Surface Chemistry

Emulsions

Did you know that the word “emulsion” is derived from the Latin word  “mulgeo” which means “to milk”? Milk is an emulsion of fat and water, along with several other components. But what exactly are these Emulsions and what is their significance in our daily lives as well as their applicability in industries?

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What is an Emulsion?

Emulsions are biphasic liquid systems consisting of two immiscible liquid phases. These phases are present in such as way that one phase is dispersed in the other continuous phase. Emulsions are part of a more general class of two-phase systems of matter called colloids.

Although the terms colloid and emulsion are sometimes used interchangeably, the term emulsion is used only when both the phases are in a liquid state. Examples of emulsions include mayonnaise, milk, lotions, etc.

Types of Emulsions

Emulsion

Emulsions can exist as “oil in water” or ” water in oil” of emulsions. The type of emulsion depends upon the properties of the dispersed phase and continuous phase. If the oil phase is dispersed in a continuous aqueous phase the emulsion is known as “oil in water”. If the aqueous phase is the dispersed phase and the oil phase is the continuous phase, then its known as “water in oil”

Whether an emulsion of oil and water turns into a “water-in-oil” emulsion or an “oil-in-water” emulsion depends on the volume fraction of both phases and the type of emulsifier used to emulsify them.

Relationship Between Concentration and Appearance

A boundary that exists between the dispersed phase and continuous phase is known as the interface. The emulsions tend to have a cloudy appearance because the phase interphases present in the emulsion scatter light at different wavelengths.

The color of the emulsion depends upon its concentration. Emulsions appear white when all light is scattered equally. In case of dilute emulsions, a low-wavelength light will be scattered more, and the emulsion will appear blue in colour. This phenomenon is known as the “Tyndall Effect”. If the emulsion is concentrated enough, the colour will be distorted to comparatively longer wavelengths and will appear more yellow.

In case of special classes of emulsions like nanoemulsions and microemulsions, the system appears translucent in colour. This property is due to the fact that light waves are scattered by the droplets only if their sizes exceed about one-quarter of the wavelength of the incident light. Since these systems have a particle size less than 100 nm, they have a translucent appearance.

Properties of Emulsions

  • Emulsion particles unavoidably form dynamic inhomogeneous structures on small length scale.
  • Emulsions are highly unstable systems and require an emulsifying agent or emulsifier ( These are usually surface active agents also known as “surfactants”)
  • Emulsions are prepared by continuous mixing or agitation of the two phases
  • When kept for longer periods of time or in case of absence of an emulsifying agent, the phases in the emulsion tend to separate, resulting in “cracking of emulsion” or ” phase inversion”.

Emulsifying Agents

These are surface active agents that are added to the emulsions to stabilize the two phases. It acts on the interface and increases the kinetic stability of an emulsion so that the size of the droplets does not change significantly with time, thus stabilizing the emulsion.

Emulsifiers are compounds that typically have a polar or hydrophilic (i.e. water-soluble) part and a non-polar (i.e. hydrophobic or lipophilic) part. Because of this, emulsifiers tend to have more or less solubility either in water or in oil.

According to the Bancroft rule, Emulsifiers and emulsifying particles tend to promote dispersion of the phase in which they do not dissolve very well. For example, proteins dissolve better in water than in oil, and so tend to form oil-in-water emulsions (that is, they promote the dispersion of oil droplets throughout a continuous phase of water).

Emulsifiers that are more soluble in water generally form oil-in-water emulsions, while emulsifiers that are more soluble in oil will form water-in-oil emulsions. Examples include egg yolk, sodium phosphates, sodium stearoyl lactylate, etc

Mechanisms of Emulsification

A number of different chemical and physical processes and mechanisms can be involved in the process of emulsification.

  • Surface tension theory – According to this theory, emulsification takes place by the reduction of interfacial tension between two phases
  • Repulsion theory – The theory proposes that the emulsifying agent creates a film over one phase that forms globules, which repel each other. This repulsive force causes them to remain suspended in the dispersion medium
  • Viscosity modification – Certain emulgents such as acacia, tragacanth, carboxymethylcellulose, polyethylene glycol, etc  increase the viscosity of the medium, which helps create and maintain the suspension of globules of the dispersed phase

Instabilities in Emulsions

The ability of an emulsion to retain its properties over time is known as the stability of an emulsion. Four types of instabilities generally occur in an emulsion, Let us take a look.

  • Flocculation – When the particles or droplets of the dispersed phase aggregate together on account of attractive forces, the phenomenon is known as flocculation and results in an unstable system. Flocculation is mainly observed in case of oil in water type of emulsions.
  • Coalescence – When the droplets of discontinuous phase bump into each other to form a larger droplet thus increasing the average particle size over time, it is known as coalescence which is a form of instability.
  • Creaming – When the droplets in an emulsion rise to the top of the emulsion under the influence of buoyancy or centripetal force, it results in the creaming of emulsion
  • Ostwald Ripening – It describes the change of an inhomogeneous structure over time, i.e., small crystals or sol particles dissolve and redeposit onto larger crystals or sol particles. Ostwald ripening is generally found in water-in-oil emulsions.

Uses of Emulsion

  • As Food: Oil-in-water emulsions are common in food products. Examples include butter, margarine, homogenized milk, mayonnaise, etc
  • In Healthcare Many cosmetic and pharmaceutical dosage forms are in the form of emulsions. Cosmetics such as lotions, creams, biphasic makeup removers are in fact emulsions. Many oral, as well as topical dosage forms, are emulsions. Microemulsions are used to deliver vaccines and kill microbes. Cod liver oil, cortisol, polysporin are some examples of emulsion formulations.
  • In Chemical Synthesis Emulsions are used in the manufacturing of polymer dispersions. These include primary components of glues and paints.

Solved Questions for You

Que: Which of these is not a form of instability in emulsions?

  1. Flocculation
  2. Creaming
  3. Coalescence
  4. Oxidation

Ans, The correct option is “D”. Oxidation is not a form of instability that occurs in emulsions.

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