Interfacial H-bonding Research Articles

Positive and Negative Impacts of Interfacial Hydrogen Bonds on Photocatalytic Hydrogen Evolution.

Understanding the behavior of water molecules at solid-liquid interfaces is crucial for various applications such as photocatalytic water splitting, a key technology for sustainable fuel production and chemical transformations. Despite extensive studies conducted in the past, the impact of the microscopic structure of interfacial water molecules on photocatalytic reactivity has not been directly examined. In this study, using real-time mass spectrometry and Fourier-transform infrared spectroscopy, we demonstrated the crucial role of hydrogen bond (H-bond) networks on the photocatalytic hydrogen evolution in thickness-controlled water adsorption layers on various TiO2 photocatalysts. Under controlled water vapor environments with relative humidity (RH) below 70%, we observed a monotonic increase in the H2 formation rate with increasing RH, indicating that reactive water molecules were present not only in the first adsorbed layer but also in several overlying layers. In contrast, at RH > 70%, when more than three water layers covered the catalyst surface, the H2 formation rate turned to decrease dramatically because of the structural rearrangement and hardening of the interfacial H-bond network induced during further water adsorption. This unique many-body effect of interfacial water was consistently observed for various TiO2 particles with different crystalline structures, including brookite, anatase, and a mixture of anatase and rutile. Our results demonstrated that depositing several water layers in a water vapor environment with RH ∼ 70% is optimal for photocatalytic hydrogen evolution rather than liquid-phase reaction conditions in aqueous solutions. This study provides molecular-level insights into designing interfacial water conditions to enhance photocatalytic performance.

Bioinspired H-Bonding Connected Gradient Nanostructure Actuators Based on Cellulose Nanofibrils and Graphene.

The construction of flexible actuators with ultra-fast actuation and robust mechanical properties is crucial for soft robotics and smart devices, but still remains a challenge. Inspired by the unique mechanism of pinecones dispersing seeds in nature, a hygroscopic actuator with interlayer network-bonding connected gradient structure is fabricated. Unlike most conventional bilayer actuator designs, the strategy leverages biobased polyphenols to construct strong interfacial H-bonding networks between 1D cellulose nanofibers and 2D graphene oxide, endowing the materials with high tensile strength (172MPa) and excellent toughness (6.64MJm-3). Furthermore, the significant difference in hydrophilicity between GO and rGO, along with the dense interlayer H-bonding, enables ultra-fast water exchange during water absorption and desorption processes. The resulted actuator exhibits ultra-fast driving speed (154°s-1), excellent pressure-resistant and cyclic stability. Taking advantages of these benefits, the actuator can be fabricated into smart devices (such as smart grippers, humidity control switches) with significant potential for practical applications. The presented approach to constructing interlayer H-bonding in gradient structures is instructive for achieving high performance and functionalization of biomass nanomaterials and the complex of 1D/2D nanomaterials.

Engineering Sulfur-Containing Polymeric Fire-Retardant Coatings for Fire-Safe Rigid Polyurethane Foam.

With the advantages of lightweight and low thermal conductivity properties, polymeric foams are widely employed as thermal insulation materials for energy-saving buildings but suffer from inherent flammability. Flame-retardant coatings hold great promise for improving the fire safety of these foams without deteriorating the mechanical-physical properties of the foam. In this work, four kinds of sulfur-based flame-retardant copolymers are synthesized via a facile radical copolymerization. The sulfur-containing monomers serve as flame-retardant agents including vinyl sulfonic acid sodium (SPS), ethylene sulfonic acid sodium (VS), and sodium p-styrene sulfonate (VSS). Additionally, 2-hydroxyethyl acrylate (HEA) and 4-hydroxybutyl acrylate are employed to enable a strong interface adhesion with polymeric foams through interfacial H-bonding. By using as-synthesized waterborne flame-retardant polymeric coating with a thickness of 600µm, the coated polyurethane foam (PUF) can achieve a desired V-0 rating during the vertical burning test with a high limiting oxygen index (LOI) of >31.5vol%. By comparing these sulfur-containing polymeric fire-retardant coatings, poly(VS-co-HEA) coated PUF demonstrates the best interface adhesion capability and flame-retardant performance, with the lowest peak heat release rate of 166kW m-2 and the highest LOI of 36.4vol%. This work provides new avenues for the design and performance optimization of advanced fire-retardant polymeric coatings.

Controlling Interfacial Hydrogen Bonding at a Gold Surface: The Effect of Organic Cosolvents.

Water often serves as both a reactant and solvent in electrocatalytic reactions. Interfacial water networks can affect the transport and kinetics of these reactions, e.g., hydrogen evolution reaction and CO2 reduction reaction. Adding cosolvents that influence the hydrogen-bonding (H-bonding) environment, such as dimethyl sulfoxide (DMSO), has the potential to tune the reactivity of these important electrocatalytic reactions by regulating the interfacial local environment and water network. We investigate interfacial H-bonding networks in water-DMSO cosolvent mixtures on gold surfaces by using surface-enhanced infrared absorption spectroscopy and molecular dynamics simulations. Experiments and simulations show that the gold surface is enriched with dehydrated DMSO molecules and the mixture phase-separates to form water clusters. Simulations show a "buckled" water conformation at the surface, further constraining interfacial H-bonding. The small size of these water clusters and the energetically unfavorable H-bond conformations might inhibit H-bonding with bulk water, suppressing the proton diffusion required for efficient hydrogen evolution reaction processes.

Hidden Beneath the Layers: Extending the Core/Shell Model of Reverse Micelles.

Reverse micelles (RMs) provide a unique and highly tunable model system to study water in confined environments. The complex properties of water within RMs arise from the disruption of extended hydrogen bond (H-bond) networks that mediate local and long-range dynamics in bulk aqueous systems. Modulating the water pool size influences its H-bond dynamics, with smaller RMs increasingly restricting the H-bond network rearrangements leading to slower dynamics; however, within small confined systems, the dynamics of the surfactants also influence the water dynamics. Using ultrafast two-dimensional infrared spectroscopy, we investigate the effects of RM size on the surfactant headgroup rotamer populations and picosecond interfacial H-bond dynamics of aerosol-OT surfactants. We find that the increased water penetration accelerates H-bond dynamics, with larger RMs showing faster dynamics. These results imply that the changes in the RM structure alter the physical structure of the RM interface and thus alter the solvation dynamics. The findings in this study can be used for developing models for structure-specific solvation dynamics that account for the surfactant packing and hydration at the interface.

Sponge-phase Lipid Droplets as Synthetic Organelles: An Ultrafast Study of Hydrogen Bonding and Interfacial Environments.

Bottom-up design of biomimetic organelles has gained recent attention as a route towards understanding the transition between non-living matter and life. Despite various artificial lipid membranes being developed, the specific relations between lipid structure, composition, interfacial properties, and morphology are not currently understood. Sponge-phase droplets contain dense, nonlamellar lipid bilayer networks that capture the complexities of the endoplasmic reticulum (ER), making them ideal artificial models of such organelles. Here, we combine ultrafast two-dimensional infrared (2D IR) spectroscopy and molecular dynamics simulations to investigate the interfacial H-bond networks in sponge-phase droplets composed of glycolipid and nonionic detergents. In the sponge phase, the interfacial environments are more hydrated and water molecules confined to the nanometer-scale aqueous channels in the sponge phase exhibit dynamics that are significantly slower compared to bulk water. Surfactant configurations and microscopic phase separation play a dominant role in determining membrane curvature and slow dynamics observed in the sponge phase. The studies suggest that H-bond networks within the nanometer-scale channels are disrupted not only by confinement but also by the interactions of surfactants, which extend 1-2 nm from the bilayer surface. The results provide a molecular-level description for controlling phase and morphology in the design of synthetic lipid organelles.

Efficient Electrocatalytic Nitrate Reduction to Ammonia Based on DNA-Templated Copper Nanoclusters

In alkaline solutions, the electrocatalytic conversion of nitrates to ammonia (NH3) (NO3RR) is hindered by the sluggish hydrogenation step due to the lack of protons on the electrode surface, making it a grand challenge to synthesize NH3 at a high rate and selectivity. Herein, single-stranded deoxyribonucleic acid (ssDNA)-templated copper nanoclusters (CuNCs) were synthesized for the electrocatalytic production of NH3. Because ssDNA was involved in the optimization of the interfacial water distribution and H-bond network connectivity, the water-electrolysis-induced proton generation was enhanced on the electrode surface, which facilitated the NO3RR kinetics. The activation energy (Ea) and in situ spectroscopy studies adequately demonstrated that the NO3RR was exothermic until NH3 desorption, indicating that, in alkaline media, the NO3RR catalyzed by ssDNA-templated CuNCs followed the same reaction path as the NO3RR in acidic media. Electrocatalytic tests further verified the efficiency of ssDNA-templated CuNCs, which achieved a high NH3 yield rate of 2.62 mg h-1 cm-2 and a Faraday efficiency of 96.8% at -0.6 V vs reversible hydrogen electrode. The results of this study lay the foundation for engineering catalyst surface ligands for the electrocatalytic NO3RR.

Enhanced Interfacial H-Bond Networks Promote Glycan–Glycan Recognition and Interaction

H-bond networks at heterogeneous interfaces play crucial roles in bioseparation, biocatalysis, biochip array profiling, and functional nanosystem self-assembly, but their precise modulation and enhancement remain challenging. In this study, we have discovered that interfacial hydrophobic hydration significantly enhances H-bond networks at the interface between a glycan-modified adsorbent and a methanol-water-acetonitrile ternary solution. The enhanced H-bond networks greatly promote the adsorbent-solution heterogeneous glycan-glycan recognition and interaction. This novel hydrophobic hydration-enhanced hydrophilic interaction (HEHI) strategy improves the affinity and efficiency of intact glycopeptide enrichment. Compared with the commonly used hydrophilic-interaction enrichment strategy, 23.5 and 48.5% more intact N- and O-glycopeptides are identified, and the enrichment recoveries of half of the glycopeptides are increased >100%. Further, in-depth profiling of both N- and O-glycosylation occurring on SARS-CoV-2 S1 and hACE2 proteins has been achieved with more glycan types and novel O-glycosylation information involved. Interfacial hydrophobic hydration provides a powerful tool for the modulation of hydrophilic interactions in biological systems.

Interfacial dynamics in inverted-headgroup lipid membranes.

Inverted-headgroup (choline-phosphate) lipids are synthetic lipids that are not found in nature and are used as model systems to understand the role of headgroup dipole orientation. Recently, studies revealed that the net orientation of interfacial water strongly depends on the headgroup electrostatics, i.e., the charges and dipole generated by the phosphate and the choline groups. In order to characterize interfacial H-bond dynamics, we measured two-dimensional infrared spectra of the ester carbonyl band and performed molecular dynamics simulations in fully hydrated 1,2-dioleoyl-sn-glycero-3-phosphocholine and 2-((2,3-bis(oleoyloxy)propyl)-dimethyl-ammonio)ethyl ethyl phosphate (DOCPe) lipid bilayers. The experiments and simulations suggest that the reverse dipole generated by the inverted-headgroup in DOCPe does not affect the carbonyl H-bond populations or the interfacial water H-bond dynamics. However, while phosphate-associated waters in both lipids appear to show a similar H-bond structure, carbonyl-associated waters are characterized by a slightly disrupted H-bond structure in the DOCPe bilayer, especially within the second hydration shell. Our findings show that changes in net water orientation perturb the water H-bonds at the linker region between the headgroup and the lipid tail, although this perturbation is weak.

Molecular Mechanism of Cell Membrane Protection by Sugars: A Study of Interfacial H-Bond Networks.

Sugars function as bioprotectants by stabilizing biomolecules during dehydration, thermal stress, and freeze-thaw cycles. A buildup of sugars occurs in many organisms upon their exposure to extreme conditions. Understanding sugar's bioprotective effects on membranes is achieved by characterizing the H-bond networks at the lipid-water interface. Here, we report the headgroup H-bond populations, structures, and dynamics of 1,2-dimyristoyl-sn-glycero-3-phosphocholine vesicles in concentrated glucose solutions using ultrafast two-dimensional infrared spectroscopy in conjunction with molecular dynamics simulations. H-Bond populations and dynamics at the ester carbonyl positions are largely unaffected even at very high, 600 mg/mL, sugar concentrations. In addition, dynamics exhibit a slight nonmonotonic dependence on sugar concentration. Simulations, which are in near-quantitative agreement with measured dynamics, show that the H-bond structure remains largely intact by the existence of sugar. This study shows that the bioprotection of sugar is realized through stable lipid-saccharide-water H-bond networks at the membrane interface that mimic the H-bond networks in pure water.

All-atom molecular dynamics simulation of acrylamide-based triblock copolymers with thermal responsive end blocks in a mixed salt solution containing divalent cations

In this work, a new smart copolymer based on polyacrylamide with the triblock classification was presented in order to propose an efficient associating polymeric material to create a physical network structure in saline water. To tune the desired copolymer, the effect of amide's type and hydrophobic pendant moieties of the potentially thermoresponsive end blocks on the physicochemical characteristics of the intermediate water-soluble block was studied in a solution containing Ca2+, Mg2+ and Cl− via atomistic simulations. The investigation of ions' mechanism of action on the water's perturbations via the calculation of water's orientational order parameter demonstrated that the divalent ions, as strong chaotropic agents, changed the water's tetrahedrality. Moreover, the coordination complexes created by the divalent ions as the coordination center and the negatively charged elements of the blocks' amides as the complexing agents intensified this issue in the block-water interface via increasing the interfacial concentration of the cations. This phenomenon has affected the blocks' hydrophilicity and hydration levels, capturing and ordering of water by changing the water's interfacial tetrahedral H-bond network structure. These behavioral variations in the solution led to changes in the dimensional, conformational, dynamic and interactional characteristics of the copolymers' blocks.

Insights on the Structural and Dynamic Properties of Corundum–Water Interfaces from First-Principle Molecular Dynamics

The interaction of water with hydroxylated mineral surfaces is central to numerous natural and technological processes; yet, at the atomic level much remains to be understood about the structure and dynamics of interfacial water and H-bonding networks. This work applies first-principle molecular dynamics to examine the relationship between surface structure and interfacial water behavior at four hydroxylated corundum (α-Al2O3) surfaces. The four surfaces studied include the (001), (012), (110), and (113) surfaces; they have distinct surface functional groups (−OH, μ-OH, and μ3-OH) and topography (planar, corrugated, and two-tiered). Interfacial H-bonding was investigated in detail by analyzing atomic density 1-D profiles, angular distribution of surface O–H bonds, and the dipole moment orientation of interface H2O molecules and by calculating vibrational density of states in the high-frequency region. Our results reveal distinct differences in the interaction of interfacial water and H-bonding networks at the four surfaces. Specifically, inter- and intramolecular H-bonding depends on surface site coordination geometry, neighboring functional groups, and surface topography. Moreover, intramolecular H-bonding involving in-plane surface hydroxyl groups may persist, thus limiting intermolecular H-bonding. Equally, first solvation layer water molecules are preferentially orientated at the (001), (012), and (113) surfaces, whereas water molecules at the (110) surface show minimal preferred orientation. The distinct differences in the behavior of interfacial water and H-bonding at the four surfaces may play a substantial role in determining the reactivity of the surfaces toward processes such as ion adsorption–desorption and dissolution–precipitation.

Tough hydrogels with tunable soft and wet interfacial adhesion

Hydrogels that possess adequate toughness in bulk and at soft interface are highly desirable. We report a fully physically crosslinked dual Amylopectin (Amy, the first network)/poly(N-hydroxyethyl acrylamide) (PHEAA, the second network) hydrogel, where both networks were hydrogen bonding (H-bonding) crosslinked, with super toughness in bulk and tunable adhesion at hydrogel-soft tissue interface. Through adjustment of hydrogen bond density, strong and tough hydrogels were prepared that exhibited different levels of adhesion strength and three adhesive failure mechanisms. We replaced Amy with oxidized starch (OSA) to synthesize dual and hybrid chemically/physically crosslinked OSA/PHEAA double network (DN) hydrogels that were capable of forming Schiff's bases at hydrogel-hydrogel or hydrogel-tissue interfaces, aside from interfacial H-bonding, incurring sustained, high interfacial toughness (1850 J m−2 hydrogel-hydrogel and 610 J m−2 hydrogel-tissue) in wet environments. Such tough and adhesive DN hydrogels have great potential in various applications such as engineering of artificial soft tissues or implantable devices that are intended to adhere to internal organs or tissues.

Hydrogen-Bond-Driven Chemical Separations: Elucidating the Interfacial Steps of Self-Assembly in Solvent Extraction

Chemical separations, particularly liquid extractions, are pervasive in academic and industrial laboratories, yet a mechanistic understanding of the events governing their function are obscured by interfacial phenomena that are notoriously difficult to measure. In this work, we investigate the fundamental steps of ligand self-assembly as driven by changes in the interfacial H-bonding network using vibrational sum frequency generation. Our results show how the bulk pH modulates the interfacial structure of extractants at the buried oil/aqueous interface via the formation of unique H-bonding networks that order and bridge ligands to produce self-assembled aggregates. These extended H-bonded structures are key to the subsequent extraction of Co2+ from the aqueous phase in promoting micelle formation and subsequent ejection of said micelle into the oil phase. The combination of static and time resolved measurements reveals the mechanisms underlying complexities of liquid extractions at high [Co2+]:[ligand] ratios by showing an evolution of interfacially assembled structures that are readily tuned on a chemical basis by altering the compositions of the aqueous phase. The results of this work point to new mechanistic principles to design separations through the manipulation of surface charge, electrostatic screening, and the associated H-bonding networks that arise at the interface to facilitate organization and subsequent extraction.

Ultrafast Spectroscopy of Lipid-Water Interfaces: Transmembrane Crowding Drives H-Bond Dynamics.

Biology takes place in crowded, heterogeneous environments, and it is therefore essential to account for crowding effects in our understanding of biophysical processes at the molecular level. Comparable to the cytosol, proteins occupy approximately 30% of the plasma membrane surface; thus, crowding should have an effect on the local structure and dynamics at the lipid-water interface. Using a combination of ultrafast two-dimensional infrared spectroscopy and molecular dynamics simulations, we quantify the effects of membrane peptide concentration on the picosecond interfacial H-bond dynamics. The measurements reveal a nonmonotonic dependence of water orientation and dynamics as a function of transmembrane peptide:lipid ratio. We observe three dynamical regimes: a "pure lipid-like" regime at low peptide concentrations, a bulk-like region at intermediate peptide concentrations where dynamics are faster by ∼20% compared to those of the pure lipid bilayer, and a crowded regime where high peptide concentrations slow dynamics by ∼50%.

The role of interfacial H-bonding on the electrical properties of UV-cured resin filled with hydroxylated Al2O3 nanoparticles

Surface hydroxylation of crude Al2O3 (c-Al2O3) nanoparticles by H2O2 was conducted to tailor the electrical properties of UV-cured resin. The hydroxyl groups on Al2O3 particles were designed to establish hydrogen bonding between the hydroxyl and carboxyl groups, which favors the enhancement of interfacial strength between fillers and UV-cured resin matrix. The effect of interfacial strength on the electrical properties was investigated. Owing to the improved interfacial strength, it can be conjectured that a larger volume of the interaction zone exists in UV-cured resin/hydroxylated Al2O3 (UV/h-Al2O3) composites. As a consequence, the number of deeper traps is increased, restraining the charge migration and raising the charge injection barrier. Thus, UV/h-Al2O3 composites exhibit remarkably enhanced breakdown strength, improved volume resistivity and suppressed space charge accumulation in comparison with that of UV/c-Al2O3 composites at the same filler content. It was found that the addition of 0.5 wt% h-Al2O3 increases the AC breakdown strength and volume resistivity by 15.5% and 367.9%, respectively. Our results suggest that hydroxylation is an efficient way to improve the electrical properties of UV-cured resin nanocomposites, thus promoting stereolithography 3D printing in the application of electrical and electronic fields.

One-Step Fabrication of pH-Responsive Membranes and Microcapsules through Interfacial H-Bond Polymer Complexation

Biocompatible microencapsulation is of widespread interest for the targeted delivery of active species in fields such as pharmaceuticals, cosmetics and agro-chemistry. Capsules obtained by the self-assembly of polymers at interfaces enable the combination of responsiveness to stimuli, biocompatibility and scaled up production. Here, we present a one-step method to produce in situ membranes at oil-water interfaces, based on the hydrogen bond complexation of polymers between H-bond acceptor and donor in the oil and aqueous phases, respectively. This robust process is realized through different methods, to obtain capsules of various sizes, from the micrometer scale using microfluidics or rotor-stator emulsification up to the centimeter scale using drop dripping. The polymer layer exhibits unique self-healing and pH-responsive properties. The membrane is viscoelastic at pH = 3, softens as pH is progressively raised, and eventually dissolves above pH = 6 to release the oil phase. This one-step method of preparation paves the way to the production of large quantities of functional capsules.

Effect of Ions on H-Bond Structure and Dynamics at the Quartz(101)-Water Interface.

We use ab initio molecular dynamics simulations to study the effect of ions on the structure and dynamics of the quartz(101)-water interface. We study several IA (Na+, Rb+) and IIA (Mg2+, Sr2+) cations, with Cl- as counterion, adsorbed onto acidic, neutral, and basic surface configurations at 300 and 373 K. We find that both cations and anions can bond directly to the surface and perturb the local H-bond network. The adsorbed ions promote the formation of intrasurface H-bonds, as shown by vibrational density of states and orientations of the surface silanols. Both local and global structural correlations of the interfacial H-bond network are studied using a structural definition of the H-bond and a network correlation function. We find the ions' effect on the solvent structure exhibits a complex dependence on specific surface interactions. The structure-making properties of ions are enhanced at the quartz surface, particularly for ions adsorbed without a complete hydration shell, and the structuring effect extends beyond the first solvation shell. The ions have a lesser effect on solvent structure in solution, especially in the presence of counterions. In fact, cations that are the greatest "structure makers" at the surface are the greatest "structure breakers" when in solution with a counterion. Therefore, we find the ions cannot be simply classified as "structure making" or "structure breaking". We discuss the implications of these findings for the effect of ions on the dissolution rate, surface charge, and solvent structure.

A simple AC calorimeter for specific heat measurement of liquids confined in porous materials: A study of hydrated Vycor.

Nanometric confinement of fluids in porous media is a classical way to stabilize metastable states. Calorimetric studies give insight on the behavior of confined liquids compared to bulk liquids. We have developed and built a simple quasi-adiabatic AC calorimeter for heat capacity measurement of confined liquids in porous media in a temperature range between 150 K and 360 K. Taking the fully hydrated porous medium as a reference, we address the thermal behavior of water as a monolayer on the surface of a porous silica glass (Vycor). For temperature ranging between 160 K and 325 K, this interfacial water shows a surprisingly large heat capacity. We describe the interfacial Hbond network in the framework of a mean field percolation model, to show that at 160 K interfacial water experiences a transformation from low density amorphous ice to a heterogeneous system where transient low and high density water patches coexist. The fraction of each species is controlled by the temperature. We identify the large entropy of the interfacial water molecules as the cause of this behaviour.

What interactions can distort the orientational distribution of interfacial water molecules as probed by second harmonic and sum frequency generation?

Aqueous interfaces are omnipresent in nature. Nonlinear optical methods such as second harmonic and sum frequency generation (SHG/SFG) are valuable techniques to access molecular level information from these interfaces. In the interpretation of SHG and SFG data for both scattering and reflection mode experiments, the relation between the second-order hyperpolarizability tensor β(2), a molecular property, and the surface second-order susceptibility χ(2), a surface averaged property, plays a central role. To correctly describe the molecular details of the interface, it needs to be determined how molecules are oriented, and what the influence is of interfacial electrostatic fields and H-bonding on the orientational distribution. Here, we revisit the relations between β(2) and χ(2) and show, by means of a Boltzmann average, that significant energy differences are needed to generate measurable changes in the molecular orientational distribution at the interface. In practice, H-bonding and surface pressure such as applied in a Langmuir trough can be strong enough to alter the shape of the orientational distribution function of water. In contrast, electrostatic fields, such as those present in the Stern layer, will not have a significant impact on the shape of the orientational distribution function of water molecules.