Spectroscopic investigation on the hydrophobicity in the mixtures of nonionic plus twin tail alkylammonium bromide surfactants

The mixed micelle study of C10E8, C12E4, and C14E8 nonionic surfactants with 12-0-8, 12-0-10, 12-0-12, and 12-0-16 twin tail alkylammonium bromides has been carried out with the help of pyrene fluorescence, dynamic light scatterings (DLS), and electron spin resonance (ESR) measurements. The pyrene fluorescence allowed us to determine the mixed cmc and aggregation number (Nagg), while the size of the mixed aggregates, and rotation correlation time (τc) of the spin probe have been evaluated from DLS and ESR, respectively. Application of the regular solution theory showed that the mixed micelle formation in the mixtures of C10E8/C12E4 + cationic surfactants occurred due to the synergistic interactions, and the regular solution theory interaction parameter, β, became more negative with an increase in the hydrophobicity of the cationic component. A variation in the micelle mole fraction suggested that the mixed micelles are mainly rich in the nonionic component. An increase in the hydrophobicity of the nonionic surfactant from C10E8 to C14E8 induced antagonistic mixing in the mixtures of C14E8 with cationic surfactants of longer hydrophobic tail. This was attributed to the mutual incompatibility among the longer hydrophobic tails of the unlike components. The DLS and ESR measurements fully supported the above results, where the size of the aggregates increases with the increase in hydrophobicity.