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Current Topics in Colloid & Interface Science   Volumes    Volume 7 
Ostwald ripening in nano-emulsions
Robert Finsy
Pages: 47 - 67
Number of pages: 21
Current Topics in Colloid & Interface Science
Volume 7 

Copyright © 2006 Research Trends. All rights reserved

When stabilized against coalescence/coagulation Ostwald ripening is the major mechanism causing the breakdown of nano-emulsions. A short summary of the classical Lifshitz-Slyozof-Wagner theory is presented. The experimental determination of Ostwald ripening rates, taking into account different effect such as initially non-stationary growth, the finite size of surfactant layers and for dynamic light scattering measurements, the conversion of intensity average sizes to number averages is given. Ostwald ripening can be distinguished from coagulation/coalescence by a method based on the dispersed phase solubility.  The main part deals with the effect of the presence of surfactant micelles on the Ostwald ripening.  To this end experimental results on some model emulsions stabilized by different ionic and non-ionic surfactants are discussed.  For the emulsions stabilized by the anionic surfactant sodium dodecyl benzene sulphonate the mass transport in both OR and solubilization is molecular diffusion of the oil through the continuous phase and no contribution of a micellar mediated transport is observed. For the emulsions stabilized by the non-ionic surfactants three different situations occur. For emulsions prepared in one single step under the high shear conditions of a microfluidizer, the mass transport is also mainly molecular diffusion. For already prepared emulsions to which extra surfactant is added, there is, besides molecular diffusion, a small contribution by a micellar mediated mechanism. In the solubilization studies the mass transport is a fusion–fission mechanism rather than molecular diffusion. Finally the retardation and inhibition of Ostwald ripening is shortly reviewed.
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