Emulsion Stability is Kinetic, Not Thermodynamic
Physical Chemistry
By definition, an emulsion is thermodynamically unstable. The system will always seek the lowest energy state, which is complete phase separation. As a formulator, your job is not to create a permanent emulsion, but to delay separation long enough (kinetically) to exceed the product's shelf life (typically 2-3 years).
Mechanisms of Instability
- Creaming/Sedimentation: Driven by density differences (Stokes' Law). Reversible with agitation.
- Flocculation: Droplets aggregate but maintain individual integrity. Driven by weak van der Waals forces.
- Coalescence: Droplets merge into larger droplets, reducing interfacial area. Irreversible.
- Ostwald Ripening: Small droplets dissolve and redeposit onto larger droplets due to Laplace pressure differences. Common in W/O emulsions.
Common Mistakes
1. Relying solely on HLB without considering molecular geometry (packing parameter).
2. Insufficient yield stress in the continuous phase to prevent creaming.
3. Processing at the wrong temperature, leading to phase inversion.
Stokes' Law and Creaming Rate
The velocity of creaming (v) is governed by:
Where r is droplet radius, ρ is density, g is gravity, and η is viscosity of the continuous phase. To reduce creaming velocity, you must either decrease droplet size (via high shear) or increase the viscosity/yield stress of the continuous phase (via polymers like Carbomer or Xanthan Gum).
Frequently Asked Questions
Why did my emulsion split at 45°C but not at room temp?
Higher temperatures increase the kinetic energy of the droplets and decrease the viscosity of the continuous phase, exponentially accelerating collision rates and coalescence. It reveals underlying instability early.
Is HLB still relevant?
Yes, for non-ionic ethoxylated surfactants. But it completely fails for anionic/cationic systems, polymeric emulsifiers, or silicone emulsions. Use it as a starting point, not gospel.