Specific heats Cp, Cv, Cconf and energy landscapes of glassforming liquids

Abstract

In pursuit of understanding of the paradoxical success of the Adam-Gibbs equation in both experiment and computer simulation studies, we examine the relation between liquid behavior at constant pressure and constant volume and compare the inherent structures excitation profiles for the two cases. This allows us to extend qualitatively the recent correlation of kinetic and thermodynamic measures of fragility to constant volume systems. The decreased fragility at constant volume is understood in terms of the relation C p >C c(cp) > C c(cv) > C v . In the process, we find a parallel between the range of volumes, relative to the total excess volume, that are explored in the first few orders of magnitude of relaxation time increase, and the range of amorphous state inherent structure energies, relative to the total range, that are explored in ergodic computer simulations, which also cover only this limited range of relaxation time change. The question of whether or not fragile behavior is determined in the configurational or vibrational manifold of states is left unanswered in this work. However, the approximate proportionality of the configurational and total excess entropies that is needed to interpret the success of the Adam Gibbs equation (which has been questioned by other authors) is confirmed within the needed limits, using data from three different types of investigation: experiments (on Se), simulation (of water in the SPC-E model), and analytical models of both defect crystals and configurationally excited liquids. Some consequences of the abrupt increases in vibrational heat capacity at T g implied by this proportionality, are discussed.

Prediction of entropy and dynamic properties of water below the homogeneous nucleation temperature

“…[35]. In the next section, we also show results for a model of water which shows approximate proportionality.…”

Entropy basis for the thermodynamic scaling of the dynamics ofo-terphenyl

“…The configurational entropy is usually unavailable, so the total entropy is used after subtraction of the crystal entropy. This subtraction assumes equivalent T and P dependences for the excess contributions to the total and the crystal entropies, with the expectation that the resulting "excess" entropy is at least proportional to the configurational entropy [40,41].…”

Relation between configurational entropy and relaxation dynamics of glass-forming systems under volume and temperature reduction

“…If R was 1, the expression in Eq. (2) would reduce to that originally proposed by AG or to that proposed by Angell and Borick [17] for g T =g P =const different from unity. In our case we have that g T (P atm )/g P (T)=0.70±0.02 for DPVC and 0.74±0.08 for PPGE, that seems to suggest that isothermal reduction of excess entropy is much more effective in term of reduction of configurational entropy.…”

“…Notwithstanding the above mentioned results appear as convincing tests of the AG model, the debate is still open about several issues: (a) the right way of calculating the configurational entropy in an experiment [16,19,20,21]; (b) the physical meaning of the size of a CRR [21,22,23,24]; (c) the amount of configurational part of S exc [7,17,16,25,21,26,27,28,18]; (d) the independence from pressure and temperature of the coefficient C AG in Eq.…”