Electrical transport properties and interaction of heteropolyacid in N,N-dimethylformamide by dielectric spectroscopy

We present a dielectric spectroscopy study on binary electrolyte solutions composed of N,N-Dimethylformamide (DMF) and heteropolyacid (HPA). The HPA is composed of H⁺ and Keggin polyoxometalate anion, [XW₁₂O₄₀]ⁿ⁻(X = P, Si, with n = 3, 4). Conductivity was used to calculate the transport parameters such as diffusion coefficient and hydrodynamic radius of polyoxometalate (POM) anions. The hydrodynamic radius of P₁₂W₄₀³⁻ and Si₁₂W₄₀⁴⁻ are 5.552 Å and 5.526 Å, respectively, indicating that these POM anions remain unsolvated in DMF. The temperature dependences of conductivity obeyed the Arrhenius equation, suggesting that the activation energy of proton conduction process is independent with temperature. The calculated activation energy shows that the conduction process follows Grotthuss mechanism, i.e., proton is transported by hydrogen bond between DMF molecules. One dielectric relaxation observed around 10 MHz is considered to be associated with the POM-DMFH⁺ ion-pairs formed by POM and protonated DMF. The interaction between anion and cation in POM-DMFH⁺ ion-pairs are explained theoretically based on the Bruggeman's mixture theories and dipole rotation model. From this, the distance between POM and DMFH⁺ are estimated as about 1.1 nm