Gelation of Lyotropic Liquid-Crystal Phases-The Interplay between Liquid Crystalline Order and Physical Gel Formation

Time Dependence of Gel Formation in Lyotropic Nematic Liquid Crystals: From Hours to Weeks

The combination of lyotropic liquid crystals (LLCs) and low-molecular-weight gelators (LMWGs) for the formation of lyotropic liquid crystal gels (LLC gels) leads to a versatile and complex material combining properties of both parent systems. We gelled the calamitic nematic NC phases of a binary and ternary system using the LMWG 3,5-bis-(5-hexylcarbamoyl-pentoxy)-benzoic acid hexyl ester (BHPB-6). This binary system consists of the surfactant N,N-dimethyl-N-ethyl-1-hexadecylammonium bromide (CDEAB) and water, whereas the ternary system consists of the surfactant N,N,N-trimethyl-N-tetradecylammonium bromide (C14TAB), the cosurfactant n-decanol, and water. Though containing similar surfactants, the gelled NC phases of the binary and ternary systems show differences in their visual and gel properties. The gelled NC phase of the binary system remains clear for several days after preparation, whereas the gelled NC phase of the ternary system turns turbid within 24 h. We investigated the time evolution of the gel strength with oscillation rheology measurements (a) within the first 24 h and (b) up to two weeks after gel formation. The shape of the fibers was investigated over different time scales with freeze fracture electron microscopy (FFEM). We demonstrate that despite their similarities, the two LLC gels also have distinct differences.

Manipulating supramolecular gels with surfactants: Interfacial and non-interfacial mechanisms

Gel is a class of self-supporting soft materials with applications in many fields. Fast, controllable gelation, micro/nano structure and suitable rheological properties are essential considerations for the design of gels for specific applications. Many methods can be used to control these parameters, among which the additive approach is convenient as it is a simple physical mixing process with significant advantages, such as avoidance of pH change and external energy fields (ultrasound, UV light and others). Although surfactants are widely used to control the formation of many materials, particularly nanomaterials, their effects on gelation are less known. This review summarizes the studies that utilized different surfactants to control the formation, structure, and properties of molecular and silk fibroin gels. The mechanisms of surfactants, which are interfacial and non-interfacial effects, are classified and discussed. Knowledge and technical gaps are identified, and perspectives for further research are outlined. This review is expected to inspire increasing research interest in using surfactants for designing/fabricating gels with desirable formation kinetics, structure, properties and functionalities.

Advanced Functional Liquid Crystals

Liquid crystals have been intensively studied as functional materials. Recently, integration of various disciplines has led to new directions in the design of functional liquid-crystalline materials in the fields of energy, water, photonics, actuation, sensing, and biotechnology. Here, recent advances in functional liquid crystals based on polymers, supramolecular complexes, gels, colloids, and inorganic-based hybrids are reviewed, from design strategies to functionalization of these materials and interfaces. New insights into liquid crystals provided by significant progress in advanced measurements and computational simulations, which enhance new design and functionalization of liquid-crystalline materials, are also discussed.

Potential cosmeceutical lamellar liquid crystals containing black longan (Dimocarpus longan Lour.) seed extract for MMP-1 and hyaluronidase inhibition

The aims of this study were to evaluate the biological activities of black Dimocarpus longan Lour. seed extracts and develop a lamellar liquid crystal (LLC). Different solvents, including petroleum ether, ethyl acetate, and 95% v/v ethanol, were used in the maceration of black D. longan seeds. The inhibitory activities on matrix metalloproteinase-1 (MMP-1) and hyaluronidase were evaluated. The irritating potency of D. longan seed extracts was determined using the hen's egg chorioallantoic membrane test. The extract with the strongest anti-ageing activities and no irritant impact was examined for its chemical contents using high-performance liquid chromatography and incorporated into the LLC. Various factors affecting the LLC formulations, including surfactant types and amounts, thickening agent types and amounts, and various oil types, were investigated. The results demonstrated that the ethyl acetate extract (EtOAc) was the most potent inhibitor of MMP-1 (IC₅₀ = 21.7 ± 5.4 µg/mL) and hyaluronidase (oleanolic acid equivalent = 0.44 ± 0.03 g per g extract). Interestingly, its MMP-1 inhibition was equivalent to that of oleanolic acid, corilagin, and gallic acid. Furthermore, its hyaluronidase inhibition was equivalent to that of gallic acid and ellagic acid. Gallic acid, which was the most abundant compound (15.6% ± 0.06% w/w) was suggested as the compound responsible for the biological activities of EtOAc extract. Considering its potential anti-skin ageing properties with no irritation of EtOAc extract, it was incorporated into the most suitable LLC, which was composed of 5% w/w Lexfeel® D5 oil, 0.5% w/w Carbopol® U21, 2% w/w Liquid Crystal Cream Maker, and 92.5% w/w DI water. The LLC containing EtOAc extract presented birefringence under a polarizing light microscope, indicating its crystallinity. The formulation had good stability after heating-cooling cycles in terms of external appearance, crystallinity, viscosity, and pH (5.5). As a result, it is recommended as a potential cosmeceutical formulation for anti-skin wrinkling. It is proposed that more research be conducted on the safety and efficacy of the treatment on human volunteers.

Micellar Lyotropic Nematic Gels

Lyotropic liquid crystal (LLC) gels are a new class of liquid crystal (LC) networks that combine the anisotropy of micellar LLCs with the mechanical stability of a gel. However, so far, only micellar LLC gels with lamellar and hexagonal structures have been obtained by the addition of gelators to LLCs. Here, the first examples of lyotropic nematic gels are presented. The key to obtain these nematic gels is the use of gelators that have a non-amphiphilic molecular structure and thus leave the size and shape of the micellar aggregates essentially unchanged. By adding these gelators to lyotropic nematic phases, an easy and reproducible way to obtain large amounts of lyotropic nematic gels is established. These nematic gels preserve the long-range orientational order and optical birefringence of a lyotropic nematic phase but have the mechanical stability of a gel. LLC nematic gels are promising new materials for elastic and anisotropic hydrogels to be applied as water-based stimuli-responsive actuators and sensors.

Synergistic structures in lyotropic lamellar gels

In this work we present a systematic study on the microstructure of soft materials which combine the anisotropy of lyotropic liquid crystals with the mechanical stability of a physical gel. Systematic small-angle neutron (SANS) and X-ray (SAXS) scattering experiments were successfully used to characterize the lyotropic lamellar phase (L_α) of the system D₂O -n-decanol - SDS which was gelled by two low molecular weight organogelators, 1,3:2,4-dibenzylidene-d-sorbitol (DBS) and 12-hydroxyoctadecanoic acid (12-HOA). Surprisingly, a pronounced shoulder appeared in the scattering curves of the lamellar phase gelled with 12-HOA, whereas the curves of the DBS-gelled L_α phase remained almost unchanged compared to the ones of the gelator-free L_α phase. The appearance of this additional shoulder strongly indicates the formation of a synergistic structure, which neither exists in the gelator-free L_α phase nor in the isotropic binary gel. By comparing the thicknesses of the 12-HOA (25-30 nm) and DBS (4-8 nm) gel fibers with the lamellar repeat distance (7.5 nm), we suggest that the synergistic structure originates from the minimization of the elastic free energy of the lamellar phase. In the case of 12-HOA, where the fiber diameter is significantly larger than the lamellar repeat distance, energetically unfavored layer ends can be prevented, when the layers cylindrically enclose the gel fibers. Interestingly, such structures mimic similar schemes found in neural cells, where axons are surrounded by lamellar myelin sheets.

Effect of the Cationic Head Group on Cationic Surfactant-Based Surfactant Mediated Gelation (SMG)

The surfactant-mediated gelation (SMG) method allows us to formulate hydrogels using a water-insoluble organogelator. In this study, we formulated hydrogels using three cationic surfactants, hexadecyltrimethylammonium bromide (CTAB), hexadecyltrimethylammonium chloride (CTAC), and hexadecylpyridinium chloride (CPC)] and an organogelator (12-hydroxyoctadecanoic acid (12-HOA), and studied their structures and mechanical properties. A fiber-like structure similar to that found in the 12-HOA-based organogels was observed by optical microscopy. Small- and wide-angle X-ray scattering profiles showed Bragg peaks derived from the long- and short-spacing of the crystalline structures in the gel fibers and a correlation peak from the surfactant micelles in the small-angle region. Furthermore, the formation of micelles in the hydrogels was confirmed by UV-vis spectroscopic measurements of the gel samples in the presence of Rhodamine 6G. We concluded that the hydrogels prepared by the SMG method in the present systems are orthogonal molecular assembled systems in which two different molecular assembled structures coexist. Among the three surfactant systems, the CTAB system presented the lowest critical gelation concentration and highest sol-gel transition temperature and viscoelasticity. These differences in gel fiber formation and gel properties were discussed from the viewpoint of the degree of solubilization of the gelator molecules in micelles coexisting with gel fibers and diffusion of the gelator molecules in the gel formation process.

Self-assembled toron-like structures in inverse nematic gels

A novel form of nematic gel (N-gel) wherein bright flower-like domains (BFDs) rich in gelator fibres are embedded in a matrix of liquid crystal (LC) molecules has been reported. These gels which we denote as inverse N-gels are unlike typical N-gels in which the LC is encapsulated within an aggregated network of gelator molecules. The self-organization of the helical gelator fibres within the BFDs leads to the creation of localized toron-like structures that are topologically protected due to their skyrmion director profile. Optical and confocal microscopy have been used to deduce the LC director configuration, in order to understand possible intermolecular interactions that can lead to the formation of the twisted structures and the inverse N-gels.

How an organogelator can gelate water: gelation transfer from oil to water induced by a nanoemulsion

A hydrogel can be formed by an organogelator in the presence of a nanoemulsion. It is expected that this is due to a gelation transfer from oil to water. The system started with an oil-in-water nanoemulsion prepared according to a phase inversion temperature (PIT) process. Into this nanoemulsion consisting of Kolliphor® RH40 and Brij® L4 as surfactants, and Miglyol® 812 as oil and water, we introduced the organogelator 12-hydroxyoctadecanoic acid (12-HOA) in the oil phase. After cooling at room temperature, a slow reversible gelation of the water phase occurred with persistence of the nanoemulsion. This thermally reversible system was investigated using various techniques (rheology, turbidimetry, optical and electron microscopies, scattering techniques). Successive stages appeared during the cooling process after the nanoemulsion formation, corresponding to the migration and self-assembly of the organogelator from the oil nanodroplets to the water phase. According to our measurements and the known self-assembly of 12-HOA, a mechanism explaining the formation of the gelled nanoemulsion is proposed.

A new horizon for vibrational circular dichroism spectroscopy: a challenge for supramolecular chirality

The development of the solid state and time-step VCD methods opened a new horizon to reveal the mechanism of chirality amplification from microscopic to supramolecular scales.