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Introduction

Emulsion

An emulsion is a colloidal system in which both phases are liquid and the liquids are normally

immiscible. It is so called lyophobic colloids.

The typical example is water and oil. These two liquids can form different types of emulsions:

(i) Oil dispersed in water (O/W type) and

(ii) Water dispersed in oil (W/O type).

Figure 1 : Types of emulsions

In system (i), water acts as dispersion medium. Examples of this type of oil in water are milk and

vanishing cream. Milk, liquid fat is dispersed in water. Meanwhile, in system (ii), oil acts as

dispersion medium. Common examples of this type are butter and cream.

Besides that, emulsions are also classified based on the size of liquid droplets.

(i) 0.2 -50 mm - Macroemulsions (kinetically stable)

(ii) 0.01 - 0.2 mm - Microemulsions (thermodynamically stable)

Common emulsions are inherently unstable and do not tend to form spontaneously. Energy input

through stirring, shaking, or homogenizing is necessary to form an emulsion. Over time,

emulsion tends to revert to the stable state of the phases comprising the emulsion. To stabilise an

emulsion, an emulsifying agent such as surfactant is usually added. The emulsifying agent forms

an interfacial film between suspended particles and the medium, promoting the dispersion of the

phases.


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In the case of water and oil, sodium oleate, a soap reduces the water surface tension and raises

that of the oil, so that a quite stable O/W type emulsion is formed. This principle is employed for

the purpose of cleaning to remove grease.

Figure 2 : Emulsifiers such as the surfactant, polymer and solid particles are used to stabilise the

emulsion.

Emulsion applications in our daily life include the following:

o Pharmaceutical emulsions e.g. stability of parenteral nutrition and anaesthetic emulsionso Food emulsions e.g. fat droplets in dairy cream

o Industrial emulsions e.g. lubricating oilso Water-in-oil emulsions

o Fluorocarbon emulsionso Cosmetic emulsions

o Bitumen emulsions


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Zeta Potential

Zeta potential is a scientific term for electrokinetic potential in colloidal system. When a particle

is immersed in a fluid, a range of processes causes the interface to become electrically charged.

Some of the charging mechanisms include adsorption of charged surfactants to the particlesurface (for example in an emulsion stabilised by an ionic surfactant), loss of ions from the solid

crystal lattice (silver halide particles used in photographic emulsions) and ionization of surface

groups (carboxylate in polymer microspheres). These processes lead to the production of a

surface charge density, expressed in coulombs per square metre, which is the fundamental

measure of charge at the interface.

Emulsion droplets that electrically charged are surrounded by a cloud of ions which carry an

equal and opposite charge. The zeta potential is the voltage difference between the dropletsurface and the liquid beyond the charge cloud. In emulsions which are electrostatically

stabilized, zeta provides a measure of the electrical repulsive force between the particles. For

other systems where the stability comes from steric components, the zeta potential can be used as

a measure of the state of the surface. It can be used to monitor optimum levels of dispersant or

other agents to be added to the droplets. Thus zeta potential can be used for monitoring and

controlling emulsion stability and as an indicator of surface chemistry.


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Case Study: The stability of an anaesthetic emulsion

Intravenous Emulsions

Intravenous emulsions are used to provide nutrition for patients who cannot be fed orally and as

drug delivery vehicles. Medical products like anaesthetic emulsion are emulsions of vegetable

oils in water, with phospholipids as emulsifier, which give a high zeta potential, and a

correspondingly long shelf life. The emulsions are administered to patients intravenously to

induce or maintain general anaesthesia to facilitate surgery. Naturally, there is a wide scope for

interaction between the components, and in many mixtures the fat emulsion becomes unstable,

and coalesces or flocculates. An understanding of the stability of the emulsion in these systems

helps in predicting which mixtures would be unstable, and even possible in producing stable

mixtures with longer shelf life.

Propofol is a short-acting, intravenously administered hypnotic agent used to induce and

maintain a deep and pain free sleep during surgical procedures. It is also used as a sedative so

that certain surgical or diagnostic procedures can be carried out. The drug is available as an

aqueous lipid emulsion . This study is concerned with a commercial anaesthetic emulsion,

Diprivan, which is a 10 volume percent soyabean oil in water emulsion. The oil droplets contain

around 1 % of propofol anaesthetic (the active ingredient).
http://en.wikipedia.org/wiki/General_anaesthesiahttp://en.wikipedia.org/wiki/Surgeryhttp://en.wikipedia.org/wiki/Intravenous_therapyhttp://en.wikipedia.org/wiki/Hypnotichttp://en.wikipedia.org/wiki/Hypnotichttp://en.wikipedia.org/wiki/Intravenous_therapyhttp://en.wikipedia.org/wiki/Surgeryhttp://en.wikipedia.org/wiki/General_anaesthesia


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While Diprivan is a safe and reliable means of administering propofol, it is often mixed with

other drugs such as muscle relaxants and narcotics. It is crucial that the droplet size is not

increased by the extra components, due to the fact that they can render the emulsion unsuitable

for infusion. The objective of this study was to measure the stability of Diprivan in the presence

of additional drugs. Several drugs were used in this study, but we will focus here on the

effect of lignocaine hydrochloride, a local anaesthetic used to reduce the pain on injection of

Diprivan.

Calculation

One of the principal process where emulsion is destabilized is by creaming. Creaming is a

separation of the emulsion into two emulsions, one of which (the cream) is richer in the disperse

phase than the other. In other words, creaming is the process by which the disperse phase sepa-

rates from an emulsion and is typically the precursor to coalescence.

The creaming of emulsion can be best explained using the Stokes equation:

= 2 r 2 ( o) g / 9

where, = creaming rate

r = droplet radius

= density of the droplet

o = density of the dispersion medium,

= viscosity of the dispersion medium (continuous phase)

g = local acceleration due to gravity.

The Stokes equation shows that creaming is inhibited by a small droplet ra dius, a highly viscous

continuous phase and a low density difference between the oil and water phases

From Stokes equation , the droplet size of the oil is one of the factor affecting the formulation of

the emulsions. Based on the Stokes equation , the rate of creaming will be directly propotio nal

to the average radius of the oil droplet (droplet size).


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For droplets larger than 1 mm, it may settle preferentially to the top or the bottom of system

under gravitational forces. Creaming is an instability but not an actual breaking, thus not as

serious as coalescence or breaking of emulsion

Probability of creaming can be reduced if

4/3 a 3 gH kT

Where, a =droplet radius

=density difference

g =gravitational constant

H =height of the vessel,

Therefore, creaming can be prevented by reducing the globule size by homogenization and

reducing . homogenization


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Results and Discussions

Figure 3: Instability induced in anaesthetic emulsion by addition of lignocaine

Due to the fact that the zeta potential is a very good index of the magnitude of the interaction

between colloidal particles and thus the measurements of zeta potential are commonly used toassess the stability of colloidal systems, similarly applied in this study. In Figure 3, it is shown

that the AcoustoSizer measurements of the droplet size and zeta potential of the Diprivan at

different levels of lignocaine addition. In this study, particle size tended to increase whereas zeta

potential appeared to exhibit decreasing trend. It is apparent that the lignocaine decreases the

magnitude of the zeta potential, and therefore the electrostatic repulsive force between the

droplets is reduced. The droplet size was not affected until the lignocaine level reaches about 10

mg/mL of emulsion. At this point the zeta is too small to provide stability for the suspension, and

the droplets grow. The droplet size continued to grow on standing and the suspension creamed

overnight. It is clear that the simultaneous measurement of zeta and size is able to provide a clear

explanation, and a way of preventing the onset of instability.

The effect the additive has on emulsion stability could be illustrated using following explanation.

In fact, it can be said that initially the lipid droplets exh ibit an appreciable charge, w hich might


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be due to the presence of surface impurities in the oil phase such as free fatty acid. Consequently,

the addition of lignocaine reduced any electrostatic repulsion between lipid/oil droplets and the

droplets grow and lead to increased flocculation. It is suggested that the addition of lignocaine

changed the interfacial composition of lipid droplets by any interaction that free the surface

impurities like free fatty acids.

Unstable emulsions undergo coalescence, which is the separation of the emulsion into a bulk

aqueous and bulk oil phase. There are four different droplet loss mechanisms which result in

coalescence: Brownian flocculation, sedimentation flocculation, creaming and disproportionation.

These processes may occur simultaneously or not in any fixed order. In this study on Diprivan

emulsion, the flocculation could be described by creaming and Brownian flocculation

mechanism.

Creaming is the separation of the initial emulsion into two emulsions, one with a higher amount

of disperse phase than the other. Creaming usually occurs prior to coagulation. In a polydisperse

(real) emulsion, droplets of different sizes will cream at different rates. Larger droplets which

move faster will collide with smaller droplets and entrap them, creating "floc". Brownian

flocculation assumes that the particles collide randomly in Brownian motion.


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Reference :www. particlesciences.com.news/technicalbriefs/2011/emulsionstabilityandtesting.html

http://www.colloidal-dynamics.com/docs/CD_products_for_emulsions.pdf

http://en.wikipedia.org/wiki/Propofol
http://www.colloidal-dynamics.com/docs/CD_products_for_emulsions.pdfhttp://www.colloidal-dynamics.com/docs/CD_products_for_emulsions.pdfhttp://www.colloidal-dynamics.com/docs/CD_products_for_emulsions.pdf

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