Stereochemistry-dependent thermotropic liquid crystalline phases of monosaccharide-based amphiphiles

Conformational rigidity controls the bulk self-assembly and liquid crystallinity from amphiphilic block molecules to copolymers. The effects of block stereochemistry on the self-assembly have, however, been less explored. Here, we have investigated amphiphilic block molecules involving eight open-chain monosaccharide-based polyol units possessing different stereochemistries, derived from D-glucose, D-galactose, L-arabinose, D-mannose and L-rhamnose (allylated monosaccharides t-Glc*, e-Glc*, t-Gal*, e-Gal*, t-Ara*, e-Ara*, t-Man*, and t-Rha*), end-functionalized with repulsive tetradecyl alkyl chain blocks to form well-defined amphiphiles with block molecule structures. All compounds studied showed low temperature crystalline phases due to polyol crystallization, and smectic (lamellar) and isotropic phases upon heating in bulk. Hexagonal cylindrical phase was additionally observed for the composition involving t-Man*. Cubic phases were observed for e-Glc*, e-Gal*, e-Ara*, and t-Rha* derived compounds. Therein, the rich array of WAXS-reflections suggested that the crystalline polyol domains are not ultra-confined in spheres as in classic cubic phases but instead show network-like phase continuity, which is rare in bulk liquid crystals. Importantly, the transition temperatures of the self-assemblies were observed to depend strongly on the polyol stereochemistry. The findings underpin that the stereochemistry in carbohydrate-based assemblies involves complexity, which is an important parameter to be considered in material design when developing self-assemblies for different functions.

Heating-Induced Switching to Hierarchical Liquid Crystallinity Combining Colloidal and Molecular Order in Zwitterionic Molecules

Hierarchical self-assemblies of soft matter involving triggerable or switchable structures at different length scales have been pursued toward multifunctional behaviors and complexity inspired by biological matter. They require several and balanced competing attractive and repulsive interactions, which provide a grand challenge in particular in the "bulk" state, i.e., in the absence of plasticizing solvents. Here, we disclose that zwitterionic bis-n-tetradecylphosphobetaine, as a model compound, shows a complex thermally switchable hierarchical self-assembly in the solvent-free state. It shows polymorphism and heating-induced reversible switching from low-temperature molecular-level assemblies to high-temperature hierarchical self-assemblies, unexpectedly combining colloidal and molecular self-assemblies, as inferred by synchrotron small-angle X-ray scattering (SAXS). The high-temperature phase sustains birefringent flow, indicating a new type of hierarchical thermotropic liquid crystallinity. The high-temperature colloidal-level SAXS reflections suggest indexation as a 2D oblique pattern and their well-defined layer separation in the perpendicular direction. We suggest that the colloidal self-assembled motifs are 2D nanoplatelets formed by the lateral packing of the molecules, where the molecular packing frustration between the tightly packed zwitterionic moieties and the coiled alkyl chains demanding more space limits the lateral platelet growth controlled by the alkyl stretching entropy. An indirect proof is provided by the addition of plasticizing ionic liquids, which relieve the ionic dense packings of zwitterions, thus allowing purely smectic liquid crystallinity without the colloidal level order. Thus, molecules with a simple chemical structure can lead to structural hierarchy and tunable complexity in the solvent-free state by balancing the competing long-range electrostatics and short-range nanosegregations.

Metal ion-triggered Pickering emulsions and foams for efficient metal ion extraction

Emulsions and foams were constructed by using surfactant particles as stabilizers. Bis(2-ethylhexyl) phosphate, abbreviated as HDEHP, was used as both an oil in neutral form and an anionic surfactant in deprotonated form, DEHP^-. In the system of HDEHP/H₂O, upon adding NaOH, a portion of HDEHP was deprotonated to form DEHP^- as stabilizers of O/W emulsions. After introducing some certain metal ions, the O/W emulsions were transformed to W/O Pickering emulsions due to the generation of insoluble particles by DEHP^- and metal ions. In addition, DEHP^- could also combine with some metal ions to produce particles absorbed at air/water interface, forming ultrastable foams. Accompanied with the formation of Pickering emulsions and foams, the extraction of metal ions from water could be realized with high removal efficiency. The extractant, HDEHP, could be effectively recycled through convenient demulsification of Pickering emulsions or destruction of foams. This work provides new ideas for the construction of particle-stabilized dispersion systems and proposes methods with potential applications in industrial wastewater treatments.

Self-Assembly of Amphiphilic Copper Nanoclusters Driven by Cationic Surfactants

Amphiphilicity is an excellent physicochemical property, which is yet to be explored from traditional surfactants to nanoparticles. This article shows that the amphiphilicity of copper nanoclusters (CuNCs) can be readily tuned by electrostatic interactions with cationic surfactants and cetyltrimethylammonium cations (CTA⁺) with counterions Br^-, Cl^-, and C₇H₈O₃S^-. Due to the role of surface ligands, the complexes of glutathione-capped CuNCs (GSH-CuNCs) and the surfactants exhibit good amphiphilicity, which enables them to self-assemble like a molecular amphiphile. This could significantly increase the utility of metal nanoclusters in basic and applied research. As the concentration of the surfactant changes, the aggregates change from nanoparticles to network-like structures. After the formation of supramolecular self-assemblies by hydrophobic interactions, the enhancement of fluorescence intensity was observed, which can be ascribed to the suppression of intramolecular vibrations based on aggregation-induced emission (AIE) and combined with the compactness of GSH-CuNCs in self-assemblies. Our study provides a facile way to generate solid fluorescent materials with excellent fluorescence performance, which may find applications in light-emitting diodes (LEDs).

Functionalized Ionic Liquid-Crystal Additive for Perovskite Solar Cells with High Efficiency and Excellent Moisture Stability

Organic-inorganic hybrid perovskite solar cells (PSCs) have emerged as a promising candidate for next-generation solar cells. However, the limited stability of PSCs hampers their practical applications. In this work, for the first time, a functionalized π-conjugated ionic liquid crystal (ILC), 4'-(N,N,N-trimethyl ammonium bromide hexyloxy)-4-cyanobiphenyl (6CNBP-N), is developed as a novel chemical additive to obtain CH₃NH₃PbI₃ (MAPbI₃) PSCs with high efficiency and excellent moisture stability. This 6CNBP-N ILC possesses the characteristics of ionic liquids and liquid crystals. The inclusion of the 6CNBP-N ILC can effectively improve the quality and stability of perovskite films, reduce the trap-state densities, and promote the carrier transport induced by the cyano group (C≡N), a rod-like π-conjugated biphenyl mesogenic unit and quaternary alkylammonium cations (R₄N⁺) in 6CNBP-N. Through this functionalized ILC engineering strategy, the power conversion efficiency (PCE) of PSCs is greatly increased from 18.07% for the control PSC to 20.45% for the PSC with 6CNBP-N along with the depressed hysteresis effect and enhanced moisture stability of PSCs. Our work provides a new strategy for designing functionalized additives for high-performance PSCs.

Dendrimer-mediated columnar mesophase of surfactants

The columnar mesophase, in which the molecular or supramolecular building blocks with rod-like geometry pack into two-dimensional (2D) lattices, is an important class of mesomorphic structure having been found in various liquid crystalline materials for practical applications. The cylindrical micelles assembled by amphiphilic surfactants may also form columnar mesophases with the micelle packing symmetry being tunable by the molecular characteristics of the surfactants. In this study, we demonstrate that a positively charged tree-like polymer, poly(amidoamine) (PAMAM) G4 dendrimer, acted as an effective structure-directing agent for the columnar mesophase of a common anionic surfactant, sodium dodecyl sulfate (SDS), via their electrostatic interaction. By adjusting the dendrimer charge density and the nominal binding ratio (Xn) of SDS to dendrimer, the electrostatic complexes self-assembled to form a body-centered cubic (BCC) sphere phase, wherein the dendrimers were staggered between the interspaces of the SDS spherical micelles packed in the BCC lattice. Four types of 2D columnar mesophase composed of SDS cylindrical micelles and dendrimers were accommodated within the interstitial tunnels, including the hexagonal columnar phase (Colhex), simple rectangular columnar phase (Colsr), oblique columnar phase (Colob) and centered rectangular columnar phase (Colcr). A detailed analysis of the geometry of the dendrimer in the columnar mesophases revealed that the structural transition was governed by the interplay among the lateral and axial deformations of the dendrimer, and the deformation of the SDS micelle cross section for achieving effective charge matching and accommodation of the dendrimer. The present study demonstrated the power of the dendrimer in directing the long-range ordered packing of oppositely charged cylinders to yield a rich structural polymorphism of the columnar mesophase that may be exploited for the development of functional materials.

Water Solubility and Surface Activity of Alkoxyethyl β-d-Maltosides

Green surfactants alkyl glycosides are key to solve the inherent problem of water solubility due to their commercial application and extensive scientific research. Based on the enhancement strategy of hydrophilicity through the reconstruction of the conventional alkyl β-d-maltoside by introducing an oxyethyl group (-OCH₂CH₂-), d-maltose was used to prepare a series of nonionic disaccharide-based surfactants alkoxyethyl β-d-maltosides (4a-h, n = 6-16) so that the related water solubility was effectively improved, while the corresponding surface activity and other excellent properties were still maintained. Their physicochemical properties, including water solubility, surface activity, moisture absorption, and thermotropic liquid crystalline behavior, were investigated. The liquid crystal texture of alkoxyethyl β-d-maltosides (n = 7-16) has a fan-shaped focal conic texture. Furthermore, decoxyethyl β-d-maltoside had the strongest foaming characteristic and the best foam stability. Moreover, dodecoxyethyl β-d-maltoside (4f, n =12) had stronger emulsifying activity in the rapeseed oil/water system. Finally, CTAC/4f binary surfactants had an obvious synergistic effect. Such β-d-maltosides should have good application prospects in the future.

DNA thermotropic liquid crystals controlled by positively charged catanionic bilayer vesicles

We report DNA thermotropic liquid crystal (TLC) formation by positively charged catanionic surfactant bilayer vesicles. The properties of DNA TLCs were found to be manipulated by both the chemical structures of cationic and anionic surfactants and the DNA amount. Positively charged catanionic bilayer vesicles bond to negative DNA sites resulting in the transition from vesicles to long range ordered lamellar crystals of DNA-catanionic surfactants, as confirmed by cryo- and freeze-fracture (FF) TEM observations and small-angle X-ray scattering (SAXS) measurements.

Magnetic polymerizable surfactants: thermotropic liquid crystal behaviors and construction of nanostructured films

Two polymerizable surfactants, 3-undecylene-1-vinylimidazolium bromide (C11VIMBr) and 3-dodecyl-1-vinylimidazolium bromide (C12VIMBr), were chosen to prepare magnetic surfactant monomers by introducing Mn2+, Gd3+ and Ho3+.