Electrostatics at the oil–water interface, stability, and order in emulsions and colloids

doi:10.1073/pnas.0610589104

Electrostatics at the oil–water interface, stability, and order in emulsions and colloids

*Soft Condensed Matter, Debye Institute, Utrecht University, Princetonplein 5, 3584 CC, Utrecht, The Netherlands; and
^‡Department of Chemical Engineering,
^§Princeton Institute for the Science and Technology of Materials, and
^‖Department of Physics, Princeton University, Princeton, NJ 08534

Contributed by Paul M. Chaikin, December 4, 2006 (received for review August 12, 2006)

Abstract

Oil–water mixtures are ubiquitous in nature and are particularly important in biology and industry. Usually additives are used to prevent the liquid droplets from coalescing. Here, we show that stabilization can also be obtained from electrostatics, because of the well known remarkable properties of water. Preferential ion uptake leads to a tunable droplet charge and surprisingly stable, additive-free, water-in-oil emulsions that can crystallize. For particle-stabilized (“Pickering”) emulsions we find that even extremely hydrophobic, nonwetting particles can be strongly bound to (like-charged) oil–water interfaces because of image charge effects. These basic insights are important for emulsion production, encapsulation, and (self-)assembly, as we demonstrate by fabricating a diversity of structures in bulk, on surfaces, and in confined geometries.

Footnotes

^†To whom correspondence may be addressed. E-mail: m.e.leunissen{at}phys.uu.nl, a.vanblaaderen{at}phys.uu.nl, or chaikin{at}physics.nyu.edu
Author contributions: A.v.B. and P.M.C. designed research; M.E.L., A.D.H., and M.T.S. performed research; A.D.H. contributed new reagents/analytic tools; M.E.L., A.v.B., and P.M.C. analyzed data; and M.E.L. and P.M.C. wrote the paper.
↵ ^¶Present address: Department of Physics, Center for Soft Matter Research, New York University, New York, NY 10003.
↵**Present address: Department of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138.
The authors declare no conflict of interest.
↵ †† In our earlier work on similar suspensions, though with less extreme screening lengths (10), the behavior was well described by the solution of the Poisson-Boltzmann equation for low, constant surface potentials: V(r)=(Z ² e ²/4πεε₀)[e^Ka_c/(1 + Ka_c)]²(e^−Kr/r), with r the interparticle distance, ε₀ the dielectric permittivity of vacuum, and K ⁻¹ = (ne ²/εε₀ k_BT)^−1/2 the Debye screening length, where n is the (monovalent) ion concentration, k_B is Boltzmann's constant, and T is the absolute temperature. For the present suspensions the real situation likely lies in between the constant surface charge and constant surface potential limits.
This article contains supporting information online at www.pnas.org/cgi/content/full/0610589104/DC1.
Abbreviations:

PMMA,

poly(methylmethacrylate);

CHB,

cyclohexyl bromide;

RITC,

rhodamine isothiocyanate.
Freely available online through the PNAS open access option.