Tetrahedral Hydrogen Bond Research Articles

Raman and IR spectra of water under graphene nanoconfinement at ambient and extreme pressure-temperature conditions: a first-principles study.

The nanoconfinement of water can result in dramatic differences in its physical and chemical properties compared to bulk water. However, a detailed molecular-level understanding of these properties is still lacking. Vibrational spectroscopy, such as Raman and infrared, is a popular experimental tool for studying the structure and dynamics of water, and is often complemented by atomistic simulations to interpret experimental spectra, but there have been few theoretical spectroscopy studies of nanoconfined water using first-principles methods at ambient conditions, let alone under extreme pressure-temperature conditions. Here, we compute the Raman and IR spectra of water nanoconfined by graphene at ambient and extreme pressure-temperature conditions using ab initio simulations. Our results revealed alterations in the Raman stretching and low-frequency bands due to the graphene confinement. We also found spectroscopic evidence indicating that nanoconfinement considerably changes the tetrahedral hydrogen bond network, which is typically found in bulk water. Furthermore, we observed an unusual bending band in the Raman spectrum at ∼10 GPa and 1000 K, which is attributed to the unique molecular structure of confined ionic water. Additionally, we found that at ∼20 GPa and 1000 K, confined water transformed into a superionic fluid, making it challenging to identify the IR stretching band. Finally, we computed the ionic conductivity of confined water in the ionic and superionic phases. Our results highlight the efficacy of Raman and IR spectroscopy in studying the structure and dynamics of nanoconfined water in a large pressure-temperature range. Our predicted Raman and IR spectra can serve as a valuable guide for future experiments.

Uniqueness of Nanoscale Confinement for Fast Water Transport: Effect of Nanotube Diameter and Hydrophobicity.

Inspired by the enhanced water permeability of carbon nanotubes (CNTs), molecular dynamics simulations were performed to investigate the transport behavior through nanotubes made of boron nitride (BNNT), silicon carbide (SiC), and silicon nitride (SiN) alongside carbon nanotubes (which have different hydrophobic attributes) considering their implication for reverse osmosis (RO) membranes under different practical environments. According to our findings, not only do CNTs but also other kinds of nanotubes exhibit transition anomalies with increasing diameter. Utilizing the robust two-phase thermodynamic (2PT) methods, the current examinations shed light on thermodynamic origin of favorable water filling of these nanotubes. The results show that regardless of the nanotube material, the filling of water inside small nanopores (d < 10 Å) as well as within pores of diameter larger than 15 Å will always be favored by the entropy of filling. However, the entropic preference for filling nanotubes with a diameter of 10-15 Å depends on the constituent material. In particular, the enhancement in total entropy of confined water was mainly due to the increased rotational freedom of confined water molecules. The thermodynamic origin of water transport was correlated with the structural and fluidic behavior of water inside these nanotubes. The observed data for density, flow, structure correlation functions, water-water coordination, tetrahedral order parameter, hydrogen bonds, and density of states functions quantitatively support the observed entropy behavior. Of critical importance is that the present study demonstrates the effectiveness of RO filtration using nanotubes of boron nitride rather than carbon. Furthermore, it was found that one should avoid the use of silicon nanotubes unless filtration needs to be performed under harsh environments where nanotube of other materials cannot survive. Specifically, the results show that both the structural and dynamic properties of water confined in BNNTs are similar to those of CNT's, and for SiNT it is similar as SiC. Our results show that besides the nanotube material, the chirality index of the nanotube also plays a significant role in determining the structure, dynamics and thermodynamics of confined water molecules.

Comparative Investigation of the Microstructure of MgCl2 Aqueous Solutions Using Different X-ray Scattering Sources, Raman Spectroscopy, and Atomistic Simulations.

Aqueous solutions of magnesium chloride (MgCl2(aq)) are often used to test advances in the theory of electrolyte solutions because they are considered an ideal strong 2:1 electrolyte. However, there is evidence that some ion association occurs in these solutions, even at low concentrations. Even a small ion-pairing constant can have a significant impact on the chemical speciation of ions, so it is important to determine whether ion pairing actually occurs. In this study, MgCl2(aq) with concentrations ranging from 1 to 35% was studied using three methods: X-ray scattering (XRS) with the Shanghai Synchrotron Radiation Facility (SSRF) and silver-anode laboratory sources, Raman spectroscopy, and molecular dynamics (MD) simulations with the COMPASS-II and Madrid force fields. XRS results were analyzed in the framework of PDF theory to obtain the reduced structure function F(Q) and the reduced pair distribution function G(r). The F(Q) values from synchrotron radiation and laboratory sources both showed that the tetrahedral hydrogen bonds in bulk water were destroyed with the increased MgCl2 concentration. The results of G(r) indicated that the main peaks centered at 2.05 and 2.80 Å can be ascribed to the interactions of Mg-O and O-O, respectively. The peak at 3.10 Å is attributed to the combined effect of O-O and Cl-O. By comparing the structural information on MgCl2 solution obtained from the two light sources, it was found that both SSRF and silver-anode laboratory sources can reflect the above-mentioned structural information on MgCl2 solution. The radial distribution function (RDF) obtained from MD simulations of MgCl2 solutions assigned the peaks at 2.0, 2.8, and 3.2 Å to the Mg-O, O-O, and Cl-O interatomic pairs, respectively. The decrease in the O-O coordination number confirms that the hydrogen-bonding network of water is disrupted by increasing MgCl2 observed by X-ray scattering. The proportion of Mg-Cl contact ion pairs gradually increases with MgCl2 concentration as does the coordination number. Raman spectroscopy results show that the bond type changes from double donor double acceptor (DDAA) to single donor-single acceptor (DA) with increasing concentration, providing explicit details of the hydrogen-bond evolution in the aqueous solution.

Water hydration of polyethylene glycol dimethyl ether.

In this work, GHz and THz complex dielectric spectra of a polyethylene glycol dimethyl ether (2000 g/mol) aqueous solution were studied. The reorientation relaxation of water in this kind of macro-amphiphilic molecule solution can be well described by three Debye models, including under-coordinated water, bulk-like water [water molecules in a tetrahedral hydrogen bond network (bulk water) and bulk water molecules affected by hydrophobic groups], and slow hydrating water (water molecules donating hydrogen bonds to hydrophilic ether groups). The reorientation relaxation timescales of bulk-like water and slow hydration water both show increases with concentration from 9.8 to 26.7ps and from 46.9 to 100.1ps, respectively. By estimating the ratios of the dipole moment of slow hydration water to the dipole moment of bulk-like water, we calculated the experimental Kirkwood factors of bulk-like and slow hydrating water. The experimental Kirkwood factor of bulk-like water increased from 3.17 to 3.44 with concentrations, while the experimental Kirkwood factor of slow hydrating water roughly remained constant at 4.13 from concentrations of 15%-60%. The estimated water molecule numbers of three water components around monomers also confirm our sorting for water components.

Fine analysis of the component effect on the microstructure of LiCl solution

Lithium resources are abundant in Chinese salt lakes, but the high Mg/Li ratio in such aqueous natural sources has brought difficulties to the separation and purification of lithium. Crystallization processes are highly affected by the solution microstructure, which, in turn, depends on its composition. Synchrotron X-ray scattering, Raman spectroscopy, and molecular dynamics simulation methods were used to study the microstructure of aqueous LiCl solutions with different concentrations. In the reduced structure function, F(Q), the double peak around the scattering vector Q = 2.5 Å−1 becomes a single peak, which indicates the addition of LiCl changes the tetrahedral hydrogen bond structure in the aqueous solution. The deconvolution fitting of the Raman spectra show that the DDAA-type (double donor-double acceptor) hydrogen bonds tend to be transformed into DA-type (single donor–single acceptor) hydrogen bonds, confirming that the addition of LiCl breaks the tetrahedral hydrogen bond structure. This hydrogen-bond transformation was confirmed by pair distribution function analysis from molecular dynamics simulations of aqueous KCl solutions. In addition, considering the large number of magnesium ions coexisting with LiCl in the Salt Lake, the microstructure of mixed LiCl-MgCl2 solution was also studied. According to the X-ray scattering results, the peak in the reduced pair distribution function G(r) near r = 3.25 Å shifted from 3.25 Å to 3.10 Å with increased Mg/Li ratio. This change is likely to result from the Cl-O and OO interactions in the mixed aqueous solution. The deconvolution fitting results of Raman spectra show that the DDAA type hydrogen bond increases with the Mg/Li ratio, while the DA + DDA type hydrogen bond decreases. The molecular dynamics results showed that chlorine ions have a higher tendency to form contact ion pairs with Mg2+ than Li+. We propose this to be one of the processes retarding the separation of pure LiCl crystal from mixed LiCl-MgCl2 systems.

Study on the Structure of a Mixed KCl and K2SO4 Aqueous Solution Using a Modified X-ray Scattering Device, Raman Spectroscopy, and Molecular Dynamics Simulation.

The microstructure of a mixed KCl and K2SO4 aqueous solution was studied using X-ray scattering (XRS), Raman spectroscopy, and molecular dynamics simulation (MD). Reduced structure functions [F(Q)], reduced pair distribution functions [G(r)], Raman spectrum, and pair distribution functions (PDF) were obtained. The XRS results show that the main peak (r = 2.81 Å) of G(r) shifted to the right of the axis (r = 3.15 Å) with increased KCl and decreased K2SO4. The main peak was at r = 3.15 Å when the KCl concentration was 26.00% and the K2SO4 concentration was 0.00%. It is speculated that this phenomenon was caused by the main interaction changing, from K-OW (r = 2.80 Å) and OW-OW (r = 2.80 Å), to Cl−-OW (r = 3.14 Å) and K+-Cl− (r = 3.15 Å). According to the trend of the hydrogen bond structure in the Raman spectrum, when the concentration of KCl was high and K2SO4 was low, the destruction of the tetrahedral hydrogen bond network in the solution was more serious. This shows that the destruction strength of the anion to the hydrogen bond network structure in solution was Cl− > SO42−. In the MD simulations, the coordination number of OW-OW decreased with increasing KCl concentration, indicating that the tetrahedral hydrogen bond network was severely disrupted, which confirmed the results of the Raman spectroscopy. The hydration radius and coordination number of SO42− in the mixed solution were larger than Cl−, thus revealing the reason why the solubility of KCl in water was greater than that of K2SO4 at room temperature.

Investigated hydrogen-bond network kinetics of acetone-water solutions by spontaneous and stimulated Raman spectroscopy.

The hydrogen bond (HB) network structure and kinetics of the acetone-water mixed solutions were investigated by the spontaneous Raman and stimulated Raman scattering (SRS) spectra. The HB network of water molecules was enhanced when the volume fraction of acetone ranged from 0 to 0.25. Two new SRS peaks of water at 3272 and 3380 cm-1 were obtained, resulting from the cooperation of the polar carbonyl (C = O)-enhanced HB and the ice-like structure formed around the methyl groups. However, when the volume fraction went beyond 0.25, the spontaneous Raman main peak at 3445 cm-1 showed a significant blue-shift, and the corresponding SRS signal disappeared, indicating that the HB of water was weakened, which originated from the self-association of acetone. In the meantime, the fully tetrahedral HB structure among water molecules was destroyed at the higher volume fraction (≥ 0.8). Hopefully, our study here would advance the study of HB network structures and kinetics in other aqueous solutions.

The trade-off effect of KCl and NH4Cl on the hydrated structure in their mixed aqueous solutions

The hydrated structure of KCl and NH4Cl mixed solution were studied by X-ray scattering, Raman spectroscopy and molecular dynamics (MD) simulation method in the range of 0.00%-28.0% mass fraction at 25 °C. Through the analysis of reduced pair distribution function [G(r)], it was found that when the concentration of KCl in the mixed aqueous solution decreased and the concentration of NH4Cl increased, the main peak of G(r) shifted from r = 2.81 Å to the right of the axis. In the absence of KCl, the position of the main peak was r = 3.15 Å. It was speculated that this phenomenon was caused by the main interaction changed from K-OW and OW-OW (r = 2.80 Å) to NO (r = 3.06 Å) and NH4+-Cl− (r = 3.3 Å). It is indicated by the Raman spectra, when the concentration of KCl is high and the concentration of NH4Cl is low, the destruction of the tetrahedral hydrogen bond network in the solution is more serious. In the MD simulation, the coordination number of OW-OW decreases with the increase of KCl concentration, indicating that the tetrahedral hydrogen bond network is severely damaged, which is concerned with the results of Raman spectroscopy.

Temperature-dependent structure of 3.5 wt.% NaCl aqueous solution: Theoretical and Raman investigation

In the laboratory, the 3.5 wt.% NaCl aqueous solution is commonly used to simulate the seawater environment. The thermodynamic and kinetic research on the aqueous solution structure is intriguing, and it would aid in explaining the initial mechanism of the seawater-related physical and chemical phenomena. In this paper, the solution structure and evolution of the hydrogen bond network at the temperature range of the marine environment were obtained using classical molecular simulation and in-situ Raman spectroscopy. Specifically, the tetrahedral hydrogen bond structure was decomposed into two types: DDAAa affected by ions and DDAAn unaffected by ions. Furthermore, the enthalpy and entropy change of hydrogen bond evolution in the temperature range were calculated to be 14.05 ± 0.75 J·mol−1 and 53.38 ± 2.49 J·mol−1·K−1. This work lays the groundwork for preliminary mechanism research of life phenomena, physical properties, and chemical reactions in the marine environment.

Synergism of dynamics of tetrahedral hydrogen bonds of liquid water

We used modified Arrhenius approximations to analyze known temperature dependences (TDs) of water microstructure parameters and its dielectric and dynamic characteristics. Analysis of activation energies showed a significant difference in the molecular dynamics of water in ranges 273–298 and 300–373 K. Features of TDs in the first range were associated with the dynamics of a metastable ice-like phase of water, in which hexagonal clusters with tetrahedral hydrogen bonds (HBs) predominate. Based on ratios of signs and values of activation energies of HBs' fluctuations and parameters of a microstructure, it was assumed that fluctuations of HBs' dipoles play a key role in the mechanism of resonant activation by a thermal energy of concertedly reactions of deformation, rupture, and formation of tetrahedral HBs in water clusters. Synergism of these reactions and interaction of charges of the vacant acceptor and donor tetrahedral orbitals of an oxygen atom trigger at 298 K an explosive transition of a metastable ice-like phase of water into an argon-like phase. Synergy of water dynamics above 298 K is adequately characterized by the product Dη, from which TDs follow the activation energies of reactions that determine the form of Stokes–Einstein relation in temperature ranges below and above 298 K point.

Understanding how water models affect the anomalous pressure dependence of their diffusion coefficients

The self-diffusion coefficient of water shows an anomalous increase with increasing hydrostatic pressure up to a broad maximum (PmD) near 1 kbar at 298 K, which has been attributed to pressure effects on the tetrahedral hydrogen bond network of water. Moreover, the ability of a water model to reproduce anomalous properties of water is a signature that it is reproducing the network. Here, water was simulated between 1 bar and 5 kbar using three water models, two four-site (with all charges in the molecular plane) and one single-site multipole (which accounts for out-of-molecular plane charge), that have reasonable pressure-temperature properties. For these three models, the diffusion coefficients display a maximum in the pressure dependence and the radial distribution functions show good agreement with the limited experimental structural data at high pressure that are available. In addition, a variety of properties associated with the network are examined, including hydrogen bond lifetimes and occupancies, three-body angle distributions, and tetrahedral order parameters. Results suggest that the initial increasing diffusion with pressure is because hydrogen bonds are distorted and thus weakened by pressure, but above PmD, the hydrogen bonds are weakened to the point it behaves more like a normal liquid. In other words, the PmD may be a measure of the angular strength of hydrogen bonds. In addition, since the four-site models over-predict the values of PmD while the multipole model under-predicts it, out-of-plane charge may improve four-site models.

Stimulated Raman scattering investigation of isotopic substitution H2O/D2O system

Stimulated Raman scattering（SRS）was used to study H2O, D2O, and their mixtures with different volume ratios. The peaks of the ice-like tetrahedral hydrogen bond structure and the peaks associated with the interaction between OH/OD groups and excess electrons in the hydrated electron clusters were observed in the forward SRS spectra. Due to the influence of isotopic substitution, the hydrogen bond structure is transformed from ordered to disordered and the binding of OH groups to electrons is weakened, the Raman peaks move toward high wavenumbers and new peaks arise. Influenced by the SRS excitation mechanism, the SRS peaks of the isotopic substitution H2O/D2O system show dependence on the substitution ratio, and the vibration mode of a single stretching region can be selected for excitation. Furthermore, the hydrogen bond structure differences in H2O and D2O are further discussed.

Spectroscopic fingerprints in the low frequency spectrum of ice (Ih), clathrate hydrates, supercooled water, and hydrophobic hydration reveal similarities in the hydrogen bond network motifs.

Solid phases of water, such as ice (Ih) and clathrate hydrates, form characteristic hydrogen bond network motifs, such as hexagonal ice, pentagons, and dodecahedrons. The same motifs might be present in supercooled water and in the hydration structure around hydrophobes. Here, we present the characteristic low frequency fingerprints of ice (Ih), tetrahydrofuran (THF) clathrate hydrates, and tetrabutyl-ammonium bromide (TBAB) semiclathrate close to their melting point, as well as supercooled water at 266.6 K and aqueous alcohol solutions. Interestingly, we find in all these cases two characteristic resonances in the THz frequency range: at least, one intensive band in the frequency range between 190 cm-1 and 220 cm-1 which is a characteristic of a tetrahedral hydrogen bond network configuration and a second band in the frequency range between 140 cm-1 and 170 cm-1, indicating a component with weaker hydrogen bonds. For solvated alcohols, we find spectroscopic fingerprints of a clathratelike structure at 164 cm-1 as well as a tetrahedral network structure at 194 cm-1, which is close to one of ice (Ih) at 192 cm-1. We propose that in the hydration shell of hydrophobes, both structural motifs are present. In the case of supercooled water-unlike ice-only one peak was found in the frequency range between 190 cm-1 and 220 cm-1. Interestingly, the latter peak center-frequency (204 cm-1) corresponds to the average of those of the two peaks observed for ice Ih (191 cm-1 and 215 cm-1). This indicates a homogeneous intermediate hydrogen bonding, providing no evidence for any heterogeneity in two high-density and low-density phases.

Energy transformation and dynamic properties of N,N-dimethylformamide aqueous solution under E/M field using NNVE MD simulations

Nonequilibrium NVE (NNVE) molecular dynamics (MD) simulations have been performed to investigated the N,N-dimethylformamide aqueous solution in both the absence and presence of E/M field (E=5×108V/m, the microwave frequency are from 0 to 100GHz). The main target of this work is to explore the energy transformation and dynamic properties of DMF aqueous solution during the microwave heating process. The temperature variation, microwave heating rate and energy distribution have been obtained. The direct coupling of rotational motion, vibration and reorientation have been revealed during the microwave energy distribution, and there are two-thirds microwave energy are stored as the intermolecular energy. The structural characteristics are further obtained from the radial distribution functions, tetrahedral order parameters and hydrogen bond. Compare to the conventional heating, the molecular conformations show significant difference and the structure of aqueous solution is intensely disturbed. The apparent increase of diffusion coefficient discloses that high microwave frequency accelerates the molecules motion.

The Adam–Gibbs relation and the TIP4P/2005 model of water

ABSTRACTWe report a numerical test of the Adam–Gibbs relation for the TIP4P/2005 model of water. The configurational entropy is here evaluated as the logarithm of the number of different basins in the potential energy landscape sampled in equilibrium conditions. Despite the non-monotonic behaviour which characterise the density dependence of the diffusion coefficient, the Adam–Gibbs relation is satisfied within the numerical precision in a wide range of densities and temperatures. We also show that expressions based on the excess entropy (the logarithm of the number of sampled microstates in phase space) fail in the region of densities where a tetrahedral hydrogen bond network develops.

High density liquid structure enhancement in glass forming aqueous solution of LiCl.

We investigate using molecular dynamics simulations the dynamical and structural properties of LiCl:6H2O aqueous solution upon supercooling. This ionic solution is a glass forming liquid of relevant interest in connection with the study of the anomalies of supercooled water. The LiCl:6H2O solution is easily supercooled and the liquid state can be maintained over a large decreasing temperature range. We performed simulations from ambient to 200 K in order to investigate how the presence of the salt modifies the behavior of supercooled water. The study of the relaxation time of the self-density correlation function shows that the system follows the prediction of the mode coupling theory and behaves like a fragile liquid in all the range explored. The analysis of the changes in the water structure induced by the salt shows that while the salt preserves the water hydrogen bonds in the system, it strongly affects the tetrahedral hydrogen bond network. Following the interpretation of the anomalies of water in terms of a two-state model, the modifications of the oxygen radial distribution function and the angular distribution function of the hydrogen bonds in water indicate that LiCl has the role of enhancing the high density liquid component of water with respect to the low density component. This is in agreement with recent experiments on aqueous ionic solutions.

Pressure dependence of viscosity in supercooled water and a unified approach for thermodynamic and dynamic anomalies of water

The anomalous decrease of the viscosity of water with applied pressure has been known for over a century. It occurs concurrently with major structural changes: The second coordination shell around a molecule collapses onto the first shell. Viscosity is thus a macroscopic witness of the progressive breaking of the tetrahedral hydrogen bond network that makes water so peculiar. At low temperature, water at ambient pressure becomes more tetrahedral and the effect of pressure becomes stronger. However, surprisingly, no data are available for the viscosity of supercooled water under pressure, in which dramatic anomalies are expected based on interpolation between ambient pressure data for supercooled water and high pressure data for stable water. Here we report measurements with a time-of-flight viscometer down to [Formula: see text] and up to [Formula: see text], revealing a reduction of viscosity by pressure by as much as 42%. Inspired by a previous attempt [Tanaka H (2000) J Chem Phys 112:799-809], we show that a remarkably simple extension of a two-state model [Holten V, Sengers JV, Anisimov MA (2014) J Phys Chem Ref Data 43:043101], initially developed to reproduce thermodynamic properties, is able to accurately describe dynamic properties (viscosity, self-diffusion coefficient, and rotational correlation time) as well. Our results support the idea that water is a mixture of a high density, "fragile" liquid, and a low density, "strong" liquid, the varying proportion of which explains the anomalies and fragile-to-strong crossover in water.

Broadband dielectric spectroscopy of glucose aqueous solution: Analysis of the hydration state and the hydrogen bond network.

Recent studies of saccharides' peculiar anti-freezing and anti-dehydration properties point to a close association with their strong hydration capability and destructuring effect on the hydrogen bond (HB) network of bulk water. The underlying mechanisms are, however, not well understood. In this respect, examination of the complex dielectric constants of saccharide aqueous solutions, especially over a broadband frequency region, should provide interesting insights into these properties, since the dielectric responses reflect corresponding dynamics over the time scales measured. In order to do this, the complex dielectric constants of glucose solutions between 0.5 GHz and 12 THz (from the microwave to the far-infrared region) were measured. We then performed analysis procedures on this broadband spectrum by decomposing it into four Debye and two Lorentz functions, with particular attention being paid to the β relaxation (glucose tumbling), δ relaxation (rotational polarization of the hydrated water), slow relaxation (reorientation of the HB network water), fast relaxation (rotation of the non-HB water), and intermolecular stretching vibration (hindered translation of water). On the basis of this analysis, we revealed that the hydrated water surrounding the glucose molecules exhibits a mono-modal relaxational dispersion with 2-3 times slower relaxation times than unperturbed bulk water and with a hydration number of around 20. Furthermore, other species of water with distorted tetrahedral HB water structures, as well as increases in the relative proportion of non-HB water molecules which have a faster relaxation time and are not a part of the surrounding bulk water HB network, was found in the vicinity of the glucose molecules. These clearly point to the HB destructuring effect of saccharide solutes in aqueous solution. The results, as a whole, provide a detailed picture of glucose-water and water-water interactions in the vicinity of the glucose molecules at various time scales from sub-picosecond to hundreds of picoseconds.

Hydration and Hydrogen Bond Network of Water during the Coil-to-Globule Transition in Poly(N-isopropylacrylamide) Aqueous Solution at Cloud Point Temperature.

Aqueous solutions of poly(N-isopropylacrylamide), P-NIPAAm, exhibit a noticeable temperature responsive change in molecular conformation at a cloud point temperature (Tcp). As the temperature rises above Tcp, the extended coil-like P-NIPAAm structure changes into a swollen globule-like conformation as hydration levels decrease and hydrophobic interactions increase. Though water plays an important role in this coil-to-globule transition of P-NIPAAm, the behavior of water molecules and the associated hydrogen-bond (HB) network of the surrounding bulk water are still veiled in uncertainty. In this study, we elucidate changes in the hydration state and the dynamical structure of the water HB network of P-NIPAAm aqueous solutions during the coil-to-globule transition by analyzing the complex dielectric constant in the terahertz region (0.25-12 THz), where bulk water reorientations and intermolecular vibrations of water can be selectively probed. The structural properties of the water HB network were examined in terms of the population of the non-HB water molecules (not directly engaged in the HB network or hydrated to P-NIPAAm) and the tetrahedral coordination of the water molecules engaged in the HB network. We found the hydration number below Tcp (≈10) was decreased to approximately 6.5 as temperature increased, in line with previous studies. The HB network of bulk water becomes more structured as the coil-to-globule phase transition takes place, via decreases in non-HB water and reduction in the orderliness of the tetrahedral HB architecture. Together these results indicate that the coil-to-globule transition is associated with a shift to hydrophobic-dominated interactions that drive thermoresponsive structural changes in the surrounding water molecules.

Hydrogen-bond dynamics of water in presence of an amphiphile, tetramethylurea: signature of confinement-induced effects

Various experimental and simulation studies have suggested that the presence of amphiphilic molecules in aqueous solutions substantially perturbs the tetrahedral hydrogen-bond (H-bond) network of neat liquid water. Such structural perturbation is expected to impact H-bond lifetime of liquid water. Tetramethylurea (TMU) is an example of an amphiphile because it possesses both hydrophobic and hydrophilic moieties. Molecular dynamics simulations of (water+TMU) binary mixtures at various compositions have been performed in order to investigate the microscopic mechanism through which the amphiphiles influence the H-bond dynamics of liquid water at room temperature. Present simulations indicate lengthening of both water–water H-bond lifetime and H-bond structural relaxation time upon addition of TMU in aqueous solution. At the highest TMU mole fraction studied, H-bond lifetime and structural relaxation time are, respectively, ∼4 and ∼8 times longer than those in neat water. This is comparable with the slowing down of H-bond dynamics for water molecules confined in cyclodextrin cavities. Simulated relaxation profiles are multi-exponential in character at all mixture compositions, and simulated radial distribution functions suggest enhanced water–water and water–TMU interactions upon addition of TMU. No evidence for complete encapsulation of TMU by water H-bond network has been found.