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Theories of Microemulsion Formation

By: Pharma Tips | Views: 4622 | Date: 02-Jun-2011

Microemulsions form simultaneously when the interfacial tension between oil andwater is reduced to close to zero. The formation and stability of a microemulsioncan be affected by various factors, including the nature and molecular weight ofsurfactant, alcohol chain length and concentration, salinity, and temperature.


Microemulsions form simultaneously when the interfacial tension between oil and
water is reduced to close to zero. The formation and stability of a microemulsion
can be affected by various factors, including the nature and molecular weight of
surfactant, alcohol chain length and concentration, salinity, and temperature.
Three different theories have been proposed to explain the microemulsion formation and
stability (Paul and Moulik 1997).

They are the interfacial mixed film theory, solubilization
theory, and thermodynamic theory. According to the interfacial mixed
film theory, the film at the interface is assumed to be a dual film and the type (W/O
or O/W) of microemulsion formed depends upon the bending or curvature of the
interface (Schulman et al. 1959).

Solubilization theory states that oil is solubilized by normal micelles and water is
solubilized by reverse micelles (Gillberg et al.1970).

Finally, according to the thermodynamic theory, the free energy of formation
must be negative to form a thermodynamically stable microemulsion (Paul and
Moulik 1997; Attwood 1994).

Surfactants play an important role in reducing the interfacial tension in microemulsions.
They can be selected based on the HLB concept
or the critical packing parameter (CPP) concept. Surfactants with a low HLB
value (3–6) are preferred for the formation of W/O microemulsion, whereas surfactants
with high HLB value (8–18) are preferred for O/W microemulsions.

The CPP describes a ratio between the hydrophobic and hydrophilic parts of a surfactant
molecule and is useful in estimating the nature of formed aggregates. The CPP can
be calculated using CPP = n/a·l,

where n is the partial molecular volume of the surfactant, а is the optimal head group area,
l is the length of the surfactant tail.

When the CPP is between 0 and 1, O/W microemulsions are formed, and when the
CPP > 1, W/O microemulsions are formed. Bicontinous microemulsions are formed
when the CPP = 1. Brij, dioctyl sodium sulphosuccinate, and lecithin are some of
the widely used surfactants to stabilize microemulsion formulations.
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