Engineering cosmetics using the Net-Average-Curvature (NAC) model

Abstract

The hydrophilic–lipophilic balance (HLB) and the packing parameter have been guiding formulators for the last 70 years. However, these indicators look at the surfactant only without considering formulation conditions such as temperature, salinity, and oil hydrophobicity. The hydrophilic–lipophilic difference (HLD) consists of two empirical correlations that consider the entire set of formulation conditions, serving as a unified theory of hydrophobicity that connects to HLB, packing parameter, and phase inversion temperature (PIT). Target HLD values for cosmetic formulations are discussed. The HLD alone, however, does not provide formulation properties such as solubilization capacity, phase diagrams, or interfacial tension. To this end, the net-average curvature (NAC) model interprets the HLD as a dimensionless interfacial curvature, using this approach to predict formulation properties and phase diagrams. The use of the HLD-NAC model in the design of a skin cleansing formula is illustrated via a predicted ‘fish’ phase diagram required to identify preferred compositions.