Emulsification by ultrasound: drop size distribution and stability

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Abstract

The aim of this work is to compare the oil-in-water emulsions produced by mechanical agitation (Ultra-Turrax, 10 000 rpm, P=170 W) or power ultrasound (ultrasound horn, 20 kHz, 130 W) using the same model system : water/kerosene/polyethoxylated (20 EO) sorbitan monostearate. The following parameters were varied : emulsification time, surfactant concentration, consumed power and volume fraction of oil. With ultrasound, the drop size (Sauter diameter, d32) is much smaller than that given by mechanical agitation under the same conditions, which makes insonated emulsions more stable. For a given drop size (d32), less surfactant is required.

Introduction

According to P. Becher, an emulsion is “a heterogeneous system, consisting of at least one immiscible liquid intimately dispersed in another in the form of droplets, whose diameters, in general, exceed 0.1 μm. Such systems possess a minimal stability, which may be accentuated by such additives as surface-active agents, finely divided solids, etc.” [1]. In this respect, surfactants may be present as monolayers or liquid crystals; polymers are another class of emulsifiers. Those thermodynamically unstable (i.e., only kinetically stable) liquid–liquid dispersions possess a high interfacial area. The type of simple emulsion (water-in-oil or oil-in-water, commonly abbreviated as w/o or o/w) is decided mainly by the volume ratio of the two liquids, their order of addition and the nature of the emulsifier [2], characterized, for instance, along the HLB scale [3]. Emulsions are common in a large range of technologies [4] and their preparation should be carefully but easily controlled when scaled up to commercial production.

Except in special cases where spontaneous emulsification can occur [1], [5], energy must be supplied to produce such metastable mixtures. Thermodynamically speaking, surfactants reduce the surface free energy required to increase any interfacial area (ΔG=γΔA), by lowering the interfacial tension, γ, and allow finely dispersed media to be created easily. In fact, a much higher amount of energy, with respect to this thermodynamic part, is necessary, since comminution of large droplets into smaller ones involves additional shear forces, so that the viscous resistance during agitation absorbs most of the energy [2], [6]. The excess energy is dissipated as heat. Energy may be provided through various means, namely mechanical agitation (stirrer, colloid mill, mixer, valve homogenizer, …) or ultrasound generation. High-power ultrasound must be used, low-intensity acoustic waves leaving the medium unchanged. High-frequency vibrations applied to a diphasic liquid system provide a different means of breaking and dispersing a bulk phase: large drops (ca. 80 μm), produced by the instability of interfacial waves, are broken into smaller ones by acoustic cavitation [7], [8]. Ultrasound emulsification was reported for the first time by Wood and Loomis [9].The first patent was granted in 1944 in Switzerland [10]. Since then, many scientists and industrialists have used different types of ultrasound devices (whistle, horn) to make emulsions [1], [11]. Beal and Skauen have studied the influence of exposure time and sample geometry on the quality of emulsion [12]. Higgins et al. have observed that, in general, for short exposure times, higher power setting produces higher specific surface area, but an optimum amount of energy seems to exist in order to produce a maximum interfacial area [13]. Above this optimum, degradation of the surfactant may occur. In fact, it has been observed by Alegria et al. [14] that the surfactant accumulates at the interface of cavitation bubbles, where it can be chemically degraded by Hradical dot and radical dotOH radicals produced by the thermal decomposition of water [15], [16].

The ultrasound device has been found to produce o/w emulsion with very small particle sizes, more stable than those prepared, for instance, with the Eppenback Homomixer or other mechanical devices [17]. The best emulsions were prepared at optimum surfactant levels, optimum HLB [3] and high power. In the emulsions studied, the effects of HLB and surfactant parameters appear to be more important than those of the power parameters [13]. Besides, Reddy and Fogler [18] claim to have prepared very stable ‘acoustic’ o/w emulsions (however, at very low volume fraction of oil) without any surfactant: they invoke electrostatic stabilization due to preferential adsorption of OH ions at the oil/water interface.

Emulsions may remain practically unchanged to the naked eye for several months, but will eventually return to their stable state, that is, a phase-separated system. In emulsion breaking, two kinds of phenomena can be discriminated (Fig. 1): those, generally reversible, involving particle aggregation and migration and those, irreversible, related to particle size modification. On the one hand, reversible flocculation of droplets can be followed by creaming or sedimentation, according to the respective densities of dispersed and continuous phases. The migration rate of the particles of the dispersed phase is given by Stoke’s law. On the other hand, irreversible changes, through Ostwald ripening and coalescence, lead to formation of larger drops, therefore to less and less stable emulsions and eventually to phase separation. Phase inversion can occur with temperature or composition change. To summarize, emulsion stability depends on:

  • droplet size,

  • density difference between dispersed and continuous phases,

  • viscosity of the contiuous phase, and

  • above all, electrostatic and/or steric repulsion between droplets (for which the surfactant plays a major role).

Ultrasound emulsification is thus an already known phenomenon [19], but in this work our goal was to undertake a systematic phenomenological study comparing the performances of ultrasound energy with those of classical mechanical agitation, all other things being equal. Our main interests lie in the average size of droplets as well as their size distribution, and in emulsion stability.

Section snippets

Experimental

In the present work, parallel studies on batch emulsification processes were conducted through mechanical agitation and power ultrasound. An Ultra-Turrax (10 000 rpm, 170 W) and an ultrasound horn (Misonix Sonicator XL 2020, 20 kHz, 130 W) were used, respectively.

Two series of experiments were then carried out with a model o/w emulsion system. The oil was kerosene (CAS Registry No. [8008-20-6], required HLB = 12) [19] and the surfactant a polyethoxylated (20 EO) sorbitan monostearate (CAS

Results and discussion

The parameters studied were the time of emulsification, the surfactant concentration, the power supplied and the volume fraction of oil. It is important to note that there was no sonochemical degradation of sodium dodecylsulfate, a model surfactant, under our working conditions. Therefore, we conclude that over the very short time of exposure to the power source, the surfactant used in our experiments was not decomposed either.

Conclusion

With our simple, three-component, model system, the comparison of two types of emulsification processes, the first one using mechanical agitation and the second one involving power ultrasound at low frequency (20  kHz), affords several interesting results in favour of the ultrasound technique:

  • smaller average drop sizes d32 (down to 0.3 μm) can be obtained,

  • for a given desired diameter, the surfactant amount required is reduced,

  • energy consumption (through heat loss) is lower,

  • ultrasound-made

Acknowledgements

The authors thank Formulaction S.A., and especially K. Puech, for performing the stability study with their specific equipment (Turbiscan MA1000).

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