Alternative classification concepts of emulsifiers have to cope with HLB’s main flaw, which is assigning a single, “universal”, calculated number to an emulsifier, as discussed in our previous paper. For emulsions, the influence of the oil phase needs to be considered as well as temperature, pH or salinity. Both alternatives to HLB discussed here make use of a thermodynamic equilibrium state as a reference – in case of oil/water/surfactant this means a microemulsion. The Hydrophilic-Lipophilic Deviation (HLD) describes the deviation of an entire formulation of water, oil, and a surfactant from the optimum, balanced reference state. From this the characteristic curvature (Cc) of surfactants can be determined, which is a number that describes their hydrophilicity or hydrophobicity.
In contrast, the PIT-slope method characterizes the hydrophilicity/hydrophobicity of a surfactant in relation to C10E4 in an octane/water system; therefore, it is also able to capture all effects of molecular structure of the surfactant.