Evaluation of the Precision of Drop-Size Determination in Oil/Water Emulsions by Low-Resolution NMR Spectroscopy Pavletta S. Denkova, † Slavka Tcholakova, ‡ Nikolai D. Denkov,* ,‡ Krassimir D. Danov, ‡ Bruce Campbell, § Catherine Shawl, § and Dennis Kim § Laboratory of NMR Spectroscopy, Institute of Organic Chemistry, Bulgarian Academy of Sciences, Acad. G. Bonchev str., Bl. 9, 1113 Sofia, Bulgaria, Laboratory of Chemical Physics & Engineering, Faculty of Chemistry, Sofia University, 1164 Sofia, Bulgaria, and Kraft Foods Inc., 801 Waukegan Road, Glenview, Illinois 60025 Received June 1, 2004. In Final Form: September 15, 2004 The accuracy of the recently reported low-resolution NMR method (Goudappel, G. J. W.; et al. J. Colloid Interface Sci. 2001, 239, 535) for the determination of drop-size distribution in oil-in-water emulsions is evaluated by comparing the NMR results with precise data from video-enhanced optical microscopy. A series of 27 soybean-oil-in-water emulsions, differing in their mean drop size, polydispersity, oil volume fraction, and emulsifier, is studied. Soybean oil is selected as a typical component of food emulsions. The experimental error of our optical procedure for drop-size determination is estimated to be around 0.3 μm, which allows us to use the microscopy data as a reference for the mean drop-size and distribution width of the studied emulsions, with known experimental error. The main acquisition parameters in the NMR experiment are varied to find their optimal values and to check how the experimental conditions affect the NMR results. Comparison of the results obtained by the two methods shows that the low-resolution NMR method underestimates the mean drop size, d33, by ≈20%. For most of the samples, NMR measures relatively precisely the distribution width ((0.1 to 0.2 dimensionless units), but for ∼20% of the samples, larger systematic deviation was registered (underestimate by 0.3-0.4 units). No correlation is found between the emulsion properties and the relative difference between the microscopy and NMR data. Possible reasons for the observed discrepancy between NMR and optical microscopy are discussed, and some advantages and limitations of the low-resolution NMR method are considered. 1. Introduction Drop-size distribution is an important emulsion char- acteristic, which affects various emulsion properties, such as stability to coalescence and sedimentation, rheological behavior, texture, color, and rate of release of volatile components (fragrance and flavor). 1-4 A large number of methods, such as laser light diffraction and scattering, electric sensing, acoustic spectroscopy, dielectric spec- troscopy, centrifugal sedimentation, and optical and electron microscopy, are used in research and application laboratories for drop-size determination in emulsions. 1,4-17 The choice of an appropriate method for a particular application depends on numerous factors, such as the method accuracy, reproducibility, and sensitivity; instru- ment cost; time and cost of the individual analyses; and requirements for special operator skills. Optical microscopy is arguably the most precise among the existing general methods for drop-size determina- tion. 5,6 The major advantage of optical microscopy is that it is a direct method, with straightforward calibration and well-understood limitations, caused mainly by the wave nature of light and by optical aberrations. Optical microscopy is often combined with image analysis tech- niques, which enhance the image quality and improve the precision of drop-size determination. 5,6 On the other hand, conventional optical microscopy requires diluted samples (typically around 1 vol % of the dispersed drops), * To whom correspondence should be addressed. Present ad- dress: Laboratory of Chemical Physics & Engineering, Faculty of Chemistry, Sofia University, 1 James Bourchier Ave., 1164 Sofia, Bulgaria. Phone: (+359-2) 962 5310. Fax: (+359-2) 962 5643. E-mail: ND@LCPE.UNI-SOFIA.BG. † Bulgarian Academy of Sciences. ‡ Sofia University. § Kraft Foods Inc.. (1) Sjo ¨ blom, J., Ed. 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