Surface order at surfactant-laden interfaces between isotropic liquid crystals and liquid phases with different polarity

Abstract

We present an ellipsometry study of the interface between thermotropic liquid crystals and liquid phases consisting of various binary mixtures of water and glycerol. The liquid-crystal samples contain a small constant amount of a surfactant which induces a homeotropic anchoring at the interface. We determine the smectic or nematic order at the interface in the temperature range above the liquid-crystal-isotropic transition while the water to glycerol ratio is varied, corresponding to a systematic modification of the polarity of the liquid phase. The surface-induced order becomes less pronounced with increasing glycerol concentration in the liquid phase. The observed behavior is compared with previous studies in which the surfactant concentration in the liquid-crystal bulk phase was varied. The results indicate that in both cases the magnitude of the surfactant coverage at the interface is the key quantity which determines the liquid-crystal surface order at the interface.

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

“…In addition, we note that the use of zeta potential to infer the charged state of an interface at high ionic strength can also be misleading (as evidenced in Figure b for the data at 1 M NaCl), whereas we conclude that the orientational ordering of the LC indicates the charging of the interface at high ionic strengths. We also note that identification of the role of the electrical double layer in ordering LCs at aqueous interfaces, as reported herein, provides new guidance to the design of interfaces than can report chemical and biological interactions. − …”

“…Because the orientational ordering of the molecules in the LC phase is long-ranged, the ionic phenomena at the LC–aqueous interface are amplified into bulk ordering transitions within the LC phase that are readily imaged by using polarized light microscopy. The results reported in this paper are significant in that they offer new fundamental insights into the balance of intermolecular forces that determine the ordering of thermotropic LCs at aqueous interfaces, and they provide new guidance to the design of dynamic and responsive LC interfaces such as those that enable reporting of chemical and biological interactions. − In particular, whereas past studies of LC–aqueous interfaces have reported that a wide range of amphiphilic molecules can adsorb at LC–aqueous interfaces to induce anchoring transitions mediated by interactions involving the tails of the amphiphiles, − ,− anchoring transitions in LCs induced by simple electrolytes at aqueous interfaces have not been previously reported.…”

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

“…The measurements reported in this paper also reveal that the LC ordering transitions triggered by contact with chaotropic salt solutions are due to an anion-induced change in the easy axis of the LC and not just a decrease in the strength of the anchoring. Overall, we note that the LC–ion interactions unmasked in this paper are significant in light of past and ongoing studies using LC–aqueous interfaces for amplification of molecular interactions. ,− Further investigation of the specific ion effects revealed in this study may also lead to an experimental system that can be used to report positive surface excess ion concentrations and thus test model predictions regarding specific ion phenomena at oil–water interfaces. Finally, we note that application of an external electric field, which would be expected to influence ion adsorption, might provide additional insights into the ionic phenomena reported in this paper.…”

“…We end this introduction by noting that the interactions of LCs with aqueous electrolyte solutions, as investigated in the study reported in this paper, is of relevance to the broad topic of oil–water interfacial phenomena. It is also significant in guiding the design of dynamic and responsive LC interfaces for the sensing of chemical and biological molecules and interactions. ,− …”

Liquid Crystals in Aqueous Ionic Surfactant Solutions: Interfacial Instabilities & Optical Applications

“…The presence of the motion in the isotropic state suggests that a liquid crystalline structure in the droplet is not essential. However it was observed that surfactants induce a liquid crystalline thin film at the stationary isotropic oil-aqueous micellar solution interface [98,99]. These observations suggest that a liquid crystalline thin film may be present around a moving isotropic droplet but experimental evidence is necessary to back up this hypothesis.…”