Pablo G. Debenedetti

Supercooled liquids and the glass transition

Glasses are disordered materials that lack the periodicity of crystals but behave mechanically like solids. The most common way of making a glass is by cooling a viscous liquid fast enough to avoid crystallization. Although this route to the vitreous state — supercooling — has been known for millennia, the molecular processes by which liquids acquire amorphous rigidity upon cooling are not fully understood. Here we discuss current theoretical knowledge of the manner in which intermolecular forces give rise to complex behaviour in supercooled liquids and glasses. An intriguing aspect of this behaviour is the apparent connection between dynamics and thermodynamics. The multidimensional potential energy surface as a function of particle coordinates (the energy landscape) offers a convenient viewpoint for the analysis and interpretation of supercooling and glass-formation phenomena. That much of this analysis is at present largely qualitative reflects the fact that precise computations of how viscous liquids sample their landscape have become possible only recently.

Is Random Close Packing of Spheres Well Defined

Despite its long history, there are many fundamental issues concerning random packings of spheres that remain elusive, including a precise definition of random close packing (RCP). We argue that the current picture of RCP cannot be made mathematically precise and support this conclusion via a molecular dynamics study of hard spheres using the Lubachevsky-Stillinger compression algorithm. We suggest that this impasse can be broken by introducing the new concept of a maximally random jammed state, which can be made precise.

Supercooled and glassy water

The anomalous properties of cold and supercooled water, such as the fact that at sufficiently low temperatures it becomes more compressible and less dense when cooled, and more fluid when compressed, have attracted the attention of physical scientists for a long time. The discovery in the 1970s that several thermodynamic and transport properties of supercooled water exhibit a pronounced temperature dependence and appear to diverge slightly below the homogeneous nucleation temperature inspired a large number of experimental and theoretical studies. Likewise, an important body of work on glassy water has been stimulated by experiments, starting in the mid-1980s and continuing to this date, which suggest that vitreous water can exist in at least two apparently distinct forms. A coherent theory of the thermodynamic and transport properties of supercooled and glassy water does not yet exist. Nevertheless, significant progress towards this goal has been made during the past 20 years. This article summarizes the known experimental facts and reviews critically theoretical and computational work aimed at interpreting the observations and providing a unified viewpoint on cold, non-crystalline, metastable states of water.

Signatures of distinct dynamical regimes in the energy landscape of a glass-forming liquid

Most materials attain a glassy state at low temperatures under suitable methods of preparation. This state exhibits the mechanical properties of a solid, but shows microscopic structural disorder,. A comprehensive understanding of the glassy state is, however, still lacking. A widespread assumption is that the non-exponential relaxation processes observed in the dynamics of glasses — and also in protein dynamics, protein folding and population dynamics — are (in common with other manifestations of complex dynamics) strongly influenced by the underlying energy landscape associated with the structural configurations that the system may adopt. But concrete evidence for this in studies of glass formation has been scarce. Here we present such evidence, obtained from computer simulations of a model glass-forming liquid. We demonstrate that the onset of non-exponential relaxation corresponds to a well defined temperature below which the depth of the potential-energy minima explored by the liquid increases with decreasing temperature, and above which it does not. At lower temperatures, we observe a sharp transition when the liquid gets trapped in the deepest accessible energy basin. This transition temperature depends on the cooling rate, in a manner analogous to the experimental glass transition. We also present evidence that the barrier heights separating potential-energy minima sampled by the liquid increase abruptly at a temperature above the glass transition but well below the onset of non-exponential relaxation. This identification of a relationship between static, topographic features of the energy landscape and complex dynamics holds the promise of a clearer, possibly thermodynamic, understanding of the glass transition.

Supercritical Fluids: Fundamentals and Applications

Fluids in the approximate range of temperature and pressure 1 to 1.1 and 1 to 2, respectively (referred to critical conditions) are commonly called supercritical. The thermophysical properties in the supercritical region can be exploited in such applications as selectivity enhancement in enzymatic reactions [1] to formation of biologically active protein powders [2]. Supercritical mixtures of practical interest are characterized by a pronounced asymmetry between conditions in the bulk, and in the solvation region around solute molecules. This solvation region tends to be solvent-rich with respect to the bulk. One of the most promising applications of supercritical fluids is particle formation [3]. Two techniques exist: rapid expansion [3] and anti-solvent precipitation [2]. We have used both methods to produce devices for the controlled release of therapeutic drugs and biologically active protein powders.

Particle formation with supercritical fluids—a review

Abstract The rapid expansion of supercritical solutions (RESS) is a new and promising method of particle formation. The distinguishing features of this process are the fast attainment of uniform conditions and of high supersaturations in the carrier fluid, which favor the formation of small, monodisperse particles. The technique has been applied to inorganic, organic, pharmaceutical, and polymeric materials. RESS can be used to comminute shock-sensitive solids, to produce intimate mixtures of amorphous materials, to form polymeric microspheres, and to deposit thin films. In this paper, we discuss the fundamentals, experimental methods, applications, and available results from studies of particle formation with supercritical solutions.

Supercooled and Glassy Water

Cold, noncrystalline states play an important role in understanding the physics of liquid water. From recent experimental and theoretical investigations, a coherent interpretation of water’s properties is beginning to emerge.

Metastable liquid-liquid transition in a molecular model of water

Liquid water’s isothermal compressibility and isobaric heat capacity, and the magnitude of its thermal expansion coefficient, increase sharply on cooling below the equilibrium freezing point. Many experimental, theoretical and computational studies have sought to understand the molecular origin and implications of this anomalous behaviour. Of the different theoretical scenarios put forward, one posits the existence of a first-order phase transition that involves two forms of liquid water and terminates at a critical point located at deeply supercooled conditions. Some experimental evidence is consistent with this hypothesis, but no definitive proof of a liquid–liquid transition in water has been obtained to date: rapid ice crystallization has so far prevented decisive measurements on deeply supercooled water, although this challenge has been overcome recently. Computer simulations are therefore crucial for exploring water’s structure and behaviour in this regime, and have shown that some water models exhibit liquid–liquid transitions and others do not. However, recent work has argued that the liquid–liquid transition has been mistakenly interpreted, and is in fact a liquid–crystal transition in all atomistic models of water. Here we show, by studying the liquid–liquid transition in the ST2 model of water with the use of six advanced sampling methods to compute the free-energy surface, that two metastable liquid phases and a stable crystal phase exist at the same deeply supercooled thermodynamic condition, and that the transition between the two liquids satisfies the thermodynamic criteria of a first-order transition. We follow the rearrangement of water’s coordination shell and topological ring structure along a thermodynamically reversible path from the low-density liquid to cubic ice. We also show that the system fluctuates freely between the two liquid phases rather than crystallizing. These findings provide unambiguous evidence for a liquid–liquid transition in the ST2 model of water, and point to the separation of time scales between crystallization and relaxation as being crucial for enabling it.

The evaporation rate, free energy, and entropy of amorphous water at 150 K

Measurement of the rates of evaporation of amorphous water (a) and ice (i) near 150 K can be interpreted as giving a measure of their free energy difference, ΔaiG (150 K)=1100±100 J/mol, which, together with the known enthalpy difference and heat capacity data, suggests a residual entropy difference of ΔaiS (0)=−0.7±2.2 J/(K mol) at absolute zero. Previous theoretical estimates of ΔaiS (0), which are much larger, did not allow the amorph to be connected with normal liquid water by a reversible thermodynamic path at atmospheric pressure. The present value allows such a connection.

Hydrophobicity of protein surfaces: Separating geometry from chemistry.

To better understand the role of surface chemical heterogeneity in natural nanoscale hydration, we study via molecular dynamics simulation the structure and thermodynamics of water confined between two protein-like surfaces. Each surface is constructed to have interactions with water corresponding to those of the putative hydrophobic surface of a melittin dimer, but is flattened rather than having its native “cupped” configuration. Furthermore, peripheral charged groups are removed. Thus, the role of a rough surface topography is removed, and results can be productively compared with those previously observed for idealized, atomically smooth hydrophilic and hydrophobic flat surfaces. The results indicate that the protein surface is less hydrophobic than the idealized counterpart. The density and compressibility of water adjacent to a melittin dimer is intermediate between that observed adjacent to idealized hydrophobic or hydrophilic surfaces. We find that solvent evacuation of the hydrophobic gap (cavitation) between dimers is observed when the gap has closed to sterically permit a single water layer. This cavitation occurs at smaller pressures and separations than in the case of idealized hydrophobic flat surfaces. The vapor phase between the melittin dimers occupies a much smaller lateral region than in the case of the idealized surfaces; cavitation is localized in a narrow central region between the dimers, where an apolar amino acid is located. When that amino acid is replaced by a polar residue, cavitation is no longer observed.