Langmuir 2002, 18, 1985-1990
1985
Molecular Diffusion of Oil/Water Emulsions in Surfactant-Free Conditions Toshio Sakai,† Keiji Kamogawa,‡,§ Katsuhiro Nishiyama,†,§ Hideki Sakai, and Masahiko Abe*,†,§ Faculty of Science and Technology, Tokyo University of Science, 2641, Yamazaki, Noda, Chiba 278-8510, Japan, Elementary & Secondary Education Bureau, The Ministry of Education, Sports, Culture and Science, Kasumigaseki, Chiyoda, Tokyo 100-0013, Japan, and Institute of Colloid and Interface Science, Tokyo University of Science, 1-3, Kagurazaka, Shinjuku, Tokyo 162-8601, Japan Received July 18, 2001. In Final Form: December 31, 2001 Growth processes of hydrocarbon droplets (C6-C16: n-hexane, cyclohexane, n-octane, n-decane, n-dodecane, n-tetradecane, and n-hexadecane) in oil/water emulsions under surfactant-free conditions were examined by dynamic light scattering (DLS) and freeze-fracture electron microscopy (FFEM). DLS results showed that the growth rate of droplet size decreased with increase in hydrocarbon chain length. For example, n-hexane droplets grew within 1 h from submicrometer to micrometer droplets, while n-hexadecane droplets with sizes of several tens of nanometers kept their dispersibility for 24 h. We determined the growth processes as coalescence and molecular diffusion (Ostwald ripening) in terms of the Lifshitz-Slyozov-Wagner theory and the Smoluchowski equation. Furthermore, FFEM was used to examine the growth mechanism in detail. Direct imaging of n-hexane and cyclohexane droplets by FFEM allowed us to observe very fine oil droplets (∼10 nm in diameter) though DLS could not detect these droplets, suggesting that fine droplets of shorter hydrocarbons such as n-hexane and cyclohexane grow via both molecular diffusion and coalescence processes in a very short time after preparation.
Introduction Destabilizing processes in emulsions can occur by two distinct mechanisms: coalescence1-7 and Ostwald ripening.8-12 Coalescence is the process by which two or more droplets fuse to form a single larger droplet. Conversely, molecuar diffusion (also referred to as Ostwald ripening or isothermal distillation) is the process by which larger droplets grow at the expense of smaller ones due to differences in their chemical potential. In molecular diffusion, the growth occurs by diffusion of the dispersed phase through the continuous phase. We have investigated the stability and growth processes of oil droplets in oil/water emulsions prepared by ultrasonic irradiation without addition of surfactant.13-20 Surfactantfree oil/water emulsion is the simplest system of emulsions * To whom correspondence should be addressed. Phone and fax: +81-471-24-8650. E-mail:
[email protected]. † Faculty of Science and Technology, Tokyo University of Science. ‡ Elementary & Secondary Education Bureau, The Ministry of Education, Sports, Culture and Science. § Institute of Colloid and Interface Science, Tokyo University of Science. (1) Deshikan, S. R.; Papadopoulos, K. D. J. Colloid Interface Sci. 1995, 174, 302. (2) Deshikan, S. R.; Papadopoulos, K. D. J. Colloid Interface Sci. 1995, 174, 313. (3) Chen, J. D.; Hahn, P. S.; Slattery, J. C. ALChE J. 1984, 30, 622. (4) Chen, J. D. J. Colloid Interface Sci. 1985, 107, 209. (5) Chen, J. D.; Hahn, P. S.; Slattery, J. C. ALChE J. 1988, 34, 140. (6) Derjaguin, B. V.; Landau, L. D. Acta Physicochim. URSS 1941, 14, 633. (7) Verwey, E. J. W.; Overbeek, J. Th. G. Theory of the Stability of Lyophobic Colloids; Elsevier: Amsterdam, 1948. (8) Ostwald, W. Z. Phys. Chem. (Leipzig) 1900, 34, 295. (9) Taylor, P. Colloids Surf., A 1995, 99, 175. (10) Taylor, P. Adv. Colloids Interface Sci. 1998, 75, 107. (11) Higuchi, W. I.; Misra, J. J. Pharm. Sci. 1962, 51, 459. (12) Thomson, W. (Lord Kelvin) Philos. Mag. 1871, 42, 448. (13) Kamogawa, K.; Sakai, T.; Momozawa, N.; Shimazaki, M.; Enomura, M.; Sakai, H.; Abe, M. J. Jpn. Oil Chem. Soc. 1998, 47 (2), 159.
capable of helping us clarify formation, disappearance (molecular diffusion), and growth mechanism of oil droplets. In our recent investigation, discrete droplet size distributions and stepwise growth of the oil droplets were observed by dynamic light scattering (DLS) measurements.13,16 For example, benzene/water emulsion had three size distributions appearing one by one with the lapse of time: 20-100 nm (nanometer size, 1000 nm).13 However, DLS data alone may not be possible to distinguish the growth processes such as flocculation, coalescence, and molecular diffusion. One of the principal concerns of emulsion scientists is to better understand the factors that influence these processes so that they can control the rate at which the processes proceed in a more systematic fashion. In this paper, evolutionary changes of oil droplets in water are examined using hydrocarbons (C6-C16: nhexane, cyclohexane, n-octane, n-decane, n-dodecane, n-tetradecane, n-hexadecane) under surfactant-free conditions, especially focusing on the molecular diffusion (Ostwald ripening) of oil droplets. We evaluate the growth (14) Kamogawa, K.; Matsumoto, M.; Kobayashi, T.; Sakai, T.; Sakai, H.; Abe, M. Langmuir 1999, 15 (6), 1913. (15) Kamogawa, K.; Akatsuka, H.; Matsumoto, M.; Yokoyama, S.; Sakai, T.; Sakai, H.; Abe, M. Colloids Surf., A 2001, 180, 41. (16) Sakai, T.; Kamogawa, K.; Harusawa, F.; Momozawa, N.; Sakai, H.; Abe, M. Langmuir 2001, 17 (2), 255. (17) Sakai, T.; Kamogawa, K.; Harusawa, F.; Momozawa, N.; Sakai, H.; Abe, M. Studies in Surface Science and Catalysis; Iwasawa, Y., Oyama, N., Kunieda, H., Eds.; Elsevier Science B.V.: Amsterdam, 2001; Vol. 132, pp 157-160. (18) Kamogawa, K.; Akatsuka, H.; Matsumoto, M.; Sakai, T.; Kobayashi, T.; Sakai, H.; Abe, M. Studies in Surface Science and Catalysis; Iwasawa, Y., Oyama, N., Kunieda, H., Eds.; Elsevier Science B.V.: Amsterdam, 2001; Vol. 132, pp 101-104. (19) Kamogawa, K.; Abe, M. Encyclopedia of Emulsion; Marcel Dekker: New York, in press. (20) Sakai, T.; Kamogawa, K.; Kwon, K. O.; Sakai, H.; Abe, M. Colloid Polym. Sci. 2002, 280 (2), 99.
10.1021/la0111248 CCC: $22.00 © 2002 American Chemical Society Published on Web 02/22/2002
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Langmuir, Vol. 18, No. 6, 2002
processes of these oil droplets with the aid of freezefracture electron microscopy (FFEM), a very useful method to understand rapid evolutionary changes and dispersion states of droplets in emulsion, along with DLS. Experimental Section Materials and Preparation of Oil/Water Emulsions. Cyclohexane (cy-C6), n-hexane (C6), n-octane (C8), n-decane (C10), n-dodecane (C12), n-tetradecane (C14), and n-hexadecane (C16) (Tokyo Kasei Co., Ltd.) were all of GR-grade and used as received. Distilled and deionized water of injection grade (Ohtsuka Pharmaceutical Co., Ltd.) was used. Oil was mixed with water in a flask, and the mixture was kept at 25 or 30 °C. The mixture was then sonicated for 2 or 8 min in a cleaning bath (Bransonic 220, 40 kHz, 125 W, SmithKline Co.). The concentrations of droplets of cy-C6, C6, and C8-C16 alkanes (3, 4, and 1 mM, respectively) were determined taking into consideration their solubility in water measured by the static light scattering method. The light scattering intensity drastically increases at the concentrations, indicating that oil droplets form in water above the concentrations. Measurements. Measurements of oil droplet size distribution were performed periodically by a dynamic light scattering method (homodyne method) using a Submicron Particle Analyzer, System 4700 (Malvern Instruments Co.). The detectable range of the instrument is from 5 to 5000 nm diameter. We confirmed that this instrument can detect particles (droplets) in such a range by using commercial polystyrene latex particles (Japan Synthetic Rubber Co., Ltd.) of 42 (5.3), 152 (3.7), 309 (3.5), and 3248 (161) nm in mean diameter (standard deviation). Each measurement starts within 30 s after droplet preparation, but it takes several minutes to get data fit for theoretical correlation curve. The light source was an argon ion laser (5 W total power, Coherent Co. Ltd.) with a wavelength of 488 nm, and the measuring angle was 90°. Samples for electron microscopy were prepared by freezefracture replication. A small droplet (∼10 µL) of each sample was placed on a small holder plate (plate diameter 3 mm, Hitachi) and was frozen in liquid nitrogen at -190 °C within 10 s after preparation. The specimens thus prepared were transferred to a freeze fracture device (FR-7000A, Hitachi), and fractured at -120 °C and
