Ultrasound emulsification: Effect of ultrasonic and physicochemical properties on dispersed phase volume and droplet size

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Abstract

Ultrasonic emulsification of oil and water was carried out and the effect of irradiation time, irradiation power and physicochemical properties of oil on the dispersed phase volume and dispersed phase droplet size has been studied. The increase in the irradiation time increases the dispersed phase volume while decreases the dispersed phase droplets size. With an increase in the ultrasonic irradiation power, there is an increase in the fraction of volume of the dispersed phase while the droplet size of the dispersed phase decreases. The fractional volume of the dispersed phase increases for the case of groundnut oil–water system while it is low for paraffin (heavy) oil–water system. The droplet size of soyabean oil dispersed in water is found to be small while that of paraffin (heavy) oil is found to be large. These variations could be explained on the basis of varying physicochemical properties of the system, i.e., viscosity of oil and the interfacial tension. During the ultrasonic emulsification, coalescence phenomenon which is only marginal, has been observed, which can be attributed to the collision of small droplets when the droplet concentration increases beyond a certain number and the acoustic streaming strength increases.

Introduction

One of the first industrial applications of high intensity acoustic energy was in the process of emulsification. When the interface of two immiscible liquids is ultrasonically irradiated, an emulsion is formed, i.e., tiny droplets of one liquid (dispersed phase) are scattered into the other liquid, which constitutes the continuous phase. When a liquid is irradiated by ultrasound, cavitation occurs when the pressure amplitude of the applied sound source reaches a certain minimum. This is known as the cavitation threshold. In oil/water system, the process of emulsification initiates when the cavitation threshold is attained. Ultrasound can provide an excess energy for new interface formation; hence it is possible to obtain emulsions even in the absence of surfactants (emulsifiers) [1]. For any intensity above this threshold there is a corresponding maximum concentration (limiting) of emulsion (% of dispersed phase hold-up) which remain relatively stable, that can be produced. This limiting concentration of emulsion increases with an increasing ultrasound intensity [2]. Its existence is attributed to the attainment of an equilibrium between the two conflicting processes of emulsification and coagulation [3]; higher limiting concentrations are produced by traveling sound waves than by standing waves, the process of coagulation (coalescence) being more pronounced with the latter [4]. The cavitation pressure threshold in a liquid increases with an increase in the viscosity of liquid. Usually the less viscous liquid undergoes cavitation more easily [5] and becomes the continuous phase (oil–water, o/w emulsion) of the emulsion. The liquid which is higher in quantity forms the continuous phase. The choice of the continuous and disperse phase to a certain extent is also dependent on the location of the energy dissipating source. The earlier work on the process of emulsification can be categorized in the form of mainly experimental [6], [7], [8], [10] or a combination of experimental and theory [7], [8], [9]. Some of the salient observation of these can be summarized as follows:

  • 1.

    A minimum threshold intensity is required for the onset of process of emulsification.

  • 2.

    Increase in irradiation power increases the quality (stability) of emulsion.

  • 3.

    Increase in the irradiation time, decreases the dispersed phase droplet size and increases the fractional dispersed phase hold-up.

  • 4.

    The forces responsible for the process of the emulsification, even in ultrasound irradiation are the same and can be summed up in the form of Weber number (which is the ratio of the dynamic forces acting on the droplet to the surface tension forces) accounting for the hydrodynamic process, and Ohnesorge number (which is defined as the ratio of viscous force to the geometric mean of the inertial and surface tension forces) accounting for the physico-chemical properties of the system.

Some of the important semi quantitative observations from the earlier work are listed below:

Muzumdar et al. [11] carried out a study of ultrasonic emulsification of oil and water and observed the effect of position of the ultrasound source from the interface on emulsion quality using ultrasonic bath and horn. The prediction of decrease in emulsion quality due to decreased intensity of the ultrasound reaching the interface (attenuation) for cavitation was confirmed by the aqueous KI decomposition reaction which showed similar attenuation behaviour. Abismail et al. [12] carried out the comparative study of emulsification using mechanical agitation and power ultrasound. Smaller average drop sizes d3,2 (about 0.3 μm) had been obtained with the later. Behrend et al. [13], [14] studied the influence of continuous phase viscosity on the emulsification by ultrasound. Continuous phase viscosity was varied by means of water soluble stabilizers (o/w systems) and different oils (w/o systems). Table 1 illustrates the work on ultrasonic emulsification reported by various researchers, the system studied, the missing information, and the issues still unexplained. It was important to know the variation of dispersed phase holdup and dispersed phase droplet size with the parameters such as, power of ultrasound source, irradiation time and physico-chemical properties of the two fluids and hence these are the focus of the present work. Two different edible oils and two mineral oils, varying in physico-chemical properties, have been used in the present work to study the formation of emulsion and their qualities (dispersed phase hold-up and droplet size).

Section snippets

Materials

Paraffin (light) and paraffin (heavy) oil were purchased from SD Fine Chemicals Ltd. Mumbai, India. Soyabean oil and groundnut oil were purchased from local market.

Experimental procedure

To 100 ml beaker, 30 ml distilled water and 40 ml of oil were added as shown in Fig. 1. The source of ultrasound energy for emulsification was an ultrasonic horn of driving frequency 22.7 kHz, the power of which could be varied as required. For example, for soyabean oil–water system, and for power 30 W the sample was sonicated for 10 min

Results and discussion

The effect of various parameters on the dispersed phase volume and droplet size of dispersed phase has been discussed below. It was observed that there is no change in the acid value (measured but not reported) of the oil as well as the emulsions after ultrasonic irradiation. This eliminates the likely possibility of chemical effects of the ultrasonic irradiations during the emulsification as has been reported by Pandit and Joshi [15] at least over the time period of these experiments. Hence,

Conclusions

Ultrasound assisted emulsification has been carried out with oils of different physicochemical properties. The effect of irradiation time and irradiation power on dispersed phase volume and droplet size has been studied. With an increase in the irradiation time, the dispersed phase volume increases while the dispersed phase droplets size decreases. As the irradiation power is increased, there is an increase in the hold-up of the dispersed phase while the droplet size of the dispersed phase

Acknowledgement

ABP would like to acknowledge the funding from the University Grant Commission (UGC) for the work.

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