Adsorption and aggregation properties of amino acid-based N-alkyl cysteine monomeric and N, N^′-dialkyl cystine gemini surfactants

An amino acid-based gemini surfactant derived from cystine (2C_nCys, where n represents the hydrocarbon chain lengths of 8, 10, and 12) was synthesized by reacting cystine with n-alkyl bromide, and its adsorption and aggregation properties were characterized by measurements of equilibrium and dynamic surface tension and dynamic light scattering. The properties of 2C_nCys were compared with those of an amino acid-based monomeric surfactant derived from cysteine (C_nCys). For n = 8 and 10, when compared to C_nCys, 2C_nCys exhibited excellent surface activities, such as a lower critical micelle concentration (cmc), greater efficiency in lowering the surface tension of water, and smaller area occupied per molecule. Adsorption rate at air/water interface decreased with an increase in hydrocarbon chain length, chain number, and concentration of respective compounds. Further, the kinetics were discussed using the monomer diffusion coefficient obtained from short and long time scales in dynamic surface tension plots. In addition, the aggregation properties of 2C_nCys for n = 8 and 10 differed from those in the case of n = 12. In other words, relatively larger micelles with diameters of approximately 7 nm were formed by 2C_nCys for n = 8 and 10 in comparison to those formed by C_nCys (2-3 nm). On the other hand, for a 0.832 mmol dm^-3 2C₁₂Cys solution, the aggregation structure investigated by cryogenic transmission electron microscopy (cryo-TEM) and small-angle neutron scattering (SANS) revealed the coexistence of small unilamellar vesicles and small rods.