Author's Note: I became involved in emulsion science by complete accident. My main expertise is in the coating/printing industry, as well as in web-handling, formulation chemistry, nanostructures, bio-mimetics and technical software applications. Two years ago, however, I had to re-formulate an emulsion by changing the oil in it, which required additional guidance as to which surfactant to then use; here, rather than the term emulsifier, surfactant is used, in its neutral scientific sense. Since this substitution process seemed fairly routine, one might assume it would be simple to find a scientific method to carry it out.
However, in reviewing the literature, I found it surprisingly challenging. Critical micelle concentration (CMC) and hydrophile-lipophile balance (HLB) were irrelevant and useless for my needs: CMC because like most cosmetic formulations, relatively high concentrations were being used; and HLB because it is largely discredited for anything other than ethoxylate surfactants at room temperature. Also, a new (to me) idea seemed promising: critical packing parameter (CPP), but also proved impractical for a real-world formulator, although it does provide insights into mesophases. Finally, my search led to the concept of hydrophilic lipophilic difference (HLD), and its recent extension, HLD-NAC, where NAC refers to the net average curvature. This concept will be described in greater detail in the present article.
-Steven Abbott, PhD