The effect of structural rigidity in quaternary ammonium-based ionic liquids (ILs) was investigated. [EtOMe]EtMe2NCl(I) and [MeOEt]EtMe2NCl(II), both 8 atom (7 carbons, 1 oxygen) ether-substituted quaternary ammonium chlorides (Quats), were synthesized and investigated. In acetonitrile, placement of the oxygen one atom from the nitrogen (Quat I) resulted in a significant decrease in the reductive stability. While the IL form showed similar stability for the two systems, placing the oxygen close to the nitrogen (methyl ethyl ether) appears to localize a greater portion of the positive charge on the nitrogen. This results in an IL with a higher melting point (58-62 degrees C) than the nonether analog, BuEtMe2NCl (III, 51.3 degrees C) IL. In contrast, placing two carbon atoms between the nitrogen and ether group, ethyl methyl ether (Quat II), delocalizes the positive charge from the nitrogen and produces a room-temperature IL. Neutralization of the acidic IL with sodium chloride resulted in an increase in the bulk conductivity, due to the polar ether group. Upon addition of CDCl3 or SOCl2, which both form a solid-electrolyte interphase on the substrate surface, the increased free sodium ion concentration allowed sodium reduction and oxidation currents greater than 4 mA/cm(2) at room temperature. (c) 2007 The Electrochemical Society