Mechanical relaxation and the notion of time-dependent extent of ergodicity during the glass transition

Glass Transition, Crystallization of Glass-Forming Melts, and Entropy

A critical analysis of possible (including some newly proposed) definitions of the vitreous state and the glass transition is performed and an overview of kinetic criteria of vitrification is presented. On the basis of these results, recent controversial discussions on the possible values of the residual entropy of glasses are reviewed. Our conclusion is that the treatment of vitrification as a process of continuously breaking ergodicity with entropy loss and a residual entropy tending to zero in the limit of zero absolute temperature is in disagreement with the absolute majority of experimental and theoretical investigations of this process and the nature of the vitreous state. This conclusion is illustrated by model computations. In addition to the main conclusion derived from these computations, they are employed as a test for several suggestions concerning the behavior of thermodynamic coefficients in the glass transition range. Further, a brief review is given on possible ways of resolving the Kauzmann paradox and its implications with respect to the validity of the third law of thermodynamics. It is shown that neither in its primary formulations nor in its consequences does the Kauzmann paradox result in contradictions with any basic laws of nature. Such contradictions are excluded by either crystallization (not associated with a pseudospinodal as suggested by Kauzmann) or a conventional (and not an ideal) glass transition. Some further so far widely unexplored directions of research on the interplay between crystallization and glass transition are anticipated, in which entropy may play—beyond the topics widely discussed and reviewed here—a major role.

Thermodynamic Analysis of the Two-Liquid Model for Anomalies of Water, HDL-LDL Fluctuations, and Liquid-Liquid Transition

After reviewing the protocol-dependent properties of HDA, which thermally anneals to LDA, and the data gap over an unusually large T-range between HDA, LDA, and water, we investigate whether or not, despite HDA's ill-defined state and distinction from a glass, the HDL-LDL fluctuations view of the two-liquid model can explain water's anomalous behavior. An analysis of the density, ρ, compressibility, β, heat capacity, Cp, and thermal conductivity, κ, of water over a monotonic (continuous) path bridging this data gap shows the following: (i) Such a path between ρwater at 320 K and ρHDA yields an untenable thermal expansion coefficient of water. (ii) There is neither a continuous path between βwater at 353 K and βHDA, nor between Cp,water at 363 K and Cp,HDA. (iii) The same value of ρwater, of βwater, or of Cp,water at two temperatures separated by a maxima or a minima is incompatible with the HDL-LDL fluctuations view. (iv) κLDA at ∼140 K is about twice that of κ water at 253 K. (v) κHDA at 120 K is incompatible with κwater at T > 320 K. Thus, there is an internal inconsistency between the thermodynamics of HDA seen as a glass and that of water seen as an HDL-LDL mixture, which is incompatible with both the HDL-LDL fluctuations view and the liquid-liquid transition.

Structural fluctuations and orientational glass of levoglucosan--High stability against ordering and absence of structural glass

To investigate whether a non-exponential relaxation always indicates 2-4 nm-size regions of dynamic heterogeneity, we studied the kinetic freezing and unfreezing of structural fluctuations involving the rotational modes in orientationally disordered crystal (ODIC) of levoglucosan by calorimetry. The heat capacity, Cp, of levoglucosan measured over the 203 K-463 K range shows that its low-temperature, orientationally ordered crystal (ORC) transforms to ODIC phase on heating, which then melts to a low viscosity liquid. On cooling, the melt transforms to the ODIC which then does not transform to the ORC. Instead, the ODIC supercools. Fluctuations resulting from hindered (random) rotations of levoglucosan molecules confined to the lattice sites and from their conformational changes become progressively slower on cooling and an orientational glass (O-G) forms showing the sigmoid shape decrease in Cp characteristic of structural arrest like that of a glass. On heating the O-G state, rotational fluctuations begin to contribute to Cp at To-g of 247.8 K and there is an overshoot in Cp and thermal hysteresis (characteristic of physical ageing) in the temperature range of 230-260 K. The non-exponential relaxation parameter, β(cal), determined by fitting the Cp data to a non-exponential, nonlinear model for relaxation of a glass is 0.60, which is similar to β(cal) found for polymers, molecular liquids, and metal-alloy melts in which Brownian diffusion occurs. Such β(cal) < 1 are seen to indicate 2-4 nm-size dynamically heterogeneous domains in an ultraviscous liquid near the glass formation, but its value of 0.60 for ODIC levoglucosan, in which Brownian diffusion does not occur, would not indicate such domains. Despite the lack of Brownian diffusion, we discuss these findings in the potential energy landscape paradigm. Levoglucosan melt, which is believed to vitrify and to stabilize a protein's disordered structure, did not supercool even at 200 K/min cooling rate. The findings have consequences for reports on the dielectric relaxation studies that indicated that levoglucosan melt supercools to form a structural glass of Tg of ∼245 K, and for computer simulation of its dynamics. Levoglucosan is the ninth ODIC that forms O-G. It does so more easily than the other eight.

Strategies to address low drug solubility in discovery and development

Drugs with low water solubility are predisposed to low and variable oral bioavailability and, therefore, to variability in clinical response. Despite significant efforts to "design in" acceptable developability properties (including aqueous solubility) during lead optimization, approximately 40% of currently marketed compounds and most current drug development candidates remain poorly water-soluble. The fact that so many drug candidates of this type are advanced into development and clinical assessment is testament to an increasingly sophisticated understanding of the approaches that can be taken to promote apparent solubility in the gastrointestinal tract and to support drug exposure after oral administration. Here we provide a detailed commentary on the major challenges to the progression of a poorly water-soluble lead or development candidate and review the approaches and strategies that can be taken to facilitate compound progression. In particular, we address the fundamental principles that underpin the use of strategies, including pH adjustment and salt-form selection, polymorphs, cocrystals, cosolvents, surfactants, cyclodextrins, particle size reduction, amorphous solid dispersions, and lipid-based formulations. In each case, the theoretical basis for utility is described along with a detailed review of recent advances in the field. The article provides an integrated and contemporary discussion of current approaches to solubility and dissolution enhancement but has been deliberately structured as a series of stand-alone sections to allow also directed access to a specific technology (e.g., solid dispersions, lipid-based formulations, or salt forms) where required.