Emulsions
Generally emulsions have particle size 0.1 – 100 µm.
Microemulsions have particle size 0.01 µm
Fine emulsions have particle size 0.25 – 25 µm
Two immiscible phase system in which one phase is in the droplet called as internal phase or disperse phase or discontinuous phase while the 2nd phase is called as external phase or continuous phase and the system is stabilized by use of emulsifiers. The generally
An emulsifier perform three actions
1. Reduction of interfacial tension
2. Formation of interfacial film on the interface and prevent adhesion and coalescence of droplets of emulsions.
3. Electrical repulsion
Particle size of disperse phase determines the appearance of emulsion.
Emulsions tend to have a cloudy or opaque appearance, because the many phase interfaces scatter light that passes through the emulsion. The basic color of emulsions is white . If the emulsion is dilute, the Tyndall effect will scatter the light and distort the color to blue ; if it is concentrated, the color will be distorted towards yellow . This phenomenon is easily observable on comparing skimmed milk (with no or little fat) to cream (high concentration of milk fat). Microemulsions and nanoemulsions tend to appear clear or transparent due to the small size of the disperse phase.
The emulsion will be opaque if particle size is 0.25 µ – 10 µm.
If the particle size is 10 – 75 nm it will be transparent (Micro emulsions or micellar emulsions)
The internal phase in monodisperse system should not be more than 74 % of total volume the system.
Parameters influencing type emulsion formed:
1. Whether an emulsion turns into a water-in-oil emulsion or an oil-in-water emulsion depends on the volume fraction of both phases and on the type of emulsifier.
Bancroft rule applies: What makes emulsion oil-in-water or water-in-oil is not the relative percentages of oil or water, but which phase the emulsifier is more soluble in. So even though there may be a formula that’s 60% oil and 40% water, if the emulsifier chosen is more soluble in water, it will create an oil-in-water system.
The Hydrophilic-lipophilic balance (or HLB) of a surfactant can be used in order to determine whether it’s a good choice for the desired emulsion or not.
• In Oil in Water emulsions – use emulsifying agents that are more soluble in water than in oil (High HLB surfactants).
• In Water in Oil emulsions – use emulsifying agents that are more soluble in oil than in water (Low HLB surfactants).
If the emulsifier is water soluble i.e. Potassium soap, polyoxyethylene alkyl ether with more than 5 ethylene oxide units usually forms o/w emulsions.
If emulsifier is oil soluble i.e. Calcium soap, polyoxyethylene alkyl ether with less than 5 ethylene oxide units usually forms w/o emulsions.
2. The polar portion of emulsifier molecules are generally better barriers to coalescence than their hydrocarbon part. So it is easily to prepare o/w emulsions relatively high internal or disperse phase volumes. On the other hand w/o emulsions invert easily if the amount of water is significantly e.g.
water-mineral oil sorbitan monooleate system, ordinary expected to form w/o emulsion because of lack of ethylene oxide units and it does it only when water is less than 40%. If the water % is increased than 40% resulting in formation of o/w emulsions.
2. Even at 20% and 30% water, w/o emulsions form only if water is added to oil with mixing. The addition of both phases followed by mixing favors o/w emulsions at all concentrations above 10% water.
3. Viscosity of each phase sometimes determines than type of emulsion formed. An increase in viscosity of internal phase aids making the phase as external phase.
Method to determine emulsion type:
1. Dilution test: emulsion can be only diluted with continuous or external phase. If addition of water breaks emulsions then it is w/o emulsion.
2. Dye test: uses water soluble dyes like amaranth, methylene blue for o/w emulsions while oil soluble dyes e.g. Sudan III, scarlet red for w/o emulsions.
3. Fluorescence: as the oil Fluoresce under UV light. w/o Fluoresce throughout while o/w have dot pattern
4. Conductivity test: o/w emulsions gives positive test while w/o fails.
5. COCl2: dry filter paper impregnated with COCl2 is blue in color. The blue color changed to pink when o/w emulsion is added.
Phase inversion temperature or HLB temperature: w/o emulsions invert to o/w emulsions on heating and reforms emulsions on cooling. Any o/w emulsion stabilized by nonionic polyoxyethylene derived surfactant contains oil-swollen micelles of the surfactants as well as emulsified oil. When the temperature is increased the water solubility of the surfactant decreases resulting in micelles broken and the size of emulsified oil droplets begins to increase which cause separation into oil phase, surfactant phase and water phase. Near this temperature the water insoluble surfactant begins to form w/o emulsion containing both water swollen micelles and emulsified water droplets in continuous oil phase. Water solubility of polyoxyethylene ether type surfactants depends upon the ability of O-atom of ether to for H-bond with water. So the solubility of surfactant decreases if temperature is increased or additional of electrolytes giving cloudy appearance.
The temperature at which inversion takes place is called as inversion temperature and at this temperature
hydrophilic and lipophilic properties of emulsifier are balanced.
Equipments for emulsification:
1. Homogenizer
2. Pohlman whistle or Ultrasonifier
3. Colloid mill
4. Mechanical stirrer i.e. impeller mounted on shaft e.g. Paddle blade, Counter rotating blade, Planetary action blade
Foaming agitation: Foaming occurs because the water soluble surfactant required emulsification generally also reduces the interfacial tension between air-water interfaces. So to minimize foaming the emulsification must be done in closed systems or under vacuum. Another approach is use of antifoams like long chain alcohols or silicone derivatives.
Antimicrobials: The most widely used preservative in emulsions is alkyl hydroxybenzoate. But they can react with polyoxyethylene grps. These interactions make the preservative. So addition of alcohol mainly PEG activates the p-hydroxybenzoic acid derivates or Parabens in presence of non-ionic surfactant. Other includes methyl paraben which is water soluble while propyl paraben which is oil soluble so their combination is used. Chlorobutanol is used in eye preparations.
Antioxidants: BHA, BHT, L-Tocopherol
Emulsion stability:
1. Creaming: The migration of one of the substances to the top (or the bottom, depending on the relative densities of the two phases) of the emulsion under the influence of buoyancy or centripetal force when a centrifuge is used.
Stroke‟s is applicable to
1. Rate of creaming and is function of square of radius of droplet. Larger particles more rapidly than smaller ones.
2. Rate of creaming is inversely proportional to viscosity.
2. Flocculation: it is reversible aggregation of dispersed phase or internal phase come out of suspension in
flakes and. This is influenced by charge on the surface of emulsified globule. The reversibility depend upon the interaction between particles, phase volume ratio and concentration of electrolytes and emulsifier. Coalescence is the process by which two or more droplets or particles merge during contact to form a
single daughter droplet (or bubble) resulting in breaking of emulsion. The major factor that prevents the
3. calescence is the strength of the interfacial barrier. To avoid coalescence or increase the shelf life generally
macromolecules like natural gums as auxiliary emulsifier are used.
Auxiliary emulsifying agent: these agents themselves cannot form stable emulsions when used alone but act act as stabilizing agent or thickening agent e.g. acacia, tragacanth etc.
HLB of a surfactant is determined by Griffin’s method:
HLB of a surfactant is determined = mol % of hydrophilic group / 5
A completely hydrophilic molecule i.e. without any nonpolar group has HLB 20.
In HLB scale the total concentration of surfactant blend is 2% is optimum but should not be more than 5%.
