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Emulsification by Ultrasound: Relation Between Intensity and Emulsion Quality

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Emulsification by Ultrasound: Relation Between Intensity and Emulsion Quality Indian Journal of Chemical Technology Vol. 4, November 1997, pp. 277-284 Emulsification by ultrasound: Relation between intensity and emulsion quality Sukti Mujumdar, P Senthil Kumar & A B Pandit* Department of Chemical Technology, University ofMumbai, Matunga, Mumbai 400 019. Received 28 April 1997; accepted 15 September 1997 Ultrasonic emulsification of oil and water was carried out and the effect of position of the ultrasound source from the interface on emulsion quality was studied using ultrasonic bath and horn. Correlations for the effect of distance of the ultrasound source from the interface on various emulsion properties such as dispersed phase fraction, droplet diameter were developed. Large variation in the emulsion properties with small changes in the position of ultrasound source was observed and correlated with an exponential type of equation. Discrepancies in the results of heterogeneous liquid phase systems reported in the literature were attributed to the small changes in the location of ultrasound source. Severe attenuation of ultrasound intensity by the oil layer was quantitatively established using decomposition of aqueous KI solution as a model reaction. The droplet diameter was predicted using Kolmogorov eddy length model. The collapse pressure developed in cavitation was determined indirectly and compares favorably with the reported values. The highly localised nature of cavitation phenomenon is also well established as a cause for the variation in the emulsion quality. Heterogeneous liquid-liquid systems hold an cavitationI. Ultrasound as a source is the one that important place in the domain of chemical has been studied extensively and commercially engineering. The success of the above systems that exploited to a significant extent. Two types of may be reacting or non-reacting, depends on the cavitation occur based on the life of the cavity. ability to generate high interfacial area by an When the cavity life is less than one acoustic equipment economically. In precise, formation of a cycle, it is called transient cavitation. The other fine emulsion is a prerequisite. Many a times, one type that does not satisfy the above criteria is has to resort to techniques such as Phase Transfer stable cavitation. Cavitational intensity is Catalysis(PTC), to overcome the reaction rate maximum when the cavitation is transient. The limitation due to low interfacial area. These driving frequency (usually 25 kHz) of the catalysts are not only costly, but also quite ultrasonic source should be less than the natural hazardous. Ultrasound assisted cavitation frequency of the cavity for the cavitation to be generating equipment could be a better solution transienf. For water and most other liquids, the especially when fine emulsions are required. The above conditions are satisfied easily. quality of emulsion generated by sonication is far Cavitation is a way to concentrate the energy of more superior to that of conventional processes. the ultrasound locally, but at numerous places The most important advantage is that the simultaneously. Positioning of the ultrasonic generation of an emulsion do not require addition source is a vital parameter affecting the quality of of surfactants. emulsion. Extensive study of this parameter has Cavitation is the formation (when surrounding been done here. This resulted in understanding pressure falls below vapour pressure), growth, and some of the fundamental aspects of cavitation violent collapse of the microbubbleslcavities. phenomenon also. Numerous ways are available for generation of Emulisfication phenomenon *Forcorrespondence Emulsification under the conditions of • 278 INDIAN 1. CHEM. TECHNOL., NOVEMBER 1997 ultrasonic irradiation occurs in two stages. In the unity, & is the average power dissipation per unit beginning, disturbances are generated at the volume, rand Pc are interfacial tension and density interface due to the vibrations as a result of the of the continuous phase respectively. The above passage of ultrasound and this gives rise to fairly equation is used frequently, though it can predict coarse drops due to the phenomenon of Rayleigh• only the order of magnitude of the maximum drop Taylor instability mechanism3,4. For this stage, the diameter. velocity needed to rupture the large interface need not be high. The typical velocity generated in the Experimental Procedure liquid on the passage of the sound (calculated Equipment-The source of ultrasonic energy for indirectly) is of the order of 0.1 m/s and this is emulsification was an ultrasonic horn of driving sufficient to rupture large interfaces. Further frequency 22.7 kHz, rated power input of 240 W breakage of fine droplets requires high inertial and an ultrasonic bath of driving frequency 22 forces to overcome the surface tension forces kHz, rated power input of 120 W. The emulsion (2a1R) which increase as the size of the droplets was analysed using IPPLUS image ana1yser, by decrease. Cavitation plays an important role at this preparing and observing slides under microscope. stage. Near the liquid-liquid interface, the velocity Emulsification using ultrasonic horn-To a 100 of the wall of the imploding cavity could reach as mL beaker, 30 mL of distilled water and 40 mL of high as 150 m/s due to the imbalance in the rush of edible oil were added as shown in Fig. 1. The liquid5• This process is called acoustic streaming. sample was sonicated for 15 min after positioning The turbulence created by this process, along with the horn <;It3 mm from the oil-water interface, in the shock waves of the collapse, may tear off part the upper oil layer above. At the end of sonicatiol\, of the droplet when present near the collapsing two emulsions were obtained. The upper layer is cavity. Thus continuous breakage of the droplets the water dispersed in oil (Upper emulsion) and the occurs in the cavitating zone up to a critical size lower layer is the oil dispersed in water (Lower that is the characteristic of the particular system3• emulsion). The temperatures of both the emulsions were measured at the end of sonication. The The maximum SIze of the droplet can be predicted densities of both the emulsions were deterrninec with the Kolmogorov eddy length theorl and is given by, inorder to calculate the weight fraction of the dispersed phase in two emulsions. Droplet size of the dispersed phase in both the emulsions was dmax =CS-2/5,)/5pr c-!l5 ... (1) analysed using an image analyser attached to a microscope. This method directly gives the where C is a constant and usually of the order of maximum, minimum and average size of the droplets in the sample. Similar experiments were performed by increasing the distance of horn from the interface to 5 mm and 7 mm. Decomposition of aqueous K1 solution-The study was conducted to quantify the intensity of ULTRASONIC HORN e 0 WATER IN OIL EMULSION OIL IN WATER EMULSION WATER IN OIL EMULSION (UPPER EMULSION) ULTRASONIC BADI s OIL IN WATER EMULSION [ ( LOWER EMULSION) TRANSDUCERS 1 Fig. l-Emulisification with Ultrasonic Horn Fig. 2-Emulisification with Ultrasonic Bath ,,), II I II , I ~'IT1' II, 'II! I """!" I MUJUMDAR et af.: EMULSIFICATION BY ULTRASOUND 279 Emulsionarea-lower Equipment 0.123.140.922551.221.00300.061.00353.40%%WaterOilAverageAverage0.021.398.6321.441.18Dia.0.612.210.984723.651.530.92254.130.613.650.925524.870.98370.922452.49Density-Density-0.92520.9229Upper1.89DropInEmulsionEmulsionInEmulsion(g/mL)(rrr/nr)upper(m2Im3)699.52.329274.0165.612.20879.522.51DropLower1.991709.4EmulsionLoweremulsion0.98653.31*10,222.9291.66782:7area-8.7110.65Interfacial1.0021Distance*10,210901.7Dia.emulsion Upper Lower Upper Emulsion m 3 Microns HORNBATH 3(mm)7 (g/mL) (Microns) ultrasound reaching the interface and the extent of Results and Discussion cavitation occurring in the aqueous phase. Sound The intensity of ultrasound decreases with an intensity reaching the aqueous phase generates increase in distance between the ultrasonic source cavities, which collapse subsequently resulting in and liquid- liquid interface6• This is mainly due to high local temperatures and pressures. This results the attenuation of sound by the molecules of the in the cleavage of the water molecules present liquid. In the presence of dispersed phase, the inside the cavity to OHo radicals, which combines attenuation is more severe7• The decrease varies to form H202 molecules. The H202 molecules exponentially with distance and is given by the oxidise the KI present in the solution, resulting in equation of the form5•6, the liberation of iodine which can spectrophotometrically be measured. One percent ... (2) solution of KI was prepared using distilled water and this was used as aqueous phase along with few where fo is the intensity at the tip of the horn, 8 is drops of starch solution for emulsification instead the distance from the source of ultrasound and a is of distilled water. The clear aqueous phase in the the attenuation coefficient. The attenuation lower emulsion was obtained by breaking the coefficient is directly proportional to the viscosity emulsion and the absorbance of the aqueous phase of the liquid. Hence position of the ultrasound was measured using UV NIS Spectrophotometer at source is an important parameter in any acoustic 355nm. The 12 concentration in lower emulsion emulsification process. could thus be estimated. The above procedure was Table 1 summarises the effect of position of the repeated for various distances of the horn from the ultrasonic source on the emulsion quality, defined interface. in terms of phase fractions and droplet sizes. The dispersed phase fraction decreases with increase in Emulsification using ultrasonic bath- The distance (8) from the interface for both ultrasonic experimental procedure for the bath is similar to horn and bath. The droplet diameter increases with that for the ultrasonic horn (Fig. 2). Here the increase in distance for horn. The trend for the bath distance between the ultrasound source (present is altogether opposite. The detailed analysis on the beneath the bottom surface of the bath) and liquid• effect of position of ultrasonic source on each liquid interface was varied using different volume.s property is discussed below.
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