Ionic conductivity mediated by hydrogen bonding in liquid crystalline 4-n-alkoxybenzoic acids

We describe the dielectric response of a series of liquid crystalline 4-n-alkoxybenzoic acids, nOBAs, with different alkyl chains, n = 4, 5, 7 and 8, in planarly aligned cells, as potential anhydrous electrolytes for electrochemical cells. All nOBAs display two modes of dielectric relaxations and conductivity. At moderate-high frequencies, f ∼101–104 Hz, the so-called mode 1 involves fast dipole rearrangements leading to direct current, DC, conductivities in the σdc1 ∼ 10−5 S cm−1 range, which are eventually insensitive to bias fields. At lower frequencies, f ∼10−1 to 101 Hz, the so-called mode 2 is related to slower processes with lower DC conductivities, σdc2 ∼ 10−6 S cm−1, which are further facilitated under sufficiently strong bias fields. Whilst mode 1 can be associated to the presence (and motions) of asymmetric dimers stabilised by hydrogen bonding and free acids in the nematic phase, mode 2 may involve the extension of the hydrogen-bonded network to longer ranges, probably by the formation of catemeric species, and its conductivity increases on heating in both the nematic and isotropic phases. Even though the conductivity values fall below those of benchmark electrolytes used in fuel cells (σdc ∼ 0.1 S cm−1), our results are promising, particularly for non-doped/non-hydrated electrolytes, and highlight the potential of the nOBAs and other hydrogen-bonded liquid crystals as components of electrolytes for ion conductivity.