Ion-Specific Effects on Vesicle-to-Micelle Transitions of an Amino Acid Surfactant Probed by Chemical Trapping

Abstract

Ion-specific effects widely exist in biological and chemical systems and cannot be explained by classical theories. The complexity of ion-specific effects in protein systems at the molecular level necessitates the use of mimetic models involving smaller molecules, such as amino acids, oligopeptides, and other organic molecules bearing amide bonds. Therefore, it is of theoretical value to determine the effect of additional salts on the aggregation transitions of acyl amino acid surfactants. Herein, the effects of specific tetraalkylammonium ions (TAA+) on sodium lauroyl glycinate (SLG) aggregation were studied by dynamic light scattering (DLS) and transmission electron microscopy. Although previous studies have shown that the kosmotropic TAA+ ions tend to induce micellar growth or micelle-to-vesicle transitions of some anionic surfactants, TAA+ addition in the present study induced partial vesicle-to-micelle transitions in SLG solutions. The chemical trapping (CT) method was employed to estimate changes in the interfacial molarities of water, amide bonds, and carboxylate groups during such transitions. The vesicle-to-micelle transitions were accompanied by a marked rise in interfacial water molarity and a decline in interfacial amide bonds molarity, suggesting that the hydrated TAA+ entered the interfacial region and disrupted hydrogen bonding, thus preventing the SLG monomers from packing tightly. Molecular dynamic simulation was also performed to demonstrate the salt-induced cleavage of amide–amide bonds between SLG headgroups. Furthermore, both CT and DLS results show that the ability of tetraalkylammonium cations to induce such transitions increased with increasing size and hydrophobicity of the cation, which follows the Hofmeister series. The current study offers critical molecular-level evidence for understanding the specific effects of tetraalkylammonium ions on the aggregation transitions of an acyl amino acid surfactant.

Interfacial Composition of Surfactant Aggregates in the Presence of Fragrance: A Chemical Trapping Study

“…The dediazoniation products corresponded to cinnamyl alcohol (CinOH), phenyl ethanol (PEOH), phenyl methanol (PMOH), and anisyl alcohol (AniOH), which are 16-ArOCin, 16-ArOPE, 16-ArOME, and 16-ArOAni, respectively. method, established by the Romsted group, has been employed to determine the interfacial molarities of weakly basic nucleophiles, such as halide ion, water, carboxylate groups, amide bonds, and alcohols, in surfactant aggregate solutions, and the details of the method can be found in previous references [10][11][12]36,37].…”

“…The interfacial region between aggregates and bulk water is like a “black box” as it is highly difficult to focus on this specific area using general experimental methods. The CT method, established by the Romsted group, has been employed to determine the interfacial molarities of weakly basic nucleophiles, such as halide ion, water, carboxylate groups, amide bonds, and alcohols, in surfactant aggregate solutions, and the details of the method can be found in previous references [ 10 , 11 , 12 , 36 , 37 ].…”

The Spontaneous Vesicle–Micelle Transition in a Catanionic Surfactant System: A Chemical Trapping Study

“…The CT method [ 25 , 26 , 33 , 34 , 35 , 36 , 37 , 38 ] is a distinctive approach that has been utilized to determine the interfacial compositions of surfactant aggregates. Here, we provide a concise rationale behind the CT method, while additional information can be found in other published works.…”

Spherical-micelle-driven deposition of high-speed impacting water droplets on superhydrophobic surfaces

“…Introducing small organic salts into surfactant solutions is a common and simple practice to modulate the self-assembly behavior of surfactants both at the interface and in the bulk. [40][41][42][43][44][45][46] Quaternary ammonium compounds with short alkyl chains can be used as branched organic salt additives to adjust the molecular packing of surfactants in aggregates and at the interface and to induce aggregate transitions. By mixing asymmetric quaternary ammonium salts (C 5 R 3 N + Br, R = H, Me, Et) with sodium alkyl sulfates (NaC n SO 4 , n = 12, 14, 16), viscoelastic wormlike micelles are formed, and surfactants with longer alkyl chains require larger quaternary ammonium ions to match.…”