Recent progress in the hydrophilic-lipophilic difference (HLD) (G. Palazzo)

The central question in the formulation of oil-water surfactant systems is what will be the state of the interfacial film. Will it be lipophilic, i.e. curved towards the water, or hydrophilic (curved towards the oil)?

The answer is not easy and depends on the nature and concentration of the surfactants and oils used and other intensive variables such as temperature and salt concentrations. The semi-empirical approach called hydrophilic-lipophilic difference (HLD) can provide a useful guideline to the formulator.

In this lecture, the classical HLD theory will first be introduced.  The different experimental approaches proposed to determine the parameters that enter the model will then be examined.

Problems related to the use of surfactant mixtures of different natures (non-ionic or ionic) will be illustrated and recently proposed improvements to the model (such as the normalised HLD) will be discussed.

The case of commercial non-ionic surfactants will be illustrated. These are mixtures of species that differ in their degree of ethoxylation and are characterised by a surfactant distribution with different affinity for the oil phase and the aqueous phase.

This heterogeneity leads to an unexpected concentration dependency of the phase behaviour and the approaches used to handle such deviations will be explained.