Orientation Of Water Molecules Research Articles

The Role of Phosphorus on Alkaline Hydrogen Oxidation Electrocatalysis for Ruthenium Phosphides.

Transition metal/p-block compounds are regarded as the most essential materials for electrochemical energy converting systems involving various electrocatalysis. Understanding the role of p-block element on the interaction of key intermediates and interfacial water molecule orientation at the polarized catalyst-electrolyte interface during the electrocatalysis is important for rational designing advanced p-block modified metal electrocatalysts. Herein, taking a sequence of ruthenium phosphides (including Ru2P, RuP and RuP2) as model catalysts, we establish a volcanic-relation between P-proportion and alkaline hydrogen oxidation reaction (HOR) activity. The dominant role of P for regulating hydroxyl binding energy is validated by active sites poisoning experiments, pH-dependent infection-point behavior, in situ surface enhanced absorption spectroscopy, and density functional theory calculations, in which P could tailor the d-band structure of Ru, optimize the hydroxyl adsorption sites across the Ru-P moieties, thereby leading to improved proportion of strongly hydrogen-bonded water and facilitated proton-coupled electron transfer process, which are responsible for the enhanced alkaline HOR performance.

The Role of Phosphorus on Alkaline Hydrogen Oxidation Electrocatalysis for Ruthenium Phosphides

Transition metal/p‐block compounds are regarded as the most essential materials for electrochemical energy converting systems involving various electrocatalysis. Understanding the role of p‐block element on the interaction of key intermediates and interfacial water molecule orientation at the polarized catalyst‐electrolyte interface during the electrocatalysis is important for rational designing advanced p‐block modified metal electrocatalysts. Herein, taking a sequence of ruthenium phosphides (including Ru2P, RuP and RuP2) as model catalysts, we establish a volcanic‐relation between P‐proportion and alkaline hydrogen oxidation reaction (HOR) activity. The dominant role of P for regulating hydroxyl binding energy is validated by active sites poisoning experiments, pH‐dependent infection‐point behavior, in situ surface enhanced absorption spectroscopy, and density functional theory calculations, in which P could tailor the d‐band structure of Ru, optimize the hydroxyl adsorption sites across the Ru‐P moieties, thereby leading to improved proportion of strongly hydrogen‐bonded water and facilitated proton‐coupled electron transfer process, which are responsible for the enhanced alkaline HOR performance.

Electric Field-Enhanced Ion Rejection Rate in Freeze Desalination.

Freeze desalination is an appealing method for seawater desalination through freezing seawater. The percentage of ions in the liquid phase, which is termed ion rejection rate, is a critical factor affecting the performance of freeze desalination. Improving the ion rejection rate is an important topic for freeze desalination. In this work, we investigate the effects of electric fields on the ion rejection rate during the freezing of seawater through molecular dynamics simulations. It is found that the ion rejection rate increases with increasing electric field strength. The enhanced ion rejection rate is due to the reduction of the energy barrier at the ice-water interface caused by the electric field, which affects the orientation of water molecules and ion-water interactions. However, the electric field hinders the ice growth rate, which affects the productivity of freeze desalination. Nevertheless, the finding in this work offers a new idea to improve the ion rejection rate. Practically, a trade-off needs to be found to optimize the overall performance of freeze desalination.

A new approach to explaining nano-bubbles paradoxical longevity

It has been established experimentally that nano-bubbles exist in water for weeks, even months. This contradicts the theory that predicts life span for such small bubbles being on the scale of milliseconds. Nano-bubbles are charged due to adsorption of OH- ions, which creates electric double layers (EDL) at their interfaces. Suggestion has been made that EDL could be responsible for nano-bubbles longevity, but the mechanism remains unknown. We propose such a mechanism here. There is one overlooked feature of the EDL – water molecules in the first layer at interface are oriented, which is supported by many experiments and theory overviewed here. EDL electric field has a gradient component, which exerts force on these oriented molecules dipole moments. We show that it is practically equal to the Young-Laplace force but opposite in direction. It ensures balance of normal forces at interface and creates a pressure gradient that prevents leakage diffusion flux from the stable bubble. In addition, oriented water dipole moments repel each other. This effect is similar to the repulsion of surfactant polar groups, which are known to counterbalance surface tension in tangential direction and stabilize bubbles. Oriented water molecules should cause the same effect. Balance of forces acting on the bubble interface leads to the equation for the stable bubble size that agrees with experimental trends.

Cholesterol Changes Interfacial Water Alignment in Model Cell Membranes.

The nanoscopic layer of water that directly hydrates biological membranes plays a critical role in maintaining the cell structure, regulating biochemical processes, and managing intermolecular interactions at the membrane interface. Therefore, comprehending the membrane structure, including its hydration, is essential for understanding the chemistry of life. While cholesterol is a fundamental lipid molecule in mammalian cells, influencing both the structure and dynamics of cell membranes, its impact on the structure of interfacial water has remained unknown. We used surface-specific vibrational sum-frequency generation spectroscopy to study the effect of cholesterol on the structure and hydration of monolayers of the lipids 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), and egg sphingomyelin (SM). We found that for the unsaturated lipid DOPC, cholesterol intercalates in the membrane without significantly changing the orientation of the lipid tails and the orientation of the water molecules hydrating the headgroups of DOPC. In contrast, for the saturated lipids DPPC and SM, the addition of cholesterol leads to clearly enhanced packing and ordering of the hydrophobic tails. It is also observed that the orientation of the water hydrating the lipid headgroups is enhanced upon the addition of cholesterol. These results are important because the orientation of interfacial water molecules influences the cell membranes' dipole potential and the strength and specificity of interactions between cell membranes and peripheral proteins and other biomolecules. The lipid nature-dependent role of cholesterol in altering the arrangement of interfacial water molecules offers a fresh perspective on domain-selective cellular processes, such as protein binding.

Molecular dynamics insights into gas-water interfacial tension: Optimizing hydrogen storage in subsurface conditions

This study presents a comprehensive analysis of interfacial tension (IFT) between pore water and hydrogen-cushion gas mixtures in subsurface porous media, a key variable in the distribution of gas mixtures for hydrogen storage. Employing molecular dynamics simulations, we developed two IFT models at 20 MPa and 373 K: one for an H2–CO2–H2O system and another for an H2–CH4–H2O system. The models reveal that increasing cushion gas concentration reduces gas-water IFT. Notably, for H2–CO2–H2O mixtures, the IFT substantially decreases with CO2 concentration up to 40%, after which the reduction rate diminishes. In H2–CH4–H2O mixtures, the IFT decreases linearly with CH4 concentration. The study attributes these variations to local density distributions and molecular orientation effects. Specifically, CO2 adsorption at the interface up to 40% concentration significantly lowers IFT, while CH4 adsorption proportionally decreases IFT. The influence of CO2 on water molecule orientation at the interface, in contrast to the non-effect of CH4, is also critical in enhancing IFT reduction. Our results also highlight the reduction of H2 self-diffusion at the interface caused by CO2 and CH4, an essential factor in optimizing subsurface hydrogen storage operations.

Fingerprint of Dipole Moment Orientation of Water Molecules in Cu2+ Aqueous Solution Probed by X-ray Photoelectron Spectroscopy.

The electronic structure and geometrical organization of aqueous Cu2+ have been investigated by using X-ray photoelectron spectroscopy (XPS) at the Cu L-edge combined with state-of-the-art ab initio molecular dynamics and a quantum molecular approach designed to simulate the Cu 2p X-ray photoelectron spectrum. The calculations offer a comprehensive insight into the origin of the main peak and satellite features. It is illustrated how the energy drop of the Cu 3d levels (≈7 eV) following the creation of the Cu 2p core hole switches the nature of the highest singly occupied molecular orbitals (MOs) from the dominant metal to the dominant MO nature of water. It is particularly revealed how the repositioning of the Cu 3d levels induces the formation of new bonding (B) and antibonding (AB) orbitals, from which shakeup mechanisms toward the relaxed H-SOMO operate. As highlighted in this study, the appearance of the shoulder near the main peak corresponds to the characteristic signature of shakeup intraligand (1a1 → H-SOMO(1b1)) excitations in water, providing insights into the average dipole moment distribution (≈36°) of the first-shell water molecules surrounding the metal ion and its direct impact on the broadening of the satellite. It is also revealed that the main satellite at 8 eV from the main peak corresponds to (metal/1b2 → H-SOMO(1b1) of water) excitations due to a bonding/antibonding (B/AB) interaction of Cu 3d levels with the deepest valence O2p/H1s 1b2 orbitals of water. This finding underscores the sensitivity of XPS to the electronic structure and orientation of the nearest water molecules around the central ion.

Impact of frozen storage under static electric field on frozen gluten rheological properties

AbstractBackground and ObjectivesElectrostatic fields, as a new food physical processing technology, can effectively improve the quality of frozen food. The purpose of this study is to investigate the effect of electrostatic field‐assisted frozen storage (SEF) on dough rheological properties, gluten structure, and its mechanism.FindingsThe results showed that the G′ and G" of the gluten were efficiently maintained by SEF throughout frozen storage. Meanwhile, after 8 weeks of frozen storage, the relative contents of α‐helices were 25.28% and 24.76%, respectively, and the relative contents of random coils were 40.58% and 41.25%, respectively, in SEFs compared with the control group. This suggested that SEFs might affect the recrystallization of ice crystals by changing the orientation of water molecules, which, in turn, alleviated the disruption of protein secondary structural order. SEF had no effect on the gauche‐gauche‐gauche conformation. SEF reduced damage to protein senior structure by frozen storage. The glutenin macropolymer (GMP) depolymerization of the control and SEF groups was 28.55% and 28.17%, respectively, and the SEF decelerated the GMP depolymerization.ConclusionsSEF slowed the disruption of the molecular structure by reducing the depolymerization of GMP and slowing the depolymerization of the α‐helix, thus maintaining the viscoelasticity of the gluten.Significance and NoveltyExploring the mechanism of the effect of electrostatic field‐assisted frozen storage on gluten quality.

Investigation of encapsulated water wire within self-assembled hydrophilic nanochannels, in a modified γ4-amino acid crystals: Tracking thermally induced changes of intermolecular interactions within a crystalline hydrate

Nanostructures formed by the self-assembly of modified/unmodified amino acids have the potential to be useful in several biological/nonbiological applications. In that regard, the greater conformational space provided by γ-amino acids, owing to their additional backbone torsional degrees of freedom and enhanced proteolytic stability, compared to their α-counterparts, should be explored. Though, modified single amino acid-based nanomaterials such as nanobelts or hydrogels are developed by utilizing the monosubstituted γ-amino acids derived from the backbone homologation of phenylalanine (Phe). Examples of a single γ-amino acid-based porous nanostructure capable of accommodating solvent molecules are not really known. The crystal structures of a modified γ4(R)Phe residue, Boc-γ4(R)Phe-OH, at different temperatures, showed that hydrogen-bonded water molecules are forming a wire inside hydrophilic nanochannels. The dynamics of intermolecular interactions between the water wire and the inner wall of the channel with relation to the temperature change was investigated by analyzing the natural bonding orbital (NBO) calculation results performed with the single crystal structures obtained at different temperature points. The NBO results showed that from 325 K onward, the strength of water–water interactions in the water wire are getting weaker, whereas, for the water–inner wall interactions, it getting stronger, suggesting a favorable change in the orientation of water molecules with temperatures, for the latter.

Theoretical investigation on potential of zero free charge of (111) and (100) surfaces of Group 10 and 11 metals

The potential of zero free charge (PZFC) value is a crucial parameter in electrochemistry. However, the evaluations of PZFC have traditionally been difficult. To overcome this challenge, we applied a hybrid solvation method that incorporates, both an explicit water layer next to the metal surface and an implicit water layer, combined with density functional theory (DFT) to simplify the PZFC evaluation. Using the (111) and (100) surfaces of Group 10 and 11 metals as model systems, we calculated their PZFC values, which showed excellent agreement with the reported data. This great match validates the accuracy and reliability of our theoretical approach. Notably, we observed that the surface structure and the orientation of water molecules have a significant influence on the PZFC values of the metals. Our study, therefore, paves the way for efficiently and accurately calculating the PZFC values of materials, which can greatly benefit their practical applications.

Structural influence of electric field direction on water bridges in one-dimensional disjoint nanochannels

The orientation of water molecules within nanochannels is pivotal in influencing water transport, particularly under the influence of electric fields. This study delves into the effects of electric field direction on water transport through disjoint nanochannels, a structure which is of emerging significance. Molecular dynamics simulations are conducted to study the properties of water in complete nanochannel and disjoint nanochannels with gap sizes of 0.2 nm and 0.4 nm, respectively, such as occupancy, transport, water bridge formation, and dipole orientation, by systematically varying the electric field direction from 0 to 180 degrees. The simulation results disclose that the electric field direction has little influence on water flow through complete nanochannels. However, as the size of the nanogap expands, the declining trend of water transfer rate through disjoint nanochannels becomes more distinctive when the electric field direction is shifted from 0 to 90 degrees under an electric field with a strength of 1 V/nm. Notably, results also reveal distinct behaviors at 90 degrees under an electric field with a strength of 1 V/nm, where the stable water chains, unstable water bridges, and no water bridges are observed in complete nanochannels, disjoint nanochannels with 0.2 nm gap, and 0.4 nm gap, respectively. Moreover, simulations indicate that increasing the electric field strength in a polarization direction perpendicular to the tube axis facilitates water bridge breakdown in disjoint nanochannels. This research sheds light on the intricate interplay between electric field direction and water transport dynamics in disjoint nanochannels, presenting valuable insights into various applications.

In Situ Surface X-Ray Diffraction Studies of the Electrochemical Double Layer on Pt(111)

The molecular structure of the electrochemical double layer is an important fundamental topic in interfacial electrochemistry. It has received renewed interest in the last years, because of its influence on many electrochemical reactions and the emergence of ab initio molecular dynamics simulations that allow a detailed description of the interface structure and dynamics [1,2]. In the measurements presented here, we investigated the properties of the electrochemical double layer on Pt(111) via in situ surface X-ray diffraction (SXRD), a technique that is well-established for single crystal studies [3-7].Platinum electrodes are widely used as electrocatalysts, for example as cathodes in proton exchange membrane fuel cells due to their high ORR reactivity. The single-crystalline Pt(111) surface, which is a well studied model system, is especially interesting for studies of the electrochemical double layer. Recent electrochemical measurements revisited this system and found evidence for a double layer structure that strongly deviated from the traditional Gouy-Chapman-Stern model and indicated a complex structural influence of water [8,9]. The water structure within the electrochemical double layer is also of major interest in computational studies. For non-adsorbing, aqueous electrolyte the electrode surface charge can be partly compensated by a layer of oriented water molecules due to their dipole moment. For potentials around the potential of zero charge (PZC), a potential-dependent orientation of water molecules with respect to the electrode surface was suggested [1,2,10].To investigate the arrangement and orientation of the water molecules near the Pt(111) surface we used in situ SXRD methods, focussing on X-ray reflectivity (XRR) and crystal truncation rod (CTR) analysis. These methods allow to acquire detailed information about the electrode surface structure as well as the electrochemical double layer. By measuring multiple CTR sets at different surface potentials, information on the potential-dependence of the interface processes can be obtained. The experiments were performed at beamline ID31 of the European Synchrotron Radiation Facility using high photon energies of 75 keV, which provide very large data sets. This allows significantly improved structural modeling as compared to conventional SXRD. The studies employed an electrochemical hanging-meniscus X-ray diffraction cell that allows for SXRD measurements of fast dynamic systems in a well-defined electrochemical environment. We will report results on the water structure in the double layer obtained in perchloric acid at potentials around the PZC.We gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft via project no. 418603497 and the BMBF via 05K19FK3 and 05K22FK1.

Intensification of the Grain Drying Process with an Ozone-Air Mixture

Background: The use of ozone allows not only to weaken the connection of moisture with the grain, but also, interacting with its internal components, the gas contributes to an increase in the temperature of the heap. Due to this, the grain must be heated for less time before reaching optimal parameters, which reduces the energy intensity of the drying process. At the same time, the separation of moisture from the grain occurs more intensively, since the gas in small concentrations improves the permeability of cell membranes, causes structural transformations of the integumentary tissues and promotes the orientation of water molecules around atomic oxygen. As a result, the drying time is significantly reduced. After treatment, the residual ozone decays to oxygen without polluting the environment. In addition, ozonation promotes deodorization, sanitation, sterilization of grain and equipment. However, the use of ozone-air mixture in grain drying in production is too insignificant due to the small experimental base and lack of practical recommendations. Purpose: Improving the efficiency of modern grain dryers through the use of the ozonation process. Materials and Мethods: The theoretical and methodological basis of the research was the work in the field of grain drying with an ozone-air mixture by leading Russian and foreign scientists, as well as their own research. The sources under study cover the first mentions of the use of ozone in grain drying and the subsequent development of this topic. The review included publications from various databases, among which are: e-library, Scopus, Web of Science, Agris, FIPS and others. Results: Hourly preliminary ozonation of grain with a humidity of 26.3% contributed to a decrease in the moisture content by 1.3% from the initial value. While in the control sample, which was not subjected to preliminary ozone treatment, the moisture content did not change over the same period. Subsequent drying of the exposed grain allowed for an hour of operation to remove 1.6% more moisture than that of the control sample. At the same time, the drying conditions of both batches of grain were identical. This indicates that the ventilation of grain with an ozone-air mixture is more effective than its presence in a buffer silo without processing. Conclusion: Pretreatment of a wet grain heap with an ozone-air mixture before feeding it to the grain dryer intensifies the process of subsequent grain drying. At the initial grain moisture content of 26.3%, hourly ozone treatment with an ozone concentration in an ozone-air mixture of 8 mg/m3 reduces the moisture content by 5.4% per hour of drying. Under the same conditions, in the experiment without processing for the same period, the decrease in grain moisture was 3.8%. Consequently, the proposed method of grain drying increases the efficiency of modern grain dryers.

DFT study of water on graphene: Synergistic effect of multilayer p-doping.

Recent experiments related to a study concerning the adsorption of water on graphene have demonstrated the p-doping of graphene, although most of the abinitio calculations predict nearly zero doping. To shed more light on this problem, we have carried out van der Waals density functional theory calculations of water on graphene for both individual water molecules and continuous water layers with coverage ranging from one to eight monolayers. Furthermore, we have paid attention to the influence of the water molecule orientation toward graphene on its doping properties. In this article, we present the results of the band structure and the Bader charge analysis, showing the p-doping of graphene can be synergistically enhanced by putting 4-8 layers of an ice-like water structure on graphene having the water molecules oriented with oxygen atoms toward graphene.

Observation of a Two-Dimensional Hydrophobic Collapse at the Surface of Water Using Heterodyne-Detected Surface Sum-Frequency Generation.

We study the effect of sodium chloride (NaCl) on the properties of the interface of water and the surfactant dodecyl sulfate (DS-) using heterodyne-detected vibrational sum-frequency generation spectroscopy. We find that the signal of the O-H stretch vibrations of oriented water molecules at the interface is highly nonlinearly dependent on the NaCl concentration. This nonlinear dependence is explained by a combination of screening of the electric field of surface-bound DS- ions pointing into the bulk and screening of the Coulomb repulsion between the headgroups of the DS- ions in the surface plane. The latter effect strongly increases the oriented water signal within a limited NaCl concentration range of 10-100 mM, indicating a two-dimensional hydrophobic collapse of the surfactant layer. The occurrence of collapse is supported by model calculations of the surface potential and surface surfactant density.

Effects of Charged Surfactants on Interfacial Water Structure and Macroscopic Properties of the Air-Water Interface.

Surfactants are used to control the macroscopic properties of air-water interfaces. However, the link between the molecular structure of surfactant adsorption and macroscopic properties is poorly understood. Using sum-frequency generation spectroscopy and molecular dynamics simulation, we show that two ionic surfactants (dodecyl trimethylammonium bromide and sodium dodecyl sulphate) with the same carbon chain lengths and head group charge magnitude (but different signs) can interact and reorient interfacial water molecules differently. DTAB forms a thicker but sparser interfacial layer than SDS. It is due to the deep penetration into the adsorption zone of Br- counterions compared to smaller Na+ ones, and also due to the flip-flop orientation of water molecules. SDS alters two distinctive interfacial water layers into a layer where H+ points to the air, forming strong hydrogen bonding with the sulphate headgroup. In contrast, only weaker dipole-dipole interactions with the DTAB headgroup are formed as they reorient water molecules with H+ point down to water. Hence, with more molecules adsorbed at the interface, SDS builds up a higher interfacial pressure than DTAB, producing lower surface tension and higher foam stability at a similar bulk concentration. Our findings offer improved knowledge for understanding various processes in the industry and nature.

Model-Dependent Solvation of the K-18 Domain of the Intrinsically Disordered Protein Tau.

A known imbalance between intra-protein and protein-water interactions in many empirical force fields results in collapsed conformational ensembles of intrinsically disordered proteins in explicit solvent simulations that disagree with experiments. Multiple strategies have been introduced in the literature to modify protein-water interactions, which improve agreement between experiments and simulations. In this work, we combine simulations with standard and modified force fields with a spatially resolved analysis of solvation free energy contributions and compare the consequences of each strategy. We find that enhanced Lennard-Jones (LJ) interactions between protein atoms and water oxygens primarily improve the solvation of nonpolar functional groups of the protein. In contrast, modified electrostatics in the water model or strengthened LJ interactions between the protein and water hydrogens mainly affect the hydration of polar functional groups. Modified electrostatics further impact the average orientation of water molecules in the hydration shell. As a result, protein-water interactions with the first hydration layers are strengthened, while interactions with water molecules in higher hydration shells are weakened. Hence, distinct strategies to balance intra-protein and protein-water interactions in simulations have qualitatively different effects on protein solvation. These differences are not necessarily captured by comparisons to experiments that report on global parameters describing protein conformational ensembles, e.g., the radius of gyration, but will influence the tendency of a protein to form aggregates or phase-separated droplets.

Electric field as a crystallization switch of heterogeneous ice formation

Using molecular dynamics simulations, we investigated the effect of external electric field on ice formation with the present of a substrate surface. It turns out that the electric field can affect the ice formation on substrate surface by altering the dipole orientation of interfacial water molecules (IWs): a crossover from inhibiting to promoting ice formation with the increase of electric field strength. According to the influence of the electric field on ice formation, the electric field strength range of 0.0 V nm−1–7.0 V nm−1 can be divided into three regions. In the region I and region III, there are both ice formation on the substrate surface. While, the behavior of IWs in the region I and region III are distinguished, including the arrangements of oxygen atoms and the dipole orientation distribution. In region II, ice formation does not occur in the system within 5 × 200 ns simulations. The IWs show a disorder structure, preventing the ice formation process on substrate. The interfacial water molecular orientation distribution and two-dimensional free energy landscape reveals that the electric field can alter the dipole orientation of the interfacial water and lead a free energy barrier, making the ice formation process harder. Our result demonstrates the external electric field can regulate the behavior of IWs, and further affect the ice formation process. The external electric field act as a crystallization switch of ice formation on substrate, shedding light into the studies on the control of ice crystallization.

Behavior of the fullerene/water system under liquid and liquid–vapor conditions: A molecular dynamics approach

Classical molecular dynamics simulations were carried out to quantify the behavior of fullerenes in water, under liquid phase and liquid–vapor conditions. Different molalities of fullerene were considered in the study. Radial distribution functions, static dielectric constant and equation of state were evaluated in liquid phase. Direct liquid–vapor coexistence simulations of water with fullerene added were conducted to explore density profiles, liquid–vapor coexistence curve, pressure tensor components and surface tension variations with respect to water. An all-atom fullerene model studied by Monticelli and the SPC/ɛ description of water were used. At ambient conditions fullerene–fullerene and fullerene-water correlations decreased as molality increased. In turn, the static dielectric constant decreased due that fullerenes hinder the dipolar orientation of water molecules. As temperature increased, the static dielectric constant lowered. Part of the study was conducted at 373 K from 10 to 24 kbar, revealing that density of the system increased as molality was higher while fullerene-fullerene correlations decreased and fullerene-water correlations increased slightly. Concerning the static dielectric constant, a decrease was observed due that fullerenes and fullerene aggregates disturbed water structure as molality increased. As the system is compressed at a fixed molality, fullerenes become more uniformly distributed and the dielectric constant increases. Therefore, increasing the pressure is a mechanism to force the dipolar orientation of water molecules. On the other hand, liquid–vapor coexistence was studied from 280 to 650 K using molalities from 0.037 to 0.695 mol/kg. Liquid–vapor simulations revealed that the location of fullerenes and the formation of aggregates affect the behavior of surface tension. Fullerenes can locate at the liquid–vapor interfaces forming aggregates that obstruct the pass of water molecules to the vapor, so surface tension increases slightly with respect to water. At 0.695 mol/kg, larger clusters were observed but they located predominantly within the liquid phase and therefore acted in the opposite direction, that is, they favored the migration of water molecules to the vapor and surface tension dropped. In addition, a peculiar behavior was observed for temperatures above 500 K: surface tension increased its value as temperature increased. This behavior could be attributed to the fact that at such high molality the calculated surface tension might not longer correspond to liquid–vapor interfaces, instead it could be related to a sort of solid–liquid interface that have formed into the system. These calculations set the basis for improving the description of fullerene/water interactions as more experimental information becomes available and also serve as reference data in the design of interaction parameters to model water-soluble fullerenes.

Molecular characteristics of the mixed cationic/anionic collector interface probed by sum-frequency vibrational spectroscopy and molecular dynamics simulations

The molecular structure and orientation of collectors and water molecules on bubble surfaces in ore pulp is a fundamental phenomenon in the process of froth flotation. Considering the adsorption behavior and orientation of collectors and water molecules at the air/solution interface is of particular significance. In this study, the molecular structure and orientation of an aqueous solution of mixed dodecylamine hydrochloride (DDA) and sodium oleate (NaOL) with different molar ratios, and water molecules were examined at the air/solution interface and characterized using a combination of surface pressure measurements, sum frequency vibrational spectroscopy (SFVS) and molecular dynamics (MD) simulations. Both individual NaOL and DDA collectors were observed to be well ordered at the interface, while gauche defects appeared in some mixed collector systems. The ordering of interfacial water molecules in the presence of collectors with increasing molar ratios of NaOL to DDA is manifested by an enhancement of OH symmetric stretch from tetrahedrally coordinated water and a decreased bond disorder in the molecular arrangement or a “liquid-like” structure. In these mixed collector systems, the COO– group from NaOL stretches deeper into water than NH3+ groups from DDA, and Na cations originating from NaOL distribute closer to COO– groups compared with the Cl originating from DDA to NH3+ groups. The interfacial water in the single NaOL system is more strongly hydrogen bonded than those in the DDA and mixed collector systems. Conversely, the alignment of interfacial water molecules in the DDA system was more disrupted. These results provide a better understanding of the ordering of mixed NaOL/DDA collectors, as well as the orientation of water molecules at the air/water interface. This might therefore provide valuable insight into the relationship between the flotation phenomena and molecular characteristics of collectors at the air/water interface and should help to improve the froth flotation technology.