Emulsion Systems
Stability in Oil-in-Water (O/W) and Water-in-Oil (W/O) mixtures.
The Thermodynamics of Emulsions
By definition, emulsions are thermodynamically unstable. Oil and water will eventually separate to lower the interfacial tension. The goal of the formulator is to delay this separation for a commercially viable period (typically 2-3 years) through the use of emulsifiers and rheology modifiers.
The HLB System (Hydrophilic-Lipophilic Balance)
Created by Griffin in 1949, the HLB system is critical for selecting the correct emulsifier blend for an O/W emulsion. Every lipid (oil/butter/wax) has a 'Required HLB'. Every emulsifier has a given 'HLB Value' (from 1 to 20).
- HLB 3-6: Lipophilic, used for W/O emulsions (e.g., Sorbitan Oleate).
- HLB 7-9: Wetting agents.
- HLB 8-18: Hydrophilic, used for O/W emulsions (e.g., Polysorbate 80).
Calculating Required HLB
If your oil phase consists of 10% Sweet Almond Oil (Req. HLB 6) and 5% Shea Butter (Req. HLB 8), the total oil phase is 15%.
Almond Contribution: (10 / 15) * 6 = 4
Shea Contribution: (5 / 15) * 8 = 2.66
Total Required HLB = 6.66
Preventing Instability
Emulsions fail through several mechanisms:
- Creaming/Sedimentation: Driven by density differences (Stokes' Law). Prevent by reducing droplet size (high shear mixing) and increasing continuous phase viscosity (adding xanthan gum or carbomer).
- Flocculation: Droplets stick together but retain their individual walls. Prevent via steric hindrance (fatty alcohols like Cetyl Alcohol).
- Coalescence: Droplets merge to form larger drops. Indicates emulsifier failure.
- Ostwald Ripening: Small droplets dissolve and redeposit onto larger ones. Common in W/O emulsions.