Surface and Interface Chemistry

Emulsions

Valentim M. B. Nunes

Engineering Unit of IPT

2014

An emulsion is a dispersed system in which the phases are partially or immiscible liquids. In all emulsions one phase is water and the other is an organic liquid known as ' oil '. There are two fundamental types of emulsions: O/W, oil-in-water and W/O, water-in-oil.

There are several technological applications: cosmetics, pharmaceuticals, agrochemicals, food industry, asphalt emulsions, etc..

W/O emulsion: butterSource: Adamson, Physical Chemistry of Surfaces, Wiley, New York, 1997

The most important property of an emulsion is its stability. To prepare a stable emulsion it is necessary a third component-emulsifying agent.

Emulsifiers are usually surfactants, finely divided solids, or certain natural products such as casein which stabilizes the milk, a natural emulsion.

The emulsifying agents form a film around the dispersed 'drops', preventing the flocculation or coalescence – breakage of emulsion. They act in two ways: lowering the surface tension (usually high in emulsions) and forming an elastic and mechanically strong film.

Emulsifying agents and emulsion type:The type of emulsion formed for a given pair of immiscible liquids depends on:•volume of each phases•emulsifying agent

The higher the volume of a given phase the more likely is to be the dispersing medium. The emulsifying agent also has great influence: for example alkali metal soaps favor emulsions of type O/W while heavy metal soaps favor W/O type emulsions.

The type of emulsion that tends to form it will depend on the balance between hydrophilic and hydrophobic part of emulsifier.

Bancroft,s Rule – The phase at which the agent emulsifier is more soluble tends to be dispersing medium.

The amphiphilic nature of most emulsifiers can be expressed in terms of an empirical scale, the HLB numbers (hidrophile-lipophile balance). To the less hydrophilic surfactants are signaled the lower values of HLB.

The optimal number to form an emulsion depends on the particular nature of the system. Assuming, for example, that 20% of sorbitan tristearate (HLB 2.1) and 80% of polyoxyethylene (HLB 14.9) is the optimal composition for a particular emulsion O/W, the HLB number of mixing is given by:

HLB(mixture) = 0.2*2.1 + 0.8*14.9 = 12.3

In this case an emulsifier with an HLB 12.3 will be great for the emulsion concerned.

There have been several attempts to rationalize the HLB scale. In the following table is the relationship between HLB numbers and characteristics of emulsifiers.

HLB numbers of a surfactant can be calculated by the Group contribution method proposed by Davies.

The HLB total number is given by:

i

ii

i LHHLB 7

Inversion TemperatureOther important parameter to characterize an emulsion is its inversion temperature, the temperature at which an emulsion W/O becomes O/W or vice versa.

Source: Shaw, D.J., Colloid and Surface Chemistry, 4th ed , Butterworth, Oxford, 1992

To determine the inversion temperature usually we prepare an emulsion with equal amount of water and oil and about 3 to 5% of emulsifier. The emulsion is heated and stirred until the inversion occurs. This parameter is very useful for estimating temperatures to which a given emulsion can be preserved.

Usually there is a reasonable correlation between the temperature inversion and HLB numbers, for a given emulsion, with different emulsifiers, as shown in the following figure for an emulsion of cyclohexane and water.

Source: Adamson, Physical Chemistry of Surfaces, Wiley, New York, 1997

Emulsion breakingSometimes it is necessary to break an emulsion (for example break and inversion of the milk to make butter). This can be done in three ways:

Mechanical methods: centrifugationThermal methodsChemical methods: addition of O/W emulsifiers tends to break W/O emulsions and vice-versa.

Micro emulsions: Involve drops of the dispersed phase from 0.01 to 0.1 µm

time

/mN.m-1

OW 0

Example: the SDS (sodium dodecyl sulfate) is an anionic surfactant that forms micro emulsions with critical or thermodynamic stability after the addition of a co-surfactant (n-pentanol)

The micro emulsions represents an intermediate state between micelles and ordinary emulsions.