Spontaneous Formation of Lipid Structures at Oil/Water/Lipid Interfaces
The authors find that when a drop of water is placed in dodecane with phospholipids, emulsion droplets and multilamellar concentric structures called "onions" spontaneously form. A multilamellar film in a semicrystalline state forms at the interface between the dodecane and the water. As this film swells with water, it sheds the onions and emulsion droplets. The authors find that the core of the onions consists of dodecane, water, and lipids, while the shell is made of partially hydrated concentric bilayers.
Authors: Sophie Pautot, Barbara J. Frisken, Ji-Xin Cheng, X. Sunney Xie, and David Weitz.
Date: September 9, 2003.
Departments of Physics, Chemistry, Chemical Biology, and DEAS, Harvard University, Cambridge, Massachusetts 02138, USA
Department of Physics, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada
Langmuir 19, 10281-10287 (2003). 
The spontaneous formation of emulsion droplets and other structures is of great interest for a variety of applications, from agriculture to medicine. Mechanisms that had been proposed to explain this phenomenon included
- (1) diffusion of water into oil,
- (2) surface tension reduction,
- (3) local convection currents or other thermal fluctuations, or
- (4) self-assembly at the interface of a surfactant to form a mulilayer film.
Here the authors report the spontaneous formation of an inverted emulsion, or water droplets in oil, for the first time. They demonstrate that both anionic and nonionic lipids can be used to stabilize the phenomenon. They also report the first observation of the spontaneous formation of onion structures in oil, confirmed by imaging the structures between crossed polarizers, which produced evidence of defects consistent with previous observations of onion-like structures. Optical microscopy and coherent anti-Stokes Raman scattering (CARS) microscopy are used to investigate the mechanisms that have been proposed to explain the phenomena, as well as the composition of the multilamellar structures.
To investigate whether the diffusion of water is responsible for the phenomena observed, the experimentalists prepared two samples, identical except for the fact that one contained dodecane saturated with water and the other with 1% silicone oil, which prevents water from diffusing into the dodecane. A few hours after water droplets were injected into the oil in each sample, a white film was found covering each water droplet, which consisted of the onionlike structures. This provided strong evidence that the diffusion of water (1) was not the underlying mechanism for the formation of the multilamellar onionlike structures.
By measuring the surface tension over time, the authors found that the rate that the surface tension changes decreased over time more than what the proposed theories predicted. Thus, the hypothesis (2) that surface tension evolution results from the diffusion of lipids to the interface was refuted. By carefully controlling the temperatures of their samples, the authors similarly ruled out the possibility (3) that the spontaneous emulsification was due to the presence of convection currents. By imaging the formation of a droplet at the interface between the water and oil directly with an optical microscope (see Figure 1), Pautot et al concluded that fluctuations due to low surface tension were not the responsible mechanism either.
Instead, the authors determined that "spontaneous emulsification results from the presence of lyotropic liquid-crystalline phases at the dodecane-water interface." The initial concentration of lipids and the temperature determined the morphology of the onion structures, and this is consistent with the notion that the formation and swelling of the liquid-crystalline lipid structures is the underlying mechanism.
Connection to soft matter
This work is noteworthy in that it describes another structural geometry into which surfactants can assemble. In addition to micelles and lamellar surfaces, surfactants may arrange into spherical "onions" consisting of multilamellar layers and a core. This research also probes the mechanism by which these onions form and the factors that affect their growth and composition. It is concluded that random fluctuations, diffusion, convection, and surface tension are not the underlying forces driving the creation of multilamellar onions, but rather it is the formation and subsequent swelling of a liquid-crystalline multilamellar film at the interface between the oil and the water droplet.
Another noteworthy observation is that the core of the "onion" structure consists not simply of water or dodecane, but both water and dodecane, as well as lipids. It is not immediately clear why the interior of the onion may consist of all three componenets, as the interior of micelles are usually not so complex. Further work could be conducted to get a better understanding of what determines the composition of the multilamellar onion core.