Principles of Emulsification

Principles of Emulsification


Introduction to emulsions


An emulsion is most accurately defined as a dispersion of liquid droplets in a second immiscible liquid. Temporary emulsions may be formed by mixing/agitating the two normally immiscible liquids, however, the stability of temporary emulsions produced in this way is poor. Emulsifiers are surface active materials (surfactants) that are used to assist in the formation of an emulsion and to stabilise the emulsion.


There are several different types of emulsions. They are loosely described by their phase relationship and/or by their appearance:


Oil-in-water


Water-in-oil


Multiple emulsions


Macro-emulsions


Micro-emulsions

Phase location is the first classification parameter:


The basic oil-in-water and water-in-oil emulsions have different physical characteristics, and are easily distinguished from one another:


Property O/W W/O

Multiple Emulsions are complex polydispersed systems where both oil in water and water in oil emulsion exists simultaneously. They find application in areas such as foods, personal care products and agrochemical formulations.


Macroemulsions are emulsions where the particle size of the disperse phase is > 0.1 mm with the particle size typically being in the range 1 - 100 mm, their appearance is typically an opaque milky white or blue white emulsion. They can form spontaneously with limited energy input but may also require significant energy/shear input depending on the surfactant system/concentration, they are generally considered to be thermodynamically unstable but can be stabilised by the correct surfactant/emulsifier selection.


Microemulsions are emulsions where the particle size of the disperse phase is typically 0.01 - 0.1 mm, the emulsion is totally transparent, they generally form spontaneously with no need for energy input and are generally considered to be thermodynamically stable. They require significantly higher concentrations of emulsifier than a typical macroemulsion.


The basic characteristics of an emulsion system:


· Emulsions are inherently unstable

· To form emulsions, a large energy input is required

· This can be combination of mechanical and chemical energy

· One key role of the surfactant/emulsifier is to lower the energy formation

· The other key role of the surfactant/emulsifier is to stabilise the emulsion


Below is a simplistic view of an emulsion particle, protected by surfactant molecules partitioned at the interface of the internal and external phase.

Emulsifiers lower the interfacial energy between the two immiscible liquids, thereby helping the formation of an emulsion, for an emulsifier to be effective at reducing droplet size and stabilising an emulsion, it needs to be located at the interface, the emulsifier must not be too soluble in either phase, otherwise it will migrate to that phase. If the emulsifier migrates away from the interface, the emulsion is destabilised.


Emulsifier Selection


· Required emulsion type, ie O/W, W/O, W/O/W

· Nature of the oil phase, ie. polar, non-polar

· Regulatory restrictions, ie. Food approved, biodegradability


Other factors to consider


· The chemical “affinity” between surfactant hydrophobe and oil phase

· Blends of surfactants are more effective than single emulsifier of correct HLB

· Blends of anionics and nonionics can be more effective than either type alone

· Any possibility of chemical reaction between the emulsifiers and the oil phase, solutes in the oil phase, or solutes in the aqueous phase


Several processes, internal to the emulsion, indicate instability in the emulsion and ultimately lead to its breakdown


· Creaming

· Flocculation or aggregation

· Coalescence

· Phase inversion

· Ostwald ripening


Emulsion Breakdown Processes

External factors can also lead to breakdown of the emulsion


Temperature


· Alters emulsifier solubility

· Nonionics become less water-soluble with increasing temperature

· Anionics become more water soluble with increasing temperature

· HLB effectively decreases with increasing temperature, when the emulsifier system is solely nonionic

· Surfactant moves away from oil/water interface thus destabilising emulsion


Water quality


· Presence of electrolytes impact on surfactant solubility

· Salting out solutes include NaOH, CaCO3, MgSO4

· Certain anionics interact with hard water

· Solubility changes cause interface partitioning changes

· Anionic/nonionic systems resistant to hard water


Increasing Stability of Emulsions


· Each of the individual causes of emulsion breakdown can be controlled by formulation technique and surfactant selection

· Some corrections are easy to implement, some require a radical reformulation of the emulsion system


Reduce Creaming and Sedimentation by:


· Density adjustment - type of solvent or oil in disperse phase

· Lower mean particle size of the disperse phase - mode of formation of the emulsion

· Increase viscosity of the continuous phase - addition of protective colloids, attention to structure of surfactant hydrophobe


Reduce Flocculation by:


· Increasing surfactant concentration

· Reinforce the entropic barrier - nonionic/polymeric surfactant species

· Reinforce the electrostatic barrier - ionic surfactant species


Reduce Coalescence by:


· Prevention of creaming

· Increase the interfacial film thickness and elasticity

· Use of polymeric stabilisers


Reduce Phase Inversion by:


· Review and revise phase ratio

· Increase phase inversion temperature

· Review ionic/nonionic ratio

· Increase EO number of nonionic component

· Decrease carbon number of hydrophobe

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