Ion transfer voltammetry at the interface of water and low dielectric constant organic solutions

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

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, Δ G tr,O → W 0 ' . The Δ G tr,O → W 0 ' values for CH3(CH2)n-2COO- and CF3(CF2)n-2COO- increased linearly with n, allowing the estimation of Δ G tr,O → W 0 ' for methylene and difluoromethylene groups, Δ G tr,O → W 0 ' (CH2) and Δ G tr,O → W 0 ' (CF2), and for their terminal groups, Δ G tr,O → W 0 ' (COO-  + CH3) and Δ G tr,O → W 0 ' (COO- + CF3). Whereas the Δ G tr,O → W 0 ' (CH2) and Δ G tr,O → W 0 ' (CF2) hardly changed with the variation in m, the Δ G tr,O → W 0 ' (COO- + CH3) and Δ G tr,O → W 0 ' (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.