© Copyright 2001 American Chemical Society
JANUARY 23, 2001 VOLUME 17, NUMBER 2
Letters Direct Observation of Flocculation/Coalescence of Metastable Oil Droplets in Surfactant-free Oil/Water Emulsion by Freeze-Fracture Electron Microscopy Toshio Sakai,† Keiji Kamogawa,‡,§ Fuminori Harusawa,† Nobuyuki Momozawa,†,§ Hideki Sakai,†,§ and Masahiko Abe*,†,§ Faculty of Science and Technology, Science University of Tokyo, 2641, Yamazaki, Noda, Chiba 278-8510, Japan, Ele. & Sec. Ed. Bureau, the Ministry of Education, Sports, Culture and Science, Kasumigaseki, Chiyoda, Tokyo 100-0013, Japan, and Institute of Colloid and Interface Science, Science University of Tokyo, 1-3, Kagurazaka, Shinjuku, Tokyo 162-8601, Japan Received June 30, 2000. In Final Form: November 9, 2000 Direct imaging of flocculation/coalescence in surfactant-free oil/water emulsion was achieved by freezefracture electron microscopy (FFEM) on benzene oil droplets ultrasonically dispersed in water. Immediately after sonication, spherical droplets with diameters at 30-100 nm (S class) and aggregates consisting of S class droplets at 200-500 nm (M class) were observed. An hour later, we were able to obtain FFEM images, in which a number of S class droplets in flocculation coalesced into larger droplets. Additionally, two droplets of M class coalesced easily when these droplets were in contact with each other. These results support that M and L (g1000 nm) class droplets are formed through flocculation of S class droplets and coalescence of M class droplets, respectively.
Introduction Emulsions are thermodynamically unstable systems that generally break down over time through a variety of physicochemical destabilizing processes, for example, gravitational separation, flocculation, coalescence, and Ostwald ripening.1-4 These destabilizing processes cause changes in the spatial distribution and/or size of the droplets. We have investigated surfactant-free oil/water emulsions (SFEs), which can be prepared without the addition of surfactant with an ultrasonicator at extremely low oil * To whom correspondence should be addressed. Phone and fax: 81-471-24-8650. E-mail:
[email protected]. † Faculty of Science and Technology, Science University of Tokyo. ‡ Ele. & Sec. Ed. Bureau, the Ministry of Education, Sports, Culture and Science. § Institute of Colloid and Interface Science, Science University of Tokyo.
concentrations (∼0.1 vol %) slightly above oil solubility, to elucidate the mechanism of stability and growth processes of emulsions.5-7 SFE is the simplest system of emulsions capable of helping us clarify the evolution and growth processes and stabilizing factors of emulsion systems. In our recent investigation, discontinuous growth (1) Evans, D. F.; Wennerstrom, H. The Colloidal Domain, 2nd ed.; Wiley-VCH: New York, 1999. (2) McClements, D. J. Food Emulsions: Practice and Techniques; CRC Press: Boca Raton, FL, 1998. (3) Hunter, R. J. Foundations of Colloid Science; Oxford University Press: Oxford, 1986; Vol. 1. (4) Hiemenz, P. C.; Rajagopalan, R. Principles of Colloid and Surface Chemistry, 3rd ed.; Marcel Dekker: New York, 1997. (5) Kamogawa, K.; Sakai, T.; Momozawa, N.; Shimazaki, M.; Enomura, M.; Sakai, H.; Abe, M. J. Jpn. Oil Chem. Soc. 1998, 47 (2), 159. (6) Kamogawa, K.; Matsumoto, M.; Kobayashi, T.; Sakai, T.; Sakai, H.; Abe, M. Langmuir 1999, 15 (6), 1913. (7) Kamogawa, K.; Akatsuka, H.; Matsumoto, M.; Yokoyama, S.; Sakai, T.; Sakai, H.; Abe, M. Colloids Surf., A, in press.
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Figure 1. Size distributions of benzene droplets at various elapsed times at 25 °C measured by dynamic light scattering.
of oil droplets and discrete droplet size distributions were observed for SFEs by dynamic light scattering (DLS) measurements.5 There are many experimental methods to characterize the shape, droplet sizes, and properties of macroemulsion8,9 and microemulsion systems:10-12 dynamic light scattering (DLS), rheological methods, small angle X-ray scattering (SAXS), small angle neutron scattering (SANS), freezefracture electron microscopy (FFEM), electrical pulse counting, and cryogenic transmission electron microscopy (cryo-TEM). The dynamic light scattering method has usually been used to determine the droplet sizes, but the technique needs a few minutes per one measurement. Growth processes such as flocculation and coalescence occur very rapidly, and with DLS it may not be possible to see the time course of events. In this paper, we focus on the evolution and growth processes of oil droplets in a surfactant-free emulsion using benzene as the unstable oil and examine the processes by a combination of DLS and FFEM. FFEM is very useful to monitor the growth processes because it can yield direct imaging of the size, aggregates, shape, and growth processes of oil droplets dispersed in water. Although such images should be considered with care in order not to be misled by artifacts, the results obtained are in agreement with those obtained by other techniques.11,13 Furthermore, FFEM can capture the rapid evolutionary changes in surfactant-free emulsions more exactly, because this technique fixes the system at a 1000-10000 °C/s freezing rate. Experimental Section Materials and Preparation of Dispersions. Benzene (Tokyo Kasei Co., Ltd.) was at GR grade and used as received. Distilled and deionized water at injection grade (Ohtsuka Pharmacy Co., Ltd.) was used without further purification. Oil (weighed by volume with a syringe) was mixed with water in a flask and kept at 25 °C. The benzene concentration is 30 mM, (8) Weiss, J.; McClements, D. J. Langmuir 2000, 16 (5), 2145. (9) Mukesh, D.; Das, A. K.; Ghosh, P. K. Langmuir 1992, 8 (3), 807. (10) Bolzinger-Thevenin, M. A.; Grossiord, J. L.; Poelman, M. C. Langmuir 1999, 15 (7), 2307. (11) Strey, R. Colloid Polym. Sci. 1994, 272 (8), 1005. (12) Bernheim-Groswasser, A.; Tlusty, T.; Safran, S. A.; Talmon, Y. Langmuir 1999, 15 (17), 5448. (13) Nojima, S.; Sunamoto, J.; Inoue, K. The Liposomes; Nankoudo Lt. Co.: Tokyo, 1988; Chapter 3.
Letters slightly above its solubility in water (23 mM) determined by the static light scattering method. The mixture was then sonicated for 2 min in an ultrasonic cleaning bath (Bransonic 220, 125 W, Smith Kline Company). Freeze Fracture Electron Microscopy. Samples for electron microscopy were prepared by freeze-fracture replication. A small droplet (∼10 µL) of each sample was placed on a small holder plate (Φ ) 3 mm, Hitachi), and it was frozen in liquid nitrogen at -190 °C. The specimens thus prepared were transferred to a freeze-fracture device (FR-7000A, Hitachi) and fractured at -120 °C and
