Emulsifiers and HLB System

Steric Separation of Oil Droplets

• Steric separation involves physically separating oil droplets to achieve emulsion stability.
• This is enhanced by molecules with a charge, such as anionic emulsifiers, which have a negative charge associated with the polar head.
• Steric stabilization is achieved by preventing the close approach of particles, which hinders aggregation. The use of polymers or surfactants adsorbed on the particle surface can create a steric barrier.

Repulsion of Negatively Charged Oil Droplets

• Negatively charged polar heads surrounding oil droplets cause repulsion between the droplets.
• This repulsion, similar to magnets pushing away from each other, helps to separate the oil droplets within a formula.
• Electrostatic repulsion occurs when particles possess like charges, creating a repulsive force that prevents them from clumping together.

Arrangement of Emulsifiers in Emulsions

• Emulsifiers align preferentially, embedding non-polar tails into the oil and positioning polar head groups in the water.
• Mixing or agitation results in the formation of droplets surrounded by emulsifiers, which lower interfacial tension and act as a physical barrier.
• Emulsifiers reduce the interfacial tension between oil and water, allowing the formation of smaller droplets, and stabilize the emulsion kinetics by preventing coalescence (thermodynamic instability).

Liquid Crystalline Phase

• In some formulas, emulsifiers form a liquid crystalline phase around oil droplets, similar to how Stridovudine lipids arrange themselves.
• This network of lipids between oil droplets provides a barrier, making it difficult for the droplets to coalesce.
• Liquid crystalline structures formed by amphiphilic molecules can enhance emulsion stability by increasing viscosity and providing a physical barrier to droplet coalescence.

Considerations for Formulating Stable Emulsions

• Formulators need to select appropriate emulsifiers for each specific formulation.
• Multiple emulsifiers are often used in a formula to create stable emulsions.
• A minimum of three emulsifiers is often used in a mixture, which can create stable formulas.
• Factors such as temperature, pH, and the presence of other ingredients can influence emulsion stability.

Hydrophilic Lipophilic Balance (HLB)

• The HLB system is used by formulators to select non-ionic emulsifiers by characterizing their solubility in oil versus water.
• HLB helps visualize why certain emulsifiers are chosen.
• HLB is an arbitrary scale from 0 to 20, where 0 is purely non-polar (oil-soluble) and 20 is totally polar (water-soluble).
• The HLB value can guide the selection of appropriate surfactants for different emulsion types, like oil-in-water (O/W) or water-in-oil (W/O) emulsions.

HLB Values and Emulsion Types

• For water-in-oil emulsions, emulsifiers with HLB values from 4 to 6 are typically used.
• For oil-in-water emulsions, emulsifiers with HLB numbers from 8 to 18 are preferred.
• Detergents and solubilizers have even higher water compatibility.
• Emulsifiers with HLB values around 8-16 promote the formation of O/W emulsions, where oil is dispersed in a continuous water phase.

Visual Representation of HLB Scale

• Emulsifiers in the middle of the HLB scale have roughly equal polar and non-polar regions.
• Low HLB emulsifiers have longer non-polar regions and smaller polar regions, making them more compatible with oil.
• High HLB emulsifiers have larger polar head groups and smaller non-polar tails, making them more compatible with water.
• The HLB scale helps in visualizing the relative hydrophobicity or hydrophilicity of surfactants, aiding in appropriate selection.

Interaction of Emulsifiers with Oil and Water

• Low HLB emulsifiers align with polar heads in the water and non-polar tails pulling into the oil.
• Mid-range HLB emulsifiers align with polar heads in the water phase and non-polar tails in the oil phase.
• High HLB emulsifiers have large polar head regions, pulling the molecule into the water and away from the oil.
• These interactions are fundamental to reducing interfacial tension and stabilizing emulsions.

Mixed Emulsifier Systems

• Most formulas use more than one emulsifier to stabilize the formula.
• These are often purchased pre-mixed from raw material suppliers.
• Multiple different sized molecules with different sized polar head regions, and lengths of non-polar tails, can pack more closely around that oil droplet.
• Using multiple emulsifiers fills in the gaps and makes it more effective than just one emulsifier.
• The more emulsifier is essentially more stable that formula will start to become.
• Synergistic effects can occur when combining emulsifiers, leading to enhanced stability compared to single emulsifier systems..

Strategies for Keeping Droplets Separate

• Strategies are needed to reduce the chance of droplets or particles joining together.
• Steric interactions physically separate droplets.
• Electrostatic interactions use charged emulsifiers (e.g., anionic) to repel droplets.
• Increasing the viscosity of the continuous phase can slow down droplet movement and reduce the likelihood of collisions..

Role of Thickeners

• Thickeners contribute to emulsion stability, especially in liquid formulas.
• Polymer thickeners can physically separate particles.
• Thickeners increase the viscosity of the continuous phase, reducing droplet mobility and slowing down phase separation.

Chemical Features of Emulsifiers

• Anionic emulsifiers: Negatively charged in the polar head group.
• Cationic emulsifiers: Positively charged in the polar head region.
• Non-ionic emulsifiers: No specific charge in the polar head group.
• Amphoteric emulsifiers: Charge depends on pH (positive in acidic, negative in alkaline environments).
• The charge and chemical nature of the emulsifier affect its interactions with other components in the formulation.

Electrical Charges and Emulsion Stability

• Like charges repel, so using emulsifiers with the same charge can electrostatically separate oil droplets.
• Formulas do not mix cationic and anionic emulsifiers.
• The magnitude of the charge influences the effectiveness of electrostatic stabilization..

Theories Behind Creating Stable Emulsions

• Formulators consider the emulsion type when selecting emulsifiers, often basing selection on HLB values.
• They consider the preferential solubility of the emulsifier in the oil or aqueous phase.
• Paired or mixed emulsifier systems cover droplets or particles completely.
• The pH of a formula or other ingredients is also considered for compatibility.
• Minimizing interfacial tension and maintaining droplet separation are key principles of emulsion stabilization..

Oil-in-Water Emulsions

• The continuous phase is the water.
• The dispersed phase is the oil particles or droplets.
• HLB value is usually above 8, ideally about 9 to 16.
• Usually about three emulsifiers is selected from this range.
• These emulsions are commonly used for products like lotions and creams..

Water-in-Oil Emulsions

• The continuous phase is the oil.
• The dispersed phase is the water.
• Low HLB value emulsifiers are chosen (usually below 8, in the 4-6 range).
• These emulsions are often used in products like butter and certain types of moisturizers..

Strategies for Achieving Formulation Stability

• Use the correct combination of emulsifiers in blends and consider the HLB system.
• Incorporate charges, such as anionic surfactants, to help stabilize the formula.
• Thicken the continuous phase using a polymer thickener or waxes.
• Control environmental factors, such as temperature and pH, to enhance emulsion stability.

Formula Example

• A phase: Polar phase
• B phase: Non-polar phase
• C phase: Heat-sensitive ingredients added after the initial emulsion is made.
• D phase: Adjust pH at the end.
• Each phase contains ingredients that are compatible with each other, ensuring proper mixing and stability..

Emulsifiers in the Formula

• Sodium Stearoyl Glutamate: Anionic.
  • HLB: 11.
• Sorbitan Laurate: Non-lonic.
  • HLB: ~8.
• Polysorbate 60: Non-lonic.
  • HLB: 14.9.
  • These emulsifiers work synergistically to stabilize the emulsion..

Identifying Emulsion Type Based on HLB Values

• Since all HLB values (11, 8, 14.9) are towards the upper part of the scale, this is an oil-in-water emulsion.
• Polysorbate 60 is a solubilizer and very compatible with water.
• The HLB values indicate the preferential solubility of the emulsifiers in the water phase.

Stabilization Mechanisms

The mechanisms by which these emulsifiers stabilize the formula include:

• Molecular structure aligns with oil and water components.
• Steric separation via polar head groups on the exterior and non-polar tails facing inward.
• Electrostatic stabilization due to the negative charge of the anionic emulsifier.
• Closer packing around oil droplets due to the inclusion of more than one emulsifier.
• These mechanisms collectively contribute to long-term emulsion stability.

Typical Oil-in-Water Emulsion Formula

• 4-10% high HLB emulsifiers (2-3 types)
• 7-15% lipids
• 0.2-4% water-compatible thickeners
• 1-5% humectants
• Water to 100%
• Sufficient preservative
• This formula provides a balanced combination of ingredients for effective emulsion formation..

Optional Ingredients

• Actives/cosmeceuticals (often around 1%)
• Fragrance (non-irritant level)
• Antioxidants (for natural or essential oils)
• Chelating agents
• Color
• These ingredients can enhance the performance and aesthetics of the emulsion..

Room Spray Experiment

• Create a room spray with and without a solubilizer.
• With solubilizer (Polysorbate 20), the essential oils disperse more evenly and stably in the water.
• This experiment demonstrates the importance of solubilizers in dispersing oils in water-based formulations..

Method

• Add 10 drops of essential Oils to water.
• Add 2 teaspoons of Polysorbate 20 before adding water.
• Shake the solution and leave for it stabilize, solution may foam.
• This method ensures proper mixing and dispersion of essential oils in the room spray..

Micellar Waters

• Micellar waters are made using the same basis.
• Micelles