Impact of Oil Type on Nanoemulsion Formation and Ostwald

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Articles Impact of Oil Type on Nanoemulsion Formation and Ostwald Ripening Stability Tim J. Wooster,* Matt Golding, and Peerasak Sanguansri Food Science Australia (CSIRO), Werribee, Victoria 3030, Australia ReceiVed May 31, 2008. ReVised Manuscript ReceiVed July 14, 2008 The formation of stable transparent nanoemulsions poses two challenges: the ability to initially create an emulsion where the entire droplet size distribution is below 80 nm, and the subsequent stabilization of this emulsion against Ostwald ripening. The physical properties of the oil phase and the nature of the surfactant layer were found to have a considerable impact on nanoemulsion formation and stabilization. Nanoemulsions made with high viscosity oils, such as long chain triglycerides (LCT), were considerably larger (D ) 120 nm) than nanoemulsions prepared with low viscosity oils such as hexadecane (D ) 80 nm). The optimization of surfactant architecture, and differential viscosity ηD/ηC, has led to the formation of remarkably small nanoemulsions. With average sizes below 40 nm they are some of the smallest homogenized emulsions ever reported. What is more remarkable is that LCT nanoemulsions do not undergo Ostwald ripening and are physically stable for over 3 months. Ostwald ripening is prevented by the large molar volume of long chain triglyceride oils, which makes them insoluble in water thus providing a kinetic barrier to Ostwald ripening. Examination of the Ostwald ripening of mixed oil nanoemulsions found that the entropy gain associated with oil demixing provided a thermodynamic barrier to Ostwald ripening. Not only are the nanoemulsions created in this work some of the smallest reported, but they are also thermodynamically stable to Ostwald ripening when at least 50% of the oil phase is an insoluble triglyceride.

1.0. Introduction Emulsions are examples of kinetically stable multiphase colloids with a droplet size ranging from 20 nm to tens of millimeters.1 When the entire size distribution of an emulsion is below 80 nm it gains advanced properties compared to conventionally sized emulsions including: optical transparency, high colloidal stability and a large interfacial area to volume ratio.2 Such emulsions are often called nanoemulsions or miniemulsions and are interesting not only because of these physical attributes, but also because of the fundamental challenges that their creation presents. These challenges are 2-fold; the ability to form very small (sub 50 nm) emulsion droplets and the greater challenge of stabilizing these droplets against Ostwald ripening. The current study examines how the physical properties of the dispersed oil phase influences these two processes and demonstrates that it is possible to create optically transparent nanoemulsions with excellent long-term stability. The ultimate size of a homogenized emulsion is determined by the balance between two opposing processes; droplet breakup and recoalescence.1,3,4 Both of these processes are promoted by the intense shear that occurs within a high shear homogenizer such as the MicrofluidizerTM. Droplet break-up occurs when the applied shear is greater than the Laplace pressure of the emulsion. In the simple case of an emulsion with a low oil volume fraction and negligible continuous phase viscosity, Taylor predicts the * To whom correspondence should be addressed. E-mail: timothy.wooster@ csiro.au. (1) Walstra, P. Chem. Eng. Sci. 1993, 48, 333. (2) Tadros, T.; Izquierdo, R.; Esquena, J.; Solans, C. AdV. Colloid Interface Sci. 2004, 108-09, 303. (3) Vankova, N.; Tcholakova, S.; Denkov, N. D.; Ivanov, I. B.; Vulchev, V. D.; Danner, T. J. Colloid Interface Sci. 2007, 312, 363. (4) Meleson, K.; Graves, S.; Mason, T. G. Soft Mater. 2004, 2, 109.

droplet radius, r ∼ γ/(ηcγ˙ ), where γ is the interfacial tension, ηc is the continuous phase viscosity and γ˙ is the shear rate.1,4,5 Taylor’s equation highlights one of the roles the surfactant plays which is to lower the resistance to droplet break-up by reducing the interfacial tension. Intuitively, smaller emulsions should result because of the greater ease of droplet break-up; however, this is not always the case. Different surfactants have differing adsorption kinetics which influences their ability to prevent droplet recoalescence.1 Another factor that has an important influence on droplet break-up is the relative viscosities of the two phases.5-8 As with laminar flow, under turbulent flow conditions there is an optimum dispersed/continuous phase viscosity ratio (ηD/ηC between 0.1 and 5), where droplet break-up is most efficient and the droplet size reaches a minimum.9,10 This optimum viscosity ratio is related to homogenization efficiency via the critical Weber number.4,11 The most important role of a surfactant is to stabilize newly formed droplets against recoalescence via the Gibbs-Marangoni effect.1 As two newly formed droplets approach each other, they acquire more surfactant at their incompletely covered interfaces. However, this adsorption of surfactant is uneven, because the amount of surfactant available is lower in the region of closest approach, than in the bulk. This uneven adsorption creates a surface tension gradient which pulls more surfactant to this depleted region. The accompanying influx of water drives the (5) Taylor, G. I. Proc. R. Soc. 1934, A146, 501. (6) Rallison, J. M. Ann. ReV. Fluid Mech. 1984, 16, 45. (7) Grace, H. P. Chem. Eng. Commun. 1982, 14, 225. (8) Hinze, J. O. AIChE J. 1955, 1, 289. (9) Braginsky, L. M.; Belevitskaya, M. A., In Liquid-Liquid Systems; Kulov, N. N., Ed. Nova Science: Commack, 1994. (10) McClements, D. J. Food Emulsions: Principles, Practices and Techniques, 2nd ed.; CRC Press: Washington, DC, 2004. (11) Calabrese, R. V.; Chang, T. P. K.; Dang, P. T. AIChE J. 1986, 32, 657.

10.1021/la801685v CCC: $40.75 Published 2008 by the American Chemical Society Published on Web 10/14/2008

Nanoemulsion Formation and Ostwald Ripening Stability

two droplets apart, stabilizing them against recoalescence. The strength of this effect is a function of both the surfactant concentration and the Gibbs elasticity of the interface.1,12,13 When there is a low surfactant concentration there is insufficient surfactant to effectively stabilize against droplet recoalescence and the mean droplet size scales directly with the surfactant’s surface excess. If there is an excess of surfactant the rate of droplet coalescence becomes negligible and the emulsion’s size is predominantly determined by the rate of droplet break-up. Invariably the requirements of both droplet break-up and recoalescence dictate that small molecule surfactants are the most capable of forming smaller emulsions (compared to macromolecular emulsifiers) because of their greater ability to rapidly adsorb to interfaces and their much lower dynamic interfacial tensions. The small droplet size of nanoemulsions makes them resistant to physical destabilization via gravitational separation, flocculation and/or coalescence.2,14,15 Nanoemulsions are resistant to creaming because their Brownian motion is enough to overcome their low gravitational separation force. They are also resistant to flocculation because of highly efficient steric stabilization.2,15 Most nanoemulsions are stabilized by synthetic surfactants which tend to have long hydrophilic tails of the order of 2-10 nm. The high ratio of steric layer thickness to droplet diameter (δ/r ratio) means that steric stabilization is very effective and even weak flocculation is prevented.13-16 However, nanoemulsions are particularly prone to a growth in particle size over time by a process known as Ostwald ripening.2,15-17 Ostwald ripening is a process whereby the larger droplets in an emulsion grow at the expense of the smaller droplets because of molecular diffusion of oil between droplets through the continuous phase. This process is driven by the Kelvin effect where the small emulsion droplets have higher local oil solubility than the larger droplets because of the difference in Laplace pressures. The rate of Ostwald ripening is largely dictated by the solubility of the oil in the continuous phase C(∞) as described by Liftshitz and Slesov, and Wagner (LSW theory).18,19 The aqueous phase solubility of an oil decreases linearly with oil molar volume, Vm.20 Despite the great interest in nanoemulsions from both an applied and fundamental aspect, there are few reports describing the formation of physically stable transparent nanoemulsions. Nanoemulsions ranging from 50 to 100 nm have been formed using low viscosity oils such as silicone oils4 or n-alkane oils.21,22 However, the low molar volume of these oils (200-350 cm3mol-1) gives them appreciable solubility in water resulting in destabilization by Ostwald ripening. On the other hand, the large molar volume of long chain triglyceride oils (∼900 cm3 mol-1) should make them insoluble in water thus preventing Ostwald ripening. The formation of true triglyceride oil nanoemulsions appears difficult as there are few reports of triglyceride emulsions with a size smaller than 120 nm. Therefore, the present study examines how the physical properties of two families of oils, a series n-alkane oils and a series of triglyceride oils, impact on nanoemulsion formation and stability. The overall (12) Narsimhan, G.; Goel, P. J. Colloid Interface Sci. 2001, 238, 420. (13) Tcholakova, S.; Denkov, N. D.; Danner, T. Langmuir 2004, 20, 7444. (14) Solans, C.; Izquierdo, P.; Nolla, J.; Azemar, N.; Garcia-Celma, M. J. Curr. Opin. Colloid Interface Sci. 2005, 10, 102. (15) Capek, I. AdV. Colloid Interface Sci. 2004, 107, 125. (16) Taylor, P. AdV. Colloid Interface Sci. 1998, 75, 107. (17) Kabalnov, A. J. Dispersion Sci. Tech. 2001, 22, 1. (18) Lifshitz, I. M.; Slyozov, V. V. J. Phys. Chem. Solids 1961, 19, 35. (19) Wagner, C. Z. Elektrochem. 1961, 65, 581. (20) Taylor, P. AdV. Colloid Interface Sci. 2003, 106, 261. (21) De Smet, Y.; Deriemaeker, L.; Parloo, E.; Finsy, R. Langmuir 1999, 15, 2327. (22) Weiss, J.; Herrmann, N.; McClements, D. J. Langmuir 1999, 15, 6652.

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objective was to understand if it is possible to create a transparent nanoemulsion that is stable against Ostwald ripening by using a triglyceride as the oil phase.

2.0. Experimental Section 2.1. Materials. Tributyrin (98%), tricaproin (99%), tricaprylin (98%), tricaprin (99%), trilaurin (97%), dodecane (99%), tetradecane (99%), hexadecane (99%), octadecane (99%), sodium dodecylsulphate, (SDS, 98.5%), polysorbate 80 (Tween 80, 98%), and polyethylene glycol (PEG, average Mw’s of 200, 600, 1000, 2000, 3350, 4000, and 6600) were used as supplied by Sigma-Aldrich Australia (Clayton, Victoria, Australia). The peanut oil was obtained directly from the manufacturer prior to the addition of antioxidants and pigments. The oil was checked for free fatty acid, mono- and diglyceride content using HPLC and was found to be clean. The miglyol was Delios SK and was used as supplied by Cognis Health and Nutrition Australia. 2.2. Emulsion Formation. Pre-emulsions were prepared by dissolving the SDS (0.5-5.6 wt %) and PEG (0-18.9 wt %) in distilled deionized water and then adding the oil (15 vol %) using a Silverson rotor-stator mixer on its lowest speed setting for 2 min. After mixing, the pre-emulsion had an average particle size D3,2 of 8.37 ( 0.025 µm in a monomodal distribution. The initial temperature of the pre-emulsion was room temperature. Without cooling, the shear during microfluidization caused the emulsion temperature to increase to 50-60 °C. Emulsions were prepared using a Microfluidics M-110Y MicrofluidizerTM (MFIC Corporation, Newton, MA, USA) with a F20 Y 75 µm interaction chamber and H30 Z 200 µm auxiliary chamber inline. Emulsions were prepared by subjecting pre-emulsions to five passes at 1000 bar, which was the maximum practical limit with the air supply of the Microfluidizer. All nanoemulsions used in Ostwald ripening studies were prepared with a Microfluidizer outlet cooler which maintained the nanoemulsion temperature at 6 °C. The reproducibility between preparations was typically 2-6 nm. 2.3. Particle Size Measurement. Nanoemulsion particle sizes were measured using dynamic light scattering (Nano ZS, Malvern, Worcestershire, United Kingdom), at a scattering angle of 173° using a 633 nm laser with each measurement being the average of 16 runs, each of 10 s duration. All samples were measured in distilled deionized water. Nanoemulsions were diluted to give a scattering intensity of less than 500 cps (∼0.0075 wt %) to avoid the effects of multiple scattering. Samples were measured after 5 min equilibration at 25 °C and results are reported as the average of 3 measurements. The intensity average emulsion diameter, and polydispersity of each sample was obtained from the Cumulant analysis23 of each sample’s correlation function. The distribution of sizes was obtained using the CONTIN analysis of each samples correlation function.24 2.4. Emulsion Stability Measurements. The physical stability of nanoemulsions was assessed by measuring nanoemulsion size as a function of time using dynamic light scattering (as above). At each time point the sample was prepared by diluting the original emulsion by 1000-5000 times to negate the effects of multiple scattering. Other studies have demonstrated that diluting the original emulsion just prior to measurement does not affect the assessment of the Ostwald ripening rate of the original undiluted emulsion.25 The Ostwald ripening rates were obtained from the slopes of rN3 vs time. DLS measurements give a Cumulant based intensity average droplet radius rI; however, LSW theory requires a number average radius. The number average radius, rN was obtained by dividing rI by 1.18.21,26 Kabalnov has shown that for small particles 0 < r < 100 nm, rI ) 1.18rN, with the value of the coefficient varying nonmonotonically with droplet size. In the present system, the Ostwald ripening rates were assessed over a droplet size range where the coefficient was kept constant at 1.18.26 2.5. Viscosity Measurements. Viscosity measurements of polyethylene glycol and SDS-polyethylene glycol solutions were performed using a Paar Physica MCR 300, using a double gap (23) Koppel, D. E. J. Chem. Phys. 1972, 57, 4814. (24) Provencher, S. W. Comput. Phys. Commun. 1982, 27, 229.

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Figure 1. Comparison of the effect of sodium dodecylsulfate content on the particle size of 15 vol % oil-in-water nanoemulsions with (i) n-alkane (hexadecane) (ii) long chain triglyceride (peanut oil) as the oil phase. (A) Average diameter, (B) Particle size distributions for 5.6 wt % SDS. Error bars represent two times standard deviation of five measurements on one preparation.

geometry and a ramping shear rate profile from 0.1 to 1000 s-1. Data points are the average of two measurements on the same sample. The reproducibility (2 times SD) of different preparations of the same composition was found to be 2.3% for SDS-PEG solutions and 0.42% for PEG solutions.

3.0. Results and Discussion 3.1. Effect of Oil Phase on Nanoemulsion Formation. The ultimate size of a homogenized emulsion is a complex interplay between the shear conditions and surfactant chemistry. In order to understand the factors involved, some of these variables need to be fixed, in the present case the microfluidization conditions. The average size of a microfluidized emulsion decreases upon recirculation; however, this effect diminishes with pass number and becomes negligible after five passes.4 The oil volume fraction, φ can also have an impact on nanoemulsion size, below a volume fraction of 0.4 the nanoemulsion sizes are essentially the same; however, increasing φ above 0.4 leads to a steady increase in emulsion size.4 Therefore, nanoemulsions were prepared at φ of 0.15, using a Microfluidizer at 1000 bar and subjecting the emulsions to 5 passes. To compare the effect of oil class on nanoemulsion formation, emulsions were prepared using SDS as the surfactant and hexadecane or peanut oil as the oil phase. Figure 1A presents the effect of SDS concentration on droplet size for emulsions prepared using the two different oils. Example particle size distributions for each oil at 5.6 wt % SDS are presented in Figure 1B. Both emulsions exhibit the classic relationship between surfactant concentration and emulsion size each having two regimes: surfactant poor (0 to 2 wt %) and surfactant rich (above 3 wt %). In the surfactant poor regime, the rate of droplet coalescence is significant because there is insufficient surfactant to fully stabilize the newly created interface. A moderate increase in surfactant concentration causes a considerable decrease in particle size because the extra surfactant is able to stabilize more interfacial area. In the surfactant rich regime there is excess surfactant in the bulk, which rapidly adsorbs to newly formed droplets, stabilizing them against recoalescence. The rate of droplet coalescence is negligible and the ultimate size is determined by the efficiency of droplet breakup.3

Table 1. Physical Properties of the Oil and Water Phases Used to Create the Nanoemulsions with and without Polyethylene Glycol in the Aqueous Phase phase water hexadecane peanut oil (LCT)

η (25 °C) (mPa s) ηD/ηCb 0.89 3.1 50

3.5 56.2

surfactant

γ (mN/m)a

SDS SDS Polysorbate 80

72.7 54 10 31 8 ∼5

a Values for equilibrium interfacial tensions were taken from Gurkov et al.27 b Values for ηD/ηC were calculated from the measured viscosities of the various phases.

What is particularly interesting about the two emulsion series in Figure 1 is that even in the presence of a large excess of surfactant, there is a 40 nm difference in droplet diameter. The large excess of surfactant suggests that the difference in size is caused by differences in droplet break-up, not recoalescence. The efficiency of droplet break-up in turbulent shear is governed by the shear rate, surfactant type/concentration and the relative viscosities of the dispersed and continuous phases. It is unlikely that interfacial tension causes the difference in nanoemulsion size (Table 1), because the equilibrium interfacial tension of SDS at an LCT/water interface (∼8 mN m-1)27 is systematically lower than that at a hexadecane/water interface (∼10 mN m-1).27 Given that both emulsions are subjected to the same shear, the only remaining possibility is the difference in the viscosity of the two oil phases. The viscosity of peanut oil is considerably higher than that of hexadecane, Table 1. This means that the differential viscosity of the peanut oil nanoemulsion is considerably higher than the hexadecane nanoemulsion. In any homogenizer there is an optimal range of ηD/ηC where droplet disruption is most efficient. Braginsky et al. have found that for turbulent shear this range of ηD/ηC is between 0.1 and 5.9 When ηD/ηC is too high, droplets are more resistant to disruption as there is (25) De Smet, Y.; Malfait, J.; DeVos, C.; Deriemaeker, L.; Finsy, R., In Trends in Colloid and Interface Science XI; Rosenholm, J. B.; Lindman, B.; Stenius, P., Eds. 1997;Vol. 105, pp 252. (26) Kabalnov, A. S.; Colloid, J. USSR 1991, 53, 709. (27) Gurkov, T. D.; Dimitrova, D. T.; Marinova, K. G.; Bilke-Crause, C.; Gerber, C.; Ivanov, I. B. Colloids Surf., A 2005, 261, 29.

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Figure 2. Comparison of the effect of polyethylene glycol addition on the particle size of 15 vol % peanut oil-in-water nanoemulsions stabilized by (i) 5.6 wt % SDS (ii) 5.6 wt % polysorbate 80. (A) Average diameter, (B) Particle size distributions for these emulsions at 5.6 wt % SDS and 16.6 wt % PEG 6600. Error bars represent two times standard deviation of five measurements on one preparation.

insufficient time to deform before the flow field causes the droplet to rotate.10 The hexadecane nanoemulsion ηD/ηC of 3.4 is within Braginsky’s range, while the triglyceride nanoemulsion ηD/ηC of 56 is much higher. This suggests that a major reason why it is more difficult to create a triglyceride nanoemulsion, compared to an n-alkane nanoemulsion, is the higher viscosity of the oil phase. Given the influence that ηD/ηC has on droplet break-up, the effect the continuous phase viscosity has on nanoemulsion size was examined next. The continuous phase viscosity was increased via the addition of polyethylene glycol to each emulsion prior to homogenization. SDS and polysorbate 80 were chosen as the surfactants because of their different interactions with PEG. It is widely known that SDS interacts with PEG to form aggregates in solution and at interfaces.28-31 The association occurs via an ion-dipole interaction involving the ionic headgroup of SDS and the dipole of the ether linkage within the PEG backbone.32 While the polysorbate, itself having a PEG headgroup, has no real interaction with the PEG in solution. Figure 2A presents the effect of PEG addition (prior to homogenization) on the particle size of triglyceride nanoemulsions stabilized by either SDS or polysorbate 80. Example particle size distributions for each oil at 5.6 wt % SDS and 16.6 wt % PEG 6600 are presented in Figure 2B. The addition of PEG to either nanoemulsion resulted in a considerable reduction in average droplet size. When polysorbate 80 is the surfactant there is a 40 nm reduction in nanoemulsion size. The nanoemulsion size at the highest PEG concentration is equivalent to the size of the smallest hexadecane nanoemulsion shown in Figure 1. The reduction in nanoemulsion size is caused by enhanced droplet deformation that results from a lowering of the ηD/ηC ratio. Without PEG, ηD/ηC is 56 for triglyceride nanoemulsions stabilized by SDS. This decreases exponentially with increasing PEG content (Figure 3), until the ηD/ηC is 4.2, which is within the optimal range for droplet breakup under turbulent shear (ηD/ηC ) 0.5-5).9 Adding PEG to SDS (28) Goddard, E. D. J. Colloid Interface Sci. 2002, 256, 228. (29) Goddard, E. D.; Ananthapadmanaban, K. P., Interactions of Surfactants with Polymers and Proteins; CRC Press: Boca Raton, FL, 1993. (30) Penfold, J.; Thomas, R. K.; Taylor, D. J. F. Curr. Opin. Colloid Interface Sci. 2006, 11, 337. (31) Kwak, J. C. T. Surfactant Science Series 77: Polymer-surfactant systems; Marcel Dekker: New York, 1998. (32) Bernazzani, L.; Borsacchi, S.; Catalano, D.; Gianni, P.; Mollica, V.; Vitelli, M.; Asaro, F.; Feruglio, L. J. Phys. Chem. B 2004, 108, 8960.

Figure 3. Effect of polyethylene glycol content on the viscosity (open symbols) and ηd/ηc (solid symbols) of (i) PEG dissolved in water and (ii) PEG dissolved in a 8.3 wt % solution of sodium dodecylsulfate. Error bars represent 2 × standard deviation. Viscosity measured at 100 s-1, of a 0.1-1000 s-1 sweep every sample was Newtonian up to 800 s-1.

nanoemulsions prior to homogenization also causes a considerable reduction in size, about 80 nm. Part of this reduction is due to the lower ηD/ηC ratio, which is larger for SDS because its interaction with PEG causes a greater increase in continuous phase viscosity (Figure 3). However, the reduction in nanoemulsion size observed with SDS/PEG is far greater than that observed with polysorbate/PEG, even at similar ηD/ηC ratios. Clearly there is a synergistic interaction between SDS and PEG that causes a further reduction in nanoemulsion size. While the result of the synergistic interaction is apparent, the cause of the size reduction is uncertain. It is possible that the interaction between SDS and PEG results in a reduction in the oil/water interfacial tension. The association between SDS and PEG is known to reduce the electrostatic repulsion between SDS charge groups which can increase surfactant packing density and hence cause a decrease in interfacial tension. Alternatively, the presence of PEG at the interface could increase its Gibbs elasticity, possibly enhancing the recoalescence stability of the emulsion during homogenization.

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Figure 4. Comparison of (A) the volume weighted particle size distribution and (B) optical clarity through a 1 cm path length cell of long chain triglyceride nanoemulsions stabilized by (i) 5.6 wt % SDS and (ii) 5.6 wt % SDS and 16.6 wt % PEG 6600.

The final size achieved by the SDS-PEG combination is quite remarkable, even more so when one considers the volume size distribution, Figure 4A. The resulting emulsions have high transparency, Figure 4B. While some of this clarity is due to a reduction in refractive index difference between the two phases, the majority of it is a result of >95% of the emulsion having a size below 60 nm, with a volume average around 30 nm. 3.2. Ostwald Ripening of Nanoemulsions. Ostwald ripening is the major destabilization mechanism of nanoemulsions. It is the net transport of oil from small droplets to larger droplets through the continuous phase.16,17 Ostwald ripening primarily occurs via the molecular dissolution of oil in the continuous phase. However, the presence of excess nonionic surfactant in the continuous phase can result in a small enhancement (2-3 times) in the rate of Ostwald ripening, presumably by micellar dissolution and transport of the oil.17,22,33 Ostwald ripening is driven by the dependence of an emulsion droplets solubility with its size, as described by the Kelvin eq 1.34

( )

C(r) ) C(∞)exp

2γVm rRT

(1)

Where C(r) is the solubility of the emulsified oil, C(∞) is the oils bulk phase solubility, r is the emulsion radius, γ is the interfacial tension, Vm is the oils molar volume, R is the gas constant and T is the absolute temperature. The Kelvin effect arises from the difference in chemical potential between small and large emulsion droplets as a result of the different Laplace pressures. Only a small difference in droplet size is needed to create a chemical potential gradient large enough to initiate Ostwald ripening (eq 2).2 Furthermore the initial size difference becomes increasingly accentuated as Ostwald ripening progresses.

∆µ ) µr - µ∞ ≈

2γVm r

(2)

Where µr is the chemical potential of the oil in the emulsion droplet of radius r1, and µ∞ is chemical potential of the oil in the bulk phase. Analytically one can determine if Ostwald ripening is the major destabilization mechanism if there is a linear relationship between the cube of the emulsion radius (rN3) and (33) Kabalnov, A. Langmuir 1994, 10, 680. (34) Thompson (Lord Kelvin) , W. Phil. Mag. 1871, 42, 448.

Figure 5. Ostwald ripening plots (rN3 vs t) for 15 vol % alkane oil nanoemulsions stabilized by the SDS-PEG surfactant system (5.6 wt % SDS, continuous phase contains 16.6 wt % PEG Mn ≈ 6600). Alkane oils used were (i) dodecane, (ii) tetradecane, (iii) hexadecane and (iv) octadecane.

time (Lifshitz-Slesov-Wagner (LSW) Theory),18,19 the slope of which is the Ostwald ripening rate, ω.

ω)

[

drN3 8 C(∞)γVmD ) dt 9 FRT

]

(3)

Where D is the diffusion coefficient of the dispersed phase in the continuous phase and F is the density of the dispersed phase. The physical stability of a series of n-alkane oil-in-water nanoemulsions was the first to be examined. Example Ostwald ripening plots are given in Figure 5, all of which exhibit a linear relationship between rN3 and time, which is indicative of Ostwald ripening. The Ostwald ripening rates obtained show a linear relationship with the molar volume of the oil (Figure 6). Table 2 compares our present rates of Ostwald ripening with LSW

Nanoemulsion Formation and Ostwald Ripening Stability

Figure 6. Nanoemulsion Ostwald ripening rate, ω as a function of oil molar volume, Vm for different families of oils (i) n-alkane nanoemulsions stabilized by SDS-PEG (5.6 wt % SDS, continuous phase contains 16.6 wt % PEG Mn ≈ 6600), (ii) triglyceride nanoemulsions stabilized by SDS-PEG (5.6 wt % SDS, continuous phase contains 16.6 wt % PEG Mn ≈ 6600).

calculations and those measured by Kabalnov et al.35 The rates that we have obtained compare very well with those obtained by Kabalnov. Slight differences can be explained by the higher oil volume fraction used in our emulsions. The effect of volume fraction on the Ostwald ripening rate can be estimated from the work of Enomoto.36 A simple correction factor, ω(φ) ) k(φ) × ω(0), can be used to account for a difference in the oil content. If this correction factor, k(0.15) ) 1.9, is used good agreement between the experimentally derived Ostwald ripening rates and those predicted by LSW theory can be achieved. This suggests that the presence of PEG in the continuous phase has no real effect on the rate of Ostwald ripening. The Ostwald ripening experiments conducted with n-alkane oils clearly demonstrate that ω is directly proportional to Vm, which is related to the C(∞) of the oil. Most common edible triglyceride oils (e.g., peanut oil) have a much larger Vm than n-alkane oils (Table 3), therefore they should have much lower Ostwald ripening rates. To test this hypothesis the physical stability of nanoemulsions made using a series of triglyceride oils was examined. Like the n-alkanes there was a linear relationship between rN3 and time indicative of Ostwald ripening (not shown). Figure 6 presents the Ostwald ripening rates of the nanoemulsions made using different triglyceride oils as a function of oil molar volume. As with the n-alkane series the triglyceride series exhibited a linear relationship between Log ω and oil Vm. As the TAG length increases C(∞) decreases to the point where trilaurin (C12:0) and peanut oil, a long chain triglyceride, do not undergo Ostwald ripening. In fact, the peanut oil nanoemulsion is so stable that there is no change in particle size over 3 months. It is interesting to compare the Ostwald ripening of analogous oils from the two series of oils. Tributyrin and octadecane have very similar molar volumes and hence should be comparable. Tributyrin has a much faster Ostwald ripening rate, and hence higher continuous phase solubility, than octadecane despite the small difference in Vm. The higher solubility of tributyrin arises from the TAG glycerol headgroup, which gives the molecule some amphiphilicity and hence higher solubility in (35) Kabalnov, A. S.; Makarov, K. N.; Pertzov, A. V.; Shchukin, E. D. J. Colloid Interface Sci. 1990, 138, 98. (36) Enomoto, Y.; Tokuyama, M.; Kawasaki, K. Acta Metall. 1986, 34, 2119.

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water. This highlights that the relationship between Vm and C(∞), while linear within an oil family, changes considerably across oil classes. 3.3. Ostwald Ripening in Mixed Oil Nanoemulsions. Now that stable nanoemulsions have been created using an insoluble oil, we were interested to see if nanoemulsions containing soluble oils could be stabilized. Higuchi and Misra were the first to propose that Ostwald ripening of emulsions can be slowed through the incorporation of a second oil with much lower continuous phase solubility.37 This concept is based on the premise that the entropy of mixing provides a chemical potential that opposes Ostwald ripening.17,37,38 When a mixed oil nanoemulsion undergoes Ostwald ripening, the soluble oil has greater mobility between droplets. Over time, the larger droplets become enriched with the soluble oil and smaller droplets become enriched with the insoluble oil. This creates a compositional imbalance that is of higher entropy than a perfectly mixed system. In an ideal system, where: (i) the molar volumes of the components are assumed to be the same, (ii) the interfacial tension is independent of composition and (iii) the components are infinitely miscible with each other and form ideal solutions over all composition ranges, the chemical potential of the soluble “mobile” oil can be written as17

∆µ ) µ∞ +

2γVm + RTln(1 - X2) r

(4)

where X2 is the mole fraction of insoluble oil in the emulsion. The first two parts of this equation represent the normal chemical potential of a single component and are always positive. The third term represents the entropy of mixing, which is negative for any fraction of X2. It is obvious from eq 4 that any nanoemulsion made using a soluble oil can be stabilized provided the fraction of the insoluble oil is sufficiently large. The stability of mixed oil nanoemulsions was examined using two triglyceride oils, tricaprylin (soluble) and peanut oil (insoluble), of high mutual miscibility. Figure 7 shows the effect of insoluble oil addition on the stability of tricaprylin nanoemulsions. Generally incorporation of increasing amounts of peanut oil leads to decreasing rates of Ostwald ripening, with three distinct types of behavior observable depending on the insoluble oil molar fraction. At low peanut oil fraction (0-0.2), the nanoemulsions exhibit the classic steady state linear relationship between rN3 and time indicating Ostwald ripening, but at a reduced rate compared to the pure tricaprylin nanoemulsion. At intermediate contents (0.3 to 0.5), the nanoemulsions initially exhibit a linear increase in rN3 with time. This is followed by a plateau in droplet growth, the onset of which occurs earlier with increasing peanut oil content. The stability behavior exhibited by these mixed oil nanoemulsions demonstrates the effect demixing has on the excess chemical potential of a nanoemulsion and can be understood by examining Kabalnov’s theoretical treatment of mixed oil nanoemulsions.17 The excess chemical potential of a mixed oil nanoemulsion is a function of the emulsions radius:

( )

rinit ∆µ R + ) -X2 RT r r

(5)

where R ) 2γVm/RT. The stability behavior being dependent on both the size of the nanoemulsion and the value of X2. When there is a low amount of insoluble oil in the nanoemulsion, X2 (37) Higuchi, W. I.; Misra, J. J. Pharm. Sci. 1962, 51, 459. (38) Binks, B. P.; Clint, J. H.; Fletcher, P. D. I.; Rippon, S.; Lubetkin, S. D.; Mulqueen, P. J. Langmuir 1999, 15, 4495. (39) Malcolmson, C.; Satra, C.; Kantaria, S.; Sidhu, A.; Lawrence, M. J. J. Pharm. Sci. 1998, 87, 109.

12764 Langmuir, Vol. 24, No. 22, 2008

Wooster et al.

Table 2. Physical Characteristics of 15 vol % n-Alkane-in-Water Nanoemulsions Stabilized by 5.6 wt % SDS (O ) 0.15, 0.3M SDS) and 16.6 wt % PEGa ω (nm3 s-1) alkane chain length dodecane tetradecane hexadecane octadecane

C12 C14 C16 C18

3

b

b

Vm (cm /mol)

rN (init) (nm)

LSW theory

experiment

227 260 292 327

26.4 20.5 30.5 23.5

1.7 0.14 0.011 0.00084

22.6 1.50 0.169 0.0180

Kabalnov 17 1 0.087 -

a The Ostwald ripening rates used for comparison are obtained from Kabalnov (φ ) 0.1, 0.1 M SDS) are for emulsions stabilised by SDS only.35 b From Weiss et al.22

Table 3. Molar Volume and Experimentally Determined Ostwald Ripening Rate for a Series of Triglyceride Oil-in-Water Nanoemulsions Stabilized by SDS-PEG

a

oil

molecular weight

fatty acid chain length

Vm (cm3/mol)a

tributyrin tricaproin tricaprylin miglyol tricaprin trilaurin peanut oil

302 386 470 521 554 639 933

C4 C6 C8 Mixture C8 and C10 C10 C12 >C18

313 411 508 614 655 703 959

ωexp (nm3 s-1) 108 4.15 0.345 0.0483 5.78 × 10-4 none

Calculated using the procedure of Malcolmson et al.39

Figure 7. Effect of insoluble oil (peanut oil) content on the Ostwald ripening of 15 vol % tricaprylin nanoemulsions stabilized by the SDSPEG surfactant system (5.6 wt % SDS, continuous phase contains 16.6 wt % PEG Mn ≈ 6600).

< 0.17 in our case, the Laplace pressure dominates and the nanoemulsion is unstable. When there is a high amount of insoluble oil, X2> 0.4 in our case, the entropy of mixing dominates, ∆µ < 0, and the nanoemulsion is thermodynamically stable. This means that the assembled emulsified state is more favorable than the bulk separated state, as is the case with a microemulsion. At intermediate insoluble oil fractions, 0.17 < X2 < 0.4, transient behavior is observed. Initially the Laplace pressure dominates and the emulsions undergo Ostwald ripening. However, as Ostwald ripening progresses two processes combine to halt Ostwald ripening: (i) the nanoemulsions droplet size increases causing a reduction in the Laplace driving force and (ii) the enrichment of the large particles with tricaprylin increases the entropy of demixing. In his treatment of mixed oil emulsions, Kabalnov came up with three stability regimes; stable, unstable and kinetically stable and defined equations to predict the limits of Ostwald ripening stability. If X2 > R/ro then ∆µ < 0, and the nanoemulsion is thermodynamically stable. If X2 < R/3ro then nanoemulsion is

Figure 8. Effect of insoluble oil (peanut oil) content on the steady state Ostwald ripening rate of 15 vol % tricaprylin nanoemulsions stabilized by the SDS-PEG surfactant system (5.6 wt % SDS, continuous phase contains 16.6 wt % PEG Mn ≈ 6600).

unstable and undergoes Ostwald ripening. At intermediate contents the nanoemulsion is said to be meta (kinetically) stable, ∆µ is positive and the emulsion still prefers the bulk separated state. However, the system contains a kinetic energy barrier that makes this decomposition difficult to achieve, but if the polydispersity is large enough this metastability will be broken. Using Kabalnov’s equations on the present nanoemulsions, the X2 for kinetic stability is 0.18 and for thermodynamic stability is 0.53. These compare favorably with the three distinct regions of nanoemulsion stability depicted in Figure 8 highlighting that Kabalnov’s equations accurately predict the transitions in mixed oil nanoemulsion stability behavior. Whether the behavior observed by the nanoemulsions made at intermediate oil fractions represents the meta-stability described by Kabalnov remains to be seen. Certainly there appears to be an energy barrier that prevents decomposition into the bulk separated states, and even

Nanoemulsion Formation and Ostwald Ripening Stability

when the emulsions undergo Ostwald ripening they quickly relax back to a stable state rather than continuing to ripen.

4.0. Summary and Conclusions This study examines how the physical properties of the oil phase influence both nanoemulsion formation and stabilization. It was found that optimization of surfactant architecture combined with proper oil properties lead to the creation of physically stable nanoemulsions with excellent optical clarity. When examining nanoemulsion formation it was found that one of the main reasons for the difficulty in forming nanoemulsions from triglyceride oils, compared to n-alkane oils, was the much higher viscosity of the triglyceride oil. Nanoemulsion size could be dramatically reduced by the addition of polyethylene glycol to the water phase. The reduction in nanoemulsion size had two origins: (i) an enhancement in droplet deformability as a result of the reduction in ηD/ηC and (ii) a synergistic interaction between SDS and PEG that further enhanced droplet deformation, via a possible combination of lower interfacial tension and a higher Gibbs elasticity. The solubility of the oil in the continuous phase also had a major impact on physical stability, with Ostwald ripening rates being directly proportional to oil molar volume. This is because there is a direct relationship between the molar volume of an oil and its solubility in water. We found that the insolubility of triglyceride oils in water acted as a kinetic barrier to Ostwald ripening, making TAG nanoemulsions inherently stable to

Langmuir, Vol. 24, No. 22, 2008 12765

Ostwald ripening. Nanoemulsions containing soluble oils could be effectively stabilized against Ostwald ripening via the addition of an insoluble triglyceride oil. These mixed oil nanoemulsions were stable because the entropy gain associated with demixing acts as a thermodynamic barrier to Ostwald ripening. The instability behavior of the mixed oil nanoemulsions was accurately predicted using Kabalnov’s stability criterion validating his theoretical treatment of this phenomenon. Not only are the nanoemulsions created in this work some of the smallest produced, but they are also thermodynamically stable to Ostwald ripening when at least 50% of the oil phase is an insoluble triglyceride. There is of course a question as to whether, given the size distributions observed, these are in fact microemulsions rather than nanoemulsions. The fact that mechanical homogenization is required for formation, coupled with the fact that these systems are stable to dilution and unstable to Ostwald ripening, indicates a kinetically rather than thermodynamically stabilized system, albeit with a droplet size distribution far below that achievable for most conventional emulsion systems. Acknowledgment. The authors thank the “Our Rural Landscapes” initiative of the Department of Primary Industries Victoria, Australia for financial support of this work. We also thank Mr. Li Jiang Cheng and Mrs. Helen French for preparing some of the nanoemulsions. Finally, we thank Timothy Taylor for technical support. LA801685V