The impact of headgroup interactions in binary mixed monolayers comprised of fatty acids and alkyl amine-based surfactants has been investigated with a view towards understanding molecular pair formation at the air-water interface. Monolayers consisting of equimolar mixtures of cationic Stearyltrimethylammonium bro-mide with myristic acid (STAB/MA), and nominally neutral octadecyl amine with myristic acid (ODA/MA) were studied using a combination of Langmuir monolayer methods. Measurements support the formation of molecular pairs between STAB and MA in monolayers, which was not observed for the ODA/MA mixture. By studying the effect of subphase pH, measurements suggest hydrogen bonding between the headgroups and surrounding water significantly impacts film structure for the mixed systems. Notably, at pH 3.5, similarities in film morphology for the STAB/MA and the ODA/MA were observed, indicating that subphase pH can be used to enhance pair for-mation. This lower pH promoted the formation of phase-separated domains with LE and LC features and changed the kinetic stability of the monolayers. These works clarify the fundamental importance of headgroup in-teractions, hydrogen-bond formation and chemical structures in regulating intermolecular pair formation, an issue that has been largely unreported and underappreciated in the literature to date. This information can provide guidance on important intermolecular interaction considerations that must be taken into account for applied systems.
