Synthesis and adsorption at the air/solution interface of new pH-responsive dicephalic and quadruple-head cationic surfactants

Two groups of novel multifunctional surfactants branched on a nitrogen atom, i.e., dicephalic 2,2’-(alkylimino)bis(N,N,N-trimethylethanammonium) dibromides (Cn-DNNMe3Br, n = 12, 14, and 16) and quadruple N,N,N-bis(2-(N,N,N-bis(2-ammoniumethyl)amino)ethyl)dodecylamine tetrabromide (C12-DNQNNMe3Br) have been synthesized in divergent synthetic routes and characterized by 1H NMR and 13C NMR, ESI-MS as well as by elemental analysis. Surface tension isotherms of their aqueous solutions in the absence and with the addition of NaBr, HBr and NaOH (at pH = 3 and pH = 11, with ionic strength fixed by addition of NaBr), reflecting the protonation/deprotonation of the tertiary amine present in the Cn-DNNMe3Br and C12-DNQNNMe3Br structures, were evaluated using the modified surface quasi-two-dimensional electrolyte (mSTDE) model of ionic surfactant adsorption. To account for protonation/deprotonation equilibria and the formation of transient multicharged surfactant–counterion complexes, we proposed the concept of effective charge, which we substantiated with data from molecular dynamics (MD) simulations. The results indicate that the difference between surfactant nominal charge and effective charge increases with the number of charges at the hydrophilic headgroup, a combined effect of counterion complexation and enhanced screening of electrostatic interactions. The use of MD tools, combined with a thermodynamic model approach to multicharged surfactant adsorption, can explain some experimentally observed phenomena and provide a basis for the development of novel materials for various applications. This type of complementary approach, integrating experimental data, molecular dynamics simulations, and a thermodynamics-based model, is relatively scarce in the literature.