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, $\Delta G_{\text{tr,O}\to \text{W}}^{0^{\text{'}}}$ . The $\Delta G_{\text{tr,O}\to \text{W}}^{0^{\text{'}}}$ values for CH₃(CH₂)_n-2COO^- and CF₃(CF₂)_n-2COO^- increased linearly with n, allowing the estimation of $\Delta G_{\text{tr,O}\to \text{W}}^{0^{\text{'}}}$ for methylene and difluoromethylene groups, $\Delta G_{\text{tr,O}\to \text{W}}^{0^{\text{'}}}$ (CH₂) and $\Delta G_{\text{tr,O}\to \text{W}}^{0^{\text{'}}}$ (CF₂), and for their terminal groups, $\Delta G_{\text{tr,O}\to \text{W}}^{0^{\text{'}}}$ (COO^-  + CH₃) and $\Delta G_{\text{tr,O}\to \text{W}}^{0^{\text{'}}}$ (COO^- + CF₃). Whereas the $\Delta G_{\text{tr,O}\to \text{W}}^{0^{\text{'}}}$ (CH₂) and $\Delta G_{\text{tr,O}\to \text{W}}^{0^{\text{'}}}$ (CF₂) hardly changed with the variation in m, the $\Delta G_{\text{tr,O}\to \text{W}}^{0^{\text{'}}}$ (COO^- + CH₃) and $\Delta G_{\text{tr,O}\to \text{W}}^{0^{\text{'}}}$ (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.