Molecular interfacial structure, kinetics and dynamics of small droplets in emulsions and colloids in dispersions

Emulsions consist of one liquid dispersed as small droplets in another liquid, such as in shampoos, milk, vaccines and other medication. The understanding of the structure and stability of droplets (and more generally colloidal dispersions) is commonly obtained from empirical studies in which a macroscopic parameter (like temperature or concentration of constituents) is varied. Since the work of Langmuir and Harkins in the early 20th century it is well established that the stability and properties of droplets (and particles) are strongly influenced by the state of the interface. However, despite the abundance and importance of these systems in our daily lives, the molecular mechanisms that dictate the stability and properties of emulsions and colloidal dispersions are still unknown. As an example our first SFS measurements on 100 nm oil droplets in water, that were stabilized by surfactant, have shown that the surfactant does not reduce the interfacial tension, and does not go to the interface in large quantities. Also, for sodium dodecylsulfate (SDS, one of the most widely used amphiphilic molecules) we have shown that in an emulsion the molecules do not behave as amphiphiles. This illustrates how little fundamental understanding of these simple soft matter systems there is. Since our physico-chemical understanding of biological matter is mostly derived from model systems, such as emulsions and colloidal dispersions, it can be expected that our control of biological matter properties is still quite poor. By using hyper Rayleigh scattering, second harmonic scattering and sum frequency scattering with zeta potential measurements and dynamic light scattering it should be possible to obtain structural, kinetic and dynamical data on the molecular level and relate this to macroscopic properties such as charge, viscosity and stability of soft and biological matter.

H. B. de Aguiar, A. G. F. de Beer, M. L. Strader and S. Roke, The Interfacial Tension of Nanoscopic Oil Droplets in Water Is Hardly Affected by SDS Surfactant, J. Am. Chem. Soc. (2010), 132, 2122- 2123.