Pre-micellar self-assembly of amino acid surfactants detected by relaxation-NMR rigidity mapping

Relaxation NMR is established here as a quantitative probe to determine self-assembly, map segmental rigidity and study conformational locking, while revealing premicellar association that is undetected by interfacial methods. Here, we investigate the longitudinal ( T 1 ) and transversal ( T 2 ) relaxation rates over a wide concentration range for eight unique amino acid surfactants: linear iminodiacetates (hydrophobic chain lengths of C 8 , C 11 and C 13 ), branched or Guerbet-type iminodiacetates (hydrophobic chain lengths of bC 12 , bC 18 and bC 22 ), an aspartate isomer (bC 12 ASP) and a zwitterionic analogue (bC 12 IDA Na ). Five proton spin populations were analysed per surfactant, and T r values ( T 2 / T 1 relaxation time ratio) were converted into rigidity maps to compare head vs tail mobility. Relaxometry-derived CMCs were successfully determined and validated against tensiometry (e.g. C 11 IDA Na : 40.2 vs 40.3 mmol.L −1 ). Rigidity maps indicated a lock on the methylene between the amine and the hydrophobic tail group of the linear surfactants, thereby identifying it as the palisade layer. However, the branched bC 12 IDA Na displayed a premicellar increase in the T 1 relaxation rate at ∼14.5 mmol.L −1 which is ∼12 % of its CMC (123 mmol.L −1 via tensiometer), consistent with dimers or trimers that surface tension measurements cannot detect. Relaxation-based rigidity mapping provides a sensitive framework to detect pre-micellar organisation and locate conformational locking. It can also be used to relate structure dynamics to interfacial packing, emulsion stability and drug delivery efficacy. • Relaxation NMR can be used to accurately measure critical micelle concentrations (CMCs), validated against tensiometry. • Branched amino acid surfactants form pre-micellar aggregates well below their CMC. • Rigidity maps of T r values ( T ₂/ T ₁ relaxation time ratio) reveal conformational locking via hydrogen bonding and aggregation. • Relaxation NMR reveals pre-micellar aggregation undetectable by surface tension measurements. • Headgroup protons become rigid post-CMC; tail protons remain more mobile.