245

Original Technical Paper

J. Jpn. Soc. Colour Mater., 94〔9〕，245–251（2021）

Potential of High-Powered Bath-Type Ultrasonicatorfor Manufacturing of Emulsifier-Free Emulsions
Kazuo Takei＊,＊＊, Nozomi Takahashi＊＊＊ and Toshio Sakai＊＊,＊＊＊,＊＊＊＊,†＊ Tokyo Food Co., Ltd., 1687-1 Kamioshima, Tsukuba, Ibaraki 300-4351, Japan＊＊ Graduate School of Medicine, Science and Technology, Shinshu University, 4-17-1 Wakasato, Nagano, Nagano 380-8553, Japan＊＊＊ Graduate School of Science and Technology, Shinshu University, 4-17-1 Wakasato, Nagano, Nagano 380-8553, Japan＊＊＊＊ Department of Materials Chemistry, Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano, Nagano 380-8553, Japan†Corresponding Author, E-mail: tsakai@shinshu-u.ac.jp(Received June 21, 2021; Accepted August 19, 2021 )Abstract
We report here on the manufacturing methods for emulsifier-free (EF-) oil-in-water (O/W) emulsions, in which oil and water weremechanically mixed in the absence of any emulsifiers such as surfactants. A commercially available bath-type ultrasonic cleaner (Bath-US;42 kHz, 26 W) did not disperse dodecane (DD) completely in water even by the treatment for 15 min, and the average diameter of DD
droplets in the EF-dodecane-in-water (DD/W) emulsions prepared was several micrometers. Using a commercially available horn-typeultrasonic homogenizer (Horn-US; 19.5 kHz, 600 W), the EF-DD/W emulsions were prepared by treatment for ~1 min, and the average
diameter of DD droplets in EF-DD/W emulsions prepared was ~1 μm. Preparation of EF-DD/W emulsions using a high-powered bath-typeultrasonicator (HPBath-US; 28 kHz, 300 W) that we developed required the treatment period for ~1 min. The DD droplets in EF-DD/Wemulsions prepared with HPBath-US became several hundred nanometers in diameter by treatment for ~10 min. The preparation ofEF-DD/W emulsions with a commercially available rotor-stator homogenizer (RS-HG; 15,000 rpm, 800 W) required the treatment period
for ~1 min, and DD droplets in DD/W emulsions prepared were several micrometers in diameter. Colloidal stability of EF-DD/W emulsions
prepared with Horn-US and HPBath-US was higher than that of EF-DD/W emulsions prepared with Bath-US and RS-HG. These studiesindicate that HPBath-US has advantages for the manufacturing of EF-O/W emulsions. Furthermore, the continuous manufacturing ofEF-DD/W emulsions was achieved by attaching the flow chamber with HPBath-US.
Key-words: Manufacturing method, Emulsifier-free oil-in-water emulsion, High-powered bath-type ultrasonicator, Treatmentperiod, Droplet size, Colloidal stability, Continuous manufacturing

1．Introduction
An emulsion is a transient mixture of two immiscible
liquids such as oil and water prepared with the aid ofmechanical mixers (e.g., homogenizers) and surface activeagents 1 - 3 ). Since the interfacial free energy ( G ) of theemulsion (mixture of oil and water) becomes higher due tothe increase in the interface area (A) between oil and water(see equation 1 ), oil and water naturally separate from eachother with the elapsed time after preparation 1-3 ).
(1)
G = γA
where γ is the interfacial tension between oil and water.
Thus, the lower γ and the resulting lower G are required toretain the emulsion state for a longer period. Therefore, ingeneral, the emulsifiers such as surfactants are required forthe long-term colloidal stabilization of emulsions because
the emulsifiers decrease the interfacial tension (γ) between【Figures and illustrations】Figures and illustrations published in black andwhite in the journal can be seen in color at our public website「J-STAGE」.
Please make use of it.

－1－oil and water. On the other hand, we have investigated thecolloidal stability of oil-in-water (O/W) 4-12 ) and water-in-oil(W/O) emulsions 13-15 ) prepared with the ultrasonicator in theabsence of any emulsifiers such as surfactants, which werenamed as emulsifier-free (EF-) O/W and W/O emulsions,
respectively. We found that the long-term colloidal
stabilization of EF-O/W and EF-W/O emulsions preparedwith an ultrasonicator was achieved by optimization of oil
type and composition 4-15 ). Moreover, in recent years, theEF-emulsion products are strongly demanded because of thebiocompatibility of emulsion products and environmentallybenign approaches for manufacturing emulsion
products 16-24 ).
Then, in this work, the manufacturing methods of EF-O/W
emulsions were examined for practical applications. A
commercially available both-type ultrasonic cleaner( Bath-US; 42 kHz, 26 W ) , a commercially available
horn-type ultrasonic homogenizer (Horn-US; 19.5 kHz,600 W ) , a high-powered bath-type ultrasonicator(HPBath-US; 28 kHz, 300 W) that we developed and a© 2021 Journal of the Japan Society of Colour Material