Phase behaviors and self-assembled properties of ion-pairing amphiphile molecules formed by medium-chain fatty acids and l-arginine triggered by external conditions

Fatty acids, composed of an amphiphilic carbon chain and a carboxyl group, are a simple class of anion surfactants when they are protonated in the presence of an alkali. They can form ion-pairing amphiphile molecules and self-assemble into different nanostructures, which can be widely applied. However, the phase transition of these nanostructures can be easily triggered by external conditions, such as pH, temperature, the addition of salts and so on. In this work, medium-chain fatty acids (MCFAs) containing 8-12 carbons were chosen as the objects for self-assembly with organic amino counter-ions, l-arginine. We investigated the effects of chain length, pH, temperature and solvent on the phase transition based on diagrams, which could be determined by freeze fracture transmission electron microscopy (FF-TEM) and polarizing microscopy observations (PMO) combined with rheological properties. The results showed that rich phase behavior was observed in the octanoic acid (C8:0) system. However, it was insensitive to changes in pH because of the lower hydrophobicity. In contrast, for decanoic acid (C10:0) and lauric acid (C12:0) systems, phase transition from the L₁ phase (micelles) to the L_α phase (vesicles) was observed and they showed stimulus-response to the change of pH. At the same time, lauric acid was sensitive to temperature, which was ascribed to the higher Krafft point. Besides, when the solvent was replaced by glycerin, phase transitions occurred from vesicles to micelles in these three systems. We hope that this phase transition observed in the MCFA system will provide a good insight into drug delivery, material science, and other related areas.