Preparation characteristics of oil-in-water emulsions using differently charged surfactants in straight-through microchannel emulsification

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Abstract

Straight-through microchannel (MC) emulsification is a novel technique for producing monodisperse emulsions using a silicon array of micromachined through-holes, named a straight-through MC. This paper investigates the effect of surfactants on the straight-through MC emulsification behavior. The oblong straight-through MC used in this study had channel lengths of 48.7 and 9.6 μm. Using anionic and nonionic surfactants, monodisperse soybean oil-in-water (O/W) emulsions with average droplet diameters of 39 μm and coefficients of variation below 3% were successfully prepared. In contrast, the systems containing cationic surfactants resulted in the preparation of polydisperse O/W emulsions and complete wetting of the dispersed phase on the plate surface. An analysis of the above straight-through MC emulsification behavior revealed that a repulsive surfactant–plate surface interaction and a high contact angle value are required to achieve successful straight-through MC emulsification. Analysis also revealed that it is important to keep the negatively charged plate surface hydrophilic during the emulsification process. The dispersed-phase flux affected the straight-through MC emulsification behavior. A size-stable zone, a size-expanding zone, and a continuous outflow zone were observed with an increase in the dispersed-phase flux when using anionic and nonionic surfactants.

Introduction

Emulsions are important materials and products in food, pharmaceutical, cosmetics, and chemical industries. Their droplet size and distribution are the most important parameters that affect stability, rheology, chemical reactivity, and physiological efficiency of any emulsion [1], [2]. Monodisperse emulsions have received a great deal of attention due to their improved stability and facilitated control of their properties. The advantages of monodisperse emulsions can develop valuable emulsions and emulsion-based materials. Conventional emulsification devices, such as dispersing machines, colloid mills, and high-pressure homogenizers, are commonly used to produce emulsions [1], [3]. However, the resultant emulsions with droplet diameters of 0.1–100 μm exhibit considerable polydispersity.

Emulsification devices for producing monodisperse emulsions have been developed in the last decade. Membrane emulsification can produce monodisperse emulsions with coefficients of variation of about 10% by forcing the dispersed phase into the continuous phase through a microporous membrane [4], [5], [6]. The resultant droplet size, which ranges between 0.3 and 30 μm, is controlled by the membrane pore size [4], [7], [8], [9], [10]. In membrane emulsification, emulsions are produced with low mechanical stress [11]. A novel grooved microchannel (MC) emulsification [12], [13] enables producing monodisperse emulsions with droplet diameters of 5–100 μm and coefficients of variation below 5% [14], [15], [16]. The unique MC structure consists of uniformly sized channels and a flat terrace line fabricated on a silicon plate. The MC emulsification process does not require any mechanical stress to make monodisperse droplets [17]. We recently developed a novel micromachined emulsification device, which we called a straight-through MC plate [18]. A straight-through MC is a silicon array of micromachined through-holes of uniform size (Fig. 1). Straight-through MC emulsification has greater throughput capacity than conventional terrace-type MC emulsification owing to a highly integrated layout of a straight-through MC. Oblong straight-through MCs were successful in preparing monodisperse emulsions with droplets on a 10-μm scale [18], [19]. A microscope video system [20] allows microscopic observations of the emulsification process [12], [18]. Microscopic observations of the membrane emulsification process became possible by using the microscope video systems [21], [22], [23], [24], [25].

The preceding techniques for monodisperse emulsions exploit interfacial properties at the membrane and channel surfaces. Surfactant molecules containing both hydrophilic and hydrophobic head groups adsorb at an oil/water interface, then exhibit specific interfacial properties that are determined by their chemical structure. Surfactants thus play an important role in the emulsification process. The type of surfactants significantly affects the droplet formation behavior in membrane emulsification [5], [26] and grooved MC emulsification [27]. Surfactant is a key parameter for the stable preparation of monodisperse emulsions in straight-through MC emulsification as well as membrane emulsification and MC emulsification. The objective of this paper was to investigate the preparation characteristics of oil-in-water (O/W) emulsions using differently charged surfactants and straight-through MC emulsification. We also studied the effect of the dispersed-phase flux on the straight-through MC emulsification behavior using appropriate surfactants.

Section snippets

Materials

Refined soybean oil (Wako Pure Chemical Ind., Osaka, Japan) was used as the dispersed phase. De-mineralized water was used as the continuous phase. Sodium dodecyl sulfate (SDS), polyoxyethylene (20) sorbitan monolaurate (Tween 20), and cetyltrimethylammonium bromide (CTAB), purchased from Wako Pure Chemical Ind. (Osaka, Japan), were used as the surfactants dissolved in the continuous phase. Tri-n-octylmethylammonium chloride (TOMAC), purchased from Wako Pure Chemical Ind. (Osaka, Japan), was

Preparation of O/W emulsions using anionic, nonionic, and cationic surfactants

Four surfactants with different charges were used to investigate the effect of the surfactants on the preparation of O/W emulsions from the oblong straight-through MC. The preparation conditions of the experiment systems are presented in Table 1. Straight-through MC emulsification was performed at an average velocity of the continuous phase along the plate surface (Vc) (lower than 1.2 mm s−1) and dispersed-phase flux (Jd) of 2.5–10 l m−2 h−1. We first investigated the droplet formation behavior

Conclusions

Straight-through MC emulsification using differently charged surfactants revealed that the surfactant–plate surface interaction and the contact angle greatly affected the straight-through MC emulsification behavior. Monodisperse O/W emulsions with CV below 3% were stably prepared from the oblong straight-through MC when using the systems containing appropriate anionic and nonionic surfactants. Their repulsive surfactant–channel surface interaction and high contact angles helped to keep the

Acknowledgements

This work was supported by the Nanotechnology Project of the Ministry of Agriculture, Forestry and Fisheries of Japan, and a Grant-in-Aid for Science Research from the Ministry of Education, Culture, Sports, Science, and Technology. The first author gratefully acknowledges the Japan Society for the Promotion of Science for financial support.

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