Orientational Pair Correlation Function Research Articles

On the orientational correlations in the supercooled chloride lithium aqueous solution using the hybrid reverse Monte Carlo simulation

Employing the Hybrid Reverse Monte Carlo (HRMC) simulation, we compute, using the obtained three-dimensional configurations, the orientational correlations of water molecules in the supercooled 9.26 molal LiCl aqueous solution. This study aims to add relevant structural properties to those obtained in our latest studies and further support our findings. The Li/Cl pair ions hydration shells and the water molecules distribution studied using the Radial Pair Distribution Functions (RPDF), ([Formula: see text]) and ([Formula: see text]) are further described using the Orientational Pair Correlation Functions (OPCF), [Formula: see text] which describes the probability of a water molecule oriented by the Euler angles [Formula: see text], being located at the position [Formula: see text], with respect to another water molecule oriented [Formula: see text] placed at the origin. The high dimensionality of the orientational correlation functions has not presented a calculation disability, as known with several simulations, in the face of the efficiency of the HRMC and the water–water orientational correlation functions showed the dominant impact of ions on the water molecular dipole orientations within the hydration shells and in the hydrogen bonded molecules network.

Is water one liquid or two?

The idea that water is a mixture of two distinct states is analyzed in some detail. It is shown that the known compressibility of water is in fact sufficiently small that for a volume of water of size 1 nm3, the density fluctuations are of order 4% of the average density. This is much smaller than the ≈25% density fluctuations that would be required for significant regions of high and low density water to occur on this volume scale. It is also pointed out that the density fluctuations in water are, if anything, smaller than those that occur in other common liquids which do not have the anomalous properties of water. It is shown that if the distribution of density fluctuations is unimodal, the system is in the one-phase region, and if bimodal, it is in the two-phase region. None of the liquid or amorphous phases of water explored in this work give any sign of being in the two-phase region. Existing neutron and X-ray scattering data on water in the amorphous phases, and in the stable liquid phases as a function pressure and temperature, are subject to a new set of empirical potential structure refinement simulations. These simulations are interrogated for their configurational entropy, using a spherical harmonic reconstruction of the full orientational pair correlation function. It is shown that the excess pair entropy derived from this function, plus the known perfect gas contributions, give a reasonable account of the total entropy of water, within the likely errors. This estimated entropy follows the expected declining trend with decreasing temperature. Evidence that higher density water will have higher entropy than lower density water emerges, in accordance with what is expected from the negative thermal expansion coefficient of water at low temperatures. However, this entropy increase is not large and goes through a maximum before declining at yet higher densities and pressures, in a manner reminiscent of what has been previously observed in the diffusion coefficient as a function of pressure. There is no evidence that ambient water can be regarded as patches of high density, high entropy and low density, low entropy liquid, as some have claimed, since high density water has a similar entropy to low density water. There is some evidence that the distinction between these two states will become more pronounced as the temperature is lowered. Extensive discussion of the use of order parameters to describe water structure is given, and it is pointed out that these indices generally cannot be used to infer two-state behavior.

The structure and dipolar properties of CO2 adsorbed in a porous glassy chalcogel: Insights from first-principles molecular dynamics

A thorough understanding of the interactions of CO2 with the hosting porous network is crucial for the design of new adsorbent materials with enhanced gas adsorption capacity and selectivity. In this paper, first-principles molecular dynamics simulations are performed to assess the interactions of CO2 adsorbed in a nanoporous glassy chalcogenide (i.e. chalcogel) under relevant laboratory conditions. The structure and local organization of the confined CO2 molecules are analyzed in terms of atomic density, orientational order parameter and pair correlation functions. Maximally localized Wannier functions are used to unravel the electronic structure, the local molecular dipole and the nature of the chemical bonding at the interface between CO2 and the glassy surface. Our results provide a useful insight on the complex interplay between the chemical interactions competing at the buried interface made by the CO2 molecules adsorbed in g-GeS2 chalcogel.

Impact of diffusion limited aggregates of impurities on nematic ordering

Abstract We study the impact of random bond-type disorder on two-dimensional (2D) orientational ordering of nematic liquid crystal (LC) configurations. The lattice Lebwohl–Lasher pseudospin model is used to model orientational ordering perturbed by frozen-in rod-like impurities of concentration p exhibiting the isotropic orientational probability distribution. The impurities are either (i) randomly spatially distributed or (ii) form diffusion limited aggregation (DLA)-type patterns characterized by the fractal dimensions d f , where we consider cases d f ∼ 1.7 and d f ∼ 1.9 . The degree of orientational ordering is quantified in terms of the orientational pair correlation function G ( r ) . Simulations reveal that the DLA pattern imposed disorder has a significantly weaker impact for a given concentration of impurities. Furthermore, if samples are quenched from the isotropic LC phase, then the fractal dimension is relatively strongly imprinted on quantitative characteristics of G ( r ) .

3D Monte Carlo simulations of a magnetic disk-like particle dispersion

We have investigated the aggregate structures of a colloidal dispersion composed of ferromagnetic disk-like particles with a magnetic moment normal to the particle axis at the particle center, by means of 3D Monte Carlo simulations. Such disk-like particles have been modeled as a circular disk-like particle with the side section shape of spherocylinder. We have attempted to clarify the influences of the magnetic field strength, magnetic interactions between particles and volumetric fraction of particles. In order to discuss quantitatively the aggregate structures of clusters, we have focused on the radial distribution and orientational pair correlation functions, etc. For no applied magnetic field cases, long column-like clusters come to be formed with increasing magnetic particle–particle interactions. The internal structures of these clusters clearly show that the particles incline in a certain direction and their magnetic moments alternate in direction between the neighboring particles in the clusters. For applied magnetic field cases, the magnetic moment of each particle inclines in the magnetic field direction and therefore the column-like clusters are not formed straightforwardly. If the magnetic field is much stronger than magnetic particle–particle interactions, the particles do not have a tendency to form the clusters. As the influence of magnetic particle–particle interactions is significantly strong, thick chain-like clusters or column-like clusters or brick-wall-like clusters come to be formed along the magnetic field direction.

Quasi-2D Monte Carlo simulations of phase transitions and internal aggregate structures in a highly dense suspension composed of magnetic plate-like particles

The phase transitions and the internal aggregate structures of a highly dense suspension composed of magnetic plate-like particles with a magnetic moment normal to the particle axis have been investigated by means of the Monte Carlo method. The present study considered a quasi-2D system in order to clarify the influences of the volumetric fraction of particles and the magnetic field strength on particle aggregations and phase transitions. The internal structures of particle aggregates have been discussed quantitatively in terms of pair correlation functions, orientational pair correlation functions, nematic and polar order parameters. The main results obtained here are summarized as follows. When the influence of the magnetic interaction between particles is of the same order of that of the perpendicular magnetic field strength, the particles form column-like clusters, and the internal structure of the suspension shows solid-like structures. For the case of a strong applied magnetic field, the internal structure is transformed from solid-like structures into isotropic ones. However, as the volumetric fraction increases, the particles form brick wall-like structures under the situation of a strong applied magnetic field, and the internal structure exhibits solid-like ones. The brick wall-like structures also appear for a relatively weak magnetic field applied along the in-plane direction despite a slightly smaller volumetric fraction compared with the case of the perpendicular applied magnetic field.

磁性扁平粒子からなる高濃度サスペンションの相転移と内部構造発現現象に関する擬2次元モンテカルロ・シミュレーション(流体工学,流体機械)

We have investigated phase transitions and internal aggregate structures of a highly dense suspension composed of magnetic plate-like particles with a magnetic moment normal to the particle axis, by means of the Monte Carlo method. In the present study, we have considered a quasi-2D system in order to clarify the influences of the volumetric fraction of particles and magnetic field strength on particle aggregations and phase transitions. Internal structures of particle aggregates have been discussed quantitatively in terms of pair correlation and orientational pair correlation functions. The main results obtained here are summarised as follows. When the influence of magnetic interactions between particles is of the same order of the magnetic field strength, the particles form column-like clusters, and the internal structure of a dispersion shows solid-like structures. As the magnetic field strength becomes strong, the internal structure of a dispersion is transformed from solid-like structures into isotropic ones. However, as the volumetric fraction increases, the particles form brick wall-like structures under circumstances of a strong applied magnetic field, and the internal structure of a dispersion exhibits solid-like ones.

扁平粒子からなる強磁性コロイド分散系の凝集構造に関する3次元モンテカルロ・シミュレーション(流体工学,流体機械)

We have investigated aggregate structures of a colloidal dispersion composed of ferromagnetic plate-like particles with a magnetic moment normal to the particle axis at the particle center, by means of three-dimensional Monte Carlo simulations. Such plate-like particles have been modeled as disk-like particles with the side section shape of a spherocylinder. In concrete, we have attempted to clarify the influences of the magnetic field strength, magnetic interactions between particles and volumetric fraction of particles. In order to discuss quantitatively the aggregate structures of clusters, we have focused on the radial distribution and orientational pair correlation function, etc. For no applied magnetic field cases, long column-like clusters are formed as magnetic particle-particle interactions increase. Characteristics of these clusters are that particles incline in a certain direction and their magnetic moments alternate in direction between neighboring particles. For applied magnetic field cases, the particles incline in the magnetic field direction so that the column-like clusters are not formed easily. When the magnetic field has the same order of influence as the magnetic particle-particle interactions, the particles do not form the clusters. However, as the influence of the magnetic field and magnetic particle-particle interactions become significantly strong, thick chain-like clusters are formed along the magnetic field direction.

Quasi-2D Monte Carlo simulations of a colloidal dispersion composed of magnetic plate-like particles with magnetic moment normal to the particle axis

We have investigated aggregation phenomena of a colloidal dispersion composed of magnetic plate-like particles by means of Monte Carlo simulations. Such plate-like particles have been modelled as disk-like particles with magnetic moment normal to the particle axis at the particle centre, with the section shape of a spherocylinder. The main objective of the present study is to clarify the influences of the magnetic field strength and magnetic interactions between particles on particle aggregation phenomena. We have concentrated our attention on a quasi-2D system from an application point of view such as the development of surface quality changing technology using such magnetic plate-like particles. A magnetic field is applied along the direction perpendicular to the plane of the monolayer. Internal structures of particle aggregates are discussed quantitatively in terms of radial distribution and orientational pair correlation functions. For the case of strong magnetic interactions between particles, particles form long column-like clusters with their magnetic moments alternating in direction between the neighbouring particles. These tendencies appear under circumstances of a weak applied magnetic field. However, as the magnetic field strength increases, particles incline towards the magnetic field direction, so that particles do not form such clusters.

磁性扁平粒子からなるコロイド分散系の擬2次元モンテカルロ・シミュレーション(流体工学,流体機械)

We have investigated aggregation phenomena of a colloidal dispersion composed of magnetic plate-like particles by means of Monte Carlo method. Such plate-like particles have been modeled as disk-like particles with a magnetic moment normal to the particle axis at the particle center, with the side section shape of a spherocylinder. The main objective of the present study is to clarify the influences of the magnetic field strength and magnetic interactions between particles on particle aggregation phenomena. We have concentrated our attention on a quasi-2D system from an application point of view such as development of surface changing technology using such magnetic plate-like particles. Internal structures of particle aggregates have been discussed quantitatively in terms of radial distribution and orientational pair correlation functions. The main results obtained here are summarised as follows. For the case of strong magnetic interactions between particles, particles form long cookie-like clusters and alternate between magnetic moments. This tendency appears under circumstances of a weak applied magnetic field. However, as the magnetic field strength becomes significant, particle inclines toward the magnetic field direction, so that the particles do not form the clusters.

Two-dimensional Monte Carlo simulations of a colloidal dispersion composed of rod-like ferromagnetic particles in an applied magnetic field

We investigated the influence of an external magnetic field on microstructures in a colloidal dispersion composed of rod-like ferromagnetic particles using the cluster-moving Monte Carlo method. The internal microstructures obtained by simulations have been analysed in terms of the orientational distribution and pair correlation functions. The results obtained are summarized as follows. As the magnetic field increases, the particles align in the direction of the magnetic field. In the case of a relatively strong magnetic interaction between particles, chain-like clusters are formed along the magnetic field direction. However, the aspect ratio of the particles and the magnetic interaction between them do not affect their orientational distribution. Two types of structures are observed in the chain-like clusters—a straight linear structure and a step-like structure. The chain-like clusters become shorter when the area fraction of the particles decreases, and the number of step-like structures increases when the area fraction of the particles increases. The step-like structure formation can be explained by the dependence of the potential energy curves on the shape of the spherocylinder particles.

Three-dimensional Monte Carlo simulations of internal aggregate structures in a colloidal dispersion composed of rod-like particles with magnetic moment normal to the particle axis

We have treated a suspension composed of ferromagnetic rod-like particles with a magnetic moment normal to the particle axis in order to investigate aggregation phenomena of such a suspension by means of cluster-moving Monte Carlo simulations. In the present study, we have considered a three-dimensional mono-dispersed model system composed of such rod-like particles. Internal structures of self-assembled clusters have been discussed quantitatively in terms of radial distribution, pair correlation, orientational pair correlation functions, number distributions of clusters, and order functions. The main results obtained here are summarized as follows. Rod-like particles tend to aggregate to form raft-like clusters along the magnetic moment direction more significantly with magnetic particle–particle interactions. In such raft-like clusters, the direction of each particle axis has a tendency to incline in parallel formation, but is not so parallel as in a two-dimensional dispersion. As the volumetric fraction increases, longer raft-like clusters are formed, but such raft-like clusters do not aggregate further to form thicker clusters, which is in significantly contrast with a dispersion of spherical particles, where thicker chain-like clusters are observed under certain conditions. For the case of strong magnetic particle–particle interactions, sufficiently long raft-like clusters are formed along the magnetic field direction, even if the influence of an external magnetic field is of the same order of that of the thermal energy. However, rod-like particles in such clusters do not necessarily incline in significantly parallel formation along a certain direction. Self-assembled tube-like clusters are formed when magnetic particle–particle interactions are much more dominant than the rotational Brownian motion under circumstances of rod-like particles inclining in a certain direction.

Three-dimensional Monte Carlo Simulations of Internal Aggregate Structures in a Colloidal Dispersion Composed of Rod-like Particles with Magnetic Moment Normal to the Particle Axis

We have investigated aggregation phenomena of a colloidal dispersion composed of ferromagnetic rod-like particles with a magnetic moment normal to the particle axis, by means of three-dimensional cluster-moving Monte Carlo simulations. Concretely, we have treated a monodispersed model system in order to clarify the influences of the volumetric fraction of particles, magnetic interactions between particles, etc., on particle aggregation phenomena. Aggregate structures have been discussed qualitatively in terms of radial distribution, orientational pair correlation functions, etc. The main results obtained here are summarized as follows. Rod-like particles tend to aggregate to form raft-like clusters along the magnetic moment direction as magnetic particle-particle interactions increase. In such raft-like clusters, the direction of each particle axis has a tendency to align in parallel formation, but is not significantly parallel as expected. As the volumetric fraction increases, longer raft-like clusters are formed. However, such raft-like clusters do not aggregate further to form thicker clusters, which is in significantly contrast with dispersions of spherical or rod-like particles with a magnetic moment along the particle axis, where thicker chain-like clusters are observed under certain conditions.

Adsorption and structure of argon in activated porous carbons

Molecular simulations are used to investigate the adsorption and structure of argon in ordered and disordered models of porous carbons. The ordered porous carbon (model A) is an assembly of regular slit pores of different sizes, while the disordered porous carbon (model B) is a structural model that reproduces the complex pore shape and pore connectivity of saccharose-based porous carbons. The same pore size distribution is used for models A and B so that we are able to estimate, for similar confinement effects, how the disorder of the porous material affects the adsorption and structure of the confined fluid. Adsorption of argon at 77.4 K in the two models is studied using Grand Canonical Monte Carlo simulations. The structure of the confined fluid is analyzed using crystalline bond order parameters and positional or bond orientational pair correlation functions. The filling pressure for the assembly of slit pores is much lower than that for the disordered porous carbon. It is also found that the isosteric heat of adsorption for the ordered porous model overestimates that for the disordered porous model. The results suggest that the agreement between models A and B would be improved if the same density of carbon atoms were used in these two models. Strong layering of Ar is observed at all pressures for model A. The confined phase is composed of liquid-like layers at low-pressures, which crystallize into well-defined hexagonal 2D crystals after complete filling of the pores. The structure of argon in the disordered porous carbon strongly departs from that in the slit pore model. Although its structure remains liquid-like overall, argon confined in model B is composed of both crystalline clusters and amorphous (solid or liquid) nano-domains.

In search of temporal power laws in the orientational relaxation near isotropic-nematic phase transition in model nematogens.

Recent Kerr relaxation experiments by Gottke et al. have revealed the existence of a pronounced temporal power law decay in the orientational relaxation near the isotropic-nematic phase transition (INPT) of nematogens of rather small aspect ratio, kappa (kappa approximately 3-4). We have carried out very long (50 ns) molecular dynamics simulations of model (Gay-Berne) prolate ellipsoids with aspect ratio 3 in order to investigate the origin of this power law. The model chosen is known to undergo an isotropic to nematic phase transition for a range of density and temperature. The distance dependence of the calculated angular pair correlation function correctly shows the emergence of a long range correlation as the INPT is approached along the density axis. In the vicinity of INPT, the single particle second rank orientational time correlation function exhibits power law decay, (t(-alpha)) with exponent alpha approximately 2/3. More importantly, we find the sudden appearance of a pronounced power-law decay in the collective part of the second rank orientational time correlation function at short times when the density is very close to the transition density. The power law has an exponent close to unity, that is, the correlation function decays almost linearly with time. At long times, the decay is exponential-like, as predicted by Landau-de Gennes mean field theory. Since Kerr relaxation experiments measure the time derivative of the collective second rank orientational pair correlation function, the simulations recover the near independence of the signal on time observed in experiments. In order to capture the microscopic essence of the dynamics of pseudonematic domains inside the isotropic phase, we introduce and calculate a dynamic orientational pair correlation function (DOPCF) obtained from the coefficients in the expansion of the distinct part of orientational van Hove time correlation function in terms of spherical harmonics. The DOPCF exhibits power law relaxation when the pair separation length is below certain critical length. The orientational relaxation of a local director, defined in terms of the sum of unit vectors of all the ellipsoidal molecules, is also found to show slow power law relaxation over a long time scale. These results have been interpreted in terms of a newly developed mode coupling theory of orientational dynamics near the INPT. In the present case, the difference between the single particle and the collective orientational relaxation is huge which can be explained by the frequency dependence of the memory kernel, calculated from the mode coupling theory. The relationship of this power law with the one observed in a supercooled liquid near its glass transition temperature is explored.

A molecular dynamics simulation study of ionic diffusion and NMR spin–lattice relaxation in Li 2Si 4O 9 glass

The effect of ionic diffusion on Li spin–lattice relaxation (SLR) in Li 2Si 4O 9 glass has been studied using the molecular dynamics simulation technique over the temperature range 2000–3800 K. The orientational pair correlation function G( t) associated with SLR has been computed for individual Li ions from the trajectories of the constituent ions at different temperatures. The temporal decay of G( t) is found to be non-exponential and can be described by the stretched-exponential functional form exp(− t/ τ SLR) β . It is shown that the non-exponential behavior of G( t) is intrinsic to the system, possibly originating from hierarchically constrained relaxation processes. The activation energies associated with Li diffusion, τ SLR and the average conductivity relaxation time 〈 τ σ 〉 are shown to be in excellent agreement indicating that the same microscopic dynamic processes are responsible for SLR, diffusion and conductivity. However, τ SLR is found to be nearly an order of magnitude longer than 〈 τ σ 〉 over the entire temperature range of these simulations. This result reflects the fundamental differences between the correlation functions associated with SLR and conductivity relaxation. The existing experimental data in the literature on SLR in normal and fast-ion conducting glasses are shown to be completely consistent with the results obtained from the present simulations.

The structure of subcritical and supercritical methanol by neutron diffraction, empirical potential structure refinement, and spherical harmonic analysis

Pulsed neutron diffraction with isotope substitution on the hydroxyl hydrogens (H) is used to study the structure of methanol in two supercritical conditions (253 °C, 117.7 MPa, 0.700 g cm−3 and 253 °C, 14.3 MPa, 0.453 g cm−3) as well as a subcritical state (202 °C, 73.7 MPa, 0.700 g cm−3) (Tc=240 °C, Pc=8.1 MPa, ρc=0.272 g cm−3 for methanol). From three experiments on CD3OD, CD3OH, and a CD3O(H0.28+D0.72) mixture at the three thermodynamic states, the composite partial structure factors and pair correlation functions, XX, XH, and HH, are derived, where X represents a weighted sum of correlations from carbon (C), oxygen (O), and methyl hydrogen (M) atoms on the methanol molecule. The data are used in an empirical potential structure refinement (EPSR) computer simulation of methanol at the three thermodynamics states. Model distributions of molecules consistent with these data are used to estimate the individual site-site radial distribution functions, the coefficients of the spherical harmonic expansion of the orientational pair correlation function, the details of hydrogen bonding, and the three-dimensional structure of clusters formed in subcritical and supercritical methanol. In both subcritical and supercritical states of moderate density, the hydrogen bonds remain, with the average number of hydrogen bonds of 1.6±0.1 per molecule and the average chain-length of 3.1±0.4 molecules, which are less than the 1.77±0.07 per molecule and 5.5±1.0 molecules, respectively, found under ambient conditions; however, in the subcritical and supercritical methanol at moderate density the hydrogen bonds are mostly associated with clusters of 3–5 methanol molecules, in contrast with ambient methanol in which methanol molecules are involved in a nonlinear hydrogen bonded chain structure. In the low-density supercritical methanol, the large clusters are broken to generate monomers or small oligomers; the average number of hydrogen bonds per molecule decreases to 1.0±0.1, and the chain-length is 1.8±0.2 with a maximum length up to 7. The present results are compared with those recently obtained by molecular dynamics simulations, NMR, and Raman scattering of supercritical methanol.

The structure of aqueous solutions of tertiary butanol

Using neutron diffraction with isotopic substitution, the structures of aqueous solutions of tertiary butanol have been studied as a function of concentration. As the behaviour of this system is thought to be driven by hydrophobic interactions, particular attention was paid to the hydration of the non-polar headgroups and the nature of the intermolecular contacts. As concentration is increased from 0.06 to 0.16 mole fraction tertiary butanol, there is evidence for the growth of small clusters of the alcohol molecules, with butanol-butanol coordination of two to three even at the lowest concentration. Orientational pair correlation functions show that the dominant intermolecular contacts between alcohol molecules are between the non-polar head groups, as would be expected for a system driven by hydrophobic interactions. As concentration increases, however, there is evidence of mixed polar-non-polar contacts. The alcohol group's hydrogen bonding requirements appear to be fully met by polar contacts with water molecules: there is no evidence for significant butanol-butanol hydrogen bonding.

The structure of liquid methanol revisited: a neutron diffraction experiment at −80 °C and +25 °C

Pulsed neutron diffraction with isotope substitution on the hydroxyl hydrogens (H) is used to study the structure of pure liquid methanol at −80 °C and +25 °C. Although this liquid has been studied with neutrons several times in the past this is the first time that the composite partial structure factors, XX, XH and HH, are derived from the diffraction data. Here X represents a weighted sum of correlations from carbon (C), oxygen (O), and methyl hydrogen (M) atoms on the methanol molecule. The data are used in an empirical potential structure refinement (EPSR) computer simulation of the liquid at both temperatures. Model distributions of molecules consistent with these data are used to estimate the individual site—site radial distribution functions, the coefficients of the spherical harmonic expansion of the orientational pair correlation function, and the length of possible chains of methanol molecules formed in the liquid. Although the results are qualitatively similar to those of earlier computer simulation studies of this liquid, they do differ in detail from previous work. In particular, although most molecules in the liquid are found as part of chains up to 10 molecules long, the average chain length is only about 2.7 molecules at both temperatures, there are about 1.3 hydrogen bonds per molecule on average, and the chains are highly nonlinear on average.

Observation of chloride-ion hydration in high-temperature liquid and supercritical water by spherical harmonic expansion analysis

The structure of the Cl− hydration shell in high-temperature (77 and 197 °C) and supercritical (375 °C) water at moderate densities (roughly twice the critical density of 0.356 g/cm3) as well as in water at an ambient temperature, is investigated by means of neutron diffraction with isotope substitution on Cl− in aqueous 3 and 9 m solutions of LiCl in D2O. These relatively high densities (compared to the critical density) are used here to maintain the salt solubility at a sufficiently high value that a measurable signal is obtained in the isotope difference. The data are subjected to an empirical potential structure refinement which enables individual site-site radial distribution functions to be estimated. The same structure refinement is also used to estimate the spherical harmonic coefficients of the Cl-water orientational pair correlation function, and these are used in turn to reconstruct the local environment around Cl− in solution. A marked disordering of the Cl− hydration shell is observed as the critical point is approached. The orientational correlation functions of the water-molecule dipole vectors around Cl− in ambient water show a strong peak at a distance of about 3.2 Å, and centered at an angle of about 126°±30°, with respect to the Cl-O(D2O) axis, confirming an almost linear Cl⋯D-O hydrogen bond but with a significant spread of orientations about this value. At the higher-temperatures the peak remains at about the same position, but becomes progressively weaker in amplitude and with a larger spread of angles, suggesting a gradual decrease in orientational correlation of the water molecules around Cl−. At the supercritical temperature, the peak is even weaker, and there is evidence for a broader hydration shell developing at a distance of r≈3.9 Å in which the water molecules are apparently randomly oriented: they apparently become more loosely packed than at lower temperatures, without being dispelled from the ion hydration shell.