Effect of Added Tetraalkylammonium Counterions on the Dilational Rheological Behaviors of N-Cocoyl Glycinate

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

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.

Thermodynamic Properties of Cetyltrimethylammonium Bromide in Ethanol-Water Media With/without the Presence of the Divalent Salt

The physicochemical properties of cetyltrimethylammonium bromide (CTAB) in pure water and ethanol-water mixtures in the presence and absence of MnSO₄.6H₂O were studied by measuring the conductivity at room temperature. The concentration range of CTAB was ~1.00 × 10^-5 M to ~1.00 × 10^-2M and the concentration of MnSO₄.6H₂O was 0.001 M, 0.005 M, 0.01 M. With increasing ethanol content in the solvent composition, the critical micelle concentration (CMC) and the degree of micellar dissociation (α) of CTAB increased. With the help of CMC and α, the standard free energy of micellization (ΔG _m ^ο ) was evaluated. With an increase in ethanol content, the negative values of ΔG _m ^ο decreased. CTAB micellization was tested in the context of specific solvent parameters. The solvent conductivity ratio at CMC to limiting conductivity was employed as a solvophobic influence. The addition of salt (MnSO₄.6H₂O) decreases the CMC of CTAB due to the screening of the electrostatic repulsion of the head groups. Here, we report that micellization is strongly influenced by salt concentration.