Industrial Chemistry

Part VI 

Principles Emulsions

2011

Importance of Emulsions

Many foods consist either partly or wholly as food emulsions, or have been in an emulsified state sometime during their manufacture.

milk, cream, salad cream, mayonnaise, salad dressings, soups, sauces, butter, margarine, beverages, ice cream and coffee whitener.

What is Emulsion?

Emulsions are mixtures of two immiscible liquids; in which both the dispersed phase and the dispersion medium are liquids.

Dispersed phase: is the liquid present in a small amount (small droplets)

Continuous phase: is the liquid present in a large amount (medium)

The dispersed liquid is known as the internal or discontinuous phase

the continuous phase is known as the external or dispersion medium

Classification (Types) of Emulsions Based on dispersed phase Oil in Water (O/W): Oil droplets dispersed in water

Water in Oil (W/O): Water droplets dispersed in oil

OilH2O

H2OOil

More complex types consist of three or more phases, which can be achieved by e.g. dispersing a w/o-emulsion into a second watery phase, leading to a water-in-oil-in-water-(w/o/w-)emulsion.

Types Based on Size of Liquid Droplets 0.2-50 µm Macroemulsion 0.01-0.2 µm Microemulsion

1. oil-in-water (o/w)2. water-in-oil (w/o)3. water-in-oil-in-water (w/o/w)4. oil-in-water-in-oil (o/w/o)

In summary

Sodas: Oil in Water emulsion

Milk: Oil in Water emulsion

Water in oil emulsion

Mayonnaise: Oil in Water emulsion

Emulsion is Thermodynamically UnstableW/O & O/W ONLY

Most oils are less dense in water, and if oil and water are mixed then the oil will simply float to the surface. In emulsions, the oil is dispersed as liquid droplets through the continuous phase, usually but not necessarily water. Those droplets want to combine together again to form a single drop of oil this means that an emulsion is thermodynamically unstable.

A difference in the densities of the two liquids may cause undesired creaming of the dispersed droplets !!!

What we can do to prevent this unwanted physical process?

Stability of emulsions may be engineered to vary from seconds to years depending on application.

How we can do that?

Compositions of Emulsion

Internal/Discontinuous/Dispersed phase (liquid)

External/Continuous phase (Liquid)

Emulsifying agent

Stabilizers

Emulsifying Agents

An emulsifier is a molecule with one oil-friendly and one water-friendly end i.e. hydrophilic tail and hydrophobic head. (Do you remember what is that?)These agents surrounds the oil droplets in water and reduces the tension between the two liquids thus impart stability. Emulsifying agents are often used to help the formation of emulsions

Classification of Emulsifiers

Chemical Structure Mechanism of Action

Synthetic Natural Auxiliary agents dispersed solids

Monomolecular

Multimolecular

Solid Particle Films

Anionics: alkali soaps (sodium or potassium oleate)detergents (sodium lauryl sulfate, sodium dioctyl sulfosuccinate, sodium docusate).

Non-ionics: Sorbitan esters (Spans®), polyoxyethylene derivatives of sorbitan esters (Tweens®), or glyceryl esters

Cationics: benzalkonium chloride, benzethonium chloride Amphoterics

Synthetic

Natural emulsifying agents are derived from plant and animal tissues and mostly in the form of hydrated lypophilic colloids. These emulsifiers make the protective sheath around the droplets, give droplets a charge so that they repel each other and swell to step-up the viscosity of the liquid.

Although natural agents are inexpensive, safe and non toxic but these are slow in action. So large quantity of emulsifier is required for proper action. Also the natural emulsifiers need preservatives as these are subjected to microbial growth.

Natural Emulsifying Agents

1) Carbohydrate Materials:

Acacia, Tragacanth, Agar, Pectin. o/w emulsion.

2) Protein Substances:

-Gelatin, Egg yolk, Caesin o/w emulsion.

3) High Molecular Weight Alcohols:

- Stearyl Alcohol, Cetyl Alcohol, Glyceryl Mono stearate o/w emulsion, cholesterol w/o emulsion.

Finely divided solids::These agents form a particulate layer around dispersed particles. Most will swell in the dispersion medium to increase viscosity and reduce the interaction between

dispersed droplets .Most commonly they support the formation of o/w

emulsions, but some may support w/o emulsions .

ExamplesMagnesium Hydroxide, Aluminum Hydroxide o/w emulsion

magnesium trisilicate .

Mechanism of action of emulsifying agents:

Monomolecular

- Coherent monomolecular film- flexible film formed by SAA,- depend on lower the d o/w ,- can prepare o/w and w/o emulsion

Examples: Synthetic SAA, K laurate

Multimolecular

Strong rigid film formed, mostly by the hydrocolloid,- which produce o/w emulsion,- d is not reduced to any extent ,- the stability due to strength of the formed interfacial film

Examples: Hydrophilic colloid ( acacia, gelatin)

Mechanism of action of emulsifying agents:

Mechanism of action of emulsifying agents:

Solid particles

Film formed by solid particles that are small in size compared to the droplet of the dispersed phase.- Particles must be wetted by both phases in order to remain at the interface and form stable film,- can form o/w and w/o

Examples: Colloid clays(bentonite, Mg(oH)2)

Tests Used To Identify Emulsion Type:

Dilution test: based on the solubility of external phase of emulsion.- o/w emulsion can be diluted with water.- w/o emulsion can be diluted with oil.

Conductivity Test:

water is good conductor of electricity whereas oil is non-conductor. Therefore, continuous phase of water runs electricity more than continuous phase of oil.

Dye-Solubility Test:

when an emulsion is mixed with a water solubledye such as amaranth and observed under themicroscope.¡ if the continuous phase appears red, then itmeans that the emulsion is o/w type as water isthe external phase¡ if the scattered globules appear red andcontinuous phase colorless, then it is w/o type.

Fluorescence test: oils give fluorescenceunder UV light, while water doesn’t. Therefore,O/W emulsion shows spotty pattern while W/Oemulsion fluoresces.

Fluorescence test:

Theory of emulsification

Droplets can be stabilized by three methods

i. By reducing interfacial tension

ii. By preventing the coalescence of droplets.

a. By formation of rigid interfacial film

b. By forming electrical double layer.

24

Theory of emulsification –reduction of interfacial tension

Phase A

Phase B

Change from A to B increases surface area of phase A, hence the Due

to increased surface energy, the system is thermodynamically

unstable.

AB

Emulsifying agents are needed to decrease the interfacial tension and to stabilize the emulsion.

Oriented-Wedge Theory:Oriented-Wedge Theory:

- mono molecular layers of emulsifying - mono molecular layers of emulsifying agents are curved around a droplet of agents are curved around a droplet of the internal phase of the emulsion.the internal phase of the emulsion.

Multimolecular films

Theory of emulsification -Formation of electrical double layer

- -

--

-

-

++

+

+

+

-

-

-

--

-

-

+

+ +

+

Electrical double layer at oil-water interface

Emulsion made with

sodium soap.

Oil Water

Interfacial films

Solid particle film: These agents form a particulate layer around dispersed particles. Most will swell in the dispersion medium to increase viscosity and reduce the interaction

between dispersed droplets .Most commonly they support the formation of

o/w emulsions, but some may support w/o emulsions .

Methods of emulsion preparation:

On small scale:¡ Porcelain mortar and pestle¡ On large scale:Mechanical stirrer

Methods of emulsion preparation:

Emulsifier is triturated with the oil in perfectlydry porcelain mortar

water is added at once

triturate immediately, rapidly and continuously(until get a clicking sound and thick white cream is formed, this is primary emulsion)

the remaining quantity of water is slowly added to form the final emulsion

English or Wet Gum Method

triturate gum with water in a mortar to form a mucilage

oil is added slowly in portions the mixture is triturated

after adding all of the oil, thoroughly mixed for several minute to form the primary emulsion

Once the primary emulsion has been formed remaining quantity of water is added to make the final emulsion.

Bottle or Forbes Bottle Method

It is extemporaneous preparation for volatile oils or oil with low viscosity.gum + oil (dry bottle)

Shake

water (volume equal to oil) is added in portions with vigorous shaking to form primary emulsion

remaining quantity of water is added to make the final emulsion

Auxiliary Emulsifying Agents

A variety of fatty acids (e.g., stearic acid), fatty alcohols (e.g., stearyl or cetyl alcohol), and fatty esters

(e.g., glyceryl monostearate )serve to stabilize emulsions through their ability to thicken the emulsion. Because these agents have only weak emulsifying properties, they are always use in combination with other emulsifiers

stearic acid

O

OHcetyl alcohol

HO

stearyl alcohol

HO

glyceryl monostearate

O

O

OH

OH

Be stable . Be compatible with other ingredients . Be non – toxic . Bossess little odor , taste , or color . Not interfere with the stability of efficacy of the active agent .

Emulsion Stability:

The instability of pharmaceutical emulsions may be classified as the following:

a)Flocculation and creaming

b) coalescence and breaking

c) Phase inversion

d) Miscellaneous physical and chemical change

Emulsion Stability

Flocculation and creaming:

¡ Flocculation - The small spheres of oil jointogether to form clumps or flocs which rise orsettle in the emulsion more rapidly thanindividual particles.¡ Creaming - it is a concentration of the flocculesof the internal phase formed upward ordownward layer according to the density ofinternal phase.