Are polar liquids less simple?

The origin of the dramatic changes in the behavior of liquids as they approach their vitreous stateincreases of many orders of magnitude in transport properties and dynamic time scales -is a major unsolved problem in condensed matter. These changes are accompanied by greater dynamic heterogeneity, which refers to both spatial variation and spatial correlation of molecular mobilities. The question is whether the changing dynamics is coupled to this heterogeneity; that is, does the latter cause the former? To address this we carried out the first nonlinear dielectric experiments at elevated hydrostatic pressures on two liquids, to measure the third-order harmonic component of their susceptibilities. We extract from this the number of dynamically correlated molecules for various state points, and find that the dynamic correlation volume for non-associated liquids depends primarily on the relaxation time, sensibly independent of temperature and pressure. We support this result by molecular dynamic simulations showing that the maximum in the four-point dynamic susceptibility of density fluctuations varies less than 10% for molecules that do not form hydrogen bonds. Our findings are consistent with dynamic heterogeneity serving as the principal control parameter for the slowing down of molecular motions in supercooled materials. SIGNIFICANCEOn cooling or compressing a liquid, molecular motions become slower, effectively ceasing as the material becomes a glass. Coincident with this is development of reciprocity -neighboring species must make adjustments in order for a given molecule to move. Conceptually, slower motions and more cooperative movements would seem to be connected, as theoretical models often assume; however, experimental verification is lacking. We carried out nonlinear dielectric spectroscopy, which enables both the timescale and degree of cooperativity of molecular motions to be quantified. By obtaining results at elevated pressures and various temperatures, we determined that a given relaxation time is associated with a given degree of cooperativity, supporting the idea that cooperativity underlies the transition of a liquid to the glassy state.

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“…Details of the simulations can be found in ref. [47]; the difference herein is a larger system size (10,000-16,000 molecules).…”

Section: Molecular Dynamics Simulationsmentioning

confidence: 97%

Dynamic correlation length scales under isochronal conditions

Casalini

Fragiadakis

Roland

2015

The Journal of Chemical Physics

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“…Such materials are limited to those in which the interactions are restricted to van der Waals forces and Coulombic forces, with no H-bonding or other strong associations nor a network structure [34].…”

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confidence: 99%

Pressure densification of a simple liquid

Casalini

Roland

2017

Journal of Non-Crystalline Solids

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The magnitude of the high frequency, static dielectric permittivity is used to determine the density of tetramethyl tetraphenyl trisiloxane, a non-associated glass-forming liquid, as a function of temperature and pressure. We demonstrate that the properties in the glassy state are affected by the pressure applied to the liquid during vitrification. This behavior is normal for hydrogen-bonded liquids and polymers, but unanticipated by models of simple liquids.KEYWORDS: pressure densification, physical aging, glass formation, simple liquids _____________________________________________________ One of the curiosities regarding studies of the glass transition is the overriding focus on the properties of the liquid, rather than those of the glass. The main reasons for this are the equilibrium nature of the liquid state and the experimental inaccessibility of structural relaxation times, τ, below the glass transition temperature, Tg. These are, of course, the properties that define the glass transition -the material falls out of equilibrium as τα becomes very large. In response to this nonequilibrium structure, glass slowly reorganizes, a process known as physical aging [1,2,3,4,5,6]. Aging is an important technical aspect of glasses, affecting their stability and thus utility for many applications. A factor controlling the non-equilibrium structure is the condition of the liquid upon vitrification. For example, variation of the rate of cooling through Tg can be used to produce glasses with varying departures from equilibrium, and thus varying stability [7,8,9,10]. Another method, employed herein, is the application of pressure to the supercooled liquid. The glass transition is pressure-dependent, so that pressure affords a means to control the properties of the glass, including its physical aging behavior. Besides the material engineering value, understanding how the glass structure depends on the pressure during its formation is also important in discerning the principal control parameters and ultimately solving the "glass transition problem" [11,12,13,14,15,16,17].

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“…34, and using results from eqs. (35) and (36), it is possible to obtain the gas-weight parameter q at each temperature.…”

Section: Excess Entropymentioning

confidence: 99%

Free energy and entropy of a dipolar liquid by computer simulations

Palomar

Sesé

2018

The Journal of Chemical Physics

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Thermodynamic properties for a system composed of dipolar molecules are computed. Free energy is evaluated by means of the thermodynamic integration technique, and it is also estimated by using a perturbation theory approach, in which every molecule is modeled as a hard sphere within a square well, with an electric dipole at its center. The hard sphere diameter, the range and depth of the well, and the dipole moment have been calculated from properties easily obtained in molecular dynamics simulations. Connection between entropy and dynamical properties is explored in the liquid and supercooled states by using instantaneous normal mode calculations. A model is proposed in order to analyze translation and rotation contributions to entropy separately. Both contributions decrease upon cooling, and a logarithmic correlation between excess entropy associated with translation and the corresponding proportion of imaginary frequency modes is encountered. Rosenfeld scaling law between reduced diffusion and excess entropy is tested, and the origin of its failure at low temperatures is investigated.

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Are polar liquids less simple?

Cited by 17 publications

References 50 publications

Dynamic correlation length scales under isochronal conditions

Dynamic correlation length scales under isochronal conditions

Pressure densification of a simple liquid

Free energy and entropy of a dipolar liquid by computer simulations

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