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Journal ArticleDOI: 10.1063/1.4943967

Structural anomaly and dynamic heterogeneity in cycloether/water binary mixtures: Signatures from composition dependent dynamic fluorescence measurements and computer simulations.

24 Mar 2016-Journal of Chemical Physics (AIP Publishing)-Vol. 144, Iss: 12, pp 124506-124506
Abstract: We have performed steady state UV-visible absorption and time-resolved fluorescence measurements and computer simulations to explore the cosolvent mole fraction induced changes in structural and dynamical properties of water/dioxane (Diox) and water/tetrahydrofuran (THF) binary mixtures. Diox is a quadrupolar solvent whereas THF is a dipolar one although both are cyclic molecules and represent cycloethers. The focus here is on whether these cycloethers can induce stiffening and transition of water H-bond network structure and, if they do, whether such structural modification differentiates the chemical nature (dipolar or quadrupolar) of the cosolvent molecules. Composition dependent measured fluorescence lifetimes and rotation times of a dissolved dipolar solute (Coumarin 153, C153) suggest cycloether mole-fraction (X(THF)/Diox) induced structural transition for both of these aqueous binary mixtures in the 0.1 ≤ X(THF)/Diox ≤ 0.2 regime with no specific dependence on the chemical nature. Interestingly, absorption measurements reveal stiffening of water H-bond structure in the presence of both the cycloethers at a nearly equal mole-fraction, X(THF)/Diox ∼ 0.05. Measurements near the critical solution temperature or concentration indicate no role for the solution criticality on the anomalous structural changes. Evidences for cycloether aggregation at very dilute concentrations have been found. Simulated radial distribution functions reflect abrupt changes in respective peak heights at those mixture compositions around which fluorescence measurements revealed structural transition. Simulated water coordination numbers (for a dissolved C153) and number of H-bonds also exhibit minima around these cosolvent concentrations. In addition, several dynamic heterogeneity parameters have been simulated for both the mixtures to explore the effects of structural transition and chemical nature of cosolvent on heterogeneous dynamics of these systems. Simulated four-point dynamic susceptibility suggests formation of clusters inducing local heterogeneity in the solution structure.

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Topics: Aqueous solution (51%)
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Journal ArticleDOI: 10.1021/ACS.JPCB.7B05552
Abstract: 1-Alkyl-3-methylimidazolium cations in the presence of water are used as a test system to study by molecular dynamics the formation of micelles in aqueous mixtures of highly anisotropic room temperature ionic liquids (IL). Structural properties, i.e., radial distribution functions (RDF) and transport parameters, such as diffusion coefficients and conductivities, are computed as a function of the IL/water mole fraction. The concentration plots reveal a sharp change of the slope of both the cation self-diffusion coefficient and the first peak of the head–head RDF at approximately the same value of the concentration. This transition, considered as a measure of a critical micellar concentration, appears only for the most anisotropic systems, composed of longer alkyl chains. The formation of the micelles is confirmed from the analysis of the tail–tail and cation–water RDFs. As a general result, we found that the larger the anisotropy of the ionic liquid the lower the critical concentration and the larger the p...

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Topics: Critical micelle concentration (59%), Micelle (56%), Ionic liquid (55%) ...read more

29 Citations


Journal ArticleDOI: 10.1063/1.5040071
Kallol Mukherjee1, Suman Das1, Ejaj Tarif1, Anjan Barman1  +1 moreInstitutions (1)
Abstract: Dielectric relaxation (DR) measurements in the frequency window 0.2 ≤ ν(GHz) ≤ 50 for deep eutectic solvents (DESs) made of acetamide (CH3CONH2) and urea (NH2CONH2) with the general composition, [f CH3CONH2 + (1 − f) NH2CONH2] at f = 0.6 and 0.7, reveal three distinct relaxation time scales—τ1 ∼ 120 ps, τ2 ∼ 40 ps, and τ3 ∼ 5 ps. Qualitatively similar time scales have been observed for DR of neat molten urea, whereas the reported DR for neat molten acetamide in the same frequency window reflects two relaxation processes with no trace of ∼100 ps time scale. This slowest DR time scale (τ1) resembles closely to the long-time constant of the simulated structural H-bond relaxation (CHB(t)) involving urea pairs. Similarity in activation energies estimated from the temperature dependent DR measurements (335 ≤ T/K ≤ 363) and structural H-bond relaxations indicates that the structural H-bond relaxation overwhelmingly dominates the slowest DR relaxation in these DESs. Simulated collective reorientational correlation functions (Cl(t)), on the other hand, suggest that the second slower time scale (∼40 ps) derives contributions from both the single particle orientation dynamics and structural H-bond relaxation, leaving no role for hydrodynamic molecular rotations. The sub-10 ps DR time scale has been found to be connected to the fast reorientation dynamics of the component molecules (acetamide or urea). Fractional viscosity dependence for the longest DR times, τDR∝η/Tp, has been observed for these DESs with the fraction power p = 0.7. Subsequently, the temporal heterogeneity aspects of these media have been investigated by examining the simulated particle motion characteristics and substantiated by estimating the dynamically correlated time scales and length-scales through simulations of four-point susceptibilities and density correlations. These estimated dynamical time scales and length-scales assist in explaining the different inferences regarding solution heterogeneity drawn from different measurements on these DESs.

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Topics: Relaxation (NMR) (53%)

26 Citations


Journal ArticleDOI: 10.1063/1.4961586
Abstract: The paper reports a detailed simulation study on collective reorientational relaxation, cooperative hydrogen bond (H-bond) fluctuations, and their connections to dielectric relaxation (DR) in deep eutectic solvents made of acetamide and three uni-univalent electrolytes, lithium nitrate (LiNO3), lithium bromide (LiBr), and lithium perchlorate (LiClO4). Because cooperative H-bond fluctuations and ion migration complicate the straightforward interpretation of measured DR timescales in terms of molecular dipolar rotations for these conducting media which support extensive intra- and inter-species H-bonding, one needs to separate out the individual components from the overall relaxation for examining the microscopic origin of various timescales. The present study does so and finds that reorientation of ion-complexed acetamide molecules generates relaxation timescales that are in sub-nanosecond to nanosecond range. This explains in molecular terms the nanosecond timescales reported by recent giga-Hertz DR measurements. Interestingly, the simulated survival timescale for the acetamide-Li+ complex has been found to be a few tens of nanosecond, suggesting such a cation-complexed species may be responsible for a similar timescale reported by mega-Hertz DR measurements of acetamide/potassium thiocyanate deep eutectics near room temperature. The issue of collective versus single particle relaxation is discussed, and jump waiting time distributions are determined. Dependence on anion-identity in each of the cases has been examined. In short, the present study demonstrates that assumption of nano-sized domain formation is not required for explaining the DR detected nanosecond and longer timescales in these media.

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Topics: Relaxation (physics) (55%), Nanosecond (51%)

25 Citations


Journal ArticleDOI: 10.1021/ACS.JPCB.6B06511
Sandipa Indra1, Ranjit Biswas1Institutions (1)
Abstract: Heterogeneity and molecular motions in representative cryoprotectant mixtures made of trehalose and glycerol are investigated in the temperature range 298 ≤ T (K) ≤ 353, via time-resolved fluorescence Stokes shift and anisotropy measurements, and molecular dynamics simulations of four-point density–time correlations and H-bond relaxations. Mixtures containing 5 and 20 wt % of trehalose along with neat glycerol are studied. Viscosity coefficients for these systems lie in the range 0.30 < η (P) < 23. Measured solute (Coumarin 153) rotation and solvation times reveal a substantial departure from the hydrodynamic viscosity dependence, suggesting the strong microheterogeneous nature of these systems. Fluorescence anisotropy decays are highly nonexponential, reflecting a non-Markovian character of the medium friction. A complete missing of the Stokes shift dynamics in these systems at 298 K but partial detection of it at other higher temperatures (shift magnitude being ∼400–600 cm–1) indicates rigid solute envi...

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Topics: Stokes shift (52%), Solvation (51%), Viscosity (50%)

18 Citations


Journal ArticleDOI: 10.1021/ACS.JPCB.9B06783
Ejaj Tarif1, Jayanta Mondal1, Ranjit Biswas1Institutions (1)
Abstract: A new room-temperature deep eutectic solvent (DES) composed of glucose, urea, and water has been prepared and its relaxation dynamics explored via temperature-dependent time-resolved fluorescence m...

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17 Citations


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Open accessJournal ArticleDOI: 10.1016/0021-9991(77)90098-5
Abstract: A numerical algorithm integrating the 3N Cartesian equations of motion of a system of N points subject to holonomic constraints is formulated. The relations of constraint remain perfectly fulfilled at each step of the trajectory despite the approximate character of numerical integration. The method is applied to a molecular dynamics simulation of a liquid of 64 n-butane molecules and compared to a simulation using generalized coordinates. The method should be useful for molecular dynamics calculations on large molecules with internal degrees of freedom.

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Topics: Generalized coordinates (62%), Holonomic constraints (58%), Verlet integration (58%) ...read more

16,572 Citations


Journal ArticleDOI: 10.1103/PHYSREVA.31.1695
William G. Hoover1Institutions (1)
01 Mar 1985-Physical Review A
Abstract: Nos\'e has modified Newtonian dynamics so as to reproduce both the canonical and the isothermal-isobaric probability densities in the phase space of an N-body system. He did this by scaling time (with s) and distance (with ${V}^{1/D}$ in D dimensions) through Lagrangian equations of motion. The dynamical equations describe the evolution of these two scaling variables and their two conjugate momenta ${p}_{s}$ and ${p}_{v}$. Here we develop a slightly different set of equations, free of time scaling. We find the dynamical steady-state probability density in an extended phase space with variables x, ${p}_{x}$, V, \ensuremath{\epsilon}\ifmmode \dot{}\else \.{}\fi{}, and \ensuremath{\zeta}, where the x are reduced distances and the two variables \ensuremath{\epsilon}\ifmmode \dot{}\else \.{}\fi{} and \ensuremath{\zeta} act as thermodynamic friction coefficients. We find that these friction coefficients have Gaussian distributions. From the distributions the extent of small-system non-Newtonian behavior can be estimated. We illustrate the dynamical equations by considering their application to the simplest possible case, a one-dimensional classical harmonic oscillator.

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15,380 Citations


Open accessJournal ArticleDOI: 10.1063/1.447334
Abstract: Three recently proposed constant temperature molecular dynamics methods by: (i) Nose (Mol. Phys., to be published); (ii) Hoover et al. [Phys. Rev. Lett. 48, 1818 (1982)], and Evans and Morriss [Chem. Phys. 77, 63 (1983)]; and (iii) Haile and Gupta [J. Chem. Phys. 79, 3067 (1983)] are examined analytically via calculating the equilibrium distribution functions and comparing them with that of the canonical ensemble. Except for effects due to momentum and angular momentum conservation, method (1) yields the rigorous canonical distribution in both momentum and coordinate space. Method (2) can be made rigorous in coordinate space, and can be derived from method (1) by imposing a specific constraint. Method (3) is not rigorous and gives a deviation of order N−1/2 from the canonical distribution (N the number of particles). The results for the constant temperature–constant pressure ensemble are similar to the canonical ensemble case.

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  • FIG. 4. The optimized penta- and hexacoordinated-Cd structures with 24 water molecules. The Gibbs free energy of the latter is only 5.6 kcal/mol lower than that of the former.
    FIG. 4. The optimized penta- and hexacoordinated-Cd structures with 24 water molecules. The Gibbs free energy of the latter is only 5.6 kcal/mol lower than that of the former.
  • FIG. 3. The optimized structure with 12 water molecules. Note the nearly linear Cl-Cd-Cl (176.6◦) angle and the nearly flat Ow-Ow-Ow-Ow (4.8 ◦) dihedral angle of the equatorial waters around Cd.
    FIG. 3. The optimized structure with 12 water molecules. Note the nearly linear Cl-Cd-Cl (176.6◦) angle and the nearly flat Ow-Ow-Ow-Ow (4.8 ◦) dihedral angle of the equatorial waters around Cd.
  • FIG. 2. Two stable geometries of CdCl2 solvated by seven water molecules. The pentacoordinated structure is ( G0298 = 2.1 kcal/mol) more stable than the hexacoordinated one. Although stable pentacoordinated structures exist, hexacoordination around Cd was found to be slightly favored with a larger number of water molecules (see text).
    FIG. 2. Two stable geometries of CdCl2 solvated by seven water molecules. The pentacoordinated structure is ( G0298 = 2.1 kcal/mol) more stable than the hexacoordinated one. Although stable pentacoordinated structures exist, hexacoordination around Cd was found to be slightly favored with a larger number of water molecules (see text).
  • TABLE I. Main optimized B3PW91/6-31G** geometrical parameters of gaseous and solvated CdCl2-(H2O)n systems. For comparison MP2/AVTZ optimized values are given in parentheses for small n. For n > 12 only data corresponding to the lowest-energy structure are shown. The water oxygen atoms (Ow) are those directly linked to Cd. Distances in Å and angles in degrees. Ow-Cd-Ow are the angles concerning the water oxygens coordinated to Cd.
    TABLE I. Main optimized B3PW91/6-31G** geometrical parameters of gaseous and solvated CdCl2-(H2O)n systems. For comparison MP2/AVTZ optimized values are given in parentheses for small n. For n > 12 only data corresponding to the lowest-energy structure are shown. The water oxygen atoms (Ow) are those directly linked to Cd. Distances in Å and angles in degrees. Ow-Cd-Ow are the angles concerning the water oxygens coordinated to Cd.
  • FIG. 8. Evolution of the shortest Cd-Ow distances (Å) during the simulation starting from the optimized pentacoordinated Cd structure. Horizontal axis in fs. Note the transient Cd-hexacoordination.
    FIG. 8. Evolution of the shortest Cd-Ow distances (Å) during the simulation starting from the optimized pentacoordinated Cd structure. Horizontal axis in fs. Note the transient Cd-hexacoordination.
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Topics: Canonical ensemble (64%), Microcanonical ensemble (62%), Grand canonical ensemble (60%) ...read more

11,619 Citations


Journal ArticleDOI: 10.1021/J100308A038
Abstract: on carbon. These particles can interact with the active sites of the carbon to form a species a t the metal-carbon interface which is stable against high-temperature reduction. This interaction may occur via oxygen atoms bonding with both the carbon surface and iron atoms contacting the carbon. This oxygen could be provided by dissociation of the C O ligands during decomposition. The remainder of the surface Fe can exist in either a reduced or carburized state, but the small size of these particles results in either case in a superparamagnetically collapsed Mossbauer spectrum. For carbons with fewer active sites, more of the particles are unbound, and these particles sinter and carburize in a manner more similar to that of Fe particles supported on graphite.

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Topics: Dissociation (chemistry) (51%), Graphite (50%)

9,938 Citations


Open accessBook
01 Jan 1976-
Abstract: Introduction. Statistical Mechanics and Molecular Distribution Functions. Computer "Experiments" on Liquids. Diagrammatic Expansions. Distribution Function Theories. Perturbation Theories. Time-dependent Correlation Functions and Response Functions. Hydrodynamics And Transport Coefficients. Microscopic Theories of Time-Correlation Functions. Ionic Liquids. Simple Liquid Metals. Molecular Liquids. Appendices. References. Index.

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9,140 Citations