The interfacial and micellization behavior of three dicarboxylic amino acid-based anionic surfactants, abbreviated as AAS (N-dodecyl derivative of -aminomalonate, -aspartate, and -glutamate) in combination with hexadecyltrimethylammonium bromide (HTAB) were investigated by surface tension, conductance, UV−vis absorption/emission spectroscopy, dynamic light scattering (DLS), and viscosity studies. Critical micelle concentration (CMC) values of the surfactant mixtures are significantly lower than the predicted values, indicating associative interaction between the components. Surface excess, limiting molecular area, surface pressure at the CMC, and Gibbs free energy indicate spontaneity of the micellization processes compared to the pure components. CMC values were also determined from the sigmoidal variation in the plot of micellar polarity and pyrene UV−vis absorption/ emission intensities with surfactant concentration. The aggregation number, determined by static fluorescence quenching method, increases with decreasing mole fraction of the AAS (α AAS ), where the micelles are mainly dominated by the HTAB molecules. The size of the micelle increases with decreasing α AAS , leading to the formation of larger and complex aggregates, as also supported by the viscosity studies. Micelles comprising 20−40 mol % AAS are highly viscous, in consonance with their sizes. Some of the mixed surfactant systems show unusual viscosity (shear thickening and increased viscosity with increasing temperature). Such mixed surfactant systems are considered to have potential in gel-based drug delivery and nanoparticle synthesis.
Theoretical investigations on the micellization of mixtures of (i) amino acid‐based anionic surfactants [AAS: N‐dodecyl derivatives of aminomalonate, −aspartate, and ‐glutamate] and (ii) hexadecyltrimethylammonium bromide (HTAB), were carried out at different mole ratios. Variation in the theoretical values of critical micelle concentration (CMC), mole fraction of surfactants in the micellar phase (X), at the interface (Xσ), interaction parameters at the bulk/interface (βR/βσ), ideality/nonideality of the mixing processes, and activity coefficients (f) were evaluated using Rubingh, Rosen, Motomora, and Sarmoria‐Puvvada‐Blankschtein models. CMC values significantly deviate from the theroretically calculated values, indicating associative interaction. With increasing mole fraction of AAS (αAAS), the magnitude of the (βR/βσ) values gradually decreased, considered to attributable to hydrophobic interactions. With increasing αAAS, the micellar mole fraction of HTAB (X2) decreased insignificantly and X2 values were higher than those compared to AAS for all combinations, due to the dominance of HTAB in micelles. Micellar mole fraction at the ideal state of AAS (X1ideal) differed from micellar mole fraction of AAS (X1), indicating nonideality in the mixed micellization process. Gibbs free energy of micellization (
∆Gm) values are more negative than the free energy of micellization for ideal mixing (∆Gmideal), indicating the micellization process is spontaneous. With increasing αAAS, the enthalpy of micellization (ΔHm) and entropy of micellization (ΔSm) values gradually increased, which indicates micellization is exothermic. The different physicochemical parameters of the mixed micelles are correlated with the variation in the spacer length between the two carboxylate groups of AAS.
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