Pickering Emulsions, i.e. emulsions stabilized with particles rather than surfactants, have always seemed to hold promise. With a large literature to examine I assumed I would create a sophisticated app. But the basic principle is so simple that the app is simple too.
You get to control the water contact angle of your particles, which are much smaller than your oil drop. See what happens and draw your own conclusion about the optimum contact angle for a stable emulsion.
An alternative to surfactants for forming an emulsion is the use of fine particles that form the shell around the oil drop or water drop. The idea of such Pickering Emulsions is very attractive to those who meet it for the first time. The reality is somewhat less attractive, though there are plenty of such emulsions out there.
The rules for creating them are simple:
- If the contact angle of the particle with water is <90° (i.e. it is hydrophilic) the emulsion will be o/w
- If the contact angle of the particle with oil is <90° (i.e. it is hydrophobic) the emulsion will be w/o
- The particles must be considerably smaller in size than the desired emulsion drop size.
In practice the rules are:
- Use particles with a 90°contact angle or else use Janus particles with are 50% hydrophilic, 50% hydrophobic
- Worry that your nanosized particles will get classed as nanoparticles and involve you in all sorts of Health & Safety and consumer issues.
The reason for the 90° "rule" is that in practice this gives optimal emulsification efficiencies as indicated in the app. At this point whether the emulsion is o/w or w/o depends on the o:w ratio, with the larger component tending to form the continuous phase.
A great advantage of a Pickering emulsion is that they are relatively stable once made. The disadvantage is related to the advantage: the tough particle shell is slow to equilibrate during production so it is hard to get the emulsion into the final desired equilibrium drop size.
The graphic gives some idea of why a 90° contact angle is likely to be optimal. In this region, whether it is o/w or w/o the particles are nicely clustered around the drop. But as the particle gets closer to the water or the oil in terms of contact angle, it is happier to be sitting in the relevant phase and not around the drop. White coloured particles dislike both phases equally and away from 90° the graphic changes the particle colour to reflect where it likes to be.
Those who were seeking greater wisdom on Pickering emulsions, or those who work with them may be surprised at the brevity and simplicity of this page. My best efforts, however, have failed to come up with anything more profound. Please let me know if you can help me to express some more profound insights.