Effect of the side or spacer chain on the hydrophobicity of bis-quaternary ammonium ions studied by ion transfer voltammetry at the aqueous|organic solution interface

Standard Gibbs energies for the transfer ( Δ G tr ∘ ) of bis-quarternary ammonium ions, bis-A2+, such as R3N+(CH2)nN+R3 (R = ethyl, propyl or butyl; n = 3–6, 8 or 12) from water (W) to organic solvent (Org) such as nitrobenzene or 1,2-dichloroethane were evaluated by ion transfer voltammetry at both a conventional stationary W|Org interface and a micro W|Org interface. The contribution of R to Δ G tr ∘ of bis-A2+ was attributed to the energy for the cavity formation in water phase, which increased the hydrophobicity of bis-A2+, being proportional to the side chain length and almost independent of Org. A decrease of the spacer chain length decreased the hydrophobicity of the bis-A2+ beyond the effect of cavity formation especially when the dielectric constant of Org was low. The effect of the spacer chain unique to bis-A2+ was evaluated by the intramolecular electrostatic repulsion between two charged sites.