Self-dynamic Structure Factor Research Articles

Molecular dynamics insights into the dynamical behavior of structurally modified water in aqueous deep eutectic solvents (ADES).

Recent studies have demonstrated that the presence of water in deep eutectic solvents (DESs) significantly affects their dynamics, structure, and physical properties. Although the structural changes due to the addition of water are well understood, the microscopic dynamics of these changes have been rarely studied. Here, we performed molecular dynamics simulation of 30% (v/v) (∼0.57 molar fraction) water mixture of DES containing CH3CONH2 and NaSCN/KSCN at various salt fractions to understand the microscopic structure and dynamics of water. The simulated results reveal a heterogeneous environment for water molecules in aqueous DES (ADES), which is influenced by the nature of the cation. The diffusion coefficients of water in ADESs are significantly lower than that in neat water and concentrated aqueous NaSCN/KSCN solution. When Na+ ions are replaced by K+ ions in the ADES system, the diffusion coefficient increases, which is consistent with the measured nuclear magnetic resonance data. Self-dynamic structure factor for water and other simulated dynamic quantities, such as reorientation, hydrogen-bond, and residence time correlation functions, show markedly slower dynamics inside ADES than in the neat water and aqueous salt solution. Moreover, these dynamics become faster when Na+ ions in ADES are replaced by K+ ions. The results suggest that the structural environment of water in Na+-rich ADES is rigid due to the presence of cation-bound water and geometrically constrained water. The medium becomes less rigid as the KSCN fraction increases due to the relatively weaker interaction of K+ ions with water than Na+ ions, which accelerates the dynamical processes.

Crossover from gas-like to liquid-like molecular diffusion in a simple supercritical fluid.

According to textbooks, no physical observable can be discerned allowing to distinguish a liquid from a gas beyond the critical point. Yet, several proposals have been put forward challenging this view and various transition boundaries between a gas-like and a liquid-like behaviour, including the so-called Widom and Frenkel lines, and percolation line, have been suggested to delineate the supercritical state space. Here we report observation of a crossover from gas-like (Gaussian) to liquid-like (Lorentzian) self-dynamic structure factor by incoherent quasi-elastic neutron scattering measurements on supercritical fluid methane as a function of pressure, along the 200 K isotherm. The molecular self-diffusion coefficient was derived from the best Gaussian (at low pressures) or Lorentzian (at high pressures) fits to the neutron spectra. The Gaussian-to-Lorentzian crossover is progressive and takes place at about the Widom line intercept (59 bar). At considerably higher pressures, a liquid-like jump diffusion mechanism properly describes the supercritical fluid on both sides of the Frenkel line. The present observation of a gas-like to liquid-like crossover in the self dynamics of a simple supercritical fluid confirms emerging views on the unexpectedly complex physics of the supercritical state, and could have planet-wide implications and possible industrial applications in green chemistry.

Hydrogen self-dynamics in diluted liquid mixtures with neon: An inelastic neutron scattering study.

We have measured the dynamic structure factor of liquid neon-hydrogen mixtures (T=30.1 K) at two different H_{2} concentration levels (namely, 3.4% and 10%) making use of inelastic neutron scattering. This system has been selected since the presence of heavy Ne atoms strongly influences the self-dynamics of the H_{2} centers of mass via the formation of short-lived cages, which act both on the vibrational and the diffusive parts of the single-particle motion. After operating a standard data reduction and the subtraction of the Ne signal, experimental neutron spectra were analyzed through a generalization of the Young and Koppel model, and the H_{2} center-of-mass self-dynamic structure factor was finally extracted for the two liquid samples. Important physical quantities (namely, single-particle mean kinetic energy and self-diffusion coefficient) were estimated from the experimental data and then compared with quantum dynamical calculations, which also provided simulations of the velocity autocorrelation functions for Ne atoms and H_{2} centers of mass. The latter estimates, in the framework of the well-known Gaussian approximation, were used to simulate the H_{2} center-of-mass self-dynamic structure factor in the same kinematic range and thermodynamic conditions of the neutron scattering one. The comparison between measured and calculated spectra turned out to be qualitatively good, but some discrepancies, especially in the low-frequency part, seem to reinforce the idea of the existence of relevant non-Gaussian effects as in the case of pure hydrogen and H_{2}-D_{2} mixtures.

Density of states from mode expansion of the self-dynamic structure factor of a liquid metal.

We show that by exploiting multi-Lorentzian fits of the self-dynamic structure factor at various wave vectors it is possible to carefully perform the Q→0 extrapolation required to determine the spectrum Z(ω) of the velocity autocorrelation function of a liquid. The smooth Q dependence of the fit parameters makes their extrapolation to Q=0 a simple procedure from which Z(ω) becomes computable, with the great advantage of solving the problems related to resolution broadening of either experimental or simulated self-spectra. Determination of a single-particle property like the spectrum of the velocity autocorrelation function turns out to be crucial to understanding the whole dynamics of the liquid. In fact, we demonstrate a clear link between the collective mode frequencies and the shape of the frequency distribution Z(ω). In the specific case considered in this work, i.e., liquid Au, analysis of Z(ω) revealed the presence, along with propagating sound waves, of lower frequency modes that were not observed before by means of dynamic structure factor measurements. By exploiting ab initio simulations for this liquid metal we could also calculate the transverse current-current correlation spectra and clearly identify the transverse nature of the above mentioned less energetic modes. Evidence of propagating transverse excitations has actually been reported in various works in the recent literature. However, in some cases, like the present one, these modes are difficult to detect in density fluctuation spectra. We show here that the analysis of the single-particle dynamics is able to unveil their presence in a very effective way. The properties here shown to characterize Z(ω), and the information in it contained therefore allow us to identify it with the density of states (DoS) of the liquid. We demonstrate that only nonhydrodynamic modes contribute to the DoS, thus establishing its purely microscopic origin. Finally, as a by-product of this work, we provide our estimate of the self-diffusion coefficient of liquid gold just above melting.

Influence of structure in heterodyne electrophoretic light scattering

Heterodyne light scattering is employed to probe the electrophoretic flow behavior of a concentrated, deionized model colloidal suspension. Both for the fluid and crystalline regime, the power spectra are well described as velocity distributions, which let assume that the power spectra are dominated by the self-dynamic structure factor. Furthermore, all measured power spectra show an increase of the frequency integrated intensity with the electric field. Since the number of particles should remain constant this finding is unexpected. These effects are rationalized on the basis of a new theoretical model including the influence of the structure factor and of the mobility polydispersity on the power spectra.

Space-dependent self-diffusion processes in molten copper halides: A molecular dynamics study

This work is concerned with single ion dynamics in molten copper halides (CuI and CuCl) which exhibit fast ionic conduction before melting. The self-dynamic structure factor of the two ionic species in each melt have been calculated by molecular dynamics simulations and the corresponding effective wavelength-dependent self-diffusion coefficients have been studied. The results have been compared with those obtained for molten alkali halides (KCl and RbCl).

Self-dynamic structure factor of dense liquids: theory and simulation.

The self-intermediate dynamic structure factor Fs(k,t) of liquid lithium near the melting temperature is calculated by molecular dynamics. The results are compared with the predictions of several theoretical approaches, paying special attention to the Lovesey model and the Wahnström and Sjögren mode-coupling theory. To this end the results for the Fs(k,t) second memory function predicted by both models are compared with the ones calculated from the simulations.

Non-Fickian diffusion in colloidal glasses

We have studied numerically the decay of the self-dynamic structure factor (SDSF) for a small particle diffusing in a colloidal glass. We show that, in line with theoretical predictions, the super-Burnett coefficient (characterizing the deviation of the fourth moment of the single particle distribution from its Gaussian value) is finite. However, our results also show that large scale deviations from Fick’s law of diffusion should still be easy to detect experimentally. These deviations take the form of the SDSF decaying algebraically rather than exponentially as Fick’s law would predict.

Molecular dynamics study of single-particle motion in supercooled water. Memory function approach and comparison with quasi-elastic neutron scattering

The single-particle dynamics of liquid water in supercooled regime has been studied by molecular dynamics (MD) simulations and the results are compared with high-resolution quasi-elastic neutron scattering (QENS) data. The simulations are performed at 264 K using the extended simple point charge model. The MD results indicate that after less than 0.1 ps the proton dynamics practically coincides with the centre of mass dynamics, showing thus a substantial coupling between translational and reorientational motions. Simple diffusion models are found inadequate to describe the MD and QENS results. The centre of mass self-dynamic structure factor has been analyzed in terms of a second-order memory function model. The model identifies two characteristic relaxation times; the short one is probably related to the hindered motion of the particle within the nearest-neighbour “cage”, while the long one can be assigned to the dynamics of the relaxation of water clusters.

Boson peaks and slow relaxation in a simulation of a network-forming ionic melt

Abstract Results are presented for the dynamical properties of a computer simulation model which has previously been shown to reproduce the distinctive structural characteristics of the intermediate-strength liquid glass former ZnCl2. Slow structural relaxation of the density on the length scale of the intermediate-range order is demonstrated, and the implications of this finding for theories of the glass transition in network-forming liquids are discussed qualitatively. A strong boson peak develops in the self-dynamic structure factor (incoherent neutron scattering) as the fluid is supercooled. Its presence is traced to the density of states (spectral density of the velocity autocorrelation function) and also to the intermediate scattering function.

A molecular-dynamics study of the dynamic properties of liquid rubidium. II. Single-particle correlation functions

For pt. I see ibid., vol. 6, p. 10897 (1994). We present results for the dynamic single-particle correlation functions (i.e., the intermediate self-scattering function and the velocity autocorrelation function) of six liquid states of rubidium obtained in the same computer experiment as described in the preceding paper. Since the incoherent structure factor for Rb cannot be determined in a neutron-scattering experiment. Our computer results could not be compared to experimental data. Temperature effects have been studied for both correlation functions: (i) an increasing temperature causes the single-particle correlations to decay much more rapidly than at low temperatures; (ii) the velocity autocorrelation function becomes a much more slowly decaying function in time. This reflects the cage (drift) effect encountered in dense (expanded) systems. An attempt to interpret the peak height and half-width of the self-dynamic structure factor in terms of mode-coupling theories has failed: either these subtle effects are encountered in a q-range inaccessible to computer experiment, or they require a more sophisticated analysis in terms of a full mode-coupling-theory approach. The diffusion constant extracted from the single-particle correlation functions shows good agreement with data obtained from the collective correlation functions, which marks a good internal consistency of the computer experiment.

Non-Gaussian behavior of the displacement statistics of interacting colloidal particles.

A colloidal dispersion consisting of optically matched host particles and a small amount of strongly scattering tracer particles has been studied with dynamic light scattering. Analysis of the self-dynamic structure factor ${F}_{s}$(q,t) obtained from these measurements demonstrates that the displacement statistics of the interacting colloidal particles shows considerable non-Gaussian behavior.