EMULSIONS

An An emulsionemulsion is a mixture of two immiscible (unblendable) is a mixture of two immiscible (unblendable) substances. One substance (the dispersed phase) is dispersed substances. One substance (the dispersed phase) is dispersed in the other (the continuous phase). in the other (the continuous phase).

EmulsificationEmulsification is the process by which emulsions are prepared. is the process by which emulsions are prepared.

Emulsions tend to have a cloudy appearance, because the many Emulsions tend to have a cloudy appearance, because the many phase interfaces (the boundary between the phases is called phase interfaces (the boundary between the phases is called the interface) scatter light that passes through the emulsion. the interface) scatter light that passes through the emulsion.

Emulsions are unstable and thus do not form spontaneously. Emulsions are unstable and thus do not form spontaneously.

Energy input through shaking, stirring, homogenizers, Energy input through shaking, stirring, homogenizers, or spray processes are needed to form an emulsion. or spray processes are needed to form an emulsion.

Over time, emulsions tend to revert to the stable state of Over time, emulsions tend to revert to the stable state of oil separated from water.oil separated from water.

Therefore now we can defineTherefore now we can define

Emulsion is a thermodynamically unstable 2 phase Emulsion is a thermodynamically unstable 2 phase system consisting of at least 2 immiscible liquids, system consisting of at least 2 immiscible liquids, one of which is dispersed in the form of small one of which is dispersed in the form of small droplets throughout the other and an emulsifying droplets throughout the other and an emulsifying agentagent

Dispersed liquid – Internal or discontinuous phaseDispersed liquid – Internal or discontinuous phaseDispersion medium- External or continuous phaseDispersion medium- External or continuous phase

Where oils, hydrocarbons and/or waxes are Where oils, hydrocarbons and/or waxes are the dispersed phasethe dispersed phase

Water or an aqueous solution is the Water or an aqueous solution is the continuous phase- the system is called an continuous phase- the system is called an oil in water emulsion (o/w)oil in water emulsion (o/w)

An o/w type emulsion is generally formed if An o/w type emulsion is generally formed if the aqueous phase constitutes > 45 % of the aqueous phase constitutes > 45 % of the total weightthe total weight

Hydrophilic emulsifier is usedHydrophilic emulsifier is used

Where water or aqueous solutions are dispersed in an Where water or aqueous solutions are dispersed in an oleaginous medium, the system is known as oleaginous medium, the system is known as water water in oil ( w/o) emulsion.in oil ( w/o) emulsion.

Emulsions are generally formed if the aqueous Emulsions are generally formed if the aqueous phase constitutes < 45 % of the total weight and phase constitutes < 45 % of the total weight and a lipophilic emulsifier is used. a lipophilic emulsifier is used.

The consistency of the emulsions varies from easily The consistency of the emulsions varies from easily pourable liquids to semisolid creams.pourable liquids to semisolid creams.

Their consistency will depend upon:Their consistency will depend upon:

1.1. The internal volume to external phase volume The internal volume to external phase volume ratioratio

2.2. In which phase ingredients solidifyIn which phase ingredients solidify3.3. What ingredients are solidifying?What ingredients are solidifying?

w/o emulsions tend to be immiscible in water, not water w/o emulsions tend to be immiscible in water, not water washable will not absorb water, are occlusive and may washable will not absorb water, are occlusive and may be greasy.be greasy.

This is primarily because oil is the external phase, it will This is primarily because oil is the external phase, it will repel any of the actions of waterrepel any of the actions of water

Consequently, it will not be allowed to evaporate from the Consequently, it will not be allowed to evaporate from the surface of the skinsurface of the skin

o/w emulsions are water miscible, water washable o/w emulsions are water miscible, water washable will absorb water, are nonocclusive, and are nongreasy.

Here water is the external phase and will readily associate with any of the actions of water.

Surface active substances (Surface active substances (surfactantsurfactant) can ) can increase the kinetic stability of emulsions greatly increase the kinetic stability of emulsions greatly so that, once formed, the emulsion does not so that, once formed, the emulsion does not change significantly over years of storage.change significantly over years of storage.

Homemade oil and vinegar salad dressing is an Homemade oil and vinegar salad dressing is an example of an unstable emulsion that will quickly example of an unstable emulsion that will quickly separate unless shaken continuously. separate unless shaken continuously.

This phenomenon is called coalescene, and This phenomenon is called coalescene, and happens when small droplets recombine to form happens when small droplets recombine to form bigger ones. bigger ones.

Fluid emulsions can also suffer from creaming, the Fluid emulsions can also suffer from creaming, the migration of one of the substances to the top of the migration of one of the substances to the top of the emulsion under the influence of buoyancy or emulsion under the influence of buoyancy or centripetal force when a centrifuge is used.centripetal force when a centrifuge is used.

Emulsions are part of a more general class of two-phase systems of matter called colloids. Although the terms colloid and emulsion are sometimes used interchangeably, emulsion tends to imply that both the dispersed and the continuous phase are liquid.

There are three types of emulsion instability: There are three types of emulsion instability: flocculationflocculation, where the particles form clumps, where the particles form clumps

creamingcreaming, where the particles concentrate towards , where the particles concentrate towards the surface of the mixture while staying separated, the surface of the mixture while staying separated,

or or breakingbreaking, where the particles coalesce and , where the particles coalesce and form a layer of liquid.form a layer of liquid.

An Emulsifier'An Emulsifier'(also known as a(also known as a surfactant' surfactant' from from surface active materialsurface active material or or emulgentemulgent) is a ) is a substance which stabilizes an emulsion. substance which stabilizes an emulsion.

Whether an emulsion turns into a water-in-oil Whether an emulsion turns into a water-in-oil emulsion or an oil-in-water emulsion depends on emulsion or an oil-in-water emulsion depends on the volume fraction of both phases and on the type the volume fraction of both phases and on the type of emulsifier. of emulsifier.

Generally, the Bancroft rule applies: emulsifiers and Generally, the Bancroft rule applies: emulsifiers and emulsifying particles tend to promote dispersion emulsifying particles tend to promote dispersion of the phase in which they do not dissolve very of the phase in which they do not dissolve very well.well.

For example, proteins dissolve better in water than in For example, proteins dissolve better in water than in oil and so tend to form oil-in-water emulsions oil and so tend to form oil-in-water emulsions (that is they promote the dispersion of oil droplets (that is they promote the dispersion of oil droplets throughout a continuous phase of water).throughout a continuous phase of water).

Creams and lotions are made by Creams and lotions are made by combining oil-based and water-based combining oil-based and water-based ingredients, and are therefore the only ingredients, and are therefore the only skin care applications that have at the skin care applications that have at the same time moisturizing and nourishing same time moisturizing and nourishing effects. effects.

The water (or water-based ingredients) The water (or water-based ingredients) supply the "moisturizing" properties, and supply the "moisturizing" properties, and the oils or fats the "nourishing" properties. the oils or fats the "nourishing" properties.

Technically, creams and lotions are Technically, creams and lotions are emulsionsemulsions. .

In an emulsion, tiny droplets of one of the liquids In an emulsion, tiny droplets of one of the liquids are suspended in a second liquid, which would are suspended in a second liquid, which would not "naturally" mix with the first. not "naturally" mix with the first.

This is the case with water and oils: it is always This is the case with water and oils: it is always possible to mix together water and oil, but possible to mix together water and oil, but unless something is done to create an emulsion, unless something is done to create an emulsion, the oils (which are lighter than water) will always the oils (which are lighter than water) will always sit on top of the water and not "blend" with it. sit on top of the water and not "blend" with it.

This physical incompatibility of water and This physical incompatibility of water and oils can be overcome by agitating the water oils can be overcome by agitating the water and oil mix. and oil mix.

Agitation disperses the molecules, and Agitation disperses the molecules, and generates an emulsion. generates an emulsion.

However, emulsions made by simply However, emulsions made by simply agitating water and oil will separate within a agitating water and oil will separate within a short time, with the water part falling to the short time, with the water part falling to the bottom, and the oils floating on top. bottom, and the oils floating on top.

The only way to obtain a The only way to obtain a stable stable emulsionemulsion is to add an is to add an emulsifying agentemulsifying agent to the mix. to the mix.

Emulsifying agents are substances that Emulsifying agents are substances that help water and oils bind together, and help water and oils bind together, and allow to prepare stable emulsions where allow to prepare stable emulsions where water and oils do not separate (or "fall water and oils do not separate (or "fall out of emulsion", as some chemists out of emulsion", as some chemists say). say).

Emulsions are stabilized by adding an Emulsions are stabilized by adding an emulsifier or emulsifying agents. emulsifier or emulsifying agents.

These agents have both a hydrophilic and a These agents have both a hydrophilic and a lipophilic part in their chemical structure. lipophilic part in their chemical structure.

All emulsifying agents concentrate at and are All emulsifying agents concentrate at and are adsorbed onto the oil:water interface to adsorbed onto the oil:water interface to provide a protective barrier around the provide a protective barrier around the dispersed dropletsdispersed droplets

In addition to this protective barrier, emulsifiers In addition to this protective barrier, emulsifiers stabilize the emulsion by stabilize the emulsion by reducing thereducing the interfacial tensioninterfacial tension of the system. of the system.

Some agents Some agents enhance stability by imparting enhance stability by imparting a charge on the droplet surfacea charge on the droplet surface thus thus reducing the physical contact between the reducing the physical contact between the droplets and decreasing the potential for droplets and decreasing the potential for coalescence. coalescence.

Emulsifying agents can be classified according to:Emulsifying agents can be classified according to: 1) chemical structure; or 1) chemical structure; or 2) mechanism of action. 2) mechanism of action.

Classes according to chemical structure are synthetic, natural, Classes according to chemical structure are synthetic, natural, finely dispersed solids, and auxiliary agents. finely dispersed solids, and auxiliary agents.

Classes according to mechanism of action are monomolecular, Classes according to mechanism of action are monomolecular, multimolecular, and solid particle films. multimolecular, and solid particle films.

Regardless of their classification, all emulsifying agents must Regardless of their classification, all emulsifying agents must be chemically stable in the system, inert and chemically non-be chemically stable in the system, inert and chemically non-reactive with other emulsion components, and nontoxic and reactive with other emulsion components, and nontoxic and nonirritant. They should also be reasonably odorless and not nonirritant. They should also be reasonably odorless and not cost prohibitive. cost prohibitive.

Synthetic Emulsifying AgentsSynthetic Emulsifying Agents Cationic, e.g., benzalkonium chloride, benzethonium Cationic, e.g., benzalkonium chloride, benzethonium

chloride chloride Anionic, e.g., alkali soaps (sodium or potassium oleate); Anionic, e.g., alkali soaps (sodium or potassium oleate);

amine soaps (triethanolamine stearate); detergents (sodium amine soaps (triethanolamine stearate); detergents (sodium lauryl sulfate, sodium dioctyl sulfosuccinate, sodium lauryl sulfate, sodium dioctyl sulfosuccinate, sodium docusate). docusate).

Nonionic, e.g., sorbitan esters (Spans®), polyoxyethylene Nonionic, e.g., sorbitan esters (Spans®), polyoxyethylene derivatives of sorbitan esters (Tweens®), or glyceryl esters derivatives of sorbitan esters (Tweens®), or glyceryl esters

Cationic and anionic surfactants are generally limited to use in Cationic and anionic surfactants are generally limited to use in topical, o/w emulsions. topical, o/w emulsions.

Cationic agents (quarternary ammonium salts) are Cationic agents (quarternary ammonium salts) are incompatible with organic anions and are infrequently used incompatible with organic anions and are infrequently used as emulsifiers. Soaps are subject to hydrolysis and may be as emulsifiers. Soaps are subject to hydrolysis and may be less desirable than the more stable detergents. less desirable than the more stable detergents.

Natural Emulsifying AgentsNatural Emulsifying Agents A variety of emulsifiers are natural products derived from plant A variety of emulsifiers are natural products derived from plant

or animal tissue. Most of the emulsifiers form hydrated or animal tissue. Most of the emulsifiers form hydrated lyophilic colloids (called lyophilic colloids (called hydrocolloidshydrocolloids) that form ) that form multimolecular layers around emulsion droplets. multimolecular layers around emulsion droplets.

Hydrocolloid type emulsifiers have little or no effect on Hydrocolloid type emulsifiers have little or no effect on interfacial tension, but exert a protective colloid effect, interfacial tension, but exert a protective colloid effect, reducing the potential for coalescence, by: reducing the potential for coalescence, by:

providing a protective sheath around the droplets providing a protective sheath around the droplets imparting a charge to the dispersed droplets (so that they imparting a charge to the dispersed droplets (so that they

repel each other) repel each other) swelling to increase the viscosity of the system (so that swelling to increase the viscosity of the system (so that

droplets are less likely to merge) droplets are less likely to merge)

Hydrocolloid emulsifiers may be classified as: Hydrocolloid emulsifiers may be classified as: vegetable derivatives, e.g., acacia, tragacanth, agar, pectin, vegetable derivatives, e.g., acacia, tragacanth, agar, pectin,

carrageenan, lecithin carrageenan, lecithin animal derivatives, e.g., gelatin, lanolin, cholesterol animal derivatives, e.g., gelatin, lanolin, cholesterol Semi-synthetic agents, e.g., methylcellulose, Semi-synthetic agents, e.g., methylcellulose,

carboxymethylcellulose carboxymethylcellulose Synthetic agents, e.g., Carbopols® Synthetic agents, e.g., Carbopols®

Naturally occurring plant hydrocolloids have the advantages of Naturally occurring plant hydrocolloids have the advantages of being inexpensive, easy to handle, and nontoxic. Their being inexpensive, easy to handle, and nontoxic. Their disadvantages are that they require relatively large disadvantages are that they require relatively large quantities to be effective as emulsifiers, and they are quantities to be effective as emulsifiers, and they are subject to microbial growth and thus their formulations subject to microbial growth and thus their formulations require a preservative. Vegetable derivatives are generally require a preservative. Vegetable derivatives are generally limited to use as o/w emulsifiers. limited to use as o/w emulsifiers.

The animal derivatives general form w/o emulsions. Lecithin and The animal derivatives general form w/o emulsions. Lecithin and cholesterol form a monomolecular layer around the emulsion cholesterol form a monomolecular layer around the emulsion droplet instead of the typically multimolecular layers. droplet instead of the typically multimolecular layers.

Cholesterol is a major constituent of wool alcohols and it gives Cholesterol is a major constituent of wool alcohols and it gives lanolin the capacity to absorb water and form a w/o emulsion. lanolin the capacity to absorb water and form a w/o emulsion. Lecithin (a phospholipid derived from egg yolk) produces o/w Lecithin (a phospholipid derived from egg yolk) produces o/w emulsions because of its strong hydrophilic character. emulsions because of its strong hydrophilic character.

Animal derivatives are more likely to cause allergic reactions Animal derivatives are more likely to cause allergic reactions and are subject to microbial growth and rancidity. Their and are subject to microbial growth and rancidity. Their advantage is in their ability to support formation of w/o advantage is in their ability to support formation of w/o emulsions. emulsions.

Semi-synthetic agents are stronger emulsifiers, Semi-synthetic agents are stronger emulsifiers, are nontoxic, and are less subject to microbial are nontoxic, and are less subject to microbial growth. growth.

Synthetic hydrocolloids are the strongest Synthetic hydrocolloids are the strongest emulsifiers, are nontoxic, and do not support emulsifiers, are nontoxic, and do not support microbial growth. However, their cost may be microbial growth. However, their cost may be prohibitive. These synthetic agents are prohibitive. These synthetic agents are generally limited to use as o/w emulsifiers. generally limited to use as o/w emulsifiers.

Finely Divided or Finely Dispersed Solid Finely Divided or Finely Dispersed Solid Particle EmulsifiersParticle Emulsifiers

These agents form a particulate layer around These agents form a particulate layer around dispersed particles. Most will swell in the dispersed particles. Most will swell in the dispersion medium to increase viscosity and dispersion medium to increase viscosity and reduce the interaction between dispersed reduce the interaction between dispersed droplets. Most commonly they support the droplets. Most commonly they support the formation of o/w emulsions, but some may formation of o/w emulsions, but some may support w/o emulsions. support w/o emulsions.

These agents include bentonite, veegum, These agents include bentonite, veegum, hectorite, magnesium hydroxide, aluminum hectorite, magnesium hydroxide, aluminum hydroxide and magnesium trisilicate. hydroxide and magnesium trisilicate.

Auxiliary Emulsifying AgentsAuxiliary Emulsifying Agents

A variety of fatty acids (e.g., stearic acid), fatty A variety of fatty acids (e.g., stearic acid), fatty alcohols (e.g., stearyl or cetyl alcohol), and alcohols (e.g., stearyl or cetyl alcohol), and fatty esters (e.g., glyceryl monostearate) fatty esters (e.g., glyceryl monostearate) serve to stabilize emulsions through their serve to stabilize emulsions through their ability to thicken the emulsion. ability to thicken the emulsion.

Because these agents have only weak Because these agents have only weak emulsifying properties, they are always use in emulsifying properties, they are always use in combination with other emulsifiers. combination with other emulsifiers.

Most of the hydrophilic colloids used as emulsifiers Most of the hydrophilic colloids used as emulsifiers are plant gumsare plant gums

They stabilize the oil in water emulsions by forming They stabilize the oil in water emulsions by forming a strong rigid multimolecular film around each oil a strong rigid multimolecular film around each oil globule and thus coalescence is retarded by the globule and thus coalescence is retarded by the presence of a hydrophilic barrier between the oil presence of a hydrophilic barrier between the oil and water phases.and water phases.

Use of natural gums as emulsifiers is warranted Use of natural gums as emulsifiers is warranted whenever it is desired to increase the viscosity of whenever it is desired to increase the viscosity of an emulsion without a corresponding increase in an emulsion without a corresponding increase in the lipid portion of the emulsionthe lipid portion of the emulsion