Thermodynamic insights on macroemulsion stability

Experiments reported in the literature for nonionic surfactants have revealed that: (1) the interfacial tension at the planar surface between oil and water, at a constant amount of surfactant and equal volumes of water and oil, passes through a minimum when plotted against the HLB (the hydrophilic–lipophilic balance); (2) under the same conditions, the emulsion stability against HLB passes through two maxima, one in the W/O range and the other in the O/W range, and a minimum between the two at the phase inversion point; (3) the minima under (1) and (2) coincide; (4) the phase inversion temperatures of emulsions, formed of oil and water as distinct phases, or microemulsions and their excess phase, and the corresponding microemulsions are the same. These observations are explained on the basis of thermodynamics. It is also shown that the stability of emulsions can be correlated with the surface excess of surfactant, which also passes through two maxima and a minimum between the two when plotted against HLB or temperature. In addition, a two ratio approach is used to determine when the Bancroft rule is obeyed or violated in microemulsions.