Tetraalkylammonium counterion effects on lauroyl β-alanine: A chemical trapping study

Abstract

The use of acyl amino acid surfactants has gained significant interest due to increasing environmental pressure from both the government and customers. Understanding the ion-specific effects (SIE) in these surfactant systems not only contributes to their practical applications in complex systems but also provides a theoretical basis for investigating SIE in protein systems. In this study, we synthesized four types of lauroyl β-alanine tetraalkylammonium salts (TAA-LA) with tetramethylammonium (TMA-LA), tetraethylammonium (TEA-LA), tetrapropylammonium (TPA-LA), and tetrabutylammonium (TBA-LA) as counterions. The SIE on their surface activities, aggregation behaviors, and interfacial molarities were examined. The critical micelle concentration of TAA-LA decrease with an increase in the size of the counterions. Chemical trapping revealed that the interfacial molarity of water and carboxylate groups followed the order TBA-LA > TPA-LA > TEA-LA > TMA-LA, whereas the interfacial amide bond molarity followed the reversed order. These results were consistent with their differences in aggregation behaviors. Specifically, LA bearing smaller TAA+ counterions formed uni-lamellar vesicles (ULVs) spontaneously, while LA with TBA+ self-assembled into double-lamellar vesicles (DLVs) of comparable sizes. Both CT and MD confirmed that TBA-LA DLVs with relatively smaller “average curvature” were more hydrated than other TAA-LA ULVs. Our results suggested that TBA+ tends to break the intermolecular hydrogen bond between amide bonds, which results in looser interfacial packing and larger “average curvature”. Our findings provide new insights into the tetraalkylammonium counterion effects on acyl amino acid surfactants and elucidate the SIE on protein systems at the molecular level.