Emulsions stabilized by fine dust particles Yong Woo Kim a, 1 , Donghyun Lim a, 1 , Hyerin Han a , Seunghyun Lee b , Kyu Hwan Choi a, *, Bum Jun Park a, * a Department of Chemical Engineering, Kyung Hee University, Yongin 17104, South Korea b Department of Nanochemistry, Gachon University, Seongnam-si, 13120, South Korea A R T I C L E I N F O Article history: Received 30 July 2019 Received in revised form 5 September 2019 Accepted 13 October 2019 Available online 19 October 2019 Keywords: Emulsion Find dust particle Interface adsorption Optical laser tweezers Fluid–fluid interface A B S T R A C T We investigated the capability of fine dust particles (FDPs) to be used as stabilizers for preparing emulsions. With the aid of cationic surfactants, it was found that the FDPs formed oil in water emulsions that were extremely stable for a long time period. The stabilization mechanism was quantitatively analyzed via z-potential measurements of surface modified FDPs and interface adsorption experiments of FDP-coated microbeads by using optical laser tweezers. © 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. Introduction Fine dust particles (FDPs) or particulate matter have been causing serious environmental, social, and economical problems, particularly throughout East Asia and Africa [1–4]. The World Health Organization categorizes FDPs depending on size as PM 10 for particles smaller than 10 mm and PM 2.5 for particles smaller than 2.5 mm, and ultrafine denotes particles smaller than 0.1 mm. The FDPs in the air are adsorbed into lungs through the bronchi and cause respiratory diseases. If taken into the blood vessels, they may cause a stroke or heart disease [4–6]. Especially for children who are vulnerable to FDPs, chronic pulmonary dysfunction can lead to greater risks in the future. There are various national policies and support to investigate the production/reduction of FDPs and the health effects [7–10]. Related research in industry and academia has been ongoing vigorously, whereas strategies to utilize FDPs have been relatively uncommon. It has been reported that FDPs can be used in testing filtration, automotive, photovoltaic modules, and heavy equipment components [11–14]. Solid particles can stabilize emulsions (i.e., Pickering emulsions) and thus can be used as a substitute for molecular surfactants [15–19]. For instance, conventional colloidal particles (e.g., polymer, silica, and clay particles) tend to strongly and irreversibly attach to immiscible fluid–fluid interfaces [20–28]. Such interface adsorption behavior of the particles decreases the direct contact at the interface between the two fluid phases that are energetically unfavorable to each other, thus resulting in reduced interfacial tension. The interface-trapped particles also impart steric hindrance due to their finite size and the electrostatic repulsion between the inter-emulsions, improving the stability of Pickering emulsions. Owing to the merits of the colloidal particle systems, which are typically cheap and environmentally friendly in comparison to molecular surfactants, Pickering emulsions have been applied in various industrial fields, including food [29–31], cosmetics [32,33], catalysis [34–36], energy storage materials [37,38], oil recovery [39–41], and waste water treatment [41–45]. The tendency of particles to adhere to the fluid interface depends strongly on wettability and the surface charge of the particles [16,46–50]. In general, when the particles are not too hydrophilic or hydrophobic, the adsorbed state at the interface is more energetically stable than when the particles are immersed in a single fluid phase. Before the interface adsorption of the particles to the interface occurs, electrostatic interaction between the charges on the particle surface and on the fluid interface affects the particle adsorption probability [20,21]. When the external force exerted by emulsification procedures overcomes the electrostatic force, the particles attach to the interface, and the adsorption process is irreversible due to the strong attachment energy. The attachment energy can be determined by the difference between the surface free energy when the particle is immersed in either water (E W ) or oil (E O ) and the energy when it is attached to the oil–water interface (E I ); it is given by E IW ¼ E I  E W or E IO ¼ E I  E O . For example, the attachment energy of a spherical microsphere to * Corresponding authors. E-mail addresses: ckh6977@khu.ac.kr (K.H. Choi), bjpark@khu.ac.kr (B.J. Park). 1 These authors equally contributed to this work. https://doi.org/10.1016/j.jiec.2019.10.012 1226-086X/© 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. Journal of Industrial and Engineering Chemistry 82 (2020) 190–196 Contents lists available at ScienceDirect Journal of Industrial and Engineering Chemistry journal home page : www.elsevier.com/loca te/jiec