The partitioning of primary alcohols into the aggregates of gemini amphiphiles determined from diffusion NMR experiments

Abstract

The partition constants (p-values) of primary alcohols in solutions containing aggregates of symmetric gemini surfactants of the family N,N′-dimethyl, N-dialkyl-α,ω-alkanediammonium dibromide (m-s-m = symmetric gemini surfactants) have been computed from the measured values of their diffusion coefficients obtained from NMR-diffusion experiments. From the p-values, both mole-fraction and concentration-based partition coefficients and Gibbs energies of transfer for the alcohols from the bulk D₂O phase to the gemini aggregate phase have been calculated. As expected, the Gibbs energy of transfer decreased linearly with an increase in the alcohol carbon length for each of the primary alcohol/gemini amphiphile series studied. The Gibbs transfer energy increment per CH₂ for the alcohols was consistent for all the alcohol/gemini amphiphile series and was in excellent agreement with the values measured for the same primary alcohol series in conventional single-headed, single-tailed surfactants. Surprisingly, the partition coefficients of the alcohols in the symmetric gemini aggregates exhibited little, if any, dependence on the spacer length of the gemini amphiphiles and were remarkably consistent as the length of the main surfactant chain increased at constant spacer length. When these results are compared to the partition coefficients of the same alcohols in corresponding monomeric surfactants, we observe little difference in the thermodynamic driving forces governing the transfer of alcohols from water to the aggregates of either monomeric or symmetric gemini surfactants.