Emulsifying agents (also called emulsifiers or emulgents) are substances comprising both oil-soluble hydrophobic (nonpolar) and water-soluble hydrophilic (polar) portions that act as a stabilizer of the droplets (globules) of the internal phase of an emulsion [1], by inhibiting flocculation, creaming, and coalescence [2] (breaking, cracking)[3]. The key goal of an emulsifier is to form condensed films (either monomolecular or multimolecular films or solid particle films) around the droplets of the dispersed phase [2]. You may also read: Quality control tests of tablets
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Definition of Emulsion
An emulsion is a two-phase system prepared by combining two immiscible liquids, in which small globules of one liquid are dispersed uniformly throughout the other liquid [2]. Emulsions are formed and stabilized with the help of emulsifiers which are surfactants and/or viscosity-generating agents. Excipients for Tablets with examples
A stable emulsion must contain at least three components: the first one is internal (disperse or discontinuous phase), the second one is an external (continuous) phase and the third component is the emulsifying agent which makes a bridge between internal and external phases.
Three types of instability (flocculation, coalescence, and creaming) may exhibit by an emulsion. Here, flocculation is the process by which the internal phase comes out of suspension in the form of flakes. Coalescence is the process by which small droplets within the media continuously join to form progressively larger droplets. Whereas creaming is a process where one of the phases migrates to the top or the bottom, depending on the relative densities of the two phases of the emulsion. You may also read: Generic vs Brand name drugs
Desirable Properties of Emulsifying Agents (Emulsifiers or Emulgents) [2]
- Emulsifiers should be surface-active and reduce surface tension to below 10 dynes/cm.
- Emulgents should be adsorbed quickly around the dispersed drops as a condensed, non-adherent film that will prevent coalescence.
- Emulsifiers should be imparted to the droplets an adequate electrical potential so that mutual repulsion occurs.
- Emulgents should increase the viscosity of the emulsion.
- Emulsifiers should be effective in a reasonably low concentration.
- The emulsifier must be compatible with the formulation ingredients and the active pharmaceutical ingredient.
- Emulsifiers should be stable, nontoxic, and promote emulsification to maintain the stability of the emulsion for the intended shelf life of the product.
Not all emulsifiers have these properties to the same degree; actually, not every good emulsifier essentially possesses all these properties. In addition, there is no one ideal emulsifier because the desirable properties of an emulsifier depend, in part, on the properties of the two immiscible phases in the specific system under consideration [2].
Selection of Emulsifiers or Emulgents
- The first step in the preparation of an emulsion is the selection of the emulsifier. The selection of an emulsifying agent is commonly critical in developing a fruitful emulsion, and the pharmacist should be aware of [2]:
- The desirable properties of emulsifying agents.
- How different emulsifying agents act to optimize emulsion stability.
- How the type and physical properties of the emulsion can be affected by the emulsifier.
As the emulsifying agent becomes more hydrophilic, its solubility in water increases, and the development of oil-in-water (O/W) emulsion is favored. On the other hand, water in oil (W/O) emulsions are favored with the more lipophilic emulsifying agents. Griffin established a scale based on the balance between these two opposing tendencies named the HLB scale that extends from 1 to approximately 50. Also read: Manufacturing defects of tablet
Hydrophilic-Lipophilic Balance
| HLB Range | Use |
| 0-3 | Antifoaming agents |
| 4-6 | W/O emulsifying agents |
| 7-9 | Wetting agents |
| 8-18 | O/W emulsifying agents |
| 13-15 | Detergents |
| 10-18 | Solubilizing agents |
- No incompatibilities should occur between the various emulsifying agents and the several excipients usually existing in pharmaceutical emulsions.
- Emulsifying agents must be nontoxic, tasteless, odorless, and physically and chemically stable.
The concentration of Emulsifying Agents (Emulsifiers or Emulgents)
The key goal of an emulsifier is to form condensed films (either monomolecular or multimolecular films or solid particle films) around the droplets of the dispersed phase. An insufficient concentration of emulsifiers will do little to prevent coalescence. Increasing the emulsifier concentration above an optimum level attains little in terms of increased stability [2]. Generally, emulsifying 10 g of oil would require 0.33 g of the emulsifying agent [2].
Types of Emulsifying Agents (Emulsifiers or Emulgents)
Types of Emulsifying Agents (Emulsifiers or Emulgents) based on the type of film they form/Mechanism of Action
Based on the type of film form/Mechanism of Action at the interface between the two phases, emulsifying agents may be classified as:
Monomolecular Films forming Emulsifiers
Those emulsifiers are capable of stabilizing an emulsion by forming a monolayer of adsorbed molecules or ions at the oil-water interface. In accordance with Gibbs’ law, the existence of an interfacial excess necessitates a reduction in interfacial tension. If the emulsifier forming the monolayer is ionized, the presence of strongly charged and mutually repelling droplets increases the stability of the system [2]. For example, Potassium laurate and Sodium dodecyl sulfate etc.
Multimolecular Films forming Emulsifiers
They form multimolecular films around droplets of dispersed oil. They act as a coating around the droplets and render them highly resistant to coalescence, even in the absence of well-developed surface potential. Moreover, any hydrocolloid not adsorbed at the interface increases the viscosity of the continuous aqueous phase; this enhances emulsion stability. For example, hydrated lyophilic colloids.
Solid Particle Films forming Emulsifiers [2]
Small solid particles that are wetted to some degree by both aqueous and non-aqueous liquid phases act as emulsifying agents. If the particles are too hydrophilic, they remain in the aqueous phase; they are dispersed completely in the oil phase if too hydrophobic. A second prerequisite is that the particles are small in relation to the droplets of the dispersed phase. For example, Hydrophilic colloids, Acacia, and Gelatin etc.
Figure: Types of films formed by emulsifying agents at the oil-water interface. Orientations are shown for O/W emulsions
Types of emulsifying agents based on chemical structure
According to the chemical structure, emulsifying agents are divided into synthetic, natural, and finely dispersed solids.
Synthetic Emulsifying Agents
Synthetic Emulsifying Agents are very effective at lowering the interfacial tension between the oil and water phases because the molecules possess both hydrophilic and hydrophobic properties. They may be subdivided into anionic, cationic, and nonionic, depending on the charge possessed by the surfactant:
Anionic Emulsifying Agents
They carry a negative charge. For example, potassium, sodium, and ammonium salt of lauric and oleic acid are soluble in water and are good O/W emulsifying agents.
Cationic Emulsifying Agents
The surface activity in the cationic emulsifying agent resides in the positively charged cation. These compounds have marked bactericidal properties. The pH of an emulsion will be 4 to 6 ranges if you use a cationic emulsifier. Cationic agents are weak emulsifying agents and usually are formulated with stabilizing or auxiliary emulsifiers such as cetostearyl alcohol.
Nonionic Emulsifying Agents
Nonionic, un-dissociated surfactants, find widespread use as emulsifying agents when they possess the proper balance of hydrophilic and lipophilic groups within the molecule. These emulsifiers are not susceptible to pH changes and the existence of electrolytes.
Natural Emulsifying Agents
Natural Emulsifying Agents are substances derived from either vegetable sources such as acacia, tragacanth, alginates, Chondrus, xanthan, and pectin or animal sources such as gelatin, egg yolk, casein, wool fat, cholesterol, wax, and lecithin. These materials form hydrophilic colloids when added to water and generally produce o/w emulsions. Although their surface activity is low, these materials achieve their emulsifying power by increasing the viscosity of the aqueous phase [2].
Finely Dispersed Solids Emulsifying Agents
Finely dispersed solids are emulsifiers that form particulate films around the dispersed droplets, producing emulsions that are coarse-grained but have considerable physical stability. It appears possible that any solid can act as an emulsifying agent of this type, provided it is reduced to a sufficiently fine powder. Finely dispersed solids emulsifying agents are good emulsifiers and tend to be absorbed the interface, increase the viscosity in the aqueous phase, and are often used in conjunction with a surfactant to prepare oil in water (O/W) emulsions. However, both O/W and W/O preparations can be prepared by adding the clay to the external phase. They are used frequently for external purposes such as a lotion or cream [2].
Auxiliary Emulsifying Agents
Auxiliary emulsifying agents are compounds that are generally incapable of forming stable emulsions. They act as thickening agents and thus help to stabilize an emulsion. Auxiliary emulsifying agents are amphiphilic in nature. In some cases, when combined with water and oil, capable of forming gel or liquid crystalline phases with the primary emulsifying agent.
Emulsifying Agents (Emulsifiers or Emulgents) Examples
| Types | Example |
| Synthetic Emulsifiers | Anionic: Potassium laurate, Triethanolamine stearate, Sodium lauryl sulfate, Alkyl polyoxyethylene sulfates, sodium dodecyl sulfate, and Dioctyl sodium sulfosuccinate, Cationic: Quaternary ammonium compounds, Cetyltrimethyllammonium bromide, and Lauryldimethylbenzylammonium chloride etc.Nonionic: Polyoxyethylene fatty acid derivatives of the sorbitan esters (for example, Tween series), Polyoxyethylene fatty alcohol ethers Sorbitan fatty acid esters Polyoxyethylene alkyl ethers (macrogols) Polyoxyethylene sorbitan fatty acid esters, Polyoxyethylene polyoxypropylene block copolymers (poloxamers), polyethylene glycol 400 monostearate, lanolin alcohols, and ethoxylated lanolin. |
| Natural Emulsifying Agents | Plant origin: Hydrophilic colloids, Alginates, Acacia, Tragacanth, Xanthan, and Pectin. Animal origin: Gelatin, Casein, Egg yolk, Wool fat, Cholesterol, Wax, and Lecithin. |
| Finely divided solids emulgents | Colloidal clays (such as attapulgite), Bentonite, and Veegum/ Magnesium Aluminum Silicate |
| Auxiliary Emulsifying Agents | Cetyl alcohol, Glyceryl monostearate, Methylcellulose, Sodium carboxymethylcellulose, and Stearic acid etc. |
List of Emulsifying agents (Emulsifiers or Emulgents)[4]
| Name of Emulsifiers | Concentration | |
| 1. | Acacia | Generally, 10 – 20% |
| 2. | Carbomer | 0.1 – 0.5% |
| 3. | Carboxymethylcellulose Sodium / Carmellose Sodium | 0.25 – 1.0% |
| 4. | Sodium Lauryl Sulfate | An Anionic emulsifier forms self-emulsifying bases with fatty alcohols 0.5 – 2.5% |
| 5. | Carrageenan | 0.1 – 0.5% |
| Cetyl Alcohol | 2 – 5% | |
| 6. | Cetostearyl Alcohol, BP name: Emulsifying Wax | 2% for pourable emulsions, 10% for stiffer emulsions such as aqueous cream BP, 3–30% may also be mixed with soft and liquid paraffin to prepare anhydrous ointment bases, for example, emulsifying ointment BP. |
| 7. | Cholesterol | 0.3–5.0% w/w for topical pharmaceutical formulations. |
| 8. | Cetomacrogol 1000 | Generally, 5-15% for creams, but concentrations as high as 25% may be employed, for example, in chlorhexidine cream BP. |
| 9. | Methylcellulose | 1.0 – 5.0% |
| 10. | Poloxamer | Commonly, 0.3% |
| 11. | Polyoxyethylene Stearates | 0.5–5% for o/w intravenous fat emulsion as an auxiliary emulsifier and 0.5–10% for o/w creams or lotions |
| 12. | Bentonite | 1.0% |
| 13. | Colloidal clays (such as attapulgite) | 2–5% w/v |
| 14. | Veegum/ Magnesium Aluminum Silicate | As an emulsion stabilizer of 1–5% or oral emulsion and as an emulsion stabilizer of 2–5% for topical emulsion. |
| 15. | Triethanolamine | Typically 2–4% v/v and 2–5 times that of fatty acids. For mineral oils, 5% v/v will be needed, with a proper increase in the amount of fatty acid used. |
| 16. | Stearic Acid | used as an emulsifier in topical formulations |
| 17. | Hydroxypropyl Cellulose | As an emulsifier in topical gels and ointments. |
| 18. | Polycarbophil | As an emulsifying agent in topical oil-in-water systems. |
| 19. | Saponite | As an oil-in-water emulsifying agent. |
| 20. | Hydroxypropyl Starch, Oleyl, Alcohol, Palmitic Acid, Pectin, Potassium Alginate, Ammonium Alginate, Calcium, Alginate etc. | As an emulsifying agent with various concentration |
The function of the emulsifying agents or Use of the emulsifying agents in Pharmacy
- The emulsifying agent is used as a stabilizer to improve the stability of an emulsion.
- Emulsifying agent or emulgent inhibits flocculation, creaming, and coalescence (breaking, cracking) in emulsion in emulsion.
- They reduce the interfacial tension between the two phases of an emulsion.
- Some emulsifying agents create monomolecular or multimolecular films or solid particle films over one phase that forms globules that repel each other. This repulsive force causes them to remain dispersed in the dispersion medium.
- Some emulsifying agents can modify the viscosity of the medium, which helps create and maintain the suspension of globules of the dispersed phase. Prebiotics, Probiotics, Synbiotics, Psychobiotics
References
- Lachman, Lieberman, H.A. and Kanig, J.L., The Theory and Practice of Industrial Pharmacy, Lea and Febiger, New York, 15th edition; 2013.
- Remington, Joseph P, and Paul Beringer. Remington: The Science and Practice of Pharmacy. 21st edition. Philadelphia: Lippincott Williams & Wilkins; 2005.
- Aulton, M. E. Pharmaceutics: The science of dosage form design. Edinburgh: Churchill Livingstone; 2004.
- Rowe, R. C., Sheskey, P. J., Owen, S. C., & American Pharmacists Association. Handbook of pharmaceutical excipients. London: Pharmaceutical Press; 2006.