The Choice Of Emulsion Type

Published Date: 02 Nov 2017

Emulsion is a liquid dosage form consisting of two immiscible liquids that will be dispersed in one another in the form of globules by the help of emulsifying agent. Emulsion can be taken orally or can be used externally on skin. The most common example of emulsion that can be consumed is milk. For emulsion that is intended for external applications, creams are the best example to give. Oil-in-water emulsion is mostly used as water washable bases. On the other hand, the water-in-oil emulsion usually act as emollient and cleansing agent. The active ingredient is often dissolved in one or both phases, hence creating a three-phase system. Patients often prefer a water-in-oil cream to an ointment because the cream spreads more readily, less greasy, and the evaporating water soothes the inflamed tissue. Oil-in-water creams when applied onto the skin; the continuous phase which is water evaporates and increases the concentration of a water-soluble drug in the adhering film. This increases the concentration gradient for drug across the stratum corneum, and promotes percutaneous absorption.

Choice of emulsion type

There are four types of emulsion in general which are oil-in-water emulsion, water-in-oil emulsion, multiple emulsion and also microemulsion. However, only two types of emulsion that is commonly prepared that is oil-in-water emulsion and water-in-oil emulsion. The type of emulsion that we are going to prepare depends on how we are going to apply it. For internal use, oil-in-water emulsion is much more preferable because it is more readily absorbed and permits palatable administration of distasteful oil. For emulsion that needs to be applied externally, oil-in-water or water-in-oil emulsions can be used depends upon the desirability of emollient and the condition of the skin surfaces.

Choice of oil phase

The externally-used emulsion usually contain larger amount of oil concentration as carriers for its active ingredient. However, types of oil that will be used must be taken into account seriously as it effects the viscosity of the product and also the ability to spread on the skin to deliver its active ingredient. The most commonly used oil is liquid paraffin. Liquid paraffin is divided into several types which are hard paraffin, soft paraffin and also light liquid paraffin. These types of paraffin can be combined together to control emulsion consistency.

The aqueous phase

The ratio of aqueous phase used also depends on the type of emulsion that is going to be prepared. The aqueous phase will be in higher ratio if oil-in-water emulsion is about to produced. Oil-in-water emulsion is usually given orally as it is more readily absorbed and can help masking the bad taste of oil in the emulsion.

Volume concentration of dispersed phase

Every types of emulsion have different volume concentration of disperse phase. The ideal volume concentration of water should be more than 25%. This is to ensure the stability of the emulsion itself. An emulsion which contains less than 25% of disperse phase has the tendency to cream easily. Higher amount of dispersed phase will affect the flow ability of the emulsion because it will become too viscous and hence difficult to deliver its active ingredient. However, microemulsion does not have to obey this law. In micro emulsion where oil act as dispersed phase, the volume concentration of oil can be increased up to 40% and it still gives no effect to the viscosity of the emulsion due to its super fine globule size.

Particle size of dispersed phase

Size of the droplet (the dispersed phase) in an emulsion has a large influence on the quality of emulsion. The droplet size can influence stability, appearance and rheological properties of emulsion itself. Theoretically, the smaller the particle size of the dispersed phase, the higher the stability of emulsion and the better the quality is.

Viscosity of continuous phase

Theoretically, a viscous continuous phase will help in the emulsion stability and reduce the rate of creaming. However, care need to be taken as a highly viscous emulsion is not suitable for oral intake. Usually, water-in-oil emulsion has a higher viscosity compared to oil-in-water emulsion. For water-in-oil emulsion, it is usually being used topically. The viscosity plays an important role as it affects the spreadability of the emulsion and also its effectiveness in delivering the active ingredient when it is applied on the skin.

Viscosity of dispersed phase

Basically, by increasing the viscosity of the dispersed phase, the stability of the emulsion will be improved as well. This is because the viscosity of the dispersed phase acted as a damping factor for particle anchoring at the oil in water interface and plays an important role during the emulsification process. The viscous dispersed phase slows the movement between each globule in the continuous phase and reduces the tendency for coalescence to occur. However, if the viscosity of the dispersed phase is too high, that is more than 60%, phase inversion of emulsion might occur.

Choice of emulsifying agent

The choice of emulsifying agent to be used is important as it is not only going to affect the stability of emulsion, but also its route of administration. There are many factors that need to be considered when choosing an emulsifying agent such as it must be less irritant, non-toxic, capable of reducing the interfacial tension between dispersed and continuous phase, chemically stable and not to missed out that the emulsifiers chose must depends on the HLB value of the emulsion system. Most of emulsifying agents used are non-ionic as it is less irritant and less toxic as compared to the anionic and cationic emulsifiers. Ionic emulsifying agent cause irritancy on gastro-intestinal tract and consequent laxative effect, so it is not recommended to be given orally.

Nature and concentration of emulsifying agent

Basically there are many different kinds of emulsifying agent that present and can be used to prepare different types of emulsion. There are natural emulsifiers which consist of polymeric carbohydrate, example is acacia. There are also emulsifiers made up of protein, for instance is gelatin which can act as an effective emulgent but need to add preservative in order to prevent microbial growth. The third one is surfactant that functions to reduce the surface tension between two phases also act as a good emulsifying agent. Surfactant can be divided further into anionic, cationic and non-ionic surfactant. There are also emulsifying agent made up from finely divide solid which is amphiphillic in nature and preferentially be wetted by one of the phases present in an emulsion.

Ideal emulsifying agent

An ideal emulsifying agent usually exhibits the following characteristics to become effective emulsifiers. Some of the characteristics are it must not be toxic especially when emulsion is going to administered orally, possessing no sensitizing or allergic reactions for emulsions that is applied topically, capable of forming coherent film around dispersed globules and reduce interfacial tension between dispersed and continuous phase, increase viscosity of external phase, cheap and lastly suitable for a wide range of different pairs of liquid.

Formulation by the HLB method

The physically stable emulsions are indicated by the presence of a thin layer of emulsifier at the oil--water interface, and the interfacial films formed by a mixing of an oil-soluble emulsifier with a water-soluble one produces the most stable emulsions. In order to produce the most physically stable emulsions, we need to calculate the realitve quantities of emulsifying agent with water combination to make sure it form a perfect combination between them. This approach is called the hydrophilic-lipophilic balance (HLB) method. Each surfactant is given an HLB number representing the relative properties of the lipophilic and hydrophilic parts of the molecule. High numbers (up to a theoretical number of 20), indicates a surfactant exhibiting mainly polar or hydrophilic properties, whereas low numbers of HLB represent lipophilic or non-polar characteristics. Each type of oil requires an emulsifier of a certain HLB number in order to produce a stable product. For example such as oil-in-water emulsion, the higher the polarity of the oil phase, the more polar must be the emulsifier system.

Other formulation additives

Additives can be added into the emulsion to improve in order to extent it shelf-life or to increase the acceptance by consumer. Examples of additives that can be added are flavors to mask the distasteful of oil when taken orally, addition of electrolytes for example in oil-in-water emulsion to increase its stability and prevent phase inversion from occur. Other than that, preservative can also be added into the formulation to prevent bacterial growth.

Evaluation of emulsion and stability testing of emulsion

The stability testing of emulsions can be carried out by performing the several following tests.

First one is microscopic test. This test is carried out by examining the rate of creaming occurring over a period of time. In this test, we need to see whether the creaming form or not after it has been left for a certain period.

Second test is particle size analyzer. It involves determination of globule size and changes in distribution of globule size in emulsion with time. An increase in mean of globule size with time indicates tthat coalescence has occurred.

Third test is focusing on viscosity changes. Since viscosity plays a very important role in producing a stable emulsion, so any changes in it can affect the quality of emulsion being produced. Viscosity changes can be done by determining the specific interfacial area, which is the surface area or volume of globules of an aging emulsion.

For stability test, we can determine it by using accelerated stability test. One way to do it is by centrifugation where we use centrifuge at 200-300 rpm which will speed up the rate of creaming and coalescence. After centrifugation, we can carry out the process listed above to determine the stability of the emulsion and compare the relative stability with other similar products.