Coupling of the orientations of liquid crystals to electrical double layers formed by the dissociation of surface-immobilized salts

We report the orientations of thermotropic liquid crystals to be coupled through dipolar interactions to the electric fields of electrical double layers that form upon contact of the liquid crystals with model surfaces presenting sodium carboxylate salts. The surfaces were prepared by the self-assembly of HOOC(CH2)(10)SH on semi-transparent films of gold. The density of sodium carboxylate groups was controlled by pretreatment of the surfaces with aqueous solutions buffered between pH 3.2 to 10.6 and quantified by using X-ray photoelectron spectroscopy. We used this well-defined experimental system to test predictions of a model that describes the dipolar coupling of the orientations of liquid crystals to an electric field formed at a surface through dissociation of sodium ions into the liquid crystal. Four predictions of the model, namely (I) the influence of the sign of the dielectric anisotropy of the liquid crystal, (II) the effect of the thickness of the film of liquid crystal relative to the Debye length within the liquid crystal, (III) the influence of the density of sodium carboxylate groups on the surface, and (IV) the influence of the concentration of electrolyte (NaI) dissolved within the liquid crystal, were found to be consistent with our experimental observations and thus support the proposed orientational coupling of the liquid crystal to the electric field formed by dissociation of the surface immobilized sodium carboxylate salt. A comparison of the orientational behavior of nematic phases of 4-cyano-4 ' -pentylbiphenyl (5CB) to the predictions of the model also revealed the sodium carboxylate-5CB interface to be held close to a constant potential (rather than constant density of charge) upon addition of electrolye to the liquid crystal.