Effect of the alkyl chain length of α,ω-dichloroalkane on the Gibbs energy of transfer for functional groups

Abstract

Ion-transfer reactions of alkyl and perfluoroalkyl carboxylate ions (CH₃(CH₂)_n–2COO^– with n = 8–12 and CF₃(CF₂)_n–2COO^– with n = 3–9) were investigated at the polarized Cl-(CH₂)_m-Cl with m = 2, 4, 6, and 8 (O) | water (W) interface to evaluate the effects of n and m on the solvation energy of the ions, as well as on their methylene and terminal groups. These ions exhibited reversible or quasi-reversible voltammetric waves due to their transfer across the O | W interfaces, enabling the determination of formal potentials and formal Gibbs transfer energies from O to W, \(\Delta G_{{{\text{tr,O}} \to {\text{W}}}}^{0^{\prime}}\). The \(\Delta G_{{{\text{tr,O}} \to {\text{W}}}}^{0^{\prime}}\) values for CH₃(CH₂)_n–2COO^– and CF₃(CF₂)_n–2COO^– increased linearly with n, allowing the estimation of \(\Delta G_{{{\text{tr,O}} \to {\text{W}}}}^{0^{\prime}}\) for methylene and difluoromethylene groups, \(\Delta G_{{{\text{tr,O}} \to {\text{W}}}}^{0^{\prime}}\) (CH₂) and \(\Delta G_{{{\text{tr,O}} \to {\text{W}}}}^{0^{\prime}}\) (CF₂), and for their terminal groups, \(\Delta G_{{{\text{tr,O}} \to {\text{W}}}}^{0^{\prime}}\) (COO^–  + CH₃) and \(\Delta G_{{{\text{tr,O}} \to {\text{W}}}}^{0^{\prime}}\) (COO^– + CF₃). Whereas the \(\Delta G_{{{\text{tr,O}} \to {\text{W}}}}^{0^{\prime}}\) (CH₂) and \(\Delta G_{{{\text{tr,O}} \to {\text{W}}}}^{0^{\prime}}\) (CF₂) hardly changed with the variation in m, the \(\Delta G_{{{\text{tr,O}} \to {\text{W}}}}^{0^{\prime}}\) (COO^– + CH₃) and \(\Delta G_{{{\text{tr,O}} \to {\text{W}}}}^{0^{\prime}}\) (COO^– + CF₃) decreased noticeably. These results suggest that the solvation energy for ions in Cl-(CH₂)_m-Cl increases with m, regardless of hydrophilic or lipophilic nature of the ions. Based on these findings, the advantage of using Cl-(CH₂)_m-Cl with a large m as a non-aqueous solvent for ion-transfer voltammetry was discussed.

Graphical abstract