ABSTRACT
Microemulsions are mixtures of two mutually insoluble liquids whose stabil-
ity is essentially due to the presence of the surfactant, a substance character-
ized by both a hydrophilic part and a lipophilic part whose task is to dispose at
the hydrophilic liquid=lipophilic liquid interface to reduce the interfacial tension. Thus, microemulsion becomes a thermodynamically stable and
homogeneous system able to form spontaneously without requesting any
additional external energy. Accordingly, microemulsion can be defined as a system of water, oil, and amphiphile, which is a single optically isotropic and thermodynamically stable liquid solution [1]. Three different approaches have been developed to explain microemulsions’ formation mechanism and stabil-
ity [2]: (1) interfacial or mixed film theories [3,4]; (2) solubilization theories
[5]; (3) thermodynamic approaches [6]. In particular, according to a simpli-
fied thermodynamic view, the Gibbs free energy DGF associated with microemulsion formation would result from the balance between the energy
increase, due to surface area increase (DA) and oil-water surface tension (g) decrease induced by the presence of surfactant, and the energy decrease
imputable to system entropy (DS) increase:
DGF ¼ gDA TDS, (8:0)
where T is the temperature. Equation 8.0 clearly states that, for a spontaneous microemulsion formation, DGF must be negative, so that TDS must exceed gDA. As microemulsion formation implies a huge increase of the oil-water interface area (DA), g must be considerably lowered by surfactant to allow the prevalence of the entropic contribute. For this reason, very
often, also for the purpose of reducing the amount of surfactant needed, the
presence of a cosurfactant, usually an alcohol, is required. In this manner,
the interfacial tension g is generally lowered to less than 1 mN=m and the formation of lipophilic or hydrophilic domains ranging from a few to a
hundred nanometers is allowed [7]. The entropic contribution DS arises from the mixing of one phase in the other in the form of a large number of small
domains. Nevertheless, DS is also improved by other favorable entropic
contributions arising from other dynamic processes such as the surfactant
diffusion in the interfacial layer separating oil from water. In addition,
surfactant exchange between solubilized surfactant molecules (monomer
surfactant) and surfactant micelles (for sufficiently high concentration, sur-
factant molecules can organize as small micelles in equilibrium with mono-
mer surfactant) improves the entropic contribute. Obviously, this energy
balance, although strictly depending on the surfactant, cosurfactant, and oil
kind, can also be heavily affected by temperature and a difference of just
18C or less can considerably alter the equilibrium position in terms of component concentration.
