EMULSIFYING AGENTS
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EMULSIFYING AGENTS
These are the substances added to an emulsion to prevent the coalescence of the globules of the dispersed phase. They are also
known as emulgents or emulsifiers. These agents have both a hydrophilic and a lipophilic part in their chemical structure. All
emulsifying agents concentrate at and are adsorbed onto the oil/water interface to provide a protective barrier around the
dispersed droplets. In addition to this protective barrier, emulsifiers stabilize the emulsion by reducing the interfacial
tension of the system. Some agents enhance stability by imparting a charge on the droplet surface thus reducing the physical
contact between the droplets and decreasing the potential for coalescence. Thus these act in three ways:
1) Formation of a protective barrier
2) Reduction of interfacial tension
3) Decreasing the potential for coalescence by forming an electrical double layer
Interfacial phenomena
Interfacial phenomena occurs at the limit between two immiscible phases, so-called surface or interface. When one phase is fragmented into (small) pieces which are dispersed in another (continuous) phase, a so-called dispersion or dispersed system is produced.
Emulsifying agents have two fundamental properties. On one hand they tend to be located preferentially at the interface between a polar and a nonpolar phase. The phenomenon according to which a molecule comes from the bulk of a solution to place itself at the interface (with some specific orientation) is called adsorption, and is characteristic of many amphiphilic molecules. On the other hand surfactant molecules in solution exhibit a tendency to self associate to produce aggregation polymer called micelles, as well as other structures.
All the properties of surfactant solutions come from one of these fundamental properties.
Amphiphile
An amphiphile is a chemical substance that possesses some affinity for both the polar substances and the apolar ones. Generally speaking these affinities are referred to as hydrophilic and lipophilic (or hydrophobic) respectively since the polar solvents are in most cases aqueous solutions and the apolar phases are organic "oils".
Most amphiphilic substances are surfactants, i.e., substances that are preferentially located at a surface or interface, where the polarity changes drastically within a few angstroms of distance.
Adsorption
When a surfactant molecule goes to the interface and locates itself there with some preferential orientation, it is said that the molecule is adsorbed. Adsorption is a spontaneous phenomenon which is driven by a reduction of the energy when the surfactant lyophobic group is removed from the solvent, and when one or both affinities are satisfied respectively at a surface or at an interface.
Adsorption is a dynamic phenomenon which is opposed by desorption, i.e., the transfer of a surfactant molecule to a bulk phase. The adsorption and desorption steps are often very rapid; as a consequence an adsorption-desorption equilibrium is reached after some time, which depends upon the surfactant concentration in the bulk phase.
Since the surfactant molecule has a lower free energy when it is adsorbed at interface than in the solvent bulk phase, the equilibrium is very much displaced toward the adsorbed state. In fact the interface is very rapidly covered by a monolayer of surfactant molecules. In such monolayer the molecules are arranged in some specific pattern which depends upon structural and geometrical characteristics.
Self-Association
The second fundamental property of surfactant molecules is their capability of self-association in aqueous or non aqueous solutions.
The tendency of the surfactant molecules to associate depends upon the formation of an adsorbed monolayer, which is the first step of surfactant association.
When the surfactant concentration increases in the aqueous phase, the surfactant molecules first saturate the interface, and then accumulate in the solution. Each time a new surfactant molecule is added to the solution, the unfavorable interaction between the surfactant hydrophobic tail and the water molecules is increased. At some point the surfactant molecules start aggregating into the so-called micelles, a self-association structure in which the hydrophobic tail is removed from the aqueous environment. The concentration at which the first micelles are formed is called the Critical Micelle Concentration, which is abbreviated as CMC. The CMC is the concentration at which the factors which favor the formation of the micelle (for instance the hydrophobic effect) start dominating the effects which oppose it.
Micellar solutions are able to solubilize different kinds of substance, and this capacity of solubilization is one of the most important properties of the surfactant solutions. Hydrophobic substances, i.e., oils, can be solubilized inside the micelles core, sometimes in very sizeable amounts. Some extreme cases are known in which the solubilized oil volume is actually larger than the aqueous solvent volume; for such situation to happen, the solution must contain a very large number of micelles and the micelles must be considerably swollen. These micelles are no longer spherical, but cigar shaped or hexagonally packed or even degraded into lamellar liquid crystals.
The choice of selection of emulsifying agent plays a very important role in the formulation of a stable emulsion. No single emulsifying agent possesses all the properties required for the formulation of a stable emulsion therefore sometimes blends of emulsifying agents have to be taken.
CRITERIA FOR THE SELECTION OF EMULSIFYING AGENTS
An ideal emulsifying agent should posses the following characteristics:
It should be able to reduce the interfacial tension between the two immiscible liquids. It should be physically and chemically stable, inert and compatible with the other ingredients of the formulation. It should be completely non irritant and non toxic in the concentrations used. It should be organoleptically inert i.e. should not impart any colour, odour or taste to the preparation. It should be able to form a coherent film around the globules of the dispersed phase and should prevent the coalescence of the droplets of the dispersed phase. It should be able to produce and maintain the required viscosity of the preparation.
CLASSIFICATION OF EMULSIFYING AGENTS
Emulsifying agents can be classified as:
1. Natural Emulsifying agents: A large number of emulsifiers are natural products derived from plant or animal tissue. Most of the emulsifiers form hydrated lyophilic colloids (called hydrocolloids) that form multimolecular layers around emulsion droplets. Hydrocolloid type emulsifiers have little or no effect on interfacial tension, but exert a protective colloid effect, reducing the potential for coalescence, by:
· providing a protective sheath around the droplets
· imparting a charge to the dispersed droplets (so that they repel each other)
· swelling to increase the viscosity of the system (so that droplets are less likely to merge)
Natural emulsifying agents from vegetable sources: These consist of agents which are carbohydrates and include gums and mucilaginous substances. Since these substances are of variable chemical composition, these exhibit considerable variation in emulsifying properties. They are anionic in nature and produce o/w emulsions. They act as primary emulsifying agents as well as secondary emulsifying agents (emulsion stabilizers). Since carbohydrates acts a good medium for the growth of microorganism, therefore emulsions prepared using these emulsifying agents have to be suitable preserved in order to prevent microbial contamination. E.g. tragacanth, acacia, agar, chondrus (Irish Moss), pectin and starch.
Gum acacia
It is generally used in the concentration of 8-15% and gives a stable and palatable emulsion over the pH range of 2-10. Emulsions tend to cream using this as the viscosity is low.
Tragacanth
Used in the concentration range of 1-2%. Rarely used now because it forms thick and coarse emulsions.
Agar
Used in the concentration of 2%.
Starch
Used in concentration of 2-5%. Rarely used because it forms coarse emulsions.
Pectin
Used as 1%. Acts as emulsion stabiliserin acacia emulsion.
Natural emulsifying agents from animal source: The examples include gelatin, egg yolk and wool fat (anhydrous lanolin). Type A gelatin (Cationic) is generally used for preparing o/w emulsion while type B gelatin is used for o/w emulsions of pH 8 and above. Lecithin and cholesterol present in egg yolk also act as emulsifying agent. They show surface activity and are used for formulating o/w emulsions. However they are used only for extemporaneous preparation and not for commercial preparation as it darken and degrade rapidly in unpreserved systems. Wool fat is mainly used in w/o emulsions meant for external use. They absorb large quantities of water and form stable w/o emulsions with other oils and fats. Animal derivatives are more likely to cause allergic reactions and are subject to microbial growth and rancidity. Their advantage is their ability to support formation of w/o creams.
Gelatin
Used in concentration of 1%. Two grades are available-Pharmagol A which has an acidic pH and Pharmagol B, which has an alkaline pH. Emulsions prepared have agreeable taste but are prone to bacterial contamination. Gelatin is used for preparing o/w emulsions.
Egg yolk
It contains lecithin which is a phospholipids acting as an emulsifier. Used in concentration range of 12-15%. Mainly used for extemporaneous preparation as long term stability is a problem. It forms stable o/w emulsions, which needs to be refrigerated to prolong shelf life.
2. Semi-synthetic polysaccharides: Includes mainly cellulose derivatives like sodium carboxy methyl cellulose, hydroxyl propyl cellulose and methyl cellulose. They are used for formulating o/w type of emulsions. They are nontoxic, and are less subject to microbial growth. They primarily act by increasing the viscosity of the system. e.g., methyl cellulose, hydroxypropyl cellulose and sodium carboxy methyl cellulose.
Methyl cellulose
This is non ionic in nature and is stable over a wide pH range. It is mainly used for emulsification of mineral and vegetables oil. Drawback is that it gets precipitated in the presence of large quantities of electrolytes.
Sodium carboxymethyl cellulose
It is anionic in nature. Acts as an true emulsifier and emulsion stabilizer.
3. Synthetic emulsifying agents: This group contains surface active agents which act by getting adsorbed at the oil water interface in such a way that the hydrophilic polar groups are oriented towards water and lipophillic non polar groups are oriented towards oil, thus forming a stable film. This film acts as a mechanical barrier and prevents coalescence of the globules of the dispersed phase. They are classified according to the ionic charge possessed by the molecules of the surfactant e.g., anionic, cationic, non-ionic and ampholytic.
i) Anionic Surfactants: The long anion chain on dissociation imparts surface
activity, while the cation is inactive. These agents are primarily used for
external preparations and not for internal use as they have an unpleasant bitter
taste and irritant action on the intestinal mucosa. e.g., alkali soaps, amine soaps, metallic soaps, alkyl sulphates and phosphates and alkyl sulphonates.
Alkali soaps
Produce good oil in water emulsions. Unstable at pH below 10 and are incompatible with acids and polyvalent inorganic and long chain organic cations.
Amine soaps
Used for preparing o/w emulsions. Stable to the presence of calcium ions or changes in pH
Bold textMetallic soaps
Generally used for formulating w/o emulsions. They are usually insoluble in water
Alkyl sulphates and phosphates
They form o/w emulsions.
Alkyl sulponates
Used mainly as wetting agents and form stable o/w emulsions along with a secondary emulsifying agent.
ii) Cationic surfactants: The positive charge cations produced on dissociation are responsible for emulsifying properties. They are mainly used in external preparations such as lotions and creams. Quaternary ammonium compounds such as cetrimide, benzalkonium chloride and benzethonium chloride are examples of important cationic surfactants. These compounds besides having good antibacterial activity are also used in combination with secondary emulsifying agents to produce o/w emulsions for external application.
iii) Non-ionic surfactants: They are the class of surfactants widely used as emulsifying agents. They are extensively used to produce both oil in water and water in oil emulsions for internal as well as external use. The emulsions prepared using these surfactants remain stable over a wide range of pH changes and are not affected by the addition of acids and electrolytes. They also show low irritancy as compared to other surfactants. E.g. glyceryl esters such as glyceryl monostearate, propylene glycol monostearate, macrogol esters such as polyoxyl stearates and polyoxyl-castor oil derivatives, sorbitan fatty acid esters such as spans and their polyoxyethylene derivatives such as tweens (polysorbates).
iv) Ampholytic surfactants: These are the substances whose ionic charge depends on the pH of the system. Below a certain pH, these are cationic while above a defined pH, these are cationic. At intermediate pH these behave as zwitterions. e.g. lecithin.
4. Finely Divided Solids: This group consists of finely divided solids having balanced hydrophilic lipophillic properties. They accumulate at the oil/water interface and form a coherent interfacial film around the droplets of dispersed phase globules and prevent coalescence. If the solid particles are preferentially wetted by oil, a w/o emulsion is formed while if wetting is done by water then o/w emulsion is seen. e.g., bentonite, aluminium magnesium stearate, attapulgite, colloidal anhydrous silica and hectorite. The emulsions formed using finely divided solids are stable and less prone to microbial contamination.
5. Auxillary Emulsifying Agents
A variety of fatty acids (e.g., stearic acid), fatty alcohols (e.g., stearyl or cetyl alcohol), and fatty esters (e.g., glyceryl monostearate) serve to stabilize creams through their ability to thicken the emulsion. Because these agents have only weak emulsifying properties, they are always used in combination with other emulsifiers.
Selection of Emulsifying Agents using HLB method
A system was developed in 1949 by William C. Griffin to assist making systemic decisions about the amounts and types of
surfactants needed in stable products. The system is called the HLB (hydrophile-lipophile balance) system and has an arbitrary
scale of 1 - 18. HLB numbers are experimentally determined for the different emulsifiers.
HLB RANGE USE
0-3 Antifoaming agents
4-6 W/O emulsifying agent
7-9 Wetting agents
8-18 O/W emulsifying agent
13-15 Detergents
10-18 Solubilizing agents
An emulsifier having a low HLB number indicates that the number of hydrophilic groups present in the molecule is less and it has a lipophillic character. For example, spans generally have low HLB number and they are also oil soluble. Because of their oil soluble character, spans cause the oil phase to predominate and form a w/o emulsion.
A higher HLB number indicate that the emulsifier has a large number of hydrophilic groups on the molecule and therefore is more hydrophilic in character. Tweens have higher HLB numbers and they are also water soluble. Because of their water soluble character, tweens will cause the water phase to predominate and form an o/w emulsion.
DISADVANTAGE OF THE HLB SYSTEM
It does not take into account:
- the effect of temperature
- the presence of additives
- the concentration of emulsifier
HLB values of some common emulsifying agents
Emulsifying Agent : HLB Value
Acacia: 8
Polysorbate 20 (Tween 20): 16.7
Polysorbate 60: (Tween 60): 14.9
Polysorbate 80 (Tween 80): 15
Oleic acid: 4.3
Sorbitan monolaurate (Span 20): 8.6
Sorbitan monolaurate (Span 60): 4.7
Sorbitan monolaurate (Span 80): 4.3
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