Article pubs.acs.org/IECR
Thermal Effects in Nanoemulsification by Ultrasound Elijah Nazarzadeh† and Shahriar Sajjadi*,‡ †
Division of Engineering, King’s College London, WC2R 2LS London, U.K. Department of Physics, King’s College London, WC2R 2LS London, U.K.
‡
ABSTRACT: A significant part of energy in sonication is dissipated as heat, which, if not controlled, can adversely affect the sonication efficiency as well as the emulsion properties. In this research, thermal effects in nanoemulsification via ultrasound are studied. Alkanes with a wide range of volatility and water solubility were used as model oils and sodium dodecylsulfate (SDS) as surfactant. Nonisothermal sonication of oil/water/surfactant mixtures resulted in the formation of emulsions whose drop size could not easily be related to formulation and process variables. Isothermal sonication produced more reproducible and finer nanoemulsions than the nonisothermal one. The difference in the drop size between two processes became wider with increasing water solubility/volatility of oils. In order to elucidate the thermal effects during sonication of the oil/water/surfactant system, the sonicationd of pure water, an aqueous solution of SDS, and an aqueous solution of SDS saturated with an oil (hexane) were thoroughly monitored. Nanobubbles of about 180 nm in diameter were detected in the sonified pure water; however, the average size of bubbles increased to about 2 μm in the presence of SDS. Bubbles formed under extensive cavitation, by using either high power sonication or volatile oil, can increase the average oil drop size by damping the shock waves (i.e., decreasing energy transfer efficiency) and, therefore, reducing the rate of drop breakup. Bubbles can also contribute to the size of emulsion droplets, if they are not removed from the emulsion prior to measurement.
1. INTRODUCTION Emulsions are dispersions of an immiscible liquid in another liquid. Nanoemulsions are categorized as emulsions having nanoscale (below 500 nm) droplets, also referred to as miniemulsions. Miniemulsions, if polymerized, can give a wide range of hybrid polymeric materials.1,2 Due to their inherent especial characteristics, nanoemulsions have attracted significant attention for various applications, including pharmaceutical, personal care, cosmetics, and household. There are different methods available for producing nanoemulsions, including high-pressure homogenization, colloid mills, ultrasonication, and phase inversion.3−5 These methods can be categorized according to their energy consumption as low and high energy emulsification methods. The energy required for emulsification (Eemul), in order to increase the interfacial area between two phases, is Eemul = σ ΔA
into smaller eddies. This energy cascade continues until the scales are sufficiently small that energy dissipates as heat. As a result, the temperature of emulsions increases during sonication as more energy is applied. Emulsification via sonication has been the focus of many studies.6−15 Despite all recent advances in the understanding of the sonication technology, the mechanism of drop formation via sonication is not clear yet. According to Li and Fogler,16 drop formation in a sonication process starts with primary interfacial instability that leads to a rupture from the dispersed phase into the continuous phase. This is completed by a secondary transient cavitation that generates high-pressure shock waves, which then rupture the dispersed phase into very small droplets. There are two types of cavities in an ultrasound wave field: transient and stable. Stable cavities often oscillate nonlinearly and exist for many cycles, while transient ones last for less than one or at most a few acoustic cycles. During their life cycle, transient cavities expand to at least double their initial size and then implode. This results in an increase in the local temperature and pressure.17 On the other hand, stable cavities grow and shrink continuously. Stable cavities might collapse; however, their implosion is not as powerful as that of the transient cavities.17 Emulsification via sonication is usually associated with a significant rise in the temperature of emulsions.10,18,17,19 This is, in fact, the main difficulty involved in using energy-intensive sonication to produce nanoemulsions, as is well-known to academics and industrialists in the field. While adiabatic
(1)
where ΔA is the increase in interfacial area and σ interfacial tension. The drop size of an emulsion is determined by the balance between two simultaneous, opposing processes: breakup and coalescence. Applying a shear rate higher than the Laplace pressure promotes drop rupture. The Laplace pressure (cohesive force) of a spherical drop is defined by interfacial tension, σ, over drop radius, r:
ΔP =
2σ r
(2)
Equation 2 implies that small drops have a higher Laplace pressure. Therefore, a higher shear rate is required to break small drops. Shear rate in an energy-intensive method results from a velocity gradient, formed by eddies, across drops. Eddies are unstable and therefore break up and transfer their energy © 2013 American Chemical Society
Received: Revised: Accepted: Published: 9683
January 26, 2013 May 13, 2013 June 6, 2013 June 6, 2013 dx.doi.org/10.1021/ie4003014 | Ind. Eng. Chem. Res. 2013, 52, 9683−9689
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Table 1. Physical Properties of Water and Various Alkanes at 25°C: Water Solubility (C∞), Density (ρ), Viscosity (μ), Boiling Point, Latent Heat of Evaporation (λ), Specific Heat (Cp), and Interfacial Tension (σa) alkanes C∞23 (kg/m3) ρb (kg/m3) μ24 (mPa·s) boiling ptb(°C) λ25 (J/g) Cp26 (J/g·°C) σa (mN/m)
C6
C8
C10
C12
C14
C16
water
9.50 × 10−3 660 0.38 70 364.52 2.28 6.25
6.60 × 10−4 698 0.70 125 362.67 2.19 7.15
5.20 × 10−5 727 0.90 174 356.98 2.18 8.41
3.50 × 10−06 749 1.49 216 356.72 2.18 9.56
3.30 × 10−7 759 3.07 253 352.10 2.18 9.24
2.10 × 10−8 770 3.51 287 342.89 2.18 10.20
998 0.89 100 334 4.18
a Values for interfacial tension are for the oils against an aqueous solution of SDS (above CMC), measured in this work. bValues for density and boiling temperature are from the products data sheet. The vapor pressures of alkanes are not given in the table, but they increase with decreasing chain length.
Figure 1. Variations in drop size with active sonication time for different alkanes under isothermal and nonisothermal conditions (10 g/L SDS, 20 vol%Oil, 70%Amp, T = 17.0 ± 2.0 °C for isothermal process).
PURELAB Option water purification system. All chemicals were used as received. 2.2. Apparatus. A digital sonifier model 450 from Branson Ultrasonics Corporation with the maximum output of 400 W (with frequency of 19.850−20.050 kHz) was used for producing nanoemulsions. Various operational parameters can be set to control the way ultrasound waves are applied to emulsions. These parameters include duration of sonication and amplitude with a setting between 10% and 100% of the maximum amplitude. Emulsifications were carried out under either nonisothermal or isothermal conditions mostly at 17.0 ± 2.0 °C. 2.3. Measurement. A Nano ZS particle sizer (ZEN3600, Malvern Instruments) at the fixed scattering angle of 173° and with laser wavelength of 633 nm was used for measuring the zaverage drop diameter (dz) and drop size distribution (DSD) of emulsions. Interfacial tensions were measured by the pendant drop method using an FTA200 tensiometer. 2.4. Method. A primary mixture was produced using an aqueous solution of 10 g/L SDS followed by addition of 20 vol %Oil. The mixture was placed in a 50 cm3 beaker and then applied to ultrasonic waves at a given amplitude to produce a nanoemulsion. The overall volume of the emulsion was 40 cm3. To elaborate the effect of temperature rise on the emulsification process, two sets of experiments were carried out. In one set, the sonication temperature was controlled within 17.0 ± 2.0 °C
sonication is an ideal process for academic purposes, it is not widely used because of the costs involved. There are a number of reports in the literature that have briefly discussed the effect of temperature on drop formation during sonication. Some authors reported that high-temperature sonication had hardly any effect on the size of drops.14,18−21 However, Gaikwad et al.22 obtained smaller drops at higher temperatures. The literature is not clear about the extent of thermal effects, and there is a significant uncertainty regarding properties of nanoemulsions produced by high-temperature sonications. The aim of this report is to unravel thermal effects in nanoemulsification via ultrasound. A set of alkanes with different chain lengths (hexane, octane, decane, dodecane, tetradecane, and hexadecane) and different physical properties (as listed in Table 1) were used as model oils to produce nanoemulsions.
2. EXPERIMENTAL SECTION 2.1. Materials. Sodium dodecylsulfate (SDS, CH3(CH2)11OSO3Na) was obtained from Sigma-Aldrich and was of 99% purity. The alkanes, hexadecane (C16H34), tetradecane (C14H30), dodecane (C12H26), decane (C10H22), octane (C8H18), and hexane (C6H14) were obtained from Sigma-Aldrich. Deionized water was obtained from an ELGA 9684
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already reached the steady-state value for the isothermal process or the minimum value for the nonisothermal process. The physical properties of alkanes change linearly with chain length (Table 1). Viscosity, interfacial tension, boiling temperature, and latent heat of evaporation of alkanes increase with chain length, whereas their vapor pressure, specific heat capacity, and solubility in the aqueous phase decrease. This results in a systematic change in alkane behavior with chain length during sonication. There is a decrease in the drop size with increasing chain length of alkanes for the isothermal process. This can be attributed to the increase in viscosity ratio (viscosity of the dispersed over that of the continuous phase) from 0.42 to 3.96 as the chain length of the corresponding alkane increases from 6 to 16, respectively. In an earlier report,27 we have shown that the final drop size of sonified nanoemulsions shows a U-shape curve when plotted against viscosity ratio with a minimum at viscosity ratio of 3 to 5.27 One common feature in all nonisothermal emulsifications was a steeper transient interval in comparison to that for isothermal emulsification (Figure 1). The high temperatures achieved during the nonisothermal process may act favorably during the early stage of emulsification, as they reduce the viscosity of the dispersed phase, lower the interfacial tension, and therefore enhance drop rupture (see eq 2). However, the nonisothermal process practically produced larger drops than the corresponding isothermal process. The difference became more significant with increasing volatility (i.e., vapor pressure) of oils. Possible reasons for this will be discussed later. For C6 emulsion, ultrasound emulsification failed to produce submicrometer emulsion under nonisothermal conditions. Isothermal sonication of C6 at higher temperature than room temperature (i.e., 27 and 40 °C) enlarged the droplets and slightly improved the drop size evolution in comparison to that resulting from the nonisothermal process; however, fluctuations were still significant (not shown). 3.2. Energy Balance and Temperature Profile for Nonisothermal Process. Figure 3 depicts the temperature
by using cold water in the jacket of the emulsification vessel (isothermal), whereas, in the other set, emulsions were allowed to absorb the sonication energy (nonisothermal). The device was set for a 2 s active sonication interval followed by a 20 s pause. This is a common procedure for maintaining emulsion temperature constant during sonication. No significant change was seen in drop size after 180 s of active sonication for isothermal conditions. Samples, during sonication, were always taken from the vicinity of the sonifier horn tip, where the highest energy dissipation occurred.
3. RESULTS AND DISCUSSION 3.1. Isothermal versus Nonisothermal Sonication. Sonication processes are usually associated with a significant rise in the temperature of emulsions. The temperature rise in our experiments, for 40 cm3 of emulsion, was within 4−5 °C after 10 s of active sonication at 70% and 100% amplitude, respectively. Previous studies reported temperature rises of up to 39 °C after 60 s sonication at 100% amplitude.18 Figure 1 depicts the time evolution of drop size for different alkanes with the sonication amplitude of 70% under isothermal and nonisothermal conditions. The isothermal drop size evolutions with sonication time reveal two characteristic intervals of emulsification processes, namely transition period and steady state. During the transitional interval, the size of drops decreases exponentially with time as drops are exposed to ultrasound waves until they reach a stable size. During the steady state, the average size of drops remains constant either because drops cannot be further ruptured or the rates of drop coalescence and breakup become comparable. The steady-state drop size was reached within 180 s of active sonication for all oils under isothermal conditions. However, we continued sonication for the total time of 600 s in order to be able to observe thermal effects more clearly. Figure 1 also shows the drop size evolution of alkanes during nonisothermal sonication. The important difference is that no clear steady state was developed for C6, C8, and C10 under nonisothermal conditions as drops started to increase in size after going through a minimum, exhibiting a U-type behavior in terms of sonication time. This minimum was advanced with increasing volatility of oils. For longer chain alkanes, C12, C14, and C16, the steady-state drop size was maintained within a sonication time of 600 s. Figure 2 shows a plot of the average drop size versus carbon chain length of alkanes for both processes. This comparison was made at the sonication time of 180 s, when drop sizes have
Figure 3. Variations in temperature with sonication time for C6 and C16 emulsions and pure water under nonisothermal conditions (10 g/ L SDS, 20 vol%Oil, 70% Amp).
profile of C6 and C16 nanoemulsions, as two extreme model oils used in this study, during nonisothermal sonication. The temperature profiles of other oils fall between these two profiles. The temperature profile for sonication of pure water is also shown as the baseline for comparison (carried out with the same procedure as explained in the Experimental Section). The major part of the sonication energy, which is dissipated as heat, is lost from the system via heat transfer to the surroundings and via phase change (i.e., evaporation), but a part remains in the system as thermal energy. The deviation in the system
Figure 2. Comparison between final drop sizes (time = 180 s) of isothermal (17.0 ± 2.0 °C) and nonisothermal emulsifications for alkanes with different chain lengths (10 g/L SDS, 20 vol%Oil, 70% Amp). 9685
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Figure 4. Time variations in temperature, morphology, and DSD of solutions comprised of 12 intervals of 30 s during nonisothermal sonication of (A) water, (B) water/SDS, and (C) water/SDS/C6. (D) Variations in temperature of the solutions with time during active (pulse on) and nonactive (pulse-off) intervals. The temperature increased during pulse-on but slightly decreased during pulse-off intervals because of heat transfer to the environment. Arrows show regions of large cavities.
30% after 600 s of sonication. This clearly shows that water also evaporates from the system (the oil phase ratio was 20%). The emulsion temperature rose further for C16, because of its smaller Cp and higher boiling temperature (Table 1), which allowed more heat to be absorbed by the nanoemulsion, before evaporative cooling of water took over and the increase in temperature leveled off. Nanodrops exhibit boiling temperatures higher than those for the bulk phase due to their higher internal pressure.20 Another point worthy of note is that the presence of oil with a specific heat lower than that of water enhances the rise in temperature, at a given power, suggesting that dilute nanoemulsions can be produced with a better control over temperature. 3.3. Simulation of Nonisothermal Sonication and Comments on Enhanced Cavitation at Elevated Temperatures. The formation of growing entities during nonisothermal sonication is very intriguing. The concomitant formation of oil droplets together with bubbles in the mixture makes it difficult to identify the type of entities dispersed in water, as measured by DLS. To further investigate this, emulsion behavior during sonication was assimilated by a set of experiments carried out nonisothermally using pure water, an aqueous solution of SDS (water/SDS), and an aqueous solution of SDS saturated with C6 (water/SDS/C6). The concentration of SDS was 10 g/L, if used. Unlike mixtures used in the previous section, all solutions used in this part are within thermodynamically miscible regimes and, therefore, transparent within the temperature range encountered in this work (solubility of alkanes in water increases with temperature). The presence of any dispersed entities in these systems,
temperature from the set point can be better understood by considering the instantaneous energy balance of the system: ⎛ dE ⎞ d(T − Tm) Ẇ − ⎜UA + (ṁ w λ w + ṁoλo) + emul ⎟ ⎝ dt dt ⎠ dT = mCp (3) dt where Ẇ is the sonication power, U the overall heat transfer coefficient, A the heat transfer area of the sonication vessel, T temperature of the emulsion, Tm the ambient temperature, ṁ w and ṁ o the rates of mass loss due to evaporation of water and oil, respectively, m the remaining mass of emulsion, λw and λo the latent heats of water and oil, respectively, and Cp the average specific heat of the emulsion. Equation 3 can be simplified in terms of overall energy balance as W − Qloss − Eemul = Qabs. The required useful energy for generating new interfacial area, eq 1, in the systems under study, is in the range of 3 to 13 J, depending on the final size of drops and interfacial tension for different oils, which is less than 1% of the total energy supplied through sonication (W). Therefore, 99% of the total energy was either absorbed in the system as thermal energy (Qabs = 19%) or lost via heat transfer and evaporation to the surroundings (Qloss = 80%). This clearly shows the scale of difficulty involved in controlling the emulsion temperature during sonication. The initial rate of increase in temperature was similar for all oils studied (Figure 3). However, a difference developed after temperature around 50 °C was reached. At this temperature, evaporative cooling of C6 caused the C6 nanoemulsion to depict a damped rate of increase in the temperature. The overall volume of the C6 nanoemulsion was reduced by around 9686
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aqueous solution during rest times (i.e., Figure 4, B4, B6, B8, and B12) shows that bubbles were rather large, unstable, and creamed to the top quickly. It also appears that the presence of surfactant leads to the formation and accumulation of large bubbles (i.e., foam) at the top of the solution, which in turn causes rapid diffusion of vapor from nanobubbles in the solution to the ones on top (i.e., Ostwald ripening).29 3.3.3. Sonication of Aqueous Solution of SDS Saturated with Oil. Addition of both SDS and C6 to water results in further changes in the system behavior under nonisothermal conditions. The aqueous solution of the surfactant was saturated with C6. No free C6 drops existed in the solution prior to experiment. The system behaved like a water/SDS system during the early stage of sonication, that is the first 30 s interval as shown in Figure 4, C1, but a difference emerged as soon as the temperature reached around 60 °C during the third interval, where a large number of bubbles formed and turned the solution cloudy. This suggests that nanobubbles start to form at a lower temperature in the presence of C6 in the solution because of its low boiling point temperature (i.e., high vapor pressure). However, the bubbles were rather unstable and left the solution clear behind them, as they rapidly creamed to the top during the rest time (Figure 4, C4). An increase in local temperature resulted in partial evaporation of C6; however, most C6 remained dissolved in the solution due to the high positive pressure achieved during sonication. It should also be noted that nanodrops exhibit boiling temperatures that are higher than those for the bulk phase.20 These can explain why C6 oil remained in the emulsion longer than what its boiling temperature indicates. As soon as sonication stopped and the pressure inside the solution reduced to atmospheric pressure, a part of C6, which was dissolved in the solution, boiled and left the solution in the form of bubbles. During the seventh interval and when the temperature was well above 70 °C, which is the boiling temperature of C6, a cloud of cavities suddenly formed at the tip of the sonifier horn that propagated in the solution toward the top and overflew (Figure 4, C7). The solution remained foamy during this interval, and after that, the foam creamed to the top (Figure 4, C8). The cream layer acted as a sink to exploit nanobubbles in the emulsion via Ostwald ripening, whose rate was intensified with increasing temperature because of the enhanced solubility of oil in water. The size of the bubbles was quite large, 2−4 μm and within the size range encountered in the nonisothermal sonication of the (high phase-ratio) C6 emulsion, as previously described. From the discussions above, it may be concluded that stable nanobubbles, which are within the same size range as oil nanodrops, are continuously formed and imploded in the absence of surfactant. In the presence of surfactant, nanobubbles also formed, but they grew quickly via Ostwald ripening and coalescence, and creamed. The large magnitude of Ostwald ripening is due to the wide size distribution of C6 bubbles within the system,30 but more importantly because of the formation of creamed foam layer at the top, which formed only when the surfactant was present. The formation of microbubbles, and their creaming, accelerated during rest time. Therefore, a long rest time serves to make an emulsion more homogeneous by separating bubbles from the emulsion, and it allows for more accurate measurement of drop size to be made. However, emulsions never became fully free of bubbles, and this is probably the main cause for irregularity in the size of C6
therefore, can be attributed to the formation of bubbles in the course of sonication. The device was set for sequences of 30 s active sonication interval followed by 30 s rest time. This arrangement results in a fast rise in the temperature, and also makes it possible to investigate such effects in emulsions under rest conditions. In the following text, sonication intervals will be addressed according to the numbers given in Figure 4. Therefore the odd intervals represent active sonication, while even intervals represent rest times. Note that samples for drop size measurements were taken during the first 5 - 10 s into the rest times. The higher increase in temperature, compared to previous experiments (Figure 3), is due to the lower volume of samples used and the longer sonication intervals adopted. 3.3.1. Sonication of Pure Water. Figure 4 depicts the effects of temperature on the evolution of morphology and drop size of the sonicated solution with time. During the first 30 s of sonication of water, some big bubbles (stable cavities) were visible, which were oscillating around the horn (Figure 4, A1). These stable cavities disappeared after a while, as they imploded or creamed to the top. The color of the sonified water changed to bluish after 30 s of sonication (Figure 4, A2), indicating the formation of nanosized water-vapor bubbles. It is known that air dissolved in the sonicated media (i.e., water) can act as nuclei for the formation of cavities, suggesting that these water bubbles may include air as well. Size measurements confirm the presence of water nanobubbles, in the range of 70−300 nm, in the liquid water (Figure 4, A4). Further sonication of water raised the temperature and produced more nanobubbles, as judged by the solution color turning from bluish to cloudy (Figure 4, A8) but did not significantly affect the bubble size. During the seventh interval, when the temperature was around 70−75 °C, big stable cavities were not individually visible anymore but appeared in a form of cloud at the tip of the sonifier horn (Figure 4, A7), which progressively enlarged with sonication time (Figure 4, A11). The appearance of stable cavities in the form of a cloud is the result of the formation of a large number of nuclei at higher temperatures.14,19 The vapor pressure inside the bubbles increases with temperature.17,28 This hinders implosion and results in the formation of large bubbles, and thus causes the color of the emulsion to change. The color of the water was cloudy after the seventh interval, but it turned to milky after interval nine (Figure 4, A8 and A12). However, the bubbles were still relatively small (see Figure 4, A4 for DSDs of A2, A7, and A11). The results clearly indicate that bubbles formed via evaporation of water in the absence of surfactant were quite stable and small in size. 3.3.2. Sonication of Aqueous Solution of SDS. In the presence of surfactant, stable cavities turned into large white bubbles, comprised of aggregation of very smaller ones (Figure 4, B1), during the first 30 s of sonication. They oscillated around and finally (when they became big enough to overcome the turbulence created by sonication) creamed to the top to form a layer of foam, which became thicker with continued sonication (Figure 4, B1, B3, B5, and B7). The bubbles were in the range of 2−4 μm in diameter. Figure 4, B5 and B7 shows that a higher number of nuclei formed around the sonifier horn during the fifth and seventh intervals. A comparison with the previous case of the sonified water makes it clear that the surfactant was responsible for the formation of large bubbles in the solution. Surfactants act to stabilize many nanobubbles, which then undergo coalescence to form larger bubbles with increasing temperature. A closer look at the sonified SDS 9687
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boiling temperature of the oils, but almost in the vicinity of the boiling temperature of water. For C10, the nonisothermal drop size was comparable with the isothermal drop size within 300 s of sonication, indicating that change in physical properties of the oil with temperature had little effect on the drop size before the rise. We therefore attribute the change in the average drop size with temperature, after the minimum, to diminished drop rupture as a result of shock wave dissipation due to increased cavitation. 3.4. Effect of Ultrasound Amplitude. Ultrasound amplitude is a direct measure of energy applied to nanoemulsions during emulsification. A higher energy input during a sonication process usually results in a relatively higher increase in temperature. In this section the effect of amplitude, as an indicative measure of the applied energy, on the sonication process is studied. Figure 5 depicts the time variations in drop
and C8 drops, which have low boiling temperatures, resulting from nonisothermal conditions (as shown in Figure 1a and b). The size of nanodrops during sonication is determined by two virtues, namely drop break up and drop coalescence. At a steady state, coalescence takes away the same number of drops that drop break up generates, leaving the number as well as the size of drops constant. Any variation in the size of drops can only be caused by alteration in the magnitude of the rates of drop break up and coalescence. The results obtained from simulation of emulsion sonication reveal that the structure of nanoemulsions is altered during nonisothermal sonication as more bubbles are formed. Extensive cavitation, as seen for volatile oils under nonisothermal conditions (such as C6 and C8 in Figure 1a and b), can affect drop size in two ways. First, the introduction of nano- and microbubbles into the emulsions can cause a significant irregularity in the average drop size measurements and produce scattered results (precautions were taken to allow for separation of bubbles from the diluted emulsions prior to size measurements). Second and far more important, an extensive cavitation, especially around the horn, can damp shock waves inside emulsions and lower the maximum pressure reached at the implosion. This results in an inefficient energy transmission, leading to a suppressed rate of drop rupture (i.e., increase in drop size), which intensifies with further increase in temperature.10 This latter effect appears to become stronger with increasing vapor pressure (i.e., volatility) and water solubility of oils, and sonication time, as discussed before. Note that oils, as the dispersed phase, can contribute to cavitation not only by the virtue of their droplet volatility but also by the extent of their solubility in the water continuous phase, where ultrasound waves propagate. The effect of sonication power is discussed in the next section. The emulsions reached temperatures as high as 80−85 °C after 600 s of active sonication, which is well above the boiling temperature of C6. All other oils have significantly higher boiling temperature than water. As the emulsion temperature reaches the vicinity of 85 °C, the formation of water-vapor bubbles apparently becomes more important, making all nanoemulsions vulnerable to an increase in droplet size with temperature. However, it appears from the results that bubbles generated via cavitation of the water continuous phase are not as destructive to sonication as those formed via the dispersed phase. For oils with high boiling temperatures, C12, C14, and C16, the average drop size did not change significantly with temperature and time during nonisothermal sonication, suggesting that the effect of temperature on the steady-state drop size, via alteration in physical properties of oils, such as viscosity and interfacial tension, is trivial (despite the fact that the effect was shown to be significant during the transient period). For lighter oils, drop size showed an increase with temperature in such a drastic way that cannot be justified by any fair change in the physical properties of the oils, except for the vapor pressure, which varies exponentially with temperature in the vicinity of boiling temperature and is directly related to the likelihood of the formation of cavities. For oils with high volatility, C6 and C8, cavitation intensified right from the beginning of nonisothermal sonication so that the minimum drop size was much larger than the isothermal steady-state value. Figures 1 and 3 show that the rise in drop size, after the minimum, for C6 took place when the emulsion temperature reached the vicinity of the C6 boiling temperature, but for C8 and C10 this occurred at temperature much lower than the
Figure 5. Variations in drop size with sonication time for C12 at various amplitudes for (a, top) isothermal (17.0 ± 2.0 °C) and (b, bottom) nonisothermal conditions (10 g/L SDS, 20 vol%Oil).
size for C12 nanoemulsions produced at various sonication amplitudes under isothermal and nonisothermal conditions. Increasing ultrasound amplitude from 25% to 70% decreases the drop size by a fair amount under isothermal conditions. A further increase to 100% does not result in a considerable change in the drop size (Figure 5a). Such a clear trend is not obvious in the nonisothermal experiments (Figure 5b), as the variations in ultrasound amplitude are associated with temperature rise to different extents. It has been shown that conversion of input power into delivered power reduces as the power amplitude increases.10 The production of acoustic bubbles increases in the area below the horn with increasing amplitude. These bubbles cloud shield the solution from the ultrasonic energy source and hence suppress power transmission.10 Figure 5b shows clear patterns, comprising transient and steady-state intervals, for the sonication amplitude of up to 70%. Under nonisothermal conditions, however, the drop size did not remain at its so-called steady state at the amplitude of 9688
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80% and started to increase after 150 s, depicting a minimum. Note that a similar effect was observed with time for the oils with relatively higher volatility (C6, C8, and C10) under nonisothermal conditions but at 70% amplitude. The increase in the drop size during nonisothermal sonication of C12 emulsion at 80% amplitude can be attributed to the enhanced formation of bubbles at elevated temperature associated with the higher energy input. As discussed earlier, the presence of bubbles can result in a decrease in the energy transfer efficiency and drop rupture rate, leading to a sharp rise in drop size.
4. CONCLUSION The effects of temperature on nanoemulsification with ultrasound waves were investigated. It was found that isothermal sonication is a prerequisite to the formation of homogeneous and fine nanoemulsions, especially when oils with a low boiling point are used. The final drop size of emulsions made via both isothermal and nonisothermal processes increased, but the difference widened, with decreasing chain length. Heavier oils showed little sensitivity toward temperature, but lighter oils produced larger drops at higher temperatures. Ultrasound waves produce cavities (bubbles) inside emulsions that cause nonhomogenous dissipation of energy at elevated temperatures. The formation of cavities increased with increasing vapor pressure and water solubility of oils. More cavities were stabilized in the presence of surfactant, and as a result, they grew to a greater extent. The presence of many bubbles damps the shock waves and does not allow energy to transfer to drops in order to rupture them. As a result, the average drop size may increase in the course of sonication. Increasing the input power, up to an optimum value (i.e., when it is not associated with a significant rise in the emulsion temperature), reduces the final drop size. Any increase in the input power, above the optimum range, results in an increase in the average drop size.
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The authors declare no competing financial interest.
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REFERENCES
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dx.doi.org/10.1021/ie4003014 | Ind. Eng. Chem. Res. 2013, 52, 9683−9689
