Question

What is the meaning of emulsion?

Answer 1

Emulsions are defined as biphasic preparation because they are made up of more than one component. Emulsion comes under the category of colloidal preparations.
Colloidal preparation is defined as “colloids is a state of any substance in which particle size varies from $10^{-9}m to 10^-{6}m$. Colloidal solution comes between the true solutions which have particle size (<1)nm and suspensions which have particle size of (>1000)nm.

Complete answer:
Emulsions are defined as a liquid-liquid colloidal system in which one liquid is polar (aqueous) phase and other liquid is non-polar (oil) phase.
Two components of emulsion are known as dispersed phase and dispersion medium depending upon the quantity of components in emulsion.
Dispersed phase of emulsion is one which is present in smaller quantities and has a tendency to get dispersed in the other phase.
Dispersion medium of emulsion is defined as the one which is present in larger proportion and has a tendency to allow dispersed phase to be dispersed in it.
Dispersion medium is also known as the continuous phase of emulsion.
When the two phases are mixed together, one of the liquids form small droplets and get dispersed in the other liquid phase. After keeping aside, droplets of dispersed phase get fused themselves and finally separate from dispersion medium and appear as two distinct phases. Due to this separation of emulsion on standing they are termed as thermodynamically unstable preparations.
Emulsions are made stable by adding Emulsifying agents which are defined as “the substances which are added into emulsion intentionally to increase the stability of emulsions”. Emulsifying agents (for example- stearic acid, tragacanth, bentonite, span, tween, and benzalkonium chloride) have the ability to reduce the interfacial tension between the dispersed phase and dispersion medium which is responsible for instability of emulsions.
Depending upon type of dispersed phase and dispersion medium emulsion are categorizes as
$w/o$- water in oil emulsion are emulsions in which water acts as dispersed phase and dispersed in oil which acts as dispersion medium. For example- cod liver oil, cold cream etc.
$o/w$-Oil in water emulsion is an emulsion in which oil acts as a dispersed phase and dispersed in water which acts as dispersion medium. For example- vanishing cream etc.

Note:
Emulsion are also categorizes on the basis of droplet size of dispersed phase into-
Micro emulsion- particle size of dispersed phase is $10 - 75$ nm.
Macro emulsion - particle size of dispersed phase is $0.25 - 10\mu m$
The thermodynamic instability of emulsions are described by the action of cohesive forces (among same molecules) and adhesive forces (among different molecules). During the preparation of emulsion one phase is made to be into smaller droplets which increases interfacial area, and ultimately Gibbs free energy of the emulsion increases..
Instability of emulsion includes coalescence, creaming, cracking and phase inversion.
Instability of emulsion is governed by stroke’s law $v = \dfrac{{{d^2}\left( {\rho - {\rho _0}} \right)g}}{{18\eta }}$
Where $v -$rate of creaming
$d -$Diameter of dispersed phase globules
$g -$ Acceleration due to gravity
$\eta -$Viscosity of dispersion medium
$\rho -$Density of dispersed phase
${\rho _0} -$Density of dispersion medium