Colloids and Surfaces A: Physicochem. Eng. Aspects 399 (2012) 78–82 Contents lists available at SciVerse ScienceDirect Colloids and Surfaces A: Physicochemical and Engineering Aspects journa l h omepa g e: www.elsevier.com/locate/colsurfa Spontaneous emulsification at the water/oil interface Jesús Santana-Solano a,∗ , Carla M. Quezada b , Sandra Ozuna-Chacón a , José Luis Arauz-Lara b a Cinvestav Unidad Monterrey, Parque de Investigación e Innovación Tecnológica (PIIT), Apodaca, Nuevo León 66629, Mexico b Instituto de Física “Manuel Sandoval Vallarta”, Universidad Autónoma de San Luis Potosí, Alvaro Obregón 64, 78000 San Luis Potosí, S.L.P., Mexico a r t i c l e i n f o Article history: Received 22 December 2011 Received in revised form 16 February 2012 Accepted 21 February 2012 Available online 3 March 2012 Keywords: Spontaneous emulsification Water droplets Droplet growth Water–oil interface a b s t r a c t Emulsification processes usually require the application of external energy to enlarge the interfacial area between both media. We study here a case where practically no external energy is required to form an emulsion, i.e., it is produced spontaneously when both liquids are put in contact in the presence of one surfactant species. We report the observation and measurements of the kinetics of the spontaneous formation and growth of water droplets at the water/oil + lipophilic surfactant interface. The droplets can reach sizes up to several microns, and their growth is found to obey a power law for a range of the surfactant concentration. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Emulsions are systems of great scientific and technological interest, exhibiting a vast phenomenology [1–3]. Although the lit- erature addressing different aspects of the production and stability of such systems is abundant, there are still many other aspects to be understood. Emulsions are unstable systems due to the high inter- facial tension between both liquids. Nevertheless, these systems can be prevented to phase separate or slow down that process, by addition of one or more surfactant species which self-assemble at the interface providing a stabilizing barrier against coalescence. The production of emulsions in most cases requires the application of mechanical external energy to enlarge the interfacial area between both liquids. Most of that applied energy during emulsification is expended in shearing the liquids and, therefore, lost due to the vis- cous resistance, which is high in many cases of interest. Thus, in the commercial production of emulsions the amount and the way of supplying energy are important issues to consider. Remarkable, however, there are systems where practically no external energy is required to form emulsions, i.e., they are produced spontaneously when both liquids are put in contact in the presence of at least one surfactant species. These phenomenon, referred to as sponta- neous emulsification, has been reported to occur in many specific systems, for instance at interfaces such as steel-slag at high temper- atures [4], crude oil–water [5], dodecane–water [6,7], to mention a few. For further discussion on other systems where spontaneous emulsification has been observed, see reviews in the literature ∗ Corresponding author. Tel.: +52 8111561740; fax: +52 8111561741. E-mail address: jsantana@cinvestav.mx (J. Santana-Solano). [3,8,9]. Although there is an abundant work reported on sponta- neous emulsification, the phenomenon is still not well understood, mainly since spontaneous emulsification seems to occur through different mechanisms in different systems. Among the mecha- nisms discussed in the literature, the most favored are those due to interfacial turbulence, negative interfacial tension, diffusion and stranding, formation and swelling of water/surfactant aggregates, phase inversion, etc. [2,3,8,9]. Spontaneous emulsification is thus an intriguing and fascinating phenomenon whose understanding is of much scientific interest. However, due to the importance of emulsion processes in many industries such as foods, cosmetics, petroleum, paints, etc, a full understanding of the different man- ifestations of spontaneous emulsification is important in order to incorporate them in industrial processes to reduce the energetic costs. In a previous work [6], we reported the observation of spon- taneous emulsification of water in oil (w/o) at an interface water–dodecane with the aid of a lipophilic surfactant. In that sys- tem, the aqueous phase consisted of water droplets in the range 10–100 m, immersed in a continuous oil phase consisting of dode- cane (purity >99%, Sigma) containing sorbitan monooleate (Span 80, purity >99%, Sigma). After few minutes of sample prepara- tion, we observed the appearance of small water droplets on the spherical interface of the initial water droplets. The small water droplets were observed to be uniform in size (initially below 1 m), forming crystalline arrays covering the total surface of the mother water droplet. Most interestingly, the small water droplets grew with time (reaching sizes of few microns) enlarging in this way the water–oil interfacial area without the application of external energy. Naturally, the growth of the daughter water droplets is at the expense of the water of the mother droplet which was observed 0927-7757/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2012.02.032