Emulsions & Sunscreens: An UpdateKen Klein
An emulsion is a system of two (or more) immiscible materials (usually liquids) in which one material (the dispersed/internal phase) is suspended or dispersed throughout another material (the continuous/external phase) in separate droplets
Typical Emulsion Components (water phase)Water Humectant Preservative Emulsifier Thickener Emollient Whiffle dust QS 2-7% 0.05-1.0 0.5-1.5% 0.1-2% 0.5-2% QS
Typical Emulsion Components (oil phase)Emollients Actives Antioxidant Emulsifier (primary) Emulsifier (secondary) Wax Silicones 5-10% Drug level 0.05-0.2% 1-2% 1-2% 0.5-2% 0.5-5%
Typical Emulsion Components (misc.)Preservative Fragrance Color qs 0.1-1% qs
Reduce surface tension Form complex interfacial films (on the surface of emulsion droplets) Form liquid crystalline structures which act as a barrier to coalescence
All emulsions are inherently unstable (with the exception of spontaneously forming micro emulsions). All we can do is delay the day when the instability will arrive.*
Stokes LawV=d2(p)g/18Goal= Reduce the value of VV=Velocity of sedimentation
d=diameter of particles of the dispersed phase p =difference between the specific gravity of the internal and external phases g=acceleration due to gravity =viscosity of the external phase
Increase viscosity Decrease particle size and obtain a narrow distribution of particle sizes Improve strength of interfacial film!
Choice of emulsifier
Reduce particle interaction Preservation considerations
Viscosity Effects! !
Increase % (phase ratio) of internal phase Decrease particle size!
The internal phase occupies a greater volume At higher concentrations you interfere with the electrical double layer and destabilize emulsions Destabilizes O/W emulsions Stabilizes many W/O emulsions Stabilizes many Cationic emulsions
Add electrolyte (monovalent)!
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Emulsion Stabilization (on the micro level)! ! ! !
Electrical charges (double layer) Solvated layers (liquid crystals?) Surface rigidity/elasticity Steric effects (emulsifier packing)
Particle Size Influences! ! ! ! !
You MUST have small particles and a narrow distribution of particle sizes Choice of emulsifier Placement of emulsifier Concentration of emulsifier Processing! ! !
Phasing Mixing equipment Temperature effects
The phenomenon whereby smaller droplets seem to disappear and larger droplets grow in size!
Small droplets have a high radius of curvature. This leads to an increased outward pressure (Laplace pressure) and to greater solubility of the oil components in the water phase. Diffusion then occurs from the water phase into the larger oil droplets which are seen to grow in size. The result is emulsion instability!
Cognis Skin Care, Dr. R. Daniels
Fatty Alcohol Adsorption
Interfacial Film Strength
The term steric stabilization is generally used to describe stabilizing action of polymers adsorbed on the surface of the discontinuous phase (droplets) of dispersions and emulsions. It is also known as colloidal protection. I.E Heller and L. Pugh; J. Chem. Phys., 22, 1778(1956)
W/O Steric Stabilizing Emulsifier
Hydrophobically Modified Polymeric Emulsifier!
Pemulen (B.F. Goodrich) High molecular weight polyacrylate C10-30 alkyl crosspolymers Oil-in-water emulsifier Stabilizes emulsions through polyanionic (negative charge on hydrophobe)steric stabilization
Acrylic acid polymer Available in several grades! ! !
ETD (Easy To Disperse), more salt tolerant? UltrezEasy to disperse Various viscosity grades
PositivesExcellent high temperature stability/viscosity, good suspension characteristics Negatives ! Poor electrolyte tolerance ! Must be neutralized ! Not compatible with cationics!
Use level (emulsions)!
Acrylic polymer emulsion (30%)!
Acrylates/Steareth-20 Methracylate Copolymer Easy to incorporate, good high temperature viscosity Must be used (neutralized)at pH 7 and above
Polymer emulsion (30%)!
Polyacrylamide (and) C13-14 isoparaffin (and) laureth-7 Easy to incorporate, good high temperature viscosity Good compatibility with zinc oxide, good low pH tolerance Off odor, expensive, difficult to control viscosity
Salcare SC 96!
Polymer emulsion (30%)!
Polyquaternium-37 (and) propylene glycol dicaprylate/dicaprate (and) PPG-1 Trideceth-6 Easy to incorporate, good high temperature viscosity Good compatibility with zinc oxide, good low pH tolerance cationic Off odor, expensive, difficult to control viscosity Incompatible with anionic materials
Cellulose based thickeners! !
Excellent stability vs. pH High use levels needed to build viscosity (1-2%) Anionic and nonionic versions available Can thicken alcohol (hydroxypropyl cellulose)
Polysaccharide Hydrocolloid, odorless Seaweed odor Seaweed derived polysaccharide, good slip Polysaccharide, odorless Anionic polysaccharide, fermentation product Good electrolyte and pH tolerance Maintains viscosity at elevated temperatures
Locust bean gum!
Xanthan Gum! ! !
Bentonite (clay derived)!
Used as a suspending agent. Can be used to stabilize w/o emulsions. Available as preblended gels (cyclomethicone, IPM, etc.) Thixotropic agent Delicate formulation parameters Water insoluble Good suspending agent Usually combined with other thickeners Good pH, electrolyte tolerance
Laponite- Sodium magnesium silicate! !
Magnesium Aluminum Silicate! ! ! !
High melting point waxes! !
Beeswax Carnauba Cetyl Stearyl Behenyl Thickens oil phase
Fatty alcohols! ! !
Methods For Choosing Emulsifiers!
Hydrophilic Lipophilic Balance Dr. T. J. Lin Phase Inversion Temperature!
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Invented in 1949 by Griffin. Assigns a number from 1 to 20 to emulsifiers based on % weight of Hydrophobe to Lipophobe in a molecule. Used almost exclusively for nonionic emulsions HLB below 9w/o HLB 9-10..? HLB above 10o/w
Oils have a required HLB Generally one should blend high and low HLB emulsifiers to get the best emulsion HLB of ethoxylated emulsifiers!
HLB=E/5 where E= weight % of ethylene oxide
Excellent starting point Generally produces a fairly good emulsion Ignores the importance of:! ! ! ! ! ! ! !
Electrical double layer Temperature effects of ethoxylates Fatty alcohols Placement of emulsifier % of emulsifier to be used Phase volume ratios Component interactions Liquid crystals
Water Solubilization: Dr. T. J. Lin! !
A method to choose emulsifiers Addresses most of the drawbacks of the HLB system
Water Solubilization: Dr. T. J. Lin!
The ability of the oil phase (with oil phase emulsifiers) to solubilize water is inversely proportional to the final particle size of the emulsion
Water Solubilization: Dr. T. J. Lin!
The oil phase is heated to emulsification temperature and stirred. Water is slowly added until a cloudy oil phase is seen. This method can be used to:! ! !
Choose emulsifiers Determine optimal concentration of emulsifiers Determine effects of oil components on emulsion stability
Water Solubilization: A Trick
Pretreat the oil phase! Improves/reduces particle size distribution
PIT: Phase Inversion
Dr. Shinoda The temperature at which the emulsion inverts due to nonionic (ethoxylates)/ interactions.
As the temperature increases, the water solubility of ethoxylated nonionic emulsifiers becomes poorer (the HLB decreases). There is a temperature (PIT) at which the Hydrophilic and Lipophilic characteristics of the emulsifier are equal (relative to the required HLB of the oil phase). At this temperature the emulsion will exhibit a phase inversion. The PIT should be at least 20C higher than the storage temperature. Choose emulsifiers, and concentrations, to raise the PIT.
Liquid Crystals:A DefinitionHighly anisotropic fluids that exist as a result of long-range orientational ordering among constituent molecules. Also: Three dimensional association structures which stabilize emulsions.
Liquid Crystals In EmulsionsSuzuki, Tsutsumi and Ishida: The self bodying action of fatty alcohols is caused by the formation of a network structure of liquid crystalline phase in the emulsion system
Liquid CrystalsThis ordering is adequate to provide for increased viscosity, but not strong enough to prevent flow, thus viscous liquids are typical. This dualism between a solid (crystal) an