© Copyright 2004 American Chemical Society
MARCH 16, 2004 VOLUME 20, NUMBER 6
Letters Preparation of Oleic Acid/Water Emulsions in Surfactant-Free Condition by Sequential Processing Using Midsonic-Megasonic Waves Keiji Kamogawa,†,‡ Gen Okudaira,§ Mitsufumi Matsumoto,§ Toshio Sakai,§ Hideki Sakai,‡,§ and Masahiko Abe*,‡,§ Elementary and Secondary Education Bureau, The Ministry of Education, Sports, Culture, Science and Technology, Kasumigaseki, Chiyoda, Tokyo 100-8959, Japan, Institiute of Colloid and Interface Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjyuku, Tokyo 162-8601, Japan, and Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan Received April 15, 2003. In Final Form: December 17, 2003 Emulsifying action of high-frequency acoustic waves was investigated on surfactant-free oleic acid/ water mixtures using megasonic irradiation (200 kHz and 1 MHz) in baths manufactured for precision cleaning. While the droplet size distribution was bimodal for single irradiation at 40 kHz or 1 MHz, a unimodal distribution was obtained at 200 kHz irradiation. The whole amount of dispersed oil was evaluated with the total organic carbon measurement. The largest dispersed amount was obtained for the 40 kHz treatment, while the amount was quite small for 200 kHz or 1 MHz treatment. Then, a tandem emulsification method was devised for oleic acid/water emulsion, in which the first irradiation was made at 40 kHz, followed by the second one at 200 kHz over varying exposure times. A primary peak appeared at 100 nm droplet size accompanying with a subpeak at 400 nm in the first dispersion. The latter population decreased with increase in exposure time in the second dispersion until it disappeared. This indicates that the large droplets are disrupted to form or release the smaller ones as a result of sonic acceleration. The averaged droplet size showed a preliminary increase before being stabilized within 2 days in the 40 kHz f 200 kHz and 40 kHz f 200 kHz f 1 MHz sequential processing. Contrary to a low stability for the emulsion obtained at 40 kHz exposure, the turbidity for these emulsions could be observed for 1-2 years. A slight rise in the ζ potential was noticed for the droplet at the early stage. These results demonstrate that sequential processing with megasonic waves has a high potential to improve characteristics of unstable emulsions in surfactant-free condition.
1. Introduction Acoustic emulsification is one of the techniques to disperse immiscible oil in water or vice versa. It can provide an excess energy of interface formation, thereby enabling * To whom the correspondence should be addressed. † Elementary and Secondary Eduaction Bureau, The Ministry of Education, Sports, Culture, Science and Technology. ‡ Institiute of Colloid and Interface Science, Tokyo University of Science. § Faculty of Science and Technology, Tokyo University of Science.
us to prepare emulsions even without addition of any surface-active materials. As has been pointed out with waxy hydrocarbons,1,2 emulsions prepared with acoustics have the characteristic of giving narrow and metastable droplet size distributions in the submicrometer range. While high-power homogenizers at low frequencies, ∼20 kHz, were often used for emulsification before,1-5 cleaning (1) Li, M. K.; Fogler, H. S. J. Fluid. Mech. 1978, 88, 499-512 and 513-528. (2) Reddy, S. R.; Fogler, H. S. J. Phys. Chem. 1980, 84, 1570-1575.
10.1021/la030160z CCC: $27.50 © 2004 American Chemical Society Published on Web 02/07/2004
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baths at frequencies ∼40 kHz are commonly used these days. The mechanism of low-frequency acoustic emulsification is understood to consist of (1) destabilization of the oil/water (o/w) interface by surface vibration and (2) disruption of oil drops by cavitation.1 However, prolonged exposure of particulates to an intense acoustic field higher than ∼101 W/cm2 sometimes arouses damage to liquid molecules or the surface of solids such as silica particles.6,7 Then, bath-type cleaners of lower power are convenient for gentle emulsification. With the bath method at 40 kHz, we also investigated the properties of surfactant-free emulsions (SFEs) of hydrophobic oils and found the formation of droplet in the 101 nm range, monodispersed or bimodal profiles of the droplet size distribution,8 and a sufficiently high stability for oleic esters9 and their mixtures with hydrophobic oils10,11 or polymers.12 For small oil molecules, bimodal size distribution is usually observed. On the other hand, cavitation hardly occurs in submegasonic-megasonic irradiation due to an elevation in the threshold13,14 power that produces cavitation, and so sonic waves cannot be applied for emulsification. Instead, they impose acceleration on solvent molecules and particulates.15 The effect becomes more significant for smaller particulates and at higher frequencies, so that its application has widely been developed as precision or megasonic cleaning in semiconductor wafer manufacturings.13-15 Meanwhile, megasonic waves as well as being a high power source at low frequencies are known to induce sonochemical effects including radical formation and molecular degradation, which have been used to open up a new field in sonochemistry, such as the sonolysis of methanol at 1 MHz,16 a new synthetic or degradation strategy of compounds,17,18 etc. While ultrasonic initiation has also been developed for radical polymerization with high-power probes at 20 kHz in the pioneering studies,19-22 submegasonic baths can give a more gentle condition for the initiation.23 We have paid attention to the acceleration effect of megasonic waves on SFE from a viewpoint of acoustic (3) Abismail, B.; Canselier, J. P.; Wilhelm, A. M.; Delmas, H.; Gourdon, C. Ultrason. Sonochem. 2000, 7, 187-192. (4) Behrend, O.; Schubert, H. Ultrason. Sonochem. 2001, 8, 271276. (5) Eberth, K.; Merry J. Int. J. Pharm. 1983, 14, 349-353. (6) Lu, Y.; Riyanto, N.; Weavers, L. K. Ultrason. Sonochem. 2002, 9, 181-188. (7) Farmer, A. D.; Collings, A. F.; Jameson, G. F. Int. J. Miner. Process. 2000, 60, 101-103. (8) Kamogawa, K.; Abe, M. Encyclopedia of Surface and Colloid Science; Marcel Dekker Inc.: New York, 2002; pp 5214-5229. (9) Kamogawa, K.; Akatsuka, H.; Matsumoto, M.; Yokoyama, S.; Sakai, T.; Sakai, H.; Abe, M. Colloids Surf., A 2001, 180, 41-53. (10) Kamogawa, K.; Matsumoto, A.; Kobayashi, T.; Sakai, T.; Sakai, H.; Abe M. Langmuir 1999, 15, 1931. (11) Matsumoto, A.; Kamogawa, K.; Katagiri, T.; Sakai, T.; Sakai, H.; Abe M., to be submitted. (12) Kamogawa, K.; Kuwayama, N.; Katagiri, T.; Akatsuka, H.; Sakai, T.; Sakai, H.; Abe, M. Langmuir 2003, 19 (10), 4063-4069. (13) Schwartzman, M. A.; Kern, W. RCA Rev. 1985, 46, 81-103. (14) Gale, G. W.; Busnaina, A. A. Part. Sci. Technol. 1995, 13, 197211. (15) McQueen, D. H. Ultrasonics 1986, 24, 273-280. (16) Butnner, J.; Gutierrez, M.; Henglein, A. J. Phys. Chem. 1991, 95, 1528-1530. (17) Current Trends in Sonochemistry; Price, G. J., Ed.; The Royal Society of Chemistry: Cambridge, 1992. (18) Sonochemistry and Sonoluminescence; Crum, L. A., Mason, T. J., Reisse, J. L., Suclick, K. S., Eds.; NATO ASI series C-524; Kluwer Academic Pub.: London, 1999. (19) Biggs, S.; Grieser, F. Macromolecules 1995, 28, 4877. (20) Cooper, G.; Gieser, F.; Biggs, S. J. Colloid Interface Sci. 1996, 184, 52. (21) Chou, H. C.; Liu Y.; Stoffer, O. J. Appl. Polym. Sci. 1999, 72, 797, ibid. 827. (22) Ooi, S. K.; Biggs, S. Ultrason. Sonochem. 2000, 7, 125-133. (23) Okudaira, G.; Kamogawa, K.; Sakai T.; Sakai, H.; Abe M. J. Oleo Sci. 2003, 52 (3), 167-170.
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emulsification. As noted above, megasonic waves have characteristics different from those of kilohertz waves and their effects are significant for submicrometer particles. Megasonic irradiation has a high potency to make predispersed droplets finer, even though it may not be suitable to emulsify the original o/w mixture. Thus, it will be adequate for additional processing of SFE droplets at the submicrometer level.8-12 Megasonic acceleration is expected to break up the larger droplets to form smaller oil droplets and prevent the small droplets from recoalescencing, thus generating more stable and smaller droplets with a unimodal size distribution. In this study, we investigate how submegasonic-megasonic waves influence the dispersion yield, droplet size, and growth stability of oleic acid SFE selected as a model system. 2. Experimental Section Materials. Highly purified oleic acid (purity >99.%) was a kind gift from Professor Masao Suzuki of Kyushu University. n-Hexadecane at 98% purity from Tokyo Kasei Co. and the distilled and deionized water from Otsuka Pharmaceutical Co. were used as received. Emulsion Preparation. A given volume of oil was added to 50 mL of water in a flask to make the oil concentration 50 mmol/L for oleic acid and 5 and 10 mmol/L for n-octane and n-hexadecane, respectively. The water/oil mixture was agitated with a vortex mixer for 20 s at room temperature and then treated successively with cleaning baths of 40 kHz type (Bransonic220, 125 W, SmithKline Co.), 200 kHz type (CA-66S-61, 600 W KAIJO), and 1 MHz type (W-357H.P. 600 W Honda Electric Co.). An ultrasonic homogenizer of 20 kHz probe (US-300T Nippon Seiki) was also used for comparison. Measurements. Droplet size distribution was determined by the dynamic light scattering method at 30 °C with a NICOMP380 ZLS without diluting the mixture. The minimal measurement time of 10 min was required for setting and stabilizing of sample before the first data points were obtained. The whole amount of dispersed oil in water was determined by the total organic carbon (TOC) measurement with a TOC meter (C-5000 Shimadzu), in which the dispersion was repeatedly sampled from the bottom of the flask used in emulsion preparation. Tandem Emulsification. Since cavitation becomes weaker at higher acoustic frequencies, megasonic baths have been regarded as ineffectual devices for emulsification. Instead, we planned a tandem emulsification sequence, that is, successive exposure to acoustic fields from the low-frequency irradiation at 40 kHz to the high-frequency one at submegasonic (200 kHz) to megasonic (1 MHz) regions. While simultaneous double- or bifrequency irradiation is being adopted to enhance the cavitation yield in sonochemical reactions (28 kHz + 1.7 MHz,24 28 kHz + 0.75-1.06 MHz25) in recent years, the tandem treatment will separate contributions from the two steps of irradiation. In addition, the tandem processing will be effective in preparing surfactant-free o/w emulsions since they have enough dispersion stability longer than 10 min in the submicrometer range in several hydrocarbon SFEs.8
3. Theory An ultrasonic wave with a frequency f and an oscillating amplitude ξ excited by transducer vibration, ξ(t) ) ξ sin(2πft), would induce the maximum acceleration (a) expressed by eq 1
a ) (2πf)2ξ
(1)
This equation allows the prediction that the accelerations at 200 kHz and 1 MHz will be 25 and 625 times larger, (24) Zhao, Y.; Zhuc, C.; Feng, R.; Xu, J.; Wang, Y. Ultrason. Sonochem. 2002, 9, 241-243. (25) Feng, R.; Zhao, Y.; Zhuc, C.; Mason, T. J. Ultrason. Sonochem. 2002, 9, 231-236.
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respectively, than that at 40 kHz with a fixed value of ξ. This acceleration is also related to the acoustic power I as
a ) 2πf(I/Fc)1/2
ζ ) a/r + b
(3)
The acceleration of molecules of the medium will induce breakup of particulates as a direct effect of sonic wave. Such extremely high acceleration is a prominent characteristic of precision cleaning. For a large oil droplet, this leads to its breakdown into smaller droplets. If a spherical small oil droplet with radius r is broken off from a large oil drop as in the situation observed in the drop weight method for surface tension measurement, the acoustic and interfacial forces will be balanced as eqs 4 and 5 predict
(4/3)πr3Fa ) 2πrγ
(4)
r ) (3γ/2Fa)1/2
(5)
When a 200 kHz bath is used, eq 5 gives r ) 15 µm for γ ) 15.6 mN m-1 of oleic acid/water. While the critical size is still larger than the value of d ∼ 100 nm obtained with 40 kHz irradiation,9 eq 5 qualitatively suggests that the broken off droplet will be smaller at higher frequencies. Similar droplet sizes and frequency dependence are also suggested in ultrasonic nebulization of water26,27 as given by the following
d ) constant × (8πγ/F)1/2f 2
(7)
(2)
where F is the medium density and c is the acoustic velocity. In the present experimental conditions of f ) 200 kHz, I ) ∼1.5 W/cm2, F ) 1 g/cm3, and c ) 1500 m/s at 30 °C
a ) ∼1.3 × 105 m/s2 ) 1.3 × 104 G
depending strongly on the reciprocal of diameter as expressed by9
(6)
Further smaller droplet size may be attained by another balance with adhesion force if the smaller droplets are assumed to have adhered to larger ones before their disruption. This balance is estimated to stand at submicrometer size for solid silica particles.28 Broken up and accelerated droplets will then be subjected to a frictional force given by Ff ) 6πrηv at a particle velocity of v and the viscosity of the medium η. This results in higher friction for smaller droplet, and hence these droplets will move with the acoustic wave while large ones will hardly do so. Acceleration is therefore a direct effect of the sonic wave with which we are concerned here for droplet disruption. Meanwhile, cavitaion and microstreaming around the particulate14,29 occur as an indirect effect and are also regarded essential for precision cleaning. They would assist the tearing off effects mentioned above. 4. Results and Discussion In the present study, the oil mainly used was oleic acid, which is hydrophobic and soluble in benzene but scarcely soluble in water (∼0.1 mmol/l) despite its polar group.9 Oleic acid SFE with discrete bimodal peaks at ∼100 nm, ∼400 nm, and ∼4 µm was observed during the droplet growth as in the case of benzene SFE. It is also characterized by changes in the zeta potential of droplets, (26) Lang, R. J. J. Acoust. Soc. Am. 1962, 34, 6. (27) Moon, Y. W.; Chung, H. J.; Woo, S. I.; Hwang, S.; Lee, M. Y.; Park, S. B. J. Aerosol Sci. 1997, 28, S525-526. (28) Olim, M. J. Electrochem. Soc. 1997, 144, 3657-3659. (29) Gormley, G.; Wu, J. J. Acoust. Soc. Am. 1998, 104, 3115-3118.
The dominant contribution from the a term suggests that the electrostatic nature of the droplet surface varies in proportion to the geometric curvature, instead of remaining constant. On the contrary, if the contribution from the b term is dominant as noticed for benzene,12 the electrostatic nature well resembles that of the macroscopic interface. Oleic acid may form oleate at higher pH. In this study, pH of water used is around 5.5 since CO2 is dissolved. After ultrasonication with 200 kHz capable of producing OH radical from water, pH shifts to a lower value (∼ 4.0) due to the nitric acid formation derived from dissolved N2, instead of shifting to higher value due to the degassing. On the other hand, pKa of oleic acid is around 9.0. This means that dissociation of oleic acid to oleate is negligible. Evaluation of Dispersion Efficiency. The dispersed amount of oleic acid at the initial stage was measured by the TOC method. At single irradiation at 40 kHz for 8 min, 60% of the whole amount could be dispersed, but only 30 and 4% dispersions were obtained with 200 kHz and 1 MHz irradiations, respectively, as the minimum estimation. Successive exposure to 40 kHz (8 min) f 200 kHz (8 min) and high-power 20 kHz (8 min) irradiations resulted in ∼80% yield of dispersion as the maximum efficiency. The highest efficiency attained by the tandem processing indicates that megasonic emulsification is more effective in treating predispersed droplets than untreated mixtures. Optimal Condition for Emulsification. Appropriate time length for the second irradiation (with higher frequencies) was estimated with the averaged droplet size of n-hexadecane SFE obtained from the size distribution. The averaged diameter in the volume-mode reached a value as low as 155 and 128 nm at 40 kHz and 200 kHz, respectively, in single exposure for 8 min. Such a saturating tendency of droplet size is similar to that found in low-frequency emulsification.3 The saturation behavior was also noticed with the dispersion yield by the TOC method. Megasonic irradiation for 8 min was therefore taken as the standard condition in the present processing. Effect of Single Irradiation on Droplet Size. Droplet size and its change with time were monitored for single frequency exposure to compare the disrupting efficiency of respective frequencies. In 40 kHz irradiation, the initial droplet size distribution of oleic acid SFE consisted of a principal peak at 100 nm and a subpeak at ∼400 nm in the volume-mode. The principal population jumped to ∼1 µm in a week after growing slowly for 1-3 days. In 200 kHz irradiation, a single peak of d ∼130 nm appeared and grew continuously, whereas 1 MHz irradiation gave a bimodal size distribution with peaks at 150 and 400 nm and the latter main peak grew continuously. Figure 1 shows the mean droplet size calculated from the distribution function. The smallest size was obtained again in 200 kHz irradiation. The mean size at 200 kHz processing was larger than that at 1 MHz processing after standing a day due to the contribution of larger droplets. The growth was slower in both cases than that in 40 kHz single irradiation. Effect of Tandem Emulsification. Figure 2 shows the initial size distributions of the oil droplets after the second ultrasonic irradiation at 200 kHz following the pretreatment with 40 kHz for 8 min.
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Figure 1. Changes in mean particle size with single frequency irradiation: (b) 40 kHz, 8 min; (2) 200 kHz, 8 min; (9) 1 MHz, 8 min.
Figure 2. Droplet size distributions after various times with 200 kHz irradiation in tandem processing. Exposure time at 200 kHz is (a) 0 min, (b) 3 min, (c) 5 min, and (d) 8 min. Water/ oleic acid mixture was pretreated with 40 kHz sonication for 8 min.
After 200 kHz processing, the height of the subpeak relative to the principal one decreased as the exposure time (with 200 kHz) increased from 3 to 8 min and finally disappeared. This suggests that 400 nm droplets produced at 40 kHz were disrupted by the acceleration to release 100-nm-sized droplets. However, the disruption was not
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Figure 3. Temporal changes in mean droplet size in 40 kHz and sequential processing. Irradiating condition is (b) 40 kHz, 8 min, (4) 40 kHz, 8 min f 200 kHz, 8 min, and (0) 40 kHz, 8 min f 200 kHz, 8 min f 1 MHz, 8 min.
gained in the reversed order of 200 kHz f 40 kHz. This droplet size is consistent with that observed at lower oil contents, and hence the released droplets seem to be metastable. The mean size in the volume mode decreased from 232 nm in single 40 kHz irradiation to ca. 100 nm in two-step irradiation at 40 kHz f 200 kHz, and to 140 nm in threestep irradiation at 40 kHz f 200 kHz f 1 MHz. Figure 3 shows temporal changes in the mean droplet size. The mean size converged in 2 days to 440 nm at 40 kHz f 200 kHz and to 340 nm at 40 kHz f 200 kHz f 1 MHz. This converging behavior of the mean size resembles that in the preliminary growth of droplets induced on polymer addition in the polystyrene/benzene system.12 The initial droplet size of 100 or 140 nm in diameter is therefore unlikely to be most stable. The overall stability including creaming was monitored with photographic recording as shown in Figure 4. Although the turbidity reduced by half in 1 month, the dispersed oil clearly remained in tandem irradiation while no dispersed oil was observed in 40 kHz single irradiation at the time. The observed enhancement in stability would be related with the surface properties, reduced size, and unimodal size distribution of droplets. Since a surface cleaning effect on the shape and chemical composition of solid particles was found in the low-frequency sonolysis of silica particles,7,8 megasonic waves may have some potential to affect the droplet surface. If a megasonic wave modifies the surface of oleic acid droplets, changes would appear in their zeta potential. The observed potential was -39.9 mV in 40 kHz single processing, while -34.2 mV in 40 kHz f 200 kHz double processing and -33.7 mV in 40 kHz f 200 kHz f 1 MHz triple processing. This contradicts with a potential change regulated by the size reduction because eq 7 predicts that the ζ potential should become a more negative value. This potential shift is analogous to the shift observed when the value for watersoluble benzene was compared with that for n-alkane droplets. So change in hydrophobicity of the droplet surface of oleic acid may affect the change in ζ potential. A similar phenomenon is also known to occur when hydrophobic polyisobutylene is introduced to benzene SFE.12 From the results mentioned so far, we can conclude that the sequential emulsification with 200 kHz and 1 MHz significantly improved (1) the averaged droplet size, (2) monodispersity of droplets, and (3) short- and longterm stabilities of oleic acid SFE. The limited yield for single irradiation at high frequencies is due to the low cavitation power for large drop disruption. These advantages in sequential processing will be also favorable to
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Figure 4. Photographic observations of demulsification of oleic acid dispersion.
acceleration or control in sonochemistry and miniemulsion polymerization, which have been performed mainly with high-power probe systems.19-22 The improvements are ascribable to tearing off and breakup of smaller droplets of d ∼ 100 nm from larger drops or flocculates formed in the first 40 kHz irradiation step, while making their surface rather hydrophilic. The latter modification of surface seems to assist the former effects through “surface activation”, and as well as microstreaming would push out the released droplets randomly, thus preventing them from recoalescence.14,29 Yet, turning tje droplet surface hydrophilic is opposing the hydrophobic strategy developed to make the surface hydrophobic for droplet stabilization through addition of hydrophobic oils8-11 and polymers.12 A certain, unknown mechanism might be involved in the acoustic processing. Among the improvements achieved (1)-(3), a monodispersed profile seems to be the most characteristic of sequential emulsification using megasonic baths. It is in contrast to a unimodal but wide size
distribution which resulted from intense irradiation at low frequencies, such as in the range 0.05-2.65 µm in the presence of surfactant.3 We suppose that the observed monodispersity of polymers synthesized at 200 kHz initiation with the present experimental setup23 would be partly due to the monodispersed generation of monomer droplets by the submegasonic wave applied, in addition to the radical chain kinetics so far concerned. Consequently, high frequency or megasonic irradiation significantly contributed to the stabilization of oleic acid droplets over a short and long period. Such monodispersity was also observed for n-octane SFE in the sequential processing. The microscopic mechanism of megasonic emulsification is still under detailed investigation.30 LA030160Z (30) Okudaira, G.; Kamogawa, K.; Sakai, T.; Sakai, H.; Abe, M. In preparation for submission.
