L-cysteine-based trimeric surfactants with hexahydro-1,3,5-triazine as the central core: Synthesis and self-assembly study

ABSTRACT

Novel trimeric surfactants consisting of three units of sodium N-acyl-L-cysteine linked with hexahydro-1,3,5-triazine as the central core ((C_nCy)₃Na₃, where n represented alkyl chain carbon number of 8, 10, and 12) were synthesized. Their conjugated acids were analyzed using FTIR, NMR, and ESI-MS to elucidate their chemical structures. The surface activity properties of (C_nCy)₃Na₃ were determined using surface tension and electrical conductivity measurements. It was found that (C₁₂Cy)₃Na₃ had a critical aggregation concentration (CAC) two orders of magnitude lower than that of the corresponding monomeric surfactant sodium N-lauroyl-L-cysteine (C₁₂CyNa). The value of the minimum surface area per molecule (A_min) of (C₁₂Cy)₃Na₃ was almost three times greater than that of C₁₂CyNa due to the increase in the degree of oligomerization. With the alkyl chain length increasing from 8 to 12, the CAC values decreased, and the values of the surface tension at CAC (γ_CAC) and the efficiency of the surface adsorption (pC₂₀) increased. Furthermore, dynamic light scattering (DLS) and transmission electron microscopy (TEM) were used to evaluate the size and morphology of the aggregates at 10CAC. The results indicate that (C_nCy)₃Na₃ can spontaneously self-assemble into spherical or ellipsoid aggregates, and the surfactants with longer chain length can form larger aggregates due to greater hydrophobic interactions.

GRAPHICAL ABSTRACT

KEYWORDS:

• Aggregation behavior
• L-cysteine
• surface-active property
• trimeric surfactant