Influence of Simple Electrolytes on the Orientational Ordering of Thermotropic Liquid Crystals at Aqueous Interfaces

Abstract

We report orientational anchoring transitions at aqueous interfaces of a water-immiscible, thermotropic liquid crystal (LC; nematic phase of 4′-pentyl-4-cyanobiphenyl) that are induced by changes in pH of the aqueous solution and the addition of simple electrolytes (NaCl) to the aqueous phase. Whereas measurements of the zeta potential on the aqueous side of the interface of LC-in-water emulsions prepared with 5CB confirm pH-dependent formation of an electrical double layer extending into the aqueous phase, quantification of the orientational ordering of the LC leads to the proposition that an electrical double layer is also formed on the LC-side of the interface with an internal electric field that drives the LC anchoring transition. Further support for this conclusion is obtained from measurements of the dependence of LC ordering on pH and ionic strength, as well as a simple model based on the Poisson-Boltzmann equation from which we calculate the contribution of an electrical double layer to the orientational anchoring energy of the LC. Overall, the results presented herein provide new fundamental insights into ionic phenomena at LC-aqueous interfaces, and expand the range of solutes known to cause orientational anchoring transitions at LC-aqueous interfaces beyond previously examined amphiphilic adsorbates.

Hierarchical organization in liquid crystal-in-liquid crystal emulsions

“…29,30 Here we note that electric fields within double layers can generate torques on LCs due to the anisotropic dielectric properties (Δε) of the LC phases. 31,32 In particular, past studies have demonstrated that, due to the positive Δε of 5CB (Δε = +13) 26 , formation of an electrical double layer at an interface of 5CB can cause the anchoring of 5CB to change from planar to homeotropic. 31,32 We tested the possible role of electrical double layers in the observed coupling of the orientations of the 5CB droplets and DSCG by creating dispersions of nematic N-(4-methoxybenzylidene)-4-butylaniline (MBBA) droplets within nematic DSCG.…”

“…This leads us to conclude that the barrier to coalescence arises from either elastic forces mediated by the DSCG or potentially other colloidal forces such as electrical double layer forces arising from charging of the interface of the 5CB in the DSCG. 32 …”

Bienzyme liquid-crystal-based cholesterol biosensor

“…The free electrons produced by the enzymatic reaction could lead towards the deprotonation of the PAA chains, generating a high density of negative charges. This may lead to form an electric field on the 5CB side, causing a homeotropic orientation, because 5CB is a negative dielectric anisotropic material and favours homeotropic orientation in the presence of an electric field [15]. H 2 O 2 could cause a homeotropic orientation without the use of HRP.…”

“…4-Cyano-4 -pentylbiphenyl (5CB), a nematic liquid crystal (LC) at room temperature, has been aligned on an octadecyltricholorosilane (OTS) coated glass and used to amplify the molecular events at 5CB/aqueous interfaces in the presence of proteins [9], polymers [10,11], surfactants [12], organophosphate [13], urea [14], and simple electrolytes [15]. Popov had to be converted to cholic acid by additional chemical reactions prior to detection [18].…”

Applications of metal ions and liquid crystals for multiplex detection of DNA

“…Some possible mechanisms have been proposed to explain how ionic species affect the orientation of LC. Firstly, when LC 4-cyano-4 0 -pentylbiphenyl (5CB) is in contact with an aqueous solution, simple electrolytes such as sodium chloride are able to partition into the interfacial region of LC and create an electrical double layer which aligns LC homeotropically [15]. Meanwhile, some chaotropic anions such as perchlorate can also interact with the nitrile group of 5CB and triggers an ordering transition [16].…”