Vlad Sadtchenko

Glass transition in pure and doped amorphous solid water: An ultrafast microcalorimetry study

Using an ultrafast scanning microcalorimetry apparatus capable of heating rates in excess of 10(5) Ks, we have conducted the first direct measurements of thermodynamic properties of pure and doped amorphous solid water (also referred to as low density amorphous ice) in the temperature range from 120 to 230 K. Ultrafast microcalorimetry experiments show that the heat capacity of pure amorphous solid water (ASW) remains indistinguishable from that of crystalline ice during rapid heating up to a temperature of 205+/-5 K where the ASW undergoes rapid crystallization. Based on these observations, we conclude that the enthalpy relaxation time in pure ASW must be greater than 10(-5) s at 205 K. We argue that this result contradicts the assignment of glass transition temperature to 135 K and that ASW may undergo fragile to strong transition at temperatures greater than 205 K. Unlike pure ASW, we observe an approximately twofold rise in heat capacity of CH3COOH doped ASW at 177+/-5 K. We discuss results of past studies taking into account possible influence of impurities and confinement on physical properties of ASW.

The vaporization rate of ice at temperatures near its melting point

The first study of free vaporization kinetics of ice at temperatures near its melting point is reported. The experimental approach employed is based on a unique combination of thermal desorption spectroscopy, microcalorimetry, and time-of-flight mass spectrometry, making it possible to overcome challenges associated with the introduction of volatile solids into a high vacuum environment. Measurements of the vaporization rate of polycrystalline ice demonstrate that the vaporization kinetics deviate dramatically from those predicted by a simple mobile precursor mechanism. The vaporization rate follows Arrhenius behavior from -40 to 0 degrees C with an effective activation energy of 50+/-4 kJ/mol, which is significantly higher than the value predicted by the simple mobile precursor mechanism. Extrapolation of earlier measurements conducted below -40 degrees C yields a value of approximately 0.02 at 0 degrees C for the vaporization coefficient alphav. In contrast, experimentally determined vaporization coefficient is found to be 0.7+/-0.3 and shows a weak dependence on temperature up to the bulk melting point. The role of possible surface phase transitions in the mechanisms of release and uptake of H2O and other chemical species by ice surfaces is discussed.

Enthalpy and high temperature relaxation kinetics of stable vapor-deposited glasses of toluene

Stable non-crystalline toluene films of micrometer and nanometer thicknesses were grown by vapor deposition at distinct rates and probed by fast scanning calorimetry. Fast scanning calorimetry is shown to be extremely sensitive to the structure of the vapor-deposited phase and was used to characterize simultaneously its kinetic stability and its thermodynamic properties. According to our analysis, transformation of vapor-deposited samples of toluene during heating with rates in excess 10(5) K s(-1) follows the zero-order kinetics. The transformation rate correlates strongly with the initial enthalpy of the sample, which increases with the deposition rate according to sub-linear law. Analysis of the transformation kinetics of vapor-deposited toluene films of various thicknesses reveal a sudden increase in the transformation rate for films thinner than 250 nm. The change in kinetics seems to correlate with the surface roughness scale of the substrate. The implications of these findings for the formation mechanism and structure of vapor-deposited stable glasses are discussed.

H/D exchange kinetics in pure and HCl doped polycrystalline ice at temperatures near its melting point: Structure, chemical transport, and phase transitions at grain boundaries

We report the results of a fast thermal desorption spectroscopy study of the H/D isotopic exchange kinetics in a few micrometer thick, pure polycrystalline ice film and in ice films doped with HCl. Using the isotopic exchange reaction as a probe of transport processes in ice, we determined the effective H/D interdiffusion coefficients, D(eff), in pure and doped polycrystalline ice at temperatures ranging from -18 to -1 degree C. In the case of pure polycrystalline ice, D(eff) demonstrates an Arrhenius dependence on temperature with an effective activation energy of 69+/-3 kJ mol(-1) and a pre-exponential of 10(9+/-0.5) microm(2) ms(-1) up to -2 degrees C. According to our analysis, H/D interdiffusion coefficient at the grain boundaries also shows an Arrhenius dependence on temperature with an activation energy of 69+/-3 kJ mol(-1) and a pre-exponential of 10(11+/-1) microm(2) ms(-1). However, the addition of 0.04% of HCl results in a marked deviation of D(eff) from Arrhenius law at -8 degrees C, which is attributed to premelting at intersections of grain boundaries. We discuss the structure and transport properties of condensed aqueous phase at grain boundaries in polycrystalline ice at various temperatures.

Vapor-deposited non-crystalline phase vs ordinary glasses and supercooled liquids: Subtle thermodynamic and kinetic differences

Vapor deposition of molecules on a substrate often results in glassy materials of high kinetic stability and low enthalpy. The extraordinary properties of such glasses are attributed to high rates of surface diffusion during sample deposition, which makes it possible for constituents to find a configuration of much lower energy on a typical laboratory time scale. However, the exact nature of the resulting phase and the mechanism of its formation are not completely understood. Using fast scanning calorimetry technique, we show that out-of-equilibrium relaxation kinetics and possibly the enthalpy of vapor-deposited films of toluene and ethylbenzene, archetypical fragile glass formers, are distinct from those of ordinary supercooled phase even when the deposition takes place at temperatures above the ordinary glass softening transition temperatures. These observations along with the absolute enthalpy dependences on deposition temperatures support the conjecture that the vapor-deposition may result in formation of non-crystalline phase of unique structural, thermodynamic, and kinetic properties.

Fast thermal desorption spectroscopy study of H∕D isotopic exchange reaction in polycrystalline ice near its melting point

Using fast thermal desorption spectroscopy, a novel technique developed in our laboratory, we investigated the kinetics of HD isotopic exchange in 3 microm thick polycrystalline H2O ice films containing D2O layers at thicknesses ranging from 10 to 300 nm at a temperature of -2.0+/-1.5 degrees C. According to our results over the duration of a typical fast thermal desorption experiment (3-4 ms), the isotopic exchange is confined to a 50+/-10 nm wide reaction zone located at the boundary between polycrystalline H2O and D2O ice. Combining these data with a theoretical analysis of the diffusion in polycrystalline medium, we establish the range of possible values for water self-diffusion coefficients and the grain boundary widths characteristic of our ice samples. Our analysis shows that for the grain boundary width on the order of a few nanometers, the diffusivity of D2O along the grain boundaries must be at least two orders of magnitude lower than that in bulk water at the same temperature. Based on these results, we argue that, in the limit of low concentrations of impurities, polycrystalline ice does not undergo grain boundary premelting at temperatures up to -2 degrees C.

Fast thermal desorption spectroscopy study of morphology and vaporization kinetics of polycrystalline ice films

Fast thermal desorption spectroscopy was used to investigate the vaporization kinetics of thin (50-100 nm) H(2)O(18) and HDO tracer layers from 2-5 microm thick polycrystalline H(2)O(16) ice films at temperatures ranging from -15 to -2 degrees C. The isothermal desorption spectra of tracer species demonstrate two distinct peaks, alpha and beta, which we attribute to the vaporization of H(2)O(18) initially trapped at or near the grain boundaries and in the crystallites of the polycrystalline ice, respectively. We show that the diffusive transport of the H(2)O(18) and HDO tracer molecules in the bulk of the H(2)O(16) film is slow as compared to the film vaporization. Thus, the two peaks in the isothermal spectra are due to unequal vaporization rates of H(2)O(18) from grain boundary grooves and from the crystallites and, therefore, can be used to determine independently the vaporization rate of the single crystal part of the film and rate of thermal etching of the film. Our analysis of the tracer vaporization kinetics demonstrates that the vaporization coefficient of single crystal ice is significantly greater than those predicted by the classical vaporization mechanism at temperatures near ice melting point. We discuss surface morphological dynamics and the bulk transport phenomena in single crystal and polycrystalline ice near 0 degrees C.

Response to “Comment on ‘Glass transition in pure and doped amorphous solid: An ultrafast microcalorimetry study’ [J. Chem. Phys. 125, 094501 (2006)]”

We report results of ultrafast scanning calorimetry (USC) measurements of enthalpy relaxation time for propanol, toluene, pentanol, decalin, and 2-ethyl-1-hexanol in a temperature range from 120to180K. These new data show that the enthalpy relaxation times measured in USC experiments are within one order of magnitude of those derived from dielectric spectroscopy studies. Thus, we demonstrate that Johari’s critique of the USC study of pure and doped amorphous solid water is without merit, and that USC experiments do provide evidence against assignment of the glass transition temperature of amorphous solid water to 136K.

Melting of superheated molecular crystals

Melting dynamics of micrometer scale, polycrystalline samples of isobutane, dimethyl ether, methyl benzene, and 2-propanol were investigated by fast scanning calorimetry. When films are superheated with rates in excess of 105 K s-1, the melting process follows zero-order, Arrhenius-like kinetics until approximately half of the sample has transformed. Such kinetics strongly imply that melting progresses into the bulk via a rapidly moving solid-liquid interface that is likely to originate at the samples surface. Remarkably, the apparent activation energies for the phase transformation are large; all exceed the enthalpy of vaporization of each compound and some exceed it by an order of magnitude. In fact, we find that the crystalline melting kinetics are comparable to the kinetics of dielectric α-relaxation in deeply supercooled liquids. Based on these observations, we conclude that the rate of non-isothermal melting for superheated, low-molecular-weight crystals is limited by constituent diffusion into an abnormally dense, glass-like, non-crystalline phase.

Communication: Surface-facilitated softening of ordinary and vapor-deposited glasses

A common distinction between the ordinary glasses formed by melt cooling and the stable amorphous films formed by vapor deposition is the apparent mechanism of their devitrification. Using quasi-adiabatic, fast scanning calorimetry that is capable of heating rates in excess of 105 K s-1, we have investigated the softening kinetics of micrometer-scale, ordinary glass films of methylbenzene and 2-propanol. At the limit of high heating rates, the transformation mechanism of ordinary glasses is identical to that of their stable vapor-deposited counterparts. In both cases, softening is likely to begin at the sample surface and progress into its bulk via a transformation front. Furthermore, such a surface-facilitated mechanism complies with zero-order, Arrhenius rate law. The activation energy barriers for the softening transformation imply that the kinetics must be defined, at least in part, by the initial thermodynamic and structural state of the samples.