Image Charge Effects on the Formation of Pickering Emulsions
Original Entry: Aaron Goldfain, AP 225, Fall 2012
Authors: Hongzhi Wang, Virendra Singh, and Sven Holger Behrens.
Publication: Hongzhi Wang, Virendra Singh, and Sven Holger Behrens, Image Charge Effects on the Formation of Pickering Emulsions. J. Phys. Chem. Let. (2012) vol. 3 page 2986.
When micron sized colloids adsorb to an oil-water interface they significantly decrease the interfacial energy and become bound to the interface. This binding is used in many applications, the most famous of which is Pickering emulsions in which colloids stabilize an emulsion by binding to the oil-water interfaces. This Letter describes investigations of a barrier to adsorption, which is due to a repulsive image charge force. The authors claim this is the first time a barrier due to an image charge force has been observed.
The authors first test to see under what conditions colloids would adsorb to the interface using the following procedure. Colloids were dispersed in a saltwater solution and an equal volume of hexadecane dyed red was added. These were then mixed in a rotor-stater homogenizer at a known rate, creating an emulsion. The authors then saw if the emulsion was stable or if the droplets coalesced. If stable, then it was assumed that particles had adsorbed to the interface, and if unstable it was assumed they had not. Figure 1 summarizes their results for different particles and salt concentration. It shows that having highly charged particles with little electric screening prevents emulsions from forming, implying that the barrier is electrostatic in origin.
The authors argue that the relevant forces are an image charge force, which is always repulsive, and Van der Waals and electric double layer forces, which are always attractive. The total force on the particle is assumed to be the sum of these three forces. The authors calculate theses forces as function of particle-interface separation distance using Hamaker and Lifshitz theories, as described in class. The results of their calculations are shown in Figure 2.
To test their predictions for the barrier height, the authors give an equation to approximate the force on a colloid due to shaking by the homogenizer. Then, they mix at varying rates, and find that the threshold mixing force (the force where particles adsorb at higher forces and don't for smaller forces) agrees with their calculated force barrier to adsorption. However, they only claim agreement within a factor of 2 or 3.
This Letter provides strong experimental evidence indicating that an electrostatic barrier to the adsorption of a colloid to an oil-water interface exists, when the colloid is approaching from the water side. The authors' claim that barrier is due to an image charge force is very convincing, as there are no other apparent repulsive forces that could cause this. Their experiment is mainly qualitative, however, so their quantitative comparison of the measured magnitude of the barrier to the calculated value is not very convincing, even though they achieve decent agreement. It would be very interesting to see a more quantitative examination of this barrier, as adsorption of colloids to interfaces is incredibly common in soft matter.
 Hongzhi Wang, Virendra Singh, and Sven Holger Behrens, Image Charge Effects on the Formation of Pickering Emulsions. J. Phys. Chem. Let. (2012) vol. 3 page 2986.