Langmuir 1999, 15, 7937-7939
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Ultrasonic Propagation in Highly Concentrated Oil-in-Water Emulsions N. Herrmann and D. J. McClements* Biopolymers and Colloids Research Laboratory, Department of Food Science, University of Massachusetts, Amherst, Massachusetts 01003 Received October 5, 1998. In Final Form: July 6, 1999 The dependence of the ultrasonic attenuation spectra (0.4-160 MHz) of n-hexadecane oil-in-water emulsions on droplet concentration (1-92 vol %) was measured. In dilute emulsions the attenuation coefficient increased with droplet concentration, but in concentrated emulsions it decreased. Traditional multiple scattering theory (Allegra and Hawley-Waterman and Truell) greatly overestimated the attenuation, even at moderate concentrations (>10%), because it ignores thermal overlap effects. On the other hand, an extended multiple scattering theory that takes these effects into account gives good agreement with experimental measurements up to much higher droplet concentrations (50-70%). Neither theory could be used to accurately predict the ultrasonic properties of highly concentrated emulsions (>90%)
Introduction Instruments based on ultrasonic spectroscopy have recently been developed to measure the disperse phase volume fraction and droplet size distribution of emulsions.1-6 These instruments have major advantages over traditional methods, such as light scattering, electrical pulse counting, and microscopy,7,8 because they can be used to analyze fairly concentrated and optically opaque emulsions without the need for sample dilution. Recent studies have shown that ultrasonic attenuation spectroscopy can be used to analyze emulsions with droplet concentrations as high as 50%, provided that thermal overlap effects are incorporated into the ultrasonic scattering theory used to interpret the attenuation spectra.9,10 In practice, there are a number of emulsion-based products that have droplet concentrations higher than this value; e.g., mayonnaise is an oil-in-water emulsion with a droplet concentration around 80%.7 It would therefore be useful to have an analytical technique that could be used to characterize the properties of these highly concentrated emulsions. The purpose of this study is to measure the ultrasonic attenuation spectra of highly concentrated oil-in-water emulsions and to determine whether their properties can be adequately described by ultrasonic multiple scattering theory (MST). We will compare our experimental measurements with predictions made by the traditional MST * Corresponding author. (1) Riebel, U.; Loffler, F. Part. Part. Syst. Charact. 1989, 6, 135. (2) Dukhin, A. S.; Goetz, P. J.; Hamlet, C. W. Langmuir 1996, 12, 4998. (3) McClements, D. J. Adv. Colloid Interface Sci. 1991, 37, 33. (4) McClements, D. J. Langmuir 1996, 12, 3454. (5) Povey, M. J. W. Ultrasonic Techniques for Fluid Characterization; Academic Press: San Diego, CA, 1997. (6) Hackely, V.; Texter J. Handbook of Ultrasonic and Dielectric Characterization Techniques for Suspended Particulates; The American Ceramic Society: Westerville, OH, 1998. (7) McClements, D. J. Emulsions: Principles, Practice and Techniques; CRC Press: Boca Raton, FL, 1998. (8) Mikula, R. J. In Emulsions: Fundamentals and Applications in the Petroleum Industry; Schramm, L. L., Ed.; Americal Chemical Society: Washington, DC, 1992; p 72. (9) McClements, D. J.; Chanamai, R.; Herrmann, N. J. Acoust. Soc. Am. 1998, in press. (10) Herrmann, N. Application de Techniques Ultrasonores a l’Etude de Dispersions. Ph.D. Thesis, Universite Louis Pasteur, Strassbourg, France, 1996.
of Allegra and Hawley-Waterman and Truell4 and with an extended MST that includes thermal overlap effects.9 The results of this study will help enable us to establish the range of applicability of the ultrasonic technique to concentrated emulsions. Experimental Procedures Materials. Reagent grade n-hexadecane and sodium dodecyl sulfate (SDS) were purchased from the Sigma Chemical Co. (St. Louis, MO). Distilled and deionized water was used for the preparation of all solutions and emulsions. Emulsion Preparation and Characterization. A 50 vol % oil-in-water emulsion was prepared by homogenizing n-hexadecane oil and surfactant solution (20 mM SDS) using a highpressure valve homogenizer (3 passes at 3500 psi) (Model MiniLab 8.30H, APV-Gaulin, Wilmington, MA). This emulsion was then either diluted with surfactant solution (7 mM SDS) or concentrated by centrifugation (1000-5000 rpm, model K, International Equipment Co., Needham Heights, MA) to form a range of emulsions with different droplet concentrations (10-92 vol %). The disperse phase volume fraction of the emulsions was determined to within 0.3 vol % by measuring their density with a pycnometer (φ ) [Femulsion - Fwater]/[Foil - Fwater]). The highly concentrated emulsions were too viscous to place directly in the pycnometer, and so they were diluted with a known amount of aqueous phase prior to making the density measurements. Their disperse phase volume fraction was then deduced from the measured density of the diluted samples using the known dilution factor. The particle size of the emulsions was measured using a commercially available instrument based on laser diffraction (LA-900, Horiba Instruments, Irvine, CA). The emulsions were diluted to a concentration of 70%). The most likely
Ultrasonic Propagation in Oil-in-Water Emulsions
reason for this is that droplets in highly concentrated emulsions are usually nonspherical and have thin layers of aqueous phase separating them. Consequently, it is not possible to treat the droplets as isolated spherical particles. In addition, there may be additional scattering and absorption phenomenon associated with the thin layers and the junction points between the droplets. Conclusions Attenuation spectra of concentrated oil-in-water emulsions are appreciably different from those predicted by traditional multiple scattering theory because of thermal overlap effects that are not accounted for in the theory. When this phenomenon is taken into account, the agreement between theory and experiment is greatly improved, and the theory can be extended to considerably higher
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droplet concentrations (50-70%). Nevertheless, neither theory could successfully describe the ultrasonic properties of highly concentrated emulsions (>70%). It is clear that a different conceptual and mathematical approach is needed to describe the ultrasonic properties of these systems. This study has important implications for the further development and application of the analytical instruments based on ultrasonic spectroscopy that are being used to analyze emulsion properties. Acknowledgment. This material is based upon work supported by the Cooperative State Research, Education, and Extension Service, U.S. Department of Agriculture, under Agreement Number 97-35503-4371. LA981480Z
