Application of HLD Framework to the pH-Dependent Behavior of Amino Acid Surfactants

With consumer demands for cleansing products that are safe and effective, together with preferences for products free of label ingredients such as sulfates, amino acid-derived surfactants play an important role in the personal care product formulator’s toolbox. One of the challenges of effectively formulating with these surfactants is their strong pH-dependent behavior, a consequence of surfactant head group equilibration between ionized (anionic) and protonated (nonionic) forms. In this presentation, we report on our efforts to apply the HLD framework to the description and characterization of pH-dependent viscosity in aqueous amino acid surfactant formulations. Using n-alkane as a proxy for the hydrophobic interior of low-curvature aggregated states, acid-base titration of surfactant-oil-water (SOW) systems has been conducted to identify system HLD = 0 conditions via phase inversion (O/W → W/O emulsions) and microemulsion formation. In the case of sodium lauroyl sarcosinate in 1% aqueous NaCl and n-octane, phase inversion has been found to occur at about 67 mol% acidulation of COO^-Na⁺, corresponding to an approximately 2:1 molar ratio of nonionic to anionic form of the sarcosinate surfactant.  SOW phase inversion endpoints have been observed to shift toward increasing mole fraction of acidified form (COOH) with increasing oil EACN. The addition of cetyl betaine or cocamidopropyl hydroxysultaine co-surfactant has been found to have no significant effect on phase inversion endpoint, indicating that these amphoteric surfactants have comparable hydrophobicity to each other and to the state of sarcosinate surfactant at HLD = 0. This conclusion is further supported by observations for surfactant-water (SW) systems without added oil, where lauroyl sarcosinate formulations with and without added amphoteric surfactant display comparable evolution of phase behavior and rheological properties over the course of titration. This methodology has been extended to additional co-surfactants to map the effects of surfactant hydrophobicity on pH-dependent formulation viscosity.