Phase Inversion Video

Phase Inversion when HLD = 0

Under the microscope at 30°C we have an O/W emulsion (HLD ~ -1) with cyclohexane (EACN=3.4) containing a red dye, and Lutensol® TDA-6 Surfactant (Cc = -0.74). In general the emulsion drops are small (the scale bar is 100 μm), because it was properly vortexed before use.

As the temperature rises, the emulsion drops move around and above 40° we start to see bigger drops forming - the interfacial energy is decreasing (HLD is becoming less negative) so they have a chance to join up.

By 46° the oil drops have fully merged - there is no interfacial energy to keep them apart. This is when HLD = 0. If you feed the EACN and Cc parameters into the HLD equation you find that HLD = 0 at 46°! When I first saw the video I didn't know the EACN and Cc values but identified 46° as HLD = 0. So I was surprised and delighted at the perfect match.

Above 46° we have HLD > 0 and start to see W/O drops appear

At the maximum temperature in this run (51.6°), the W/O conversion is near complete as HLD ~ 0.35.

On cooling we see a mixture of kinetic and thermodynamic effects before O/W takes over once more, though the oil drops are rather larger than the well-dispersed ones at the start - there's no mixing energy to break up larger drops.

What about Type III?

In test-tube experiments, we are used to seeing a middle phase when HLD = 0. Why can't we see one here? Although it's not obvious from this single video, the BASF team were able to confirm that a middle phase is created.

Can we measure EACN and Cc values using microscopes?

Given that we all find it frustrating using classic test tube experiments, the video suggests that with some ingenuity other methods, using small volumes, could become viable ...