Refractive index in relation to solvent effects on the amphiphilic association of n-alkylammonium carboxylates

Abstract

Measurements of the refractive index of isotropic solutions of n-alkylammonium carboxylates [CH3(CH2)7 +NH3 −O2CR, R = H, n-tridecyl, n-heptadecyl, and 9-anthryl] display concentration dependence related to the degree of surfactant association in pure and mixed solvents of static relative permittivity in the range er = 2.28–110 at 298 K. Results for n-octylammonium formate comply with: (i) small reverse micelle type aggregates in low er ≤ 10 solvents, e.g., benzene; (ii) monomers and/or small association complexes in solvents of intermediate relative permittivity (er ∼ 15–45, e.g., methanol, and acetone:water (50:50 v:v)); and (iii) charged normal micelles in high er ≥ 78 solvents, e.g., water, and N-methylformamide. In water, the critical micelle concentration (cmc) of n-octylammonium formate (0.27–0.29 mol dm−3) is an order of magnitude higher than in benzene. A large anion group does not inhibit micellization in the case of n-octylammonium 9-anthracene carboxylate in chloroform. Refractometry, along with measurements of the electric conductance of some solutions, yield concordant information on micellar parameters and equilibrium processes compared to results from NMR and dielectric measurements. The solubility and Hamaker constants of n-octylammonium carboxylates in different solvents (er = 1.8–182) are described, and experimental data are considered in the context of a micelle transition model.