Synthesis of two biofriendly anionic surfactants (N-n-decanoyl-l-valine and N-n-decanoyl-l-leucine) and their mixed micellization with nonionic surfactant Mega-10 in Tris-buffer medium at pH 9

Abstract

Two biofriendly anionic amino acid surfactants (AAS), N-n-decanoyl-L-valine (C₁₀-val) and N-n-decanoyl-L-leucine (C₁₀-leu) were synthesized and mixed micellization of them with nonionic surfactant N-decanoyl-N-methyl-glucamine (Mega-10) was investigated by tensiometry and fluorimetry in 50 mM Tris-buffer (pH = 9) medium at 298 K. The critical micelle concentration (cmc), surface properties, e.g., Gibbs surface excess (Γ_max), area of exclusion per surfactant monomer (A_min) and surface pressure at cmc (Π_cmc) were determined. Gibbs free energy of micellization (ΔG⁰_m) and Gibbs free energy of adsorption (ΔG⁰_ads) were also determined. Both the free energy values are negative indicating the spontaneity and stability of the mixed micelles. The size of the micelles was determined by dynamic light scattering (DLS) measurements. The deviation of mixed micelles from the ideal behavior was discussed on the basis of Clint, Motomura, Rubingh (regular solution theory), Rosen and Maeda's theory. Rubingh–Holland theory was applied on the ternary systems made by these three surfactants. The compositions of mixed micelle, the activity coefficients and the corresponding interaction parameters were evaluated from these theories. The interaction parameters (β) are negative, indicating attractive interaction between the ionic and nonionic surfactants. The micellar aggregation number (N_agg) and micropolarity were evaluated using steady state fluorescence measurements and the packing parameter (P) was determined on the basis of Israelachvili's theory. The standard free energy changes associated with the transfer of surfactant tail due to micellization of pure, binary and ternary combinations of surfactants were determined from Nagarajan's model. We report for the first time in detail the binary and ternary combinations using amino acid based surfactants.