Standard Gibbs energies for the transfer (DeltaG(tr)degrees) of bis-quarternary ammonium ions, bis-A, such as R3N+(CH2)(n)N+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 DeltaG(tr)degrees of bis-A(2+) was attributed to the energy for the cavity formation in water phase, which increased the hydrophobicity of bis-A(2+), 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-A(2+) beyond the effect of cavity formation especially when the dielectric constant of Org was low. The effect of the spacer chain unique to bis-A(2+) was evaluated by the intramolecular electrostatic repulsion between two charged sites. (C) 2004 Elsevier B.V. All rights reserved.