Formulating clear oil in water dispersions can be one of the most challenging areas in cosmetic science. It also is an area that has probably had the most interest and follow up questions of any article I have published in the last three years.1
Technically, there are two types of dispersions that are referred to as “solutions” by cosmetic chemists: isotropic and micellular.
Isotropic solutions are homogeneous mixtures of two or more ingredients in which the particles of the oil or solute are solubilized in a solvent without the use of surfactants.
Clear micellular solutions are aqueous dispersions which use surfactants and/or solvents to reduce the oil droplet size to <50 nanometers.
Micelles are organized structures which surfactants form in water. This point is referred to as the critical micelle concentration or CMC.
Above the CMC, these structures surround hydrophobic ingredients, like fragrances and oils, and can form clear dispersions if the right surfactant is used at a high enough concentration.
These emulsions typically have a particle size of approximately 5-10 nanometers and should only be used when emulsifying a highly polar oil phase like a fragrance.
Micellular dispersions can also be formulated using lower levels of surfactant, if high shear mechanical energy is used to reduce the particle size. However, they normally have a larger particle size.
Microemulsions are a third type of micellular dispersion that require higher concentrations of surfactant versus typical micellular solutions.2
Microemulsions, unlike typical micellular solutions, are thermodynamically stable dispersions with a particle size of <50 nanometers that instantly form with simple mixing.
They are low viscosity systems containing an aqueous phase, oil phase, surfactant, and a cosurfactant. The cosurfactant, typically a water insoluble polyol or alkanol, helps reduce the interfacial tension which decreases the size of the dispersed droplets.
Other ways of formulating clear oil in water emulsions include matching the refractive index of the oil with that of the aqueous phase and molecular encapsulation using cyclodextrin (CD). Refractive index matching normally requires an emulsifier and the use of high concentrations of polyol. This only works well with certain types of oils that have a refractive index close to that of water.
These emulsions also require a thickener to help improve stability since their particle size is typically >100 nanometers. Clear water soluble CD complexes require the use of highly water soluble CD forms like alpha, gamma, or Hydroxypropyl CD and only work for ingredients that will bind inside the donut-like structure. The typical ratio of CD to complexed ingredient is 5-10 parts CD for every 1% complexed ingredient, depending on the type of ingredient used and the molecular weight.
Key factors influencing emulsion formation/stability
- Oil polarity-high polarity oils like fragrances are much easier to solubilize than emollients. For example, 10 grams of surfactant can solubilize two to three grams of fragrance oils and but only .2-.5 grams of non-polar oils.
- When solubilizing oils, start with a ratio of 20 parts solubilizer to one part oil.
- For fragrances, start with five parts solubilizer to one part fragrance oil and reduce the solubilizer until the emulsion becomes cloudy.
- Test clear solutions made using various ratios of solubilizer to oil at 4°C and 45°C to determine the optimum stability.
- Surfactant type and concentration: Use combinations of surfactants for better efficiency.
- Solvents-solvents like Ethanol, Propanediol, and PEG 8 can often reduce the amount of surfactant needed.
- Ostwald Ripening describes particle size growth over time and is a leading cause of clear oil in water emulsion instability. Typically, the emulsion starts out clear and becomes cloudy over time. This phenomenon results from the difference in oil solubility immediately outside of small and large micelles.
- To reduce Ostwald Ripening, a narrow particle size distribution is required. As micelles undergo Ostwald Ripening, the concentration of the insoluble oil increases as the more soluble oil migrates out of the micelle.
- Equilibrium is reached at some point, stopping additional Ostwald Ripening. Typically, the addition of a small amount of nonpolar oil is sufficient to limit Ostwald Ripening.
- Hydrophobically modified polymers can also be useful in stabilizing the interface against Ostwald Ripening.
Recommended solubilizers for fragrances
- The key historic benchmark solubilizer is PEG 40 Hydrogenated Castor Oil (Cremophor RH 40-BASF)
- Eumulgin HPS (PEG-40 Hydrogenated Castor Oil, PPG-1-PEG-9 Lauryl Glycol Ether-BASF, Coceth 7-BASF)
- COSMACOL N II-9 (C12-C13 Pareth 9-Sasol)
- Brij IC20 (Isoceteth 20-Croda)
Recommended green options
The following do not contain ethylene oxide-based surfactants for fragrances.
- TEGO Solve 61 (Polyglyceryl-6 Caprylate, Polyglyceryl-4 Caprylate, Polyglyceryl-4 Cocoate, Polyglyceryl-6 Ricinoleate-Evonik)
- TEGO Solve 55 (Polyglyceryl-3 Caprylate/Caprate/ Succinate, Propylene Glycol-Evonik)
- Sepiclear G7 (Heptyl Glucoside, 70% solids-Seppic)
- Poly Suga Mulse D9 (Sorbitan Oleate Decylglucoside Crosspolymer, 64% solids-Colonial Chemical)
- Caprol Micro Express (PEG 6 Caprylic/Capric Glycerides, Polyglyceryl-6 Dioleate, Glyceryl Caprylate/Caprate-Abitec) – just add your emollient and water to form a microemulsion.
- Base microemulsion formulation – DI water 16%, Glycerin 15%, emollient 47%, Polyglyceryl 10 Diisostearate 12%, Polyglyceryl 10 Diisostearate 10% (Dr Straetmans base formulation)
- 30% Sucrose Laurate (Ryoto Sugar Ester L-1695-Mitsubishi Kagaku Foods), 20% Glycerol, 50% oil forms a clear microemulsion.