Gels and lyotropic liquid crystals: using an imidazolium-based catanionic surfactant in binary solvents

Formation of asymmetric belt-like aggregates from a bio-based surfactant derived from dehydroabietic acid

The morphology and physicochemical properties of ordered molecular aggregates are closely related to surfactant molecules. Herein, a rosin-based amine oxide surfactant containing a large hydrophobic group (abbreviated R-10-AO) was synthesized from dehydroabietic acid, which is an important derivative of rosin. Cryogenic transmission electron microscopy (cryo-TEM) images and small-angle X-ray scattering (SAXS) showed that at a concentration of ∼5 mM, R-10-AO molecules formed flexible nanobelts with a thickness of only 2-3 nm. The width of these nanobelts was 50-150 nm and the length was more than 1 μm. The formation of the stable nanobelts arose from the strong van der Waals forces of the bulky hydrophobic portions of R-10-AO in solution, facilitating the stability of the asymmetrical aggregates. Rheological tests showed that the formed nanobelts were thermodynamically stable. The entanglement of these nanobelts led to significant viscoelasticity of the solutions. The zero-shear viscosity (η₀) of the R-10-AO solution reached 10 Pa s at a concentration of 5 mM, which is much greater than that of most wormlike micellar solutions. This work provides the inspirations of preparing aggregates with novel properties using natural products.

Influence of the Alkyl Chain Length of the Imidazole Ionic Liquid-Type Surfactants on Their Aggregation Behavior with Sodium Dodecyl Sulfate

The influence of the alkyl chain length of the ionic liquid surfactants 1-hexadecyl-3-alkyl imidazolium bromide [C₁₆imC_n]Br (n = 2-16) on their aggregation behavior with sodium dodecyl sulfate (SDS) in water was studied. The rheological properties, thermostability, and microstructure of the samples were characterized via a combination of rheology, cryo-transmission electron microscopy, polarization optical microscopy, and small-angle X-ray scattering. Upon the addition of SDS, the [C₁₆imC_n]Br (n = 2, 4, 6) rodlike micelles transit into the gels with high water content. The effects of molar ratio and alkyl chain length on the viscoelasticity and thermal stability of the SDS/[C₁₆imC_n]Br (n = 2, 4, 6) gels were studied. However, the [C₁₆imC_n]Br (n = 8, 10, 12, 14, 16) rodlike micelles precipitate with the addition of SDS. The [C₁₆imC_n]Br (n = 10, 12, 14, 16) gels transit to the rodlike micelles with the proper addition of SDS. The mechanism of the influence of the alkyl chain length of the [C₁₆imC_n]Br on their aggregation behavior with SDS was proposed.

Aqueous systems of a surface active ionic liquid having an aromatic anion: phase behavior, exfoliation of graphene flakes and its hydrogelation

Surface active ionic liquid (SAIL) induced hydrogelation, in the absence of additives, is important considering the properties of soft-hydrogels that can be utilized in different applications. The present study is concerned with the phase behavior and hydrogelation of a SAIL, 1-hexadecyl-3-methylimidazolium p-toluenesulfonate, [C16mim][PTS]. The obtained information about the phase behavior along with the surfactant like behavior of the SAIL was exploited for effective exfoliation of graphene-flakes from graphite in aqueous medium that remain stable for at least one month. Thus the obtained dispersion of graphene-flakes was subsequently hydrogelated exploiting the observations made from the phase behavior of the SAIL, via entanglement of long worm-like micelles of the SAIL formed at higher concentration. The obtained graphene-flake based hydrogels were found to be equally stable as compared to the blank hydrogel as well as against centrifugation. The low melting point of hydrogel facilitates the extraction of graphene-flakes from the hydrogel matrix by heating and diluting the gel and there is no sign of agglomeration in the extracted graphene-flakes even if the extraction is carried out after a period of three months. The present work is an exemplary study on exfoliation, hydrogelation and extraction of graphene-flakes from a hydrogel, when required, using a SAIL and is expected to provide a new platform for utilization of SAILs for efficient graphene exfoliation and subsequent preparation of functional materials.

Viscoelastic lyotropic liquid crystals formed in a bio-based trimeric surfactant system

Exploring the self-assembly of oligomeric surfactants is expected to bridge the gap between conventional and polymeric surfactants. Using the natural resource rosin as the starting material, a bio-based star-shaped trimeric quaternary ammonium surfactant (abbreviated tri-R-4-Phe) was synthesized. With three bulky dehydroabietic acid units in the hydrophobic group, tri-R-4-Phe has a molecular weight of 1684.9 and shows strong affinity towards both water and nonpolar organic compounds. In the presence of tri-R-4-Phe, C12EO3 was able to form lamellar lyotropic liquid crystals over a wide concentration range in water. The tri-R-4-Phe/C12EO3/water tertiary system was investigated by polarizing optical microscopy (POM), small angle X-ray scattering (SAXS) and rheological measurements. The investigated samples with different formulations all showed strong viscoelasticity, and the viscosity increased with the surfactant content. All samples showed interesting shear banding phenomena due to the shear induced mesoscale phase transition in tri-R-4-Phe/C12EO3/water systems. The present work reveals the unique behaviour of trimeric surfactant involved LLC systems and the result may be helpful in investigating delicate molecular self-assembly using natural resources.

Relaxation dynamics of Sierpinski hexagon fractal polymer: Exact analytical results in the Rouse-type approach and numerical results in the Zimm-type approach

In this paper, we focus on the relaxation dynamics of Sierpinski hexagon fractal polymer. The relaxation dynamics of this fractal polymer is investigated in the framework of the generalized Gaussian structure model using both Rouse and Zimm approaches. In the Rouse-type approach, by performing real-space renormalization transformations, we determine analytically the complete eigenvalue spectrum of the connectivity matrix. Based on the eigenvalues obtained through iterative algebraic relations we calculate the averaged monomer displacement and the mechanical relaxation moduli (storage modulus and loss modulus). The evaluation of the dynamical properties in the Rouse-type approach reveals that they obey scaling in the intermediate time/frequency domain. In the Zimm-type approach, which includes the hydrodynamic interactions, the relaxation quantities do not show scaling. The theoretical findings with respect to scaling in the intermediate domain of the relaxation quantities are well supported by experimental results.

Mechanically-sensitive hydrogels formed from β-cyclodextrin and an anionic surfactant containing a biphenyl group

Hydrogels are important soft materials with intriguing properties. By taking advantage of the host-guest interactions and multiple molecular interactions, it is expected that novel hydrogel systems can be formed. This strategy has been implemented here, transparent hydrogels were formed by using a newly-synthesized anionic surfactant, sodium 2-(4-phenylphenoxy)dodecanoate (C12biphNa), and β-cyclodextrin (β-CD) in different proportions and their properties were investigated. Gelation of water occurs at extremely low surfactant concentrations (5 mM for a 1:3 C12biphNa: β-CD system), and a single C12biphNa with its associated β-CDs can trap about 11,000 water molecules on average. In addition, the systems are fragile to mechanical stimulus and thus show mechanical sensitivity. Cryo-TEM reveals that the hydrogels have a microstructure consisting of rigid nanowire-like aggregates (with cross-sectional diameters of about 7-8 nm) locally distributed in a parallel manner in solution. These microstructural features are responsible for the peculiar properties of the hydrogel systems presented. The inclusion complexes formed by C12biphNa and β-CD were investigated using (1)H NMR and 2D nuclear overhauser effect spectroscopy and their aggregation state was proposed. This work enriches the connotation of nonamphiphilic self-assembly and provides inspiration for constructing new functional soft materials.

Formation of large nanodomains in liquid solutions near the phase boundary

Large nanodomains were formed in liquid solutions near the phase separation point where the size of nanodomains increased dramatically.

A new strategy to prepare giant vesicles from surface active ionic liquids (SAILs): a study of protein dynamics in a crowded environment using a fluorescence correlation spectroscopic technique

A simple procedure for the preparation of giant vesicles using surface active ionic liquids (SAILs) has been provided in this paper.

Aggregation Behavior of Imidazolium-Based Surface-Active Ionic Liquids with Photoresponsive Cinnamate Counterions in the Aqueous Solution

Two imidazolium-based surface active ionic liquids (SAILs) with photoresponsive cinnamate aromatic counterions, viz. 1-dodecyl-3-methylimidazolium cinnamate ([C12mim][CA]) and 1-dodecyl-3-methylimidazolium para-hydroxy-cinnamate ([C12mim][PCA]), were newly synthesized, and their self-assembly behaviors in aqueous solutions were systematically explored. Results of surface tension and conductivity measurements show that both [C12mim][CA] and [C12mim][PCA] display a superior surface activity in aqueous solutions compared to the common imidazolium-based SAIL, 1-dodecyl-3-methylimidazolium bromide (C12mimBr), which implies the incorporation of cinnamate aromatic counterions can promote the micellar formation. Furthermore, [C12mim][CA] shows higher surface activity due to the higher hydrophobicity of its counterion in comparison to [C12mim][PCA] that has a hydroxyl group. Both hexagonal liquid-crystalline phase (H1) and cubic liquid-crystalline phase (V2) were constructed in the [C12mim][CA] aqueous solutions. In contrast, the [C12mim][PCA]/H2O system only exhibits a single hexagonal liquid-crystalline phase (H1) in a broad concentration region. These lyotropic liquid crystal (LLC) phases were comprehensively characterized by polarized optical microscopy (POM), small-angle X-ray scattering (SAXS), and rheometer. Investigation on the temperature-dependent self-assembly nanostructures demonstrates that the higher temperature leads to a looser arrangement. Under UV irradiation, trans-cis photoisomerization of the phenylalkene group results in inferior surface activity of the prepared SAILs in aqueous solution with higher cmc values. Moreover, UV light irradiation induces obvious change of the structural parameters without altering the LLC phases. This work is expected to enrich the investigations of phase behaviors formed in SAILs systems and receive particular attention due to their unique properties and potential applications in drug delivery, biochemistry, materials science, etc.

Solvent and Substituent Effects on the Aggregation Behavior of Surface-Active Ionic Liquids with Aromatic Counterions and the Dispersion of Carbon Nanotubes in their Hexagonal Liquid Crystalline Phase

The aggregation behavior of surface-active ionic liquids (SAILs) 1-dodecyl-3-methylimidazolium m- and p-hydroxybenzoate (m-C12mimHB and p-C12mimHB) in water and ethylammonium nitrate (EAN) was investigated. Surface tension measurements indicate that the cmc values of SAILs in EAN are much higher than those in water, resulting from the weaker solvophobic effect of EAN, and the stronger stability of SAILs/EAN complexes proven by DFT calculations. Compared to 1-dodecyl-3-methylimidazolium salicylate (C12mimSal), the effect of substituent position leads to weaker interactions between aromatic counterions and headgroups. The hexagonal liquid crystal (H1) phase formed by C12mimHB in water or EAN at a higher concentration was determined by polarized optical microscopy (POM), small-angle X-ray scattering (SAXS), and rheology techniques. Structural parameters estimated from SAXS curves suggest that the higher SAILs concentration or temperature leads to a smaller lattice parameter (a0) and a denser arrangement of cylinders. For C12mimHB, the formation of the H1 phase in H2O is easier than that in EAN. Furthermore, compared to C12mimSal, C12mimHB exists over a broad region of the hexagonal liquid crystalline (H1) phase, which is due to the different position of the substituents on the aromatic ring of counterions. Therefore, the H1 phase of the lypotropic liquid crystals (LLCs) formed in the C12mimHB/H2O system exhibits excellent performance in uniformly dispersing multiwalled carbon nanotubes (MWCNTs). Increasing the concentration of MWCNTs results in a larger lattice parameter (a0) value, indicating the integration of MWCNTs within the cylinders of the H1 phase. The rheological measurement results demonstrate that MWCNTs/LLCs composites are highly viscoelastic, and the presence of MWCNTs obviously strengthens the apparent viscosity of the H1 phase.

Self-Assembly and Rheological Properties of a Pseudogemini Surfactant Formed in a Salt-Free Catanionic Surfactant Mixture in Water

The surface and bulk properties of bola-type dicarboxylic acid (sebacic acid, SA) and zwitterionic surfactant tetradecyldimethylamine oxide (C14DMAO) mixtures in aqueous solutions were studied. Surface tension measurements indicate a pronounced synergistic effect between SA and C14DMAO. In bulk aqueous solutions, rich phase behavior was observed with a varied SA-to-C14DMAO ratio (ρ) and a total surfactant concentration. Typically at ρ = 0.5, a novel pseudogemini surfactant (C14-S-C14) forms, driven by electrostatic interaction and hydrogen bonding. The C14-S-C14/H2O system exhibits rich phase behavior induced by the transition of aggregates. With increasing concentration of C14-S-C14, one can observe a viscous L1 phase, an L1/Lα two-phase region where a birefringent Lα phase is on the top of an L1 phase, a single Lα phase, and finally a mixture of an Lα phase and a precipitate. Microstructures formed in the Lα phases were determined by freeze-fracture transmission electron microscopy (FF-TEM) and cryogenic-transmission electron microscopy (cryo-TEM) observations. Polymorphic aggregation behavior was observed with the formation of a variety of bilayer structures including unilamellar vesicles, onions, and open and hyperbranched bilayers. Rheological measurements showed that the Lα phases are viscoelastic and sensitive to temperature where a quick loss of viscoelasticity was observed at elevated temperature.

Efficient and selective removal of dyes using imidazolium-based supramolecular gels

A supramolecular gel was constructed by using an imidazolium-based surfactant, N-cetyl-N'-carboxymethyl imidazolium bromide ([N-C16, N'-CO2H-Im]Br), in the DMSO/H2O binary solvent mixtures and investigated as an adsorbent for removing dyes from aqueous solution. The self-assembled gel displays a morphology of microplatelets stacked in bilayer units with interdigitated hydrocarbon tails, and the structure remains unchanged below the sol-gel transition temperature. The gel also exhibits a strong birefringence property and excellent mechanical strength. In particular, the gels show superior performance in removal of anionic dye molecules, for example, removing 80% of eosin Y within 10 min, The constructed gels also present excellent salinity tolerance, even when the concentration of NaCl is 1000 times higher than that of the dye, and can maintain their high efficiency after 25 cycles, indicative of their promise in water treatment.

Wormlike micelles with photoresponsive viscoelastic behavior formed by surface active ionic liquid/azobenzene derivative mixed solution

The UV-light-stimulated self-assembly behavior of a surface active ionic liquid (SAIL), 1-hexadecyl-3-methylimidazolium bromide (C16mimBr), with an azobenzene derivative, sodium azobenzene 4-carboxylate (AzoCOONa), was investigated in aqueous solution. The properties and structures of the aggregates, formed at a concentration ratio equal to 2:1 ([C16mimBr]:[AzoCOONa]), were comprehensively characterized by rheometer and cryogenic transmission electron microscopy. Initially, viscoelastic wormlike micelles with a viscosity of 0.65 Pa·s were constructed in the C16mimBr/AzoCOONa system. Upon irradiation by UV light (365 nm), particularly fascinating is that the wormlike micelles become much longer and more entangled, exhibiting a high viscosity of 6.9 Pa·s. This can be attributed to photoisomerization of the AzoCOONa molecule from trans to cis form. It is the first time that, with exposure to UV or visible light, the aggregate type of the photoresponsive system has remained unchanged, with only a change of internal property parameters. The cation-π interaction prevailing over the hydrophobic interaction and electrostatic interaction between C16mimBr and AzoCOONa molecules is supposed to be responsible for this peculiar phase behavior. The wormlike micelles constructed with the SAIL and photosensitive additive exhibit controllable viscoelastic behavior in the photoresponsive process. In addition, the average contour length of wormlike micelles was found to slightly decrease with the increase of temperature. We expect this system will receive particular attention due to its unique properties and potential applications in drug delivery, biochemistry, and materials science, etc.

Experimental and DFT studies on the aggregation behavior of imidazolium-based surface-active ionic liquids with aromatic counterions in aqueous solution

Two imidazolium-based surface-active ionic liquids with aromatic counterions, namely, 1-dodecyl-3-methylimidazolium salicylate (C12mimSal) and 1-dodecyl-3-methylimidazolium 3-hydroxy-2-naphthoate (C12mimHNC), were synthesized, and their aggregate behavior in aqueous solutions was systematically explored. Surface tension and conductivity measurements indicate that both C12mimSal and C12mimHNC show superior surface activity compared to the common imidazolium-based SAIL with the same hydrocarbon chain length, 1-dodecyl-3-methylimidazolium bromide (C12mimBr). This result demonstrates that the incorporation of aromatic counterions favors the formation of micelles. C12mimHNC displays a higher surface activity than C12mimSal, resulting from the different hydrophobicities of the counterions. In comparison with C12mimBr, C12mimSal not only can form hexagonal liquid-crystalline phase (H1) in aqueous solution, but also exhibits a broad region of cubic liquid-crystalline phase (V2) at higher concentration. As for the C12mimHNC/H2O system, a lamellar liquid-crystalline (L(α)) phase was observed. These lyotropic liquid crystals (LLCs) were characterized by polarized optical microscopy (POM) and small-angle X-ray scattering (SAXS). Structural parameters calculated from SAXS patterns suggest that a higher concentration of the SAIL leads to a denser arrangement whereas a higher temperature results in the opposite effect. The rheological results manifest that the formed H1 phase in the C12mimSal/H2O system exhibits an impressive viscoelastic behavior, indicated by a modulus (G' and G″) that is 1 order of magnitude higher than that of C12mimBr. Density functional theory (DFT) calculations reveal that C12mimSal has a more negative interaction energy with a water molecule and the Sal(-) counterion presents a stronger electronegativity than the HNC(-) counterion. The specific phase behavior of the C12mimSal/H2O and C12mimHNC/H2O systems can be attributed to the strong synergic interaction between the imidazolium cation and the aromatic counterion, including electrostatic attraction, hydrophobic interaction, and especially π-π interaction.

pH sensitive smart gels of cetylpyridinium chloride in binary solvent mixtures: phase behaviour, structure and composition

Figure showing the effect of pH on CPC gel formation at 25 °C and fluorescence emission spectra of CPC solutions at pH ∼ 11.8.

Incorporation of graphene oxide into C12E4/C12mimBr hybrid lyotropic liquid crystal and its thermo-sensitive properties

Schematic illustration of the formation of GO/C₁₂E₄/C₁₂mimBr lamellar and hexagonal LLC composites.

Aggregation behavior of long-chain piperidinium ionic liquids in ethylammonium nitrate

Micelles formed by the long-chain piperidinium ionic liquids (ILs) N-alkyl-N-methylpiperidinium bromide of general formula CnPDB (n = 12, 14, 16) in ethylammonium nitrate (EAN) were investigated through surface tension and dissipative particle dynamics (DPD) simulations. Through surface tension measurements, the critical micelle concentration (cmc), the effectiveness of surface tension reduction (Πcmc), the maximum excess surface concentration (Гmax) and the minimum area occupied per surfactant molecule (Amin) can be obtained. A series of thermodynamic parameters (DG0 m, DH0 m and DS0 m) of micellization can be calculated and the results showed that the micellization was entropy-driven. In addition, the DPD simulation was performed to simulate the whole aggregation process behavior to better reveal the micelle formation process.