Hydrogen-bonding Associations Research Articles

Synergistic adsorption mechanism of novel combined collector in flotation separation of fayalite from magnetite: Experiments and MD simulations

As an iron-bearing silicate mineral, fayalite has similar surface properties to magnetite, which makes it challenging to efficiently separate fayalite from magnetite by flotation. In this study, magnetite and fayalite were separated using novel combined collector polyoxyethylene alkyl amine (PAE) and terpinol (AT) through reverse flotation. Combined with a variety of characterization analysis and molecular dynamics (MD) simulation, the synergistic adsorption mechanism of the combined collector on mineral surface was revealed from multiple perspectives. The optimum scheme and pH response range of the combined collector were determined by micro-flotation test. Compared with PAE alone, the difference value of flotation recovery between fayalite and magnetite using combined collector was increased by 10.16%, and the dosage was reduced by 20%. Additionally, an effectively adsorption of combined collector onto fayalite surface to improve its surface hydrophobicity. Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS) and MD simulations results showed synergistic adsorption was produced onto fayalite surface using combined collector mainly through non-polar interaction, hydrogen bond association and bridge adsorption. Furthermore, the spatial distribution and diffusion behavior of water molecules provide microscopic evidence for the enhancement of surface hydrophobicity of fayalite by combined collector.

Versatile photoluminescence behavior of polycyclic hydroxybenzimidazoles driven by intermolecular hydrogen bonding

Herein, the synthesis of polycyclic hydroxybenzimidazole based on 4-hydroxyphthalimide is presented and two isomeric structures are formed. The isomeric structures are capable of forming intermolecular hydrogen-bonded molecular associates. Hydroxybenzimidazole hydrogen-bonded organic frameworks have been shown to be sensitive to different solvent polarity, particularly in proton donor media, resulting in a blue shift in emission. The role of proton donor media has been evaluated using the binary mixture of acetonitrile/water and protonation by trifluoroacetic acid. The results show that by tuning the environment, the aggregation induced emission has appeared in the blue region and larger aggregates are formed compared to the less polar aprotic solvents. Under acidic conditions, the disruption of the hydrogen-bonded dimers was estimated, resulting in deep blue emission. This provides an opportunity to control the molecular associates and tune the optical behavior.

Engineering Tough Supramolecular Hydrogels with Structured Micropillars for Tunable Wetting and Adhesion Properties.

Soft-lithography is widely used to fabricate microstructured surfaces on plastics and elastomers for designable physical properties such as wetting and adhesions. However, it remains a big challenge to construct high-aspect-ratio microstructures on the surface of hydrogels due to the difficulty in demolding from the gel with low strength and stiffness. Demonstrated here is the engineering of tough hydrogels by soft-lithography to form well-defined micropillars. The mechanical properties of poly(acrylamide-co-methacrylic acid) hydrogels with dense hydrogen-bond associations severely depend on temperature, with Young's modulus increasing from 8.1MPa at 15°C to 821.8MPa at -30°C, enabling easy demolding at low temperatures. Arrays of micropillars are maintained on the surface of the gel, and can be used at room temperature when the gel restores soft and stretchable. The hydrogel also exhibits good shape-memory property, favoring tailoring the morphology with a switchable tilt angle of micropillars. Consequently, the hydrogel shows tunable wetting and adhesion properties, as manifested by varying contact angles and adhesion strengths. These surface properties can also be tuned by geometry and arrangement of micropillars. This facile strategy by harnessing tunable viscoelasticity of supramolecular hydrogels should be applicable to other soft materials, and broaden their applications in biomedical and engineering fields.

Relationship between synthesis method-crystal structure-melting properties in cocrystals: the case of caffeine-citric acid.

The influence of the crystal synthesis method on the crystallographic structure of caffeine-citric acid cocrystals was analyzed thanks to the synthesis of a new polymorphic form of the cocrystal. In order to compare the new form to the already known forms, the crystal structure of the new cocrystal (C8H10N4O2·C6H8O7) was solved by powder X-ray diffraction thanks to synchrotron experiments. The structure determination was performed using `GALLOP', a recently developed hybrid approach based on a local optimization with a particle swarm optimizer, particularly powerful when applied to the structure resolution of materials of pharmaceutical interest, compared to classical Monte-Carlo simulated annealing. The final structure was obtained through Rietveld refinement, and first-principles density functional theory (DFT) calculations were used to locate the H atoms. The symmetry is triclinic with the space group P-1 and contains one molecule of caffeine and one molecule of citric acid per asymmetric unit. The crystallographic structure of this cocrystal involves different hydrogen-bond associations compared to the already known structures. The analysis of these hydrogen bonds indicates that the cocrystal obtained here is less stable than the cocrystals already identified in the literature. This analysis is confirmed by the determination of the melting point of this cocrystal, which is lower than that of the previously known cocrystals.

Synthesis of Novel (Meth)acrylates with Variable Hydrogen Bond Interaction and Their Application in a Clear Viscoelastic Film.

Clear viscoelastic films (CVFs) have many applications in the display industry. Acrylic monomers containing a hydrogen bond (H-monomer) are often used in the preparation of CVF to increase the cohesion and form favorable interactions with the display substrate. Common H-monomers face a counterbalance between the glass transition temperature (Tg) and the hydrogen-bonding association constant (Ka). Strong hydrogen bonding often leads to a high Tg and high modulus, which are unfavorable in certain applications such as foldable display. To solve these problems, four types of hydrogen-bonding (meth)acrylic monomers (carbamate acrylate, carbamate methacrylate, urea acrylate, and urea methacrylate) with different Ka and Tg were readily synthesized. Among them, urea acrylates displayed the highest Ka while still maintaining moderate Tg. These H-monomers were copolymerized with 2-ethylhexyl acrylate (EHA) and cross-linked to obtain a series of copolymers (H-copolymers) as pressure-sensitive adhesives. After the characterization of rheology, optics, and peel adhesion, urea-acrylic H-copolymers showed the best overall performance by combining great optical property (>98% in transmittance, < 1% in haze) and mechanical performance (8-12 N/25 mm in peel adhesion, 84-92% in creep recovery). This work provides a new path to prepare acrylic CVF for flexible display application.

Polyurethane hard domain composed of mixed hard segments

AbstractThe packing of hard segment in polyurethane hard domain has not been resolved or seen clearly yet. In this work, we prepared polyurethane blends, which were composed of different weight ratios of crystallizable 1195A and non‐crystallizable EG85A, to study the structural evolution and crystallization in mixed hard domain. Time and temperature dependent differential scanning calorimetry, Fourier transform infrared spectroscopy (FTIR), and x‐ray diffraction results were used to record the micro‐phase evolution and crystallization processes. It was very surprising to find that the multiple melting temperatures in these blends were similar. We deduced that the packing of hard segment tended to separate into different groups, probably dependent on their segmental length. The crystalline nucleus was consist of 1195A hard segments with similar length, and the out‐layer structure was consisted of mixed hard segments of varying lengths. The FTIR results demonstrated that crystallinity did not affect the degree of hydrogen bonding association (HBA), and the stacking of hard segments may be only differed in the degree of ordering. Moreover, the degree of HBA of the blends was always higher than the calculated number based on weight ratio of pure samples.

Self-compliant ionic skin by leveraging hierarchical hydrogen bond association

Robust interfacial compliance is essential for long-term physiological monitoring via skin-mountable ionic materials. Unfortunately, existing epidermal ionic skins are not compliant and durable enough to accommodate the time-varying deformations of convoluted skin surface, due to an imbalance in viscosity and elasticity. Here we introduce a self-compliant ionic skin that consistently works at the critical gel point state with almost equal viscosity and elasticity over a super-wide frequency range. The material is designed by leveraging hierarchical hydrogen bond association, allowing for the continuous release of polymer strands to create topological entanglements as complementary crosslinks. By embodying properties of rapid stress relaxation, softness, ionic conductivity, self-healability, flaw-insensitivity, self-adhesion, and water-resistance, this ionic skin fosters excellent interfacial compliance with cyclically deforming substrates, and facilitates the acquisition of high-fidelity electrophysiological signals with alleviated motion artifacts. The presented strategy is generalizable and could expand the applicability of epidermal ionic skins to more complex service conditions.

Double-Network Organohydrogels Toughened by Solvent Exchange.

Double-network hydrogels based on calcium alginate are extensively exploited. Unfortunately, their low strength and unstable constitution to open environments limit their application potential. Herein, a new type of double-network organohydrogel (OHG) is proposed. By solvent exchange, a stable physical network is established based on dimethyl sulfoxide (DMSO)-alginate in the presence of a polyacrylamide network. The DMSO content endows tunable mechanical properties, with a maximum tensile strength of ≈1.7MPa. Importantly, the OHG shows much better environmental stability compared to the conventional double-network hydrogels. Due to the reversible association of hydrogen bonds, the OHG possesses some unique properties, including free-shapeability, shape-memory, and self-adhesion, that offers several promising ways to utilize alginate-based gels for wide applications.

Tough supramolecular hydrogels of poly(N,N-dimethylacrylamide)-grafted poly(methacrylic acid) with cooperative hydrogen bonds as physical crosslinks.

Incorporating associative interactions as the energy dissipation units has been recognized as an effective strategy to develop tough hydrogels. For hydrogen-bond associations, however, it is highly challenging to stabilize them under aqueous conditions. Although affording cooperativity can enhance and stabilize the hydrogen bonds, it usually requires stepwise polymerization to form these cooperative associations between different polymers and networks. Here, we report a series of tough supramolecular hydrogels with robust hydrogen-bond associations between grafted polymers that are synthesized by polymerization of a macromonomer of poly(N,N-dimethylacrylamide) (PDMAA) and a small monomer of methacrylic acid. The grafted chains of PDMAA form cooperative hydrogen bonds with the main chain of poly(methacrylic acid) (PMAAc), forming supramolecular hydrogels with high toughness and good stability. The tough and stiff hydrogels are in a glassy state, exhibit forced elastic deformation at room temperature, and remain stable over a wide pH range. In contrast, hydrogels prepared by the copolymerization of DMAA and MAAc are swollen and weak in water due to the lack of successive hydrogen donor/acceptor units and the absence of cooperative hydrogen bonds. In addition, these tough hydrogels exhibit good recyclability and shape memory properties, owing to the supramolecular nature of the network and the temperature-dependent mechanical properties. The influence of polymer structure on the associative interactions and macroscopic properties of the hydrogels should be informative for the design of tough soft materials with versatile applications.

Structure of tert-butyl alcohol in carbon tetrachloride solution. Molecular dynamics simulation study

The structure of tert-butyl alcohol (TBA) solution in carbon tetrachloride (CCl4) is studied over the entire concentration range using molecular dynamics simulation (MD). It is shown that at low alcohol concentrations (within 5% molar fraction), separate small associates of TBA appear as a result of hydrogen bonding between alcohol molecules, and cyclic structures of four TBA molecules appear among these associates. With increasing concentration, the formation of hydrogen-bonded associates quickly reaches its limit corresponding to 1.7 bonds per molecule as in pure TBA. At high alcohol concentrations, both linear and cyclic associates of TBA molecules are observed with a noticeable fraction of cyclic tetramers. Using the Voronoi method, aggregates of the nearest TBA molecules are considered. The number and size of such aggregates grows rapidly with concentration due to the presence of “nuclei” formed by hydrogen bonding. At an alcohol mole fraction of 15%, a percolation cluster of TBA molecules begins to form in the solution. On the contrary, in the local environment of CCl4 molecules no significant structural rearrangements occur, only a gradual replacement of the molecules is observed upon addition of alcohol. However at low content of CCl4 in alcohol, we can see some tendency to clustering of CCl4 molecules, which may indicate on a slight solvophobic effect in this solution.

A Numerical Study of Vapor–Liquid Equilibrium in Binary Refrigerant Mixtures Based on 2,3,3,3-Tetrafluoroprop-1-ene

The binary refrigerant mixtures containing 2,3,3,3-Tetrafluoroprop-1-ene are considered as excellent substitutes for traditional refrigerants. Weak hydrogen bonds exist in hydrofluorocarbons and hydrofluoroolefins. However, for several recently published binary refrigerant mixtures, there is no Vapor–Liquid Equilibrium calculation study considering hydrogen-bonding associations. This work presents a calculation work of the saturated properties of nine pure refrigerants using the Cubic-Plus-Association Equation of State, considering the hydrogen-bonding association in refrigerant fluids. The average relative deviations of the saturated vapor pressure, liquid, and vapor density are less than 1.0%, 1.5%, and 3.5%, respectively. The Vapor–Liquid Equilibrium of ten binary refrigerant mixtures containing 2,3,3,3-Tetrafluoroprop-1-ene is also calculated using the Cubic-Plus-Association Equation of State with the van der Waals mixing rule. The average relative deviations of the liquid-phase and vapor-phase mole fractions are less than 1.0% and 2.0%, respectively. Moreover, the Vapor–Liquid Equilibrium data and the model’s adaptability are analyzed and discussed.

Atomic insights into the heterogeneity and the interface interactions of nanoconfined aqueous electrolyte solution

The structure and properties of nanoconfined electrolyte (nano electrolyte) solutions are important subjects in biochemistry, electrochemistry, separation science, etc. In the present work, neutron scattering and data driven reverse Monte Carlo were employed to quantitatively reveal the microstructure of aqueous CaCl2 solution in mesoporous silica MCM-41 at the atomic level. The electrolyte solution distributes in-homogeneously in the pores of mesoporous silica along the radial direction, which can be divided into the core, the transition, and the interface regions. The hydrogen bond structure of water in nano electrolytes is strengthened in the core region, analogous to that of nano water confined in nanochannels, and the ion association was enhanced in the transition region. The structure of the solution in the interface region was analogous to the bulk electrolyte solution under high pressure because of the confinement and interface effects. When the package ratio of nano electrolyte solution is insufficient, small amount of solution quasi-monolayer adsorb on the internal surface of mesoporous silica, the majority solution preferentially forms clusters with different sizes to adhere to the wetted interface. The anomalous hydrogen-bonding structure and ion association found in the present work improve our understanding of nanoconfined electrolyte solutions.

Simulation of multiphase behavior in supercritical CO2–heavy oil–water sludge extraction systems using Cubic-Plus-Association Equation of State

A new methodology was proposed for predicting the multiphase behavior of a supercritical CO2–heavy oil–water–solvent system during supercritical CO2 extraction. The cubic-plus-association equation of state (CPA EoS) was used to model the hydrogen bond association between asphaltene and water and to develop a nine-step characterization method for asphaltene-containing oil. Results showed that the proposed method could represent phase behavior for three equilibrium phases: the vapor phase and high-density and low-density liquid phases. The absolute average relative deviation between the predicted and experimental phase boundary pressures was 3.6–11.8%, smaller than those (7.0–20.8%) calculated using Soave–Redlich–Kwong EoS with Huron–Vidal mixing rules. Moreover, case studies on appropriate extraction conditions showed that the extraction pressure increased with increasing molecular weight and critical pressure of each pseudocomponent. Increasing extraction pressure for H2O-containing systems was beneficial for improving extraction efficiency.

Intrinsic Anti-Freezing and Unique Phosphorescence of Glassy Hydrogels with Ultrahigh Stiffness and Toughness at Low Temperatures.

Most hydrogels become frozen at subzero temperatures, leading to degraded properties and limited applications. Cryoprotectants are massively employed to improve anti-freezing property of hydrogels; however, there are accompanied disadvantages, such as varied networks, reduced mechanical properties, and the risk of cryoprotectant leakage in aqueous conditions. Reported here is the glassy hydrogel having intrinsic anti-freezing capacity and excellent optical and mechanical properties at ultra-low temperatures. Supramolecular hydrogel of poly(acrylamide-co-methacrylic acid) with moderate water content (≈50 wt.%) and dense hydrogen-bond associations is in a glassy state at room temperature. Since hydrogen bonds become strengthened as the temperature decreases, this gel becomes stronger and stiffer, yet still ductile, with Young's modulus of 900MPa, tensile strength of 30MPa, and breaking strain of 35% at -45°C. This gel retains high transparency even in liquid nitrogen. It also exhibits unique phosphorescence due to presence of carbonyl clusters, which is further enhanced at subzero temperatures. Further investigations elucidate that the intrinsic anti-freezing property is related to a fact that most water molecules are tightly bound and confined in the glassy matrix and become non-freezable. This correlation, as validated in several systems, provides a roadmap to develop intrinsic anti-freezing hydrogels for widespread applications at extreme conditions.

Impact-Protective Bicontinuous Hydrogel/Ultrahigh-Molecular Weight Polyethylene Fabric Composite with Multiscale Energy Dissipation Structures for Soft Body Armor.

Soft body armor greatly improves the comfort and security of the wearers. Although laminates based on high-performance fabrics have been adopted, it remains an enormous challenge to develop fabric laminates having flexibility, low bulge deformation, and ballistic protection capability simultaneously. Herein, we report a bullet-proof bicontinuous hydrogel (BH)/ultrahigh-molecular weight polyethylene fabric (UPF) composite. The presence of the BH significantly improves the impact resistance performance of the UPF, without compromising its flexibility. In specific, the multiscale energy dissipation structures composed of hydrogen bond associations in the chain scale, bicontinuous phase structures in the nanoscale, and fibers in the microscale are broken to dissipate energy. As a result, the impact energy of the bullet is greatly absorbed and the bulge height of the composites is significantly reduced in contrast to the neat UPF laminates. This study indicates that the flexible BH-UPF composites with multiscale energy dissipation structures have a promising application in soft body armor.

Electronic and Crystal Packing Effects in Terms of Static and Kinetic Force Field Features: Picolinic Acid N-Oxide and Methimazole

Herein, we experimentally obtained and studied the inner-crystal scalar potential fields and the associated vector force fields of static and kinetic nature in picolinic acid N-oxide (PANO) and methimazole. The “through-bond” and “through-space” electronic effects were defined and distinguished via the vector fields and concurred with the penetration of the electron contributor’s electrostatic and kinetic force field pseudoatoms (φes- and φk-basins) into the occupier’s chemical atom (ρ-basin). A special focus was given to the dipolar N+–O– bond as well as to the thioamide N–C═S and carboxylic O═C–OH fragments. An unusual way of interatomic charge transfer was revealed between two nonbonded hydrogen atoms [COO−]H···H[−C] in the PANO crystal. The experimental electric and kinetic force fields in the molecular crystals were compared to the theoretical ones for the free molecules and hydrogen-bonded associates, which helped figure out the natural consequences of the crystal packing effect. As expected, the appearance of neighboring attractors in a dense crystal packing requires zero-flux surfaces (ZFSs) emerging in the vector fields and the compression of the outer force field pseudoatoms of a molecule. We proposed to consider a ZFS in the kinetic force field to be a turning surface for electrons in the sense that an electron passed through the boundary is immediately affected by the redirected force attributed to another attractor. The strengthening and noticeable increase observed in the covalency of the intramolecular hydrogen bond O–H···O in the PANO crystal is a direct consequence of such compression of the hydrogen atom and pseudoatoms by the inner-crystal environment. The Pauli and fermionic scalar and vector fields were applied to locate electron lone pairs and describe their involvement in noncovalent interactions within the donor–acceptor mechanism.

ТЕОРЕТИЧЕСКОЕ ИССЛЕДОВАНИЕ АССОЦИАЦИИ АРИЛОВЫХ ПРОИЗВОДНЫХ МОЛОЧНОЙ КИСЛОТЫ

A theoretical study of the association of phenyl and ortho-substituted aryl derivatives of lactic acid was carried out. Two variants of hydrogen-bonded associates in the gas phase were calculated: non-classical, actually found in the crystals, and simulated classical dimers. The energy advantage of classical dimers and the non-equivalence of diastereotopic electron lone pairs at the carbonyl oxygen atom were shown.

Experimental study on sodium acetate trihydrate/glycerol deep eutectic solvent nanofluids for thermal energy storage

With the development of electronic machinery, the traditional heat exchange working fluid can no longer be used in some extreme working environments. As a new type of fluid, deep eutectic solvent has the advantages of promising thermal stability, low cost and non-toxicity through the hydrogen bond association of hydrogen bond acceptor and hydrogen bond donor, and is also coined to be an efficient heat transfer working fluid. In this work, three kinds of nanofluids filled with TiO2, Al2O3 and Fe2O3 nanoparticles were prepared with glycerol/sodium acetate trihydrate deep eutectic solvent as the base solution, and the mechanism of thermal conductivity change of nanofluids were discussed from atomic and microscopic levels. The experimental results showed that the prepared glycerol/ sodium acetate trihydrate 2:1 deep eutectic solvent system reduced the viscosity by 58.03% compared with glycerol, and increased the thermal conductivity by 17.2% compared with glycerol at 25 ℃. The effects of nanoparticles on thermal conductivity and specific heat capacity were also investigated. Compared with deep eutectic solvent, the thermal conductivity was improved after adding Fe2O3 nanoparticles; it is worth noting that the specific heat capacity of nanofluids was more than doubled after adding nanoparticles.

Experimental evidence and computational results in the investigation of thermodynamic properties and molecular structure of hydrogen-bonded cyclohexylamine and ethyl methyl ketone system

Combining experimental and computational study is a fundamental and attractive method for analyzing intermolecular hydrogen bonds and specific interactions in binary mixtures of different liquids. An experimental and computational investigation of the structural interactions between the molecules of cyclohexylamine and ethyl methyl ketone (EMK) is presented. In the experimental part of the work, the density of mixtures of cyclohexylamine and EMK in the specific mole fractions is reported at 298.15, 303.15, 308.15, and 313.15 K and at an absolute pressure of 86.7 kPa. Densities data have been used to evaluate the excess molar volumes, the thermal expansion coefficients, and the isothermal coefficient of pressure excess molar enthalpy. The Jouyban–Acree model has been used for mathematical correlation of the densities. Using the Redlich-Kister equation, the results of excess molar volumes were fitted. Through computational technics such as density functional theory (DFT), quantum theory of atoms in molecules (QTAIM), and also molecular dynamics (MD) simulations, the analysis of intermolecular hydrogen-bonded associations has been accomplished. MD simulations computed density and radial distribution functions (RDFs) in the liquid phase at 298.15 K and at 1 atm.

Improvement of structural characteristics and in vitro digestion properties of zein by controlling postharvest ripening process of corn

The newly harvested corn was stored at constant temperature (15 and 25 °C) and relative humidity (55%). The structural characteristics and in vitro digestion properties of zein as well as their structure-function relationship during postharvest ripening of corn were analyzed. The free sulfhydryl content and surface hydrophobicity decreased and the disulfide bonds and absolute value of ζ-potential increased during postharvest ripening of corn for 56 days at two storage temperatures. The particle size and polydispersity index reached the minimum values when postharvest ripening for 28 days. The association of hydrogen bonds for zein was decreased and more β-sheets were transformed into random coils in the postharvest ripening period, leading to the reduction in thermal stability and enthalpy change of zein. In addition, the molecular weight of α-zein decreased when postharvest ripening at 15 °C for 28 days, and changed little during postharvest ripening at 25 °C. In vitro simulated digestion experiment proved that the maximum degree of hydrolysis of zein was appeared when postharvest ripening for 28 days at both two storage temperatures, which was higher at 15 °C than at 25 °C. This indicated that appropriate postharvest ripening behavior could improve the digestibility of zein. The reduction of protein aggregation extent and thermal stability as well as enhanced disordered secondary structure of zein might be important factors to improve its digestibility during the postharvest ripening. The results suggested that controlling the postharvest ripening process of corn could modify the structure of zein and improve its digestion properties. • Postharvest ripening of corn was performed at constant temperature and humidity. • Aggregation, thermal stability and ordered secondary structure of zein reduced. • Degree of hydrolysis for zein was highest by postharvest ripening for 28 days. • Improvement of zein digestibility was related to its structural modification. • Controlling postharvest ripening process of corn improved digestion property of zein.