Colloids and Surfaces A: Physicochem. Eng. Aspects 231 (2003) 11–17 On the mechanism of catastrophic phase inversion in emulsions F. Bouchama a,∗ , G.A. van Aken b , A.J.E. Autin c , G.J.M. Koper d a Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands b Wageningen Centre for Food Sciences, NIZO Food Research, P.O. Box 20, 6710 BA Ede, The Netherlands c University of Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, The Netherlands d Laboratory of Physical Chemistry, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands Received 5 June 2003; accepted 22 August 2003 Abstract This paper reports on the mechanism of catastrophic phase inversion. Experiments were carried out in a stirred vessel where phase inversion was detected by a jump in emulsion conductivity. The system studied contained paraffin oil, water and Triton X-100. The position of the phase inversion has been compared between two emulsification routes of phase inversion, namely the direct emulsification route and the wash-out route. The locus of phase inversion was found to be strongly different for both cases. In particular for the wash-out route the way of dilution appeared to be an important parameter, which to our knowledge has never been studied in detail before. Our experiments show that the locus of catastrophic phase inversion is determined by the added volume of the dispersed phase at each step of the dilution rather than the addition rate. This observation is explained on the basis of the role of multiple emulsion formation in catastrophic phase inversion. © 2003 Published by Elsevier B.V. Keywords: Emulsions; Catastrophic; Inversion; Multiple emulsion; Volume fraction 1. Introduction Emulsions are metastable colloids made out of two immiscible fluids, one being dispersed in the other in the presence of a surface-active agent. Emul- sion type, water-in-oil (w/o) emulsion or oil-in-water (o/w) emulsion, and emulsion stability are known to be associated with the equilibrium phase behaviour in surfactant-oil-water systems [1–5]. Conversion be- tween these two types of emulsion is generally called “phase inversion” or inversion. The term catastrophic ∗ Corresponding author. Tel.: +31-71-527-4237. E-mail address: f.bouchama@chem.leidenuniv.nl (F. Bouchama). phase inversion was introduced by Salager [6] to de- scribe inversion in emulsions induced by changes in the emulsion water-to-oil ratio, as opposed to transi- tional inversion induced by changing the surfactant affinity for the two phases. The term “catastrophic” was chosen after the suggestion by Dickinson [7,8] that elementary catastrophe theory may be applicable to describe phase inversion in emulsions. This sug- gestion was based on the fact that emulsion phase in- version induced by increasing the volume fraction of the dispersed phase displays the qualitative character- istics of the so-called cusp catastrophe theory devel- oped by Thom [9] showing bimodality, sudden jumps, divergence and hysteresis. However, although this ap- proach has been successfully applied to describe the 0927-7757/$ – see front matter © 2003 Published by Elsevier B.V. doi:10.1016/j.colsurfa.2003.08.011