Reorientation dynamics of nanoconfined water: power-law decay, hydrogen-bond jumps, and test of a two-state model

The reorientation dynamics of water confined within nanoscale, hydrophilic silica pores are investigated using molecular dynamics simulations. The effect of surface hydrogen-bonding and electrostatic interactions are examined by comparing with both a silica pore with no charges (representing hydrophobic confinement) and bulk water. The OH reorientation in water is found to slow significantly in hydrophilic confinement compared to bulk water, and is well-described by a power-law decay extending beyond one nanosecond. In contrast, the dynamics of water in the hydrophobic pore are more modestly affected. A two-state model, commonly used to interpret confined liquid properties, is tested by analysis of the position-dependence of the water dynamics. While the two-state model provides a good fit of the orientational decay, our molecular-level analysis evidences that it relies on an over-simplified picture of water dynamics. In contrast with the two-state model assumptions, the interface dynamics is markedly heterogeneous, especially in the hydrophilic pore and there is no single interfacial state with a common dynamics.     

2H NMR studies of glycerol dynamics in protein matrices

We use (2)H NMR spectroscopy to investigate the rotational motion of glycerol molecules in matrices provided by the connective tissue proteins elastin and collagen. Analyzing spin-lattice relaxation, line-shape properties, and stimulated-echo decays, we determine the rates and geometries of the motion as a function of temperature and composition. It is found that embedding glycerol in an elastin matrix leads to a mild slowdown of glycerol reorientation at low temperatures and glycerol concentrations, while the effect vanishes at ambient temperatures or high solvent content. Furthermore, it is observed that the nonexponential character of the rotational correlation functions is much more prominent in the elastin matrix than in the bulk liquid. Results from spin-lattice relaxation and line shape measurements indicate that, in the mixed systems, the strong nonexponentiality is in large part due to the existence of distributions of correlation times, which are broader on the long-time flank and, hence, more symmetric than in the neat system. Stimulated-echo analysis of slow glycerol dynamics reveals that, when elastin is added, the mechanism for the reorientation crosses over from small-angle jump dynamics to large-angle jump dynamics and the geometry of the motion changes from isotropic to anisotropic. The results are discussed against the background of present and previous findings for glycerol and water dynamics in various protein matrices and compared with observations for other dynamically highly asymmetric mixtures so as to ascertain in which way the viscous freezing of a fast component in the matrix of a slow component differs from the glassy slowdown in neat supercooled liquids.     

Influence of concentration and temperature on the dynamics of water in the hydrophobic hydration shell of tetramethylurea

We study the influence of the amphipilic compound tetramethylurea (TMU) on the dynamical properties of water, using dielectric relaxation spectroscopy in the regime between 0.2 GHz and 2 THz. This technique is capable of resolving different water species, their relative fractions, and their corresponding reorientation dynamics. We find that the reorientation dynamics of water molecules in the hydration shell of the hydrophobic groups of TMU is between 3 (at low concentrations) and 10 (at higher concentrations) times slower than the dynamics of bulk water. The data indicate that the effect of hydrophobic groups on water is strong but relatively short-ranged. With increasing temperature, the fraction of water contained in the hydrophobic hydration shell decreases, which implies that the overall effect of hydrophobic groups on water becomes smaller.     

Structure and dynamics of water in nonionic reverse micelles: A combined time-resolved infrared and small angle x-ray scattering study

We study the structure and reorientation dynamics of nanometer-sized water droplets inside nonionic reverse micelles (water/Igepal-CO-520/cyclohexane) with time-resolved mid-infrared pump-probe spectroscopy and small angle x-ray scattering. In the time-resolved experiments, we probe the vibrational and orientational dynamics of the O-D bonds of dilute HDO:H(2)O mixtures in Igepal reverse micelles as a function of temperature and micelle size. We find that even small micelles contain a large fraction of water that reorients at the same rate as water in the bulk, which indicates that the polyethylene oxide chains of the surfactant do not penetrate into the water volume. We also observe that the confinement affects the reorientation dynamics of only the first hydration layer. From the temperature dependent surface-water dynamics, we estimate an activation enthalpy for reorientation of 45 ± 9 kJ mol(-1) (11?±?2 kcal mol(-1)), which is close to the activation energy of the reorientation of water molecules in ice.     

Ab Initio H2O in Realistic Hydrophilic Confinement

A protocol for the ab initio construction of a realistic cylindrical pore in amorphous silica, serving as a geometric nanoscale confinement for liquids and solutions, is presented. Upon filling the pore with liquid water at different densities, the structure and dynamics of the liquid inside the confinement can be characterized. At high density, the pore introduces long‐range oscillations into the water density profile, which makes the water structure unlike that of the bulk across the entire pore. The tetrahedral structure of water is also affected up to the second solvation shell of the pore wall. Furthermore, the effects of the confinement on hydrogen bonding and diffusion, resulting in a weakening and distortion of the water structure at the pore walls and a slowdown in diffusion, are characterized.     

A comparative molecular simulation study of the glass former ortho-terphenyl in bulk and freestanding films

We performed molecular dynamics simulations of the low-molecular weight organic glass former ortho-terphenyl in bulk and freestanding films. The main motivation is to provide molecular insight into the confinement effect without explicit interfaces. Based on earlier models of ortho-terphenyl we developed an atomistic model for bulk simulations. The model reproduces literature data both from simulations and experiments starting from specific volume and diffusivity to mean square displacement and radial distribution functions. After characterizing the bulk model we form freestanding films by the elongation and expansion method. These films give us the opportunity to study the dynamical heterogeneity near the glass transition through in-plane mobility and reorientation dynamics. We finally compare the model in bulk and under confinement. We found qualitatively a lower glass transition temperature for the freestanding film compared to the bulk.     

Molecular modeling and simulation of water near model micelles: diffusion, rotational relaxation and structure at the hydration interface

This paper reports on molecular dynamics simulations of two hydrated micelles composed of C12E6 and LDAO surfactants. The simulations results provide a quantitative picture of the dynamics of the hydration water at the water/micelle interface. Both the residence time of water near the micelle surface and its retardation with respect to the bulk have been estimated. It is found that the water dynamics is radically different for the two micellar systems and depends on the physical nature of the micelle surface in contact with water. For C12E6 this interface is thicker and presents a stronger hydrophilic character than that of LDAO. Thus, in C12E6, surface water dynamics is 1-2 orders of magnitude slower than that of bulk water, compared with only 18% for the LDAO system. The simulations have also revealed the nature of the rotational landscape experienced by water at the micellar surface: In the C12E6 micelle water rotation occurs in a highly anisotropic space due to confinement of waters at the interface; in LDAO the rotational landscape is instead isotropic. These findings clearly indicate that the slowdown of interfacial water relaxation near complex micelles depends, case by case, on the structural properties of the interface itself, such as the ratio between hydrophobic/hydrophilic exposed regions and on the interface thickness and topography.     

Water dynamics at protein interfaces: ultrafast optical Kerr effect study

The behavior of water molecules surrounding a protein can have an important bearing on its structure and function. Consequently, a great deal of attention has been focused on changes in the relaxation dynamics of water when it is located at the protein surface. Here we use the ultrafast optical Kerr effect to study the H-bond structure and dynamics of aqueous solutions of proteins. Measurements are made for three proteins as a function of concentration. We find that the water dynamics in the first solvation layer of the proteins are slowed by up to a factor of 8 in comparison to those in bulk water. The most marked slowdown was observed for the most hydrophilic protein studied, bovine serum albumin, whereas the most hydrophobic protein, trypsin, had a slightly smaller effect. The terahertz Raman spectra of these protein solutions resemble those of pure water up to 5 wt % of protein, above which a new feature appears at ~80 cm(-1), which is assigned to a bending of the protein amide chain.     

Collective dynamics of a ferroelectric smectogen in geometrical confinement

We investigate the influence of geometric confinement on the dynamics in a ferroelectric smectic C* material. Molecular and collective relaxation processes are studied by means of broadband dielectric spectroscopy. In microporous material (pore size 200nm) the Goldstone mode (GM) is still observed, but it is shifted to lower frequencies compared with bulk samples. In nanoporous material (pore size 7.5 nm), the collective reorientation modes are completely suppressed. In all samples, the molecular processes seem to be uninfluenced by the confinement.     

Thermal signature of hydrophobic hydration dynamics

Hydrophobic hydration, the perturbation of the aqueous solvent near an apolar solute or interface, is a fundamental ingredient in many chemical and biological processes. Both bulk water and aqueous solutions of apolar solutes behave anomalously at low temperatures for reasons that are not fully understood. Here, we use (2)H NMR relaxation to characterize the rotational dynamics in hydrophobic hydration shells over a wide temperature range, extending down to 243 K. We examine four partly hydrophobic solutes: the peptides N-acetyl-glycine-N'-methylamide and N-acetyl-leucine-N'-methylamide, and the osmolytes trimethylamine N-oxide and tetramethylurea. For all four solutes, we find that water rotates with lower activation energy in the hydration shell than in bulk water below 255 +/- 2 K. At still lower temperatures, water rotation is predicted to be faster in the shell than in bulk. We rationalize this behavior in terms of the geometric constraints imposed by the solute. These findings reverse the classical "iceberg" view of hydrophobic hydration by indicating that hydrophobic hydration water is less ice-like than bulk water. Our results also challenge the "structural temperature" concept. The two investigated osmolytes have opposite effects on protein stability but have virtually the same effect on water dynamics, suggesting that they do not act indirectly via solvent perturbations. The NMR-derived picture of hydrophobic hydration dynamics differs substantially from views emerging from recent quasielastic neutron scattering and pump-probe infrared spectroscopy studies of the same solutes. We discuss the possible reasons for these discrepancies.     

The Hydrophobic Gap at High Hydrostatic Pressures

We have gained new insight into the so‐called hydrophobic gap, a molecularly thin region of decreased electron density at the interface between water and a solid hydrophobic surface, by X‐ray reflectivity experiments and molecular dynamics simulations at different hydrostatic pressures. Pressure variations show that the hydrophobic gap persists up to a pressure of 5 kbar. The electron depletion in the interfacial region strongly decreases with an increase in pressure, indicating that the interfacial region is compressed more strongly than bulk water. The decrease is most significant up to 2 kbar; beyond that, the pressure response of the depletion is less pronounced.     

Orientational dynamics of water trapped between two nanoscopic hydrophobic solutes: A molecular dynamics simulation study

We investigate thoroughly the effect of confinement and solute topology on the orientational dynamics of water molecule in the interplate region between two nanoscopic hydrophobic paraffinlike plates. Results are obtained from molecular dynamics simulations of aqueous solutions of paraffinlike plates in the isothermal-isobaric ensemble. An analysis of survival time auto correlation function shows that the residence time of the water molecule in the confined region between two model nanoscopic hydrophobic plates depends on solute surface topology (intermolecular distance within the paraffinlike plate). As expected, the extent of confinement also changes the residence time of water molecules considerably. Orientational dynamics was analyzed along three different directions, viz., dipole moment, HH, and perpendicular to molecular plane vectors. It has been demonstrated that the rotational dynamics of the confined water does not follow the Debye rotational diffusion model, and surface topology of the solute plate and the extent of confinement have considerable effect on the rotational dynamics of the confined water molecules.     

Simulations of the confinement of ubiquitin in self-assembled reverse micelles

We describe the effects of confinement on the structure, hydration, and the internal dynamics of ubiquitin encapsulated in reverse micelles (RM). We performed molecular dynamics simulations of the encapsulation of ubiquitin into self-assembled protein/surfactant reverse micelles to study the positioning and interactions of the protein with the RM and found that ubiquitin binds to the RM interface at low salt concentrations. The same hydrophobic patch that is recognized by ubiquitin binding domains in vivo is found to make direct contact with the surfactant head groups, hydrophobic tails, and the iso-octane solvent. The fast backbone N-H relaxation dynamics show that the fluctuations of the protein encapsulated in the RM are reduced when compared to the protein in bulk. This reduction in fluctuations can be explained by the direct interactions of ubiquitin with the surfactant and by the reduced hydration environment within the RM. At high concentrations of excess salt, the protein does not bind strongly to the RM interface and the fast backbone dynamics are similar to that of the protein in bulk. Our simulations demonstrate that the confinement of protein can result in altered protein dynamics due to the interactions between the protein and the surfactant.     

Hydration Dynamics of Water near an Amphiphilic Model Peptide at Low Hydration Levels: A Dielectric Relaxation Study

A dielectric relaxation study of aqueous solutions of the amphiphilic model peptide N‐acetyl‐leucine amide (NALA) at 298 K over a wide range of hydration levels is presented. The experiments range from states where water builds up several hydration layers to states where single water molecules or small water clusters are shared by several NALA molecules. The dielectric spectra reveal two modes on the 10 and 100 ps timescales. These are largely broadened with regard to the Lorentzian shape caused by simple Debye‐type relaxation, and are well described by the Kohlrausch–Williams–Watts stretched exponential function. The fast mode is assigned to water reorientation comprising bulk water as well as hydration water. Even when all water molecules are in contact with the solute, this fast component is dominant, and its mean relaxation time is retarded by less than a factor of two relative to neat water. The amplitude of the slow process is far higher than expected for the dipolar reorientation of the solute. The observations are consistent with results from molecular dynamics simulations for a similar model peptide reported in the literature. They suggest that the slow relaxation mode is mainly founded in peptide–water dipolar couplings, with some additional contribution from slowly reorienting hydration water molecules. The results are discussed with regard to the hydration dynamics of proteins and the interpretation of dielectric spectra of protein solutions.     

Radiowave dielectric investigation of water confined in channels of carbon nanotubes

Structure and dynamics of water confined in channels of diameter of few nanometer in size strongly differ from the ones of water in the bulk phase. Here, we present radiowave dielectric relaxation measurements on water-filled single-walled carbon nanotubes, with the aim of highlighting some aspects on the molecular electric dipole organization of water responding to high spatial confinement in a hydrophobic environment. The observed dielectric spectra, resulting into two contiguous relaxation processes, allow us to separate the confined water in the interior of the nanotubes from external water, providing support for the existence in the confinement region of water domains held together by hydrogen bonds. Our results, based on the deconvolution of the dielectric spectra due to the presence of a bulk and a confined water phase, furnish a significantly higher Kirkwood correlation factor, larger than the one of water in bulk phase, indicating a strong correlation between water molecules inside nanotubes, not seen in bulk water.     

Temperature dependence of dimerization and dewetting of large-scale hydrophobes: a molecular dynamics study

We studied by molecular dynamics simulations the temperature dependence of hydrophobic association and drying transition of large-scale solutes. Similar to the behavior of small solutes, we found the association process to be characterized by a large negative heat capacity change. The origin of this large change in heat capacity is the high fragility of hydrogen bonds between water molecules at the interface with hydrophobic solutes; an increase in temperature breaks more hydrogen bonds at the interface than in the bulk. With increasing temperature, both entropy and enthalpy changes for association strongly decrease, while the change in free energy weakly varies, exhibiting a small minimum at high temperatures. At around T=Ts=360 K, the change in entropy is zero, a behavior similar to the solvation of small nonpolar solutes. Unexpectedly, we find that at Ts, there is still a substantial orientational ordering of the interfacial water molecules relative to the bulk. Nevertheless, at this point, the change in entropy vanishes due to a compensating contribution of translational entropy. Thus, at Ts, there is rotational order and translational disorder of the interfacial water relative to bulk water. In addition, we studied the temperature dependence of the drying-wetting transition. By calculating the contact angle of water on the hydrophobic surface at different temperatures, we compared the critical distance observed in the simulations with the critical distance predicted by macroscopic theory. Although the deviations of the predicted from the observed values are very small (8-23%), there seems to be an increase in the deviations with an increase in temperature. We suggest that these deviations emerge due to increased fluctuations, characterizing finite systems, as the temperature increases.     

Communication: On the locality of hydrogen bond networks at hydrophobic interfaces

The formation of structured hydrogen bond networks in the solvation shells immediate to hydrophobic solutes is crucial for a large number of water mediated processes. A long lasting debate in this context regards the mutual influence of the hydrophobic solute into the bulk water and the role of the hydrogen bond network of the bulk in supporting the solvation structure around a hydrophobic molecule. In this context we present a molecular dynamics study of the solvation of various hydrophobic molecules where the effect of different regions around the solvent can be analyzed by employing an adaptive resolution method, which can systematically separate local and nonlocal factors in the structure of water around a hydrophobic molecule. A number of hydrophobic solutes of different sizes and two different model potential interactions between the water and the solute are investigated.     

Anisotropy on the collective dynamics of water confined in swelling clay minerals

Collective excitations of water confined in the interlayer space of swelling clay minerals were studied by means of inelastic neutron scattering. The effect of bidimensional confinement on the dynamics of the interlayer water was investigated by using a synthetic Na-saponite sample with a general formula of Si(7.3)Al(0.7)Mg(6)O(20)(OH)(4)Na(0.7) in a bilayer hydration state. Experimental results reveal two inelastic signals, different from those described for bulk water with a clear anisotropy on the low-energy excitation of the collective dynamics of interlayer water, this difference being stronger in the perpendicular direction. Results obtained for the parallel direction follow the same trend as bulk water, and the effect of the confinement is mainly manifested from the fact that clay interlayer water is more structured than bulk water. Data obtained in the perpendicular direction display a nondispersive behavior below a cutoff wavenumber value, Q(c), indicating a nonpropagative excitation below that value. Molecular dynamics simulations results agree qualitatively with the experimental results.