Zwitterionic surfactants are key components in fluorine-free firefighting foams, personal care formulations, and enhanced oil recovery, where their mildness and synergistic interactions with co-surfactants offer distinct advantages over conventional ionic surfactants. However, the relationship between headgroup chemistry and micellar architecture in these systems remains poorly characterised at the molecular level, limiting rational formulation design.
We present a systematic multi-technique investigation of three coco- and lauramidopropyl zwitterionic surfactants — hydroxysultaine (CAPHS), amine oxide (LAPAO), and betaine (LAPB) — that share a common C₁₂-amidopropyl hydrophobic tail but differ in headgroup structure. Critical micelle concentrations were determined by Wilhelmy plate tensiometry, and micellar structure was characterised by small-angle neutron scattering (SANS) and dynamic light scattering (DLS), with sodium dodecyl sulphate (SDS) as a reference standard.
SANS analysis, validated by Guinier fitting (R² > 0.95, shape factors ≈ 0.775), reveals that all surfactants form spherical micelles with packing parameters between 0.30 and 0.34. Importantly, LAPHS adopts a core-shell morphology with the highest eccentricity (e = 0.26), while LAPAO (e = 0.18) and LAPB (e = 0.17) form simple spherical micelles. All-atom molecular dynamics simulations of these three micelles corroborate the experimentally determined radii of gyration and headgroup areas, while providing atomic-scale insight into hydration shell dynamics and solvent-accessible surface areas that rationalise the observed structural differences.
This integrated experimental–computational approach establishes a direct structure–property relationship between headgroup chemistry and micellar morphology, contributing to the rational design of zwitterionic surfactant formulations for applications including next-generation fluorine-free firefighting foams.