Influence of Specific Anions on the Orientational Ordering of Thermotropic Liquid Crystals at Aqueous Interfaces

Abstract

We report that specific anions (of sodium salts) added to aqueous phases at molar concentrations can trigger rapid, orientational ordering transitions in water-immiscible, thermotropic liquid crystals (LCs; e.g., nematic phase of 4′-pentyl-4-cyanobiphenyl, 5CB) contacting the aqueous phases. Anions classified as chaotropic, specifically iodide, perchlorate and thiocyanate, cause 5CB to undergo continuous, concentration-dependent transitions from planar to homeotropic (perpendicular) orientations at LC-aqueous interfaces within 20 s of addition of the anions. In contrast, anions classified as relatively more kosmotropic in nature (fluoride, sulfate, phosphate, acetate, chloride, nitrate, bromide, and chlorate) do not perturb the LC orientation from that observed without added salts (i.e., planar orientation). Surface pressure-area isotherms of Langmuir films of 5CB supported on aqueous salt solutions reveal ion-specific effects ranking in a manner similar to the LC ordering transitions. Specifically, chaotropic salts stabilized monolayers of 5CB to higher surface pressures and areal densities (12.6 mN/m at 27 Å2/molec. for NaClO4) and thus smaller molecular tilt angles (30° from the surface normal for NaClO4) than kosmotropic salts (5.0 mN/m at 38 Å2/molec. with a corresponding tilt angle of 53° for NaCl). These results and others reported herein suggest that anion-specific interactions with 5CB monolayers lead to bulk LC ordering transitions. Support for the proposition that these ion-specific interactions involve the nitrile group was obtained by using a second LC with nitrile groups (E7; ion-specific effects similar to 5CB were observed) and a third LC with fluorine-substituted aromatic groups (TL205; weak dipole and no ion-specific effects were measured). Finally, we also establish that anion-induced orientational transitions in micrometer-thick LC films involve a change in the easy axis of the LC. Overall, these results provide new insights into ionic phenomena occurring at LC-aqueous interfaces, and reveal that the long-range ordering of LC oils can amplify ion-specific interactions at these interfaces into macroscopic ordering transitions.

Reversible Switching of Liquid Crystalline Order Permits Synthesis of Homogeneous Populations of Dipolar Patchy Microparticles

“…Previously, we showed that it is possible to trigger bipolar-to-radial ordering transitions in LC droplets under conditions of high ionic strength (> 100 mM) and alkaline pH (> 12), due to the effects of an electrical double layer (EDL) that forms on the LC-side of the aqueous—LC interface. [38] To explore the use of salts and alkaline conditions for microparticle synthesis, we first formed emulsions of bipolar 5CB droplets with PS colloids adsorbed at either one or both boojums. Next, sodium chloride and sodium hydroxide (the final concentration of sodium chloride was 1 M and the pH was 12.7) were added to the aqueous phase of the emulsions.…”

Design of Functional Materials Based on Liquid Crystalline Droplets

“…For example, recent studies have established that electrical double layers form on the LC-side of aqueous-LC interfaces. 58, 59 It is possible, for example, that the contribution of the electrical double layer to the anchoring energy of the LC droplets is size-dependent. Additional studies, both experimental and theoretical, are needed to resolve these fundamental issues that influence the design of responsive soft materials based on LCs.…”

“…14 Under these latter conditions, recently documented effects of electrical double layers 58, 59 lead to sufficiently small anchoring energies that the size-dependent changes in configuration of the LCs are observed in the micrometer-range of sizes.…”

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

“…However, all anions tested in our experiment are kosmotropic in nature. Chaotropic anions such as perchlorate might influence the orientation of LC as shown in a past study [16].…”

“…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]. Secondly, when 5CB is in contact with a solid surface decorated with copper ions, nitrile group of 5CB is able to complex with copper ions [17,18] and causes a homeotropic orientation.…”