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.

由于政府和客户日益增加的环境压力，酰基氨基酸表面活性剂的使用引起了人们的极大兴趣。了解这些表面活性剂体系中的离子特异性效应 (SIE)不仅有助于其在复杂系统中的实际应用，而且还为研究蛋白质系统中的 SIE 提供理论基础。在这项研究中，我们合成了四种类型的月桂酰β-丙氨酸四烷基铵盐（TAA-LA）与四甲基铵（TMA-LA）、四乙基铵（TEA-LA）、四丙基铵（TPA-LA）和四丁基铵（TBA-LA）：抗衡离子。SIE 的表面活性、聚集行为和界面摩尔浓度进行了检查。TAA-LA 的临界胶束浓度随着抗衡离子尺寸的增加而降低。化学捕获表明，水和羧酸盐基团的界面摩尔浓度遵循TBA-LA>TPA-LA>TEA-LA>TMA-LA的顺序，而界面酰胺键摩尔浓度遵循相反的顺序。这些结果与它们在聚合行为上的差异是一致的。具体来说，带有较小 TAA⁺抗衡离子的 LA 自发形成单层囊泡 (ULV)，而带有 TBA^{+ 的}自组装成大小相当的双层囊泡 (DLV)。CT 和 MD 均证实，“平均曲率”相对较小的 TBA-LA DLV 比其他 TAA-LA ULV 水合程度更高。我们的结果表明 TBA⁺倾向于破坏酰胺键之间的分子间氢键，从而导致界面堆积更松散，“平均曲率”更大。我们的研究结果为四烷基铵抗衡离子对酰基氨基酸表面活性剂的影响提供了新的见解，并在分子水平上阐明了蛋白质系统的 SIE。