Effects of interfacial molarity on salts inducing vesicle-to-micelle transitions in acyl taurines

Abstract

Specific ion effects (SIEs) are crucial in engineering, chemistry and biology. N-acyl taurine (NAT) exists naturally in organisms and may have various physiological functions. However, compared with other acyl amino acid-based surfactants bearing carboxylate headgroups, the SIEs on NATs have received little research. Here, sodium lauroyl taurate (SLT) and sodium methyl lauroyl taurate (SMLT) bearing secondary and tertiary amides, respectively, were synthesized by a green method. The interfacial molarities of water, amides and sulfonate groups in SLT and SMLT aggregates were estimated simultaneously in the presence of tetraalkylammonium cations (TAA+) of different chain lengths. With increasing salt concentration, the interfacial water molarity increased, whereas the interfacial amide molarity decreased in all cases. However, at a fixed salt concentration, the interfacial water molarity increased with the hydrophobicity of cations, while the interfacial amide bond molarity followed the reverse order. The combined TEM and DLS results show that the majority of SLT and SMLT molecules formed vesicles spontaneously in the absence of salts, and some of these vesicles turned into micelles with TAA+ addition. Tertiary amides of SMLT may also absorb protons at the interface and interact with each other by hydrogen bonding. Such hydrogen bonds facilitate the tight packing of surfactant molecules and induce vesicle formation, just like the intermolecular amide-amide hydrogen bonds between SLT. Added TAA+ cleaves the hydrogen bonds between NAT and induces vesicle-to-micelle transitions. This hypothesis is supported by CT results showing that vesicle-to-micelle transitions were accompanied by decreases in interfacial amide molarity.