Electrolyte Effects on the Stability of Nematic and Lamellar Lyotropic Liquid Crystal Phases: Colligative and Ion-Specific Aspects

Current Topics in Ionic Liquid Crystals

Ionic liquid crystals (ILCs), that is, ionic liquids exhibiting mesomorphism, liquid crystalline phases, and anisotropic properties, have received intense attention in the past years. Among others, this is due to their special properties arising from the combination of properties stemming from ionic liquids and from liquid crystalline arrangements. Besides interesting fundamental aspects, ILCs have been claimed to have tremendous application potential that again arises from the combination of properties and architectures that are not accessible otherwise, or at least not accessible easily by other strategies. The current review highlights recent developments in ILC research, starting with some key fundamental aspects. Further subjects covered include the synthesis and variations of modern ILCs, including the specific tuning of their mesomorphic behavior. The review concludes with reflections on some applications that may be within reach for ILCs and finally highlights a few key challenges that must be overcome prior and during true commercialization of ILCs.

Effect of head-group size of some tetradecylalkylammonium bromide surfactants on obtaining the lyotropic biaxial nematic phase

Lyotropic quaternary mixtures of some tetradecylalkylammonium bromide surfactants were prepared to examine the effect of the size of the surfactant head group on the stabilization of different lyotropic nematic phases. The lyotropic mixtures were prepared by the addition of the tetradecylalkylammonium bromides (TTAABr) in the mixture of NaBr/decanol (DeOH)/water. The uniaxial to biaxial nematic phase transitions were determined via laser conoscopy. Some micellization parameters such as critical micelle concentration, degree of counterion binding and micellization Gibbs energy were evaluated from the electrical conductivity measurements of diluted binary surfactants/water solutions. The results indicate that the head-group size of the surfactant molecules influences the amphiphilic molecular aggregate topology. Moreover, the effective area per surfactant head group is a key parameter on stabilizing the lyotropic biaxial nematic phase.

Role of kosmotrope-chaotrope interactions at micelle surfaces on the stabilization of lyotropic nematic phases

Three lyotropic quaternary systems of ionic surfactants were prepared to investigate the role of kosmotrope-chaotrope interactions at the micelle surfaces on stabilizing the different nematic phases. The ionic surfactants were potassium laurate (KL), sodium dodecylsulfate (SDS) and tetradecyltrimethylammonium bromide (TDTMABr), where KL is a kosmotrope surfactant, and others are chaotrope. The first system consisted of KL/decanol (DeOH)/water/alkali sulfate and the second of SDS/DeOH/water/alkali sulfate. The third system was prepared by adding sodium salts of chaotropic or kosmotropic anions to the primary mixture of TDTMABr/DeOH/water, separately. The characteristic textures of discotic nematic (N _D), biaxial nematic (N _B) and calamitic nematic (N _C) phases were identified under polarizing light microscope. Laser conoscopy was employed to determine the uniaxial-to-biaxial phase transitions. The kosmotrope-kosmotrope or chaotrope-chaotrope interactions between the head groups of the surfactants and the ions of the electrolytes led to the stabilization of the N _D phase. On the other hand, kosmotrope-chaotrope interactions stabilize the N _B and/or N _C phases.

Ions at hydrophobic interfaces

We review the present understanding of the behavior of ions at the air-water and oil-water interfaces. We argue that while the alkali metal cations remain strongly hydrated and are repelled from the hydrophobic surfaces, the anions must be classified into kosmotropes and chaotropes. The kosmotropes remain strongly hydrated in the vicinity of a hydrophobic surface, while the chaotropes lose their hydration shell and can become adsorbed to the interface. The mechanism of adsorption is still a subject of debate. Here, we argue that there are two driving forces for anionic adsorption: the hydrophobic cavitational energy and the interfacial electrostatic surface potential of water. While the cavitational contribution to ionic adsorption is now well accepted, the role of the electrostatic surface potential is much less clear. The difficulty is that even the sign of this potential is a subject of debate, with the ab initio and the classical force field simulations predicting electrostatic surface potentials of opposite sign. In this paper, we will argue that the strong anionic adsorption found in the polarizable force field simulations is the result of the artificial electrostatic surface potential present in the classical water models. We will show that if the adsorption of anions were as large as predicted by the polarizable force field simulations, the excess surface tension of the NaI solution would be strongly negative, contrary to the experimental measurements. While the large polarizability of heavy halides is a fundamental property and must be included in realistic modeling of the electrolyte solutions, we argue that the point charge water models, studied so far, are incompatible with the polarizable ionic force fields when the translational symmetry is broken. The goal for the future should be the development of water models with very low electrostatic surface potential. We believe that such water models will be compatible with the polarizable force fields, which can then be used to study the interaction of ions with hydrophobic surfaces and proteins.

Ion specificity and micellization of ionic surfactants: a Monte Carlo study

We developed a simulation method that allows us to calculate the critical micelle concentrations for ionic surfactants in the presence of different salts. The results are in good agreement with the experimental data. The simulations are performed on a simple cubic lattice. The anionic interactions with the alkyl chains are taken into account based on the previously developed theory of the interfacial tensions of hydrophobic interfaces: the kosmotropic anions do not interact with the hydrocarbon tails of ionic surfactants, while chaotropic anions interact with the alkyl chains through a dispersion potential proportional to the anionic polarizability.

Effect of cations on condensation of a mesogenic amphiphilic molecule at the air-aqueous electrolyte interface

We report the interactions of a mesogenic molecule, 4'-octyl-4-biphenyl-carbonitrile (8CB), with some cations (Na(+), Cu(2+), Ni(2+), La(3+) and Al(3+)) dissolved in the aqueous subphase. Surface manometry studies show that the di- (Ni(2+) and Cu(2+)) and trivalent (La(3+)) ions promote condensation in the area per molecule and enhance the stability of the monolayer. This is inferred from the increase in the values of collapse pressure and the compression elastic modulus. The specific ion effect is seen between perchlorate and chloride anions with respect to the Al(3+) cation. The presence of monovalent ions (Na(+)) in the subphase does not influence the isotherm of 8CB. However, in this case, with pH (>6), the isotherm shifts to a higher area per molecule. The excess Gibbs free energy calculated for the 8CB monolayer indicates repulsive interaction for monovalent ions and attractive interaction for multivalent ions in the subphase. Kinetic studies of the monolayer in an ion-enriched subphase have yielded an additional characteristic time constant indicative of reorganization of the monolayer. Ellipsometric adsorption isotherm measurements carried out for representative ions show a reduction in the value of the ellipsometric angle with increasing valency. Our studies indicate that the interaction of ions with the 8CB monolayer at the air-electrolyte interface can be promoted by choosing cations of higher valency and anions of larger size, higher polarizability and chaotropic nature. These factors play an important role and can potentially affect the anchoring transition.

Electrolyte effects on the chiral induction and on its temperature dependence in a chiral nematic lyotropic liquid crystal

We present a study on the effect of added CsCl and of temperature variation on the chiral induction in a chiral nematic lyotropic liquid crystal (LC) composed of the surfactant cesium perfluorooctanoate (CsPFO), water, and the chiral dopant d-Leucine (d-Leu). The chiral induction was measured as the helical pitch P. The role of the additives CsCl and d-Leu on the phase behavior is investigated and discussed. The thermal stabilization effect of CsCl is shown to lead to an apparent salt effect on the pitch when the pitch is compared at a constant temperature. This apparent effect is removed by comparing the pitch measured for different salt concentrations at a temperature relative to the phase-transition temperatures; thus, the real salt effect on the pitch is described. High salt concentrations are shown to increase the pitch, that is, hinder the chiral induction. The effect is discussed in terms of a decreased solubilization of the amphiphilic chiral solute d-Leu in the micelles due to the salt-induced screening of the surfactant head groups and the consequential denser packing of the surfactants. The temperature variation of the pitch is investigated for all CsCl concentrations and is found to be essentially independent of the salt concentration. The temperature variation is analyzed and discussed in the context of a theoretical model taking into account specific properties of lyotropic liquid crystals. A hyperbolic decrease of the pitch is found with increasing temperature, which is known, from thermotropic liquid crystals, to stem from pretransitional critical fluctuations close to the lamellar phase. However, the experimental data confirmed the theoretical prediction that, at high temperature, that is, far away from the transition into the lamellar phase, the pitch is characterized by a linear temperature dependence which is determined by a combination of steric and dispersion chiral interactions. The parameters of the theoretical expression for the pitch have been determined by fitting the experimental data. The analysis of the salt concentration dependence of these parameters indicates that the chiral induction mechanism of d-Leu is dominated by chiral steric interactions.