Room-Temperature Super Hydrogel as Dye Adsorption Agent

Supramolecular hydrogels were prepared in the mixtures of a chiral amphiphilic lithocholic acid (LCA) and a nonionic surfactant, dodecyldimethylamine oxide (C₁₂DMAO), in water. With the addition of LCA to C₁₂DMAO micellar solutions, a transition from micelles to gels occurs at room temperature. Hydrogels can form at very low concentrations (below 0.1 wt %), exhibiting a super gelation capability. The rheological measurements show a strong mechanical strength with an elastic modulus exceeding 5000 Pa and a yield stress exceeding 100 Pa. Microstructures determined by TEM, SEM, and AFM observations demonstrate that the gels are formed by intertwined helical fibrils. The formation of fibrils is induced by enormous cycles of units composed of two LCA molecules and four C₁₂DMAO molecules driven by comprehensive noncovalent interaction, especially the hydrogen bonds produced in two reversed LCA molecules and the C₁₂DMAOH⁺–C₁₂DMAO pairs. The xerogels show excellent adsorption capability of the toxic dye with a maximum adsorption value of 202 mg·g^–1

Effect of Cationic Surfactants with Different Counterions on the Growth of Au Nanoclusters

The influence of a series of cationic surfactants composed of cetyltrimethylammonium cations with different counterions (Br^–, Cl^–, OH^–, C₇H₈O₃S^–, [CeCl₃Br]⁻, and NO₃^–) on the aging process of gold nanoclusters (Au NCs) was studied. The finely different points of Au NCs treated by different surfactants were demonstrated by UV–vis and fluorescence spectra, transmission electron microscopy images, etc. Because of the difference of counterions, these surfactants have diverse physicochemical properties in surface activity, specific conductivity, pH, and viscosity, which may account for the difference of Au NCs in the aging process. In addition, the affinity of the counterions in surfactants to the surface of Au has also been demonstrated completely. This affinity may further guide the difference of the synthesized Au nanomaterials

A Systematic Investigation and Insight into the Formation Mechanism of Bilayers of Fatty Acid/Soap Mixtures in Aqueous Solutions

Vesicles are the most common form of bilayer structures in fatty acid/soap mixtures in aqueous solutions; however, a peculiar bilayer structure called a “planar sheet” was found for the first time in the mixtures. In the past few decades, considerable research has focused on the formation theory of bilayers in fatty acid/soap mixtures. The hydrogen bond theory has been widely accepted by scientists to explain the formation of bilayers. However, except for the hydrogen bond, no other driving forces were proposed systematically. In this work, three kinds of weak interactions were investigated in detail, which could perfectly demonstrate the formation mechanism of bilayer structures in the fatty acid/soap mixtures in aqueous solutions. (i) The influence of hydrophobic interaction was detected by changing the chain length of fatty acid (C_<i>n</i>H_2<i>n</i>+1COOH), in which <i>n</i> = 10 to 18, the phase behavior was investigated, and the phase region was presented. With the help of cryogenic transmission electron microscopy (cryo-TEM) observations, deuterium nuclear magnetic resonance (²H NMR), and X-ray diffraction (XRD) measurements, the vesicles and planar sheets were determined. The chain length of C_<i>n</i>H_2<i>n</i>+1COOH has an important effect on the physical state of the hydrophobic chain, resulting in an obvious difference in the viscoelasticity of the solution samples. (ii) The existence of hydrogen bonds between fatty acids and their soaps in aqueous solutions was demonstrated by Fourier transform infrared (FT-IR) spectroscopy and molecule dynamical simulation. From the pH measurements, the pH ranges of the bilayer formation were at the p<i>K</i>_a values of fatty acids, respectively. (iii) Counterions can be embedded in the stern layer of the bilayers and screen the electrostatic repulsion between the COO^– anionic headgroups. FT-IR characterization demonstrated a bidentate bridging coordination mode between counterions and carboxylates. The conductivity measurements provided the degree of counterion binding (β = 0.854), indicating the importance of the counterions

Hydrogels of Superlong Helices to Synthesize Hybrid Ag-Helical Nanomaterials

The gelation behavior of mixtures of sodium deoxycholate (NaDC) and glutathione (GSH) in water is investigated. The system exhibits a structural transition of self-assembled hydrogels from nanofibers to nanohelix structures, and then to helical ribbons with increasing GSH concentration. Superlong helical nanofibers with left- and right-handed orientations are produced by tuning the concentration of GSH at a fixed concentration of NaDC. Random coil and β-sheet structures are significant for the formation of the helical structures, and are indicated by circular dichroism (CD) and Fourier transform infrared (FT-IR) spectra. The mechanical strength of the “weak” hydrogels is enhanced by the introduction of appropriate suitable amount of AgNO₃. Furthermore, the controlled growth of Ag nanoparticles at spatially arranged locations along the nanohelices (hybrid Ag-helical nanomaterial) is readily achieved by UV reduction of Ag (I) ions on the supramolecular helical templates

Transition of Phase Structures in Mixtures of Lysine and Fatty Acids

Aggregation behaviors of the mixtures of lysine and fatty acids (FAs) with different chain lengths in aqueous solutions were investigated, and the self-assembled structural transition was determined in detail. Aggregates including micelles, vesicles, sponge structures, and fibers were observed by varying the compositions and the chain length of fatty acids. The sponge phase found in mixtures of octanoic acid and lysine was determined by freeze fracture-transmission electron microscope (FF-TEM). Circular dichroism (CD) signals were detected in the self-assembled structures due to the chirality of lysine molecules. The rheological properties of samples consisting of different aggregates formed by mixtures of lysine and fatty acids were measured, which provided the controlling factor of the chain length. The combined effect of noncovalent interactions including electrostatic interactions, hydrogen bonding, and hydrophobicity is supposed to be responsible for the aggregation behaviors, in which the hydrogen bonding acts as the main driving force in the self-assembled process

Hydrogelation and Crystallization of Sodium Deoxycholate Controlled by Organic Acids

The gelation and crystallization behavior of a biological surfactant, sodium deoxycholate (NaDC), mixed with l-taric acid (L-TA) in water is described in detail. With the variation of molar ratio of L-TA to NaDC (<i>r</i> = <i>n</i>_L‑TA/<i>n</i>_NaDC) and total concentration of the mixtures, the transition from sol to gel was observed. SEM images showed that the density of nanofibers gradually increases over the sol–gel transition. The microstructures of the hydrogels are three-dimensional networks of densely packed nanofibers with lengths extending to several micrometers. One week after preparation, regular crystallized nanospheres formed along the length of the nanofibers, and it was typical among the transparent hydrogels induced by organic acids with p<i>K</i>_a₁ value <3.4. Small-angle X-ray diffraction demonstrated differences in the molecular packing between transparent and turbid gels, indicating a variable hydrogen bond mode between NaDC molecules

Hydrogels Facilitated by Monovalent Cations and Their Use as Efficient Dye Adsorbents

Gelation behavior of lithocholate (LC^–) mixed with different monovalent cations in water was detected. The hydrogels consisting of tubular networks were formed by introducing alkali metal ions and NH₄⁺ to lithocholate aqueous solutions at room temperature. The formation of tubular structures was considered to be mainly driven by the electrostatic interaction with the assistance of a delicate balance of multiple noncovalent interactions. It is interesting that the increase in temperature can induce a significant enhancement in strength of the hydrogels, accompanied by the formation of bundles of tubules and larger size aggregates. The mechanism of the temperature-induced transition can be explained by the “salting-out” effect and the electric double layer model. The hydrogels showed very high adsorption efficiency and adsorption capability for the cationic dyes and were promising to act as toxic substance adsorbents

Fluorescent Hydrogels with Tunable Nanostructure and Viscoelasticity for Formaldehyde Removal

Hydrogels with ultrahigh water content, ∼99 wt %, and highly excellent mechanical strength were prepared by 4′-<i>para</i>-phenylcarboxyl-2,2′:6′,2″-terpyridine (PPCT) in KOH aqueous solution. The self-assembled structure, rheological properties, and the gel–sol transformation temperature (<i>T</i>_gel–sol) of PPCT/KOH hydrogels that depend on PPCT and KOH concentrations were studied, indicating easily controllable conditions for producing hydrogels in PPCT and KOH mixtures. An important finding was that the hydration radius (<i>R</i>_h) of cations (M⁺ = Li⁺, Na⁺, K⁺, Cs⁺, NH₄⁺, (CH₃)₄N⁺, (CH₃CH₂)₄N⁺, (CH₃CH₂CH₂)₄N⁺, (CH₃CH₂CH₂CH₂)₄N⁺) plays a vital role in gelation of PPCT/MOH systems. To produce hydrogels in PPCT/MOH systems, the <i>R</i>_h of M⁺ must be in a suitable region of 3.29 to 3.58 Å, e.g., K⁺, Na⁺, Cs⁺, and the capability of M⁺ for inducing PPCT to form hydrogels is K⁺ > Na⁺ > Li⁺, which is followed by the Hofmeister series. The hydrogels of PPCT and KOH mixtures are responsive to external stimuli including temperature and shearing force, and present gelation-induced enhanced fluorescence emission property. The states of being sensitive to the stimuli can readily recover to the original hydrogels, which are envisaged to be an attracting candidate to produce self-healing materials. A typical function of the hydrogels of PPCT and KOH mixtures is that formaldehyde (HCHO) can speedily be adsorbed via electrostatic interaction and converted into nontoxic salts (HCOOK and CH₃OK), making it a promising candidate material for HCHO removal in home furnishings to reduce indoor environmental pollutants

Influence of Counterions on Lauric Acid Vesicles and Theoretical Consideration of Vesicle Stability

The counterions, including inorganic cations, Na⁺ and Cs⁺, and organic cation, (C₂H₅)₄N⁺, influence the phase behavior and self-assembled structures of the lauric acid (LA) in water. Dissolving LA in NaOH, CsOH, and (C₂H₅)₄NOH (tetraethylammonium hydroxide, TeAOH) solutions, respectively, we observed that the three systems totally exhibited the same phase behavior, from birefringent L_α phase/precipitates (P) → L_α phase → L_α phase/L₁ (micelles) → L₁. The temperature influence on phase behavior was investigated, and with an increase of temperature, we observed that less phase behavior change occurred in the systems of LA/CsOH/H₂O and LA/TeAOH/H₂O, while the phase behavior of the LA/NaOH/H₂O system exhibited an obvious change. Cryogenic transmission electron microscopy (cryo-TEM) images demonstrated that the different microstructures of L_α phase samples in the three systems existed. For the systems of LA/NaOH/H₂O and LA/TeAOH/H₂O, uni- and multilamellar vesicles coexist for L_α phase samples, as both the morphology and size of these vesicles are polydisperse. The curvatures of the bilayer membranes of the two systems are considered to vary from positive, zero, and even negative. However, only spherically unilamellar vesicles exist in the system of LA/CsOH/H₂O, indicating that the bilayers are more rigid than those in the LA/NaOH/H₂O and LA/TeAOH/H₂O systems. Through the combination of the Helfrich curvature energy theory and the mass-action model, the effective bending constant <i>K</i> = 0.5 <i>k</i>_B<i>T</i> in the LA/CsOH/H₂O system was obtained, demonstrating that the unilamellar vesicles are stabilized by thermal fluctuations. A primary discussion for the effect of the nature of counterions on the stability and deformation of the vesicles is presented

Balance of Coordination and Hydrophobic Interaction in the Formation of Bilayers in Metal-Coordinated Surfactant Mixtures

Metal–ligand coordination and hydrophobic interaction are two significant driving forces in the aggregation of mixtures of M^<i>n</i>+ surfactants and alkyldimethylamine oxide (C_<i>n</i>DMAO) in aqueous solutions. The coordinated systems exhibit rich aggregation behavior. This study investigated the effect of M^<i>n</i>+ ions (Zn²⁺, Ca²⁺, Ba²⁺, Al³⁺, Fe³⁺, La³⁺, Eu³⁺, and Tb³⁺) and hydrophobic chains (hydrocarbon and fluorocarbon) on the formation of metal-coordinated bilayers. We found that fluorocarbon chains and branched hydrocarbon chains are preferable to the corresponding linear hydrocarbon chains for the formation of an L_α phase. Moreover, L_α phases formed by fluorocarbon chains exhibited higher viscoelasticity than ones formed by the hydrocarbons, and the bilayers formed by branched chains were rather flexible, revealing obvious undulation. The construction of bilayers was also strongly affected by metal ions due to their variable coordination ability with C_<i>n</i>DMAO. Our results contribute to the understanding of the formation of metal-coordinated bilayers, which is driven by the interplay of noncovalent forces