Partition constants of alcohols and surfactants in mixed alcohol/10-carbon dimeric amphiphile aggregates from NMR-diffusion experiments

Abstract

The partition constants (p-values) of primary alcohols in solutions containing aggregates of some symmetric dimeric (gemini) surfactants N,N′-dimethyl,N-N′-didecyl-α,ω-alkanediammonium dibromide (10-s-10 gemini surfactants) have been computed from the diffusion coefficients via NMR spectroscopy. From the partition constants, thermodynamic partition coefficients and Gibbs energies of transfer for the alcohols from the bulk D₂O phase to the dimeric aggregate phase have been calculated. For 10-6-10 and 10-8-10 surfactants, the partition constants for two primary alcohols, 1-butanol (C₄OH) and 1-pentanol (C₅OH), increased with increasing the amount of the surfactant in the solution, while the thermodynamic partition coefficients and the calculated Gibbs transfer energies were constant with increasing surfactant concentration. The partition constants and the thermodynamic partition coefficients for a series of homologous alcohols in the 10-series dimerics were determined at a surfactant concentration corresponding to 100 mg mL⁻¹; the Gibbs energy of transfer (obtained from the thermodynamic partition coefficients) decreased linearly with the alcohol carbon length for each of the primary alcohol/gemini amphiphile series studied. Finally, the diffusion coefficients for the surfactants were used along with the diffusion coefficients of the aggregates to obtain partition constants of the dimeric surfactants in mixed aggregates composed of C₄OH and C₅OH in both 10-6-10 and 10-8-10; our results indicated the surfactant partition constants increased with increasing surfactant concentration in excellent agreement with our previous work and the literature. All these results were used to obtain a comprehensive description of the alcohol/surfactant mixed micelles as a function of the composition of the system, and to examine the applicability of the pseudo-phase separation model to describe the phenomenon of solubilization in the formation of mixed micelles.