion and re-dissolution into an emulsion droplet. For hydrocarbons the molar solubility ratio of oil in water x at 25 ◦ C decreases with alkane chain length according to [43]: ln x = −24.63 +

34.61 N

(2)

where N is the number of carbon atoms in the alkane. The stability of the hexadecane emulsions may however not only be attributed only to the low solubility of hexadecane in water but also to the presence of small amounts of surface active impurities as indicated by interfacial tension data. For purified hexadecane the initial droplet size was similar to the hexadecane used as received, and there were no sign of aging for at least 2 weeks. However, after 1.5 months larger droplets appear and the main peak in the DSD is shifted. The coarsening is thus not affected by the presence of water soluble surface active impurities. Literature values of oil in water for some of the oils used in this study are listed in Table 8. It is known that squalene is very water insoluble, and has therefore been used in an interesting study of compositional ripening of Pickering stabilized emulsions [44]. Thus, the extrapolated solubility data shown in Table 8 is likely too high.

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Table 8 Solubilities at 25 ◦ C. Numbers without references are calculated from reference values. Oil Cis-decalin Octane Decane Dodecane Tetradecane Hexadecane Squalene a b

Mole fraction −7

∼1 × 10 [45,46] 1.5 × 10−9 a 6.4 × 10−10 a 5.0 × 10−9 [42] 3.6 × 10−10 a 3.6 × 10−10 [42] 2.4 × 10−10 a 4.22 × 10−11 [42] 1.7 × 10−10 a ∼8 × 10−13 [42] 1.2 × 10−11 [43] b

[kg/m3 ] ∼7.7 × 10−4 5 × 10−6 a 3.96 × 10−5 [42] 3.4 × 10−6 a 3.40 × 10−6 [42] 3.86 × 10−7 [42] 2.2 × 10−6a ∼1 × 10−8 [42] ∼1.2 × 10−8 [43] b

Extrapolated from Eq. (2). Extrapolated from high temp, high pressure.

The squalene emulsions stabilized by CC30 particles proved to be very stable, showing limited, if any, droplet size increase 3 years after preparation (Figs. 9 and 10). This clearly demonstrates that Ostwald ripening was effectively prevented, as was coalescence. 5. Conclusion Oil-in-water nanoemulsions have been produced using modified silica particles with a mean diameter of ∼7 nm. The main finding was that MF processing produces droplets of similar size as those stabilized by surfactants [34], and that the interfacial tension has limited effect on the size of the droplets created and stabilized. The maximum stabilized surface area depended on the particle/oil ratio. However, there were always some excess particles, which were explained by the limited energy of adsorption to the oil/aqueous interface of the particles at the lower end of the particle size distribution. By increasing the particle concentrations above what is needed to stabilize the maximum surface area created by the Microfluidizer, the time needed for microfluidization, is reduced, while the droplet size is not. The rate of Ostwald ripening was studied for a series of alkanes with chain lengths from C8 to C16, and found to, as expected, increase with increasing oil solubility in the aqueous phase. By choosing an oil with very limited solubility in the aqueous phase (e.g. squalene), the droplet size increase was minimal even after 3 years. Acknowledgements Johan Andersson, Lukas Boge and Hans Ringblom, SP Technical Research Institute of Sweden, are acknowledged for the preparation and particle sizing of the emulsions. Cryo- and ns-TEM (Fig. 10) were performed by Lars Haag and Emmanuel Tupin at Vironova AB. RISE Research Institutes of Sweden AB and PERFORM, “A competence platform in formulation science coordinated by SP” are acknowledged for financing. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.colsurfa. 2014.06.034. References [1] B.P. Binks, S.O. Lumsdon, Influence of particle wettability on the type and stability of surfactant-free emulsions, Langmuir 16 (2000) 8622–8631. [2] S. Arditty, C.P. Whitby, B.P. Binks, V. Schmitt, F. Leal-Calderon, Some general features of limited coalescence in solid-stabilized emulsions, Eur. Phys. J. E 11 (2003) 273–281.

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