Studies on the growth of ice crystal templates during the synthesis of a monolithic silica microhoneycomb using the ice templating method

Porous materials with a wide variety of functions can be obtained through sol-gel synthesis. Recently, we found that sol-gel based materials can be molded into a monolithic microhoneycomb structure by simply freezing their parent hydrogels unidirectionally. The main feature of the monoliths obtained through this method, which we named the Ice Templating Method, is that they have straight and aligned macropores, the sizes of which are in the micrometer range. As these macropores are the traces of the ice crystals which are formed during freezing and which practically act as the template, the sizes as well as the shape of them depend on how the template ice crystals are formed and how they grow. Therefore in this work, the growth behavior of the ice crystals formed during the unidirectional freezing of a silica hydrogel was examined and the influences of this growth behavior on the properties of the resulting monoliths were verified.     

Ultrastructure in frozen/etched saline solutions: on the internal cleansing of ice

Seawater, with its 3.5% salt content, freezes into hexagonal ice (Ih) that encloses concentrated brine within its matrix. When unsubmerged sea ice reaches a certain height and temperature, the brine drains downward through narrow channels. This mechanism was now modeled by frozen 2-3.5% saline as investigated by cryo-etch high-resolution secondary electron microscopy. Thus, saline was either plunge-frozen in liquid ethane at -183 degrees C or else high-pressure frozen to -105 degrees C in 5-6 ms. Ice from a freshly exposed surface was then subjected to a high-vacuum sublimation ("etching"), a procedure that removes pure bulk ice in preference to ice from frozen hydrated salt. After chromium-coating the etched surface with a 2-nm film, the sample was examined by cryo-HRSEM. Granular icy "fences" were seen surrounding empty areas where amorphous ice had originally resided. Since the fences, about 1-2 mum high, survived the etching, it is likely that they consist of frozen brine. The presence of such fences suggests that, during freezing, saline can purge itself of salt with remarkable speed (5-6 ms). Alternatively, channels (perhaps routed around submicroscopic crystallites of cubic ice (Ic) embedded in the amorphous ice at -105 degrees C) can guide the migration of salt to the periphery of ice patches. Macromolecules fail to form fences because they diffuse too slowly or because they are too large to pass through the channels.     

Adsorption study of acetone on acid-doped ice surfaces between 203 and 233 K

Adsorption studies of acetone on pure ice surfaces obtained by water freezing or deposition or on frozen ice surfaces doped either with HNO3 or H2SO4 have been performed using a coated wall flow tube coupled to a mass spectrometric detection. The experiments were conducted over the temperature range 203-233 K and freezing solutions containing either H2SO4 (0.2 N) or HNO3 (0.2-3 N). Adsorption of acetone on these ice surfaces was always found to be totally reversible whatever were the experimental conditions. The number of acetone molecules adsorbed per ice surface unit N was conventionally plotted as a function of acetone concentration in the gas phase. For the same conditions, the amount of acetone molecules adsorbed on pure ice obtained by deposition are about 3-4 times higher than those measured on frozen ice films, H2SO4-doped ice surfaces lead to results comparable to those obtained on pure ice. On the contrary, N increases largely with increasing concentrations of nitric acid in ice surfaces, up to about 300 times under our experimental conditions and for temperatures ranging between 213 and 233 K. Finally, the results are discussed and used to reestimate the partitioning of acetone between the ice and gas phases in clouds of the upper troposphere.     

Temperature dependence of particle-particle adherence forces in ice and clathrate hydrates

Particle-particle pulloff adherence forces were measured as a function of temperature in the ice/n-decane/ice and tetrahydrofuran (THF) hydrate/n-decane/THF hydrate systems using a newly developed micromechanical testing technique. Experiments using approximately 200 microm radius particles were performed at atmospheric pressure over the temperature range 263-275 K. The ice and hydrate particles displayed very similar behavior. While the measured adherence forces had significant variation, the shapes of the cumulative force distribution curves were similar among the different sets of experiments. The measured adherence forces distributions shifted to lower force values as the temperature was decreased from the solid melting temperature. The observed forces and trends were explained by the capillary cohesion of rough surfaces, with the capillary bridging liquid being stabilized below its freezing point by the negative curvature of the bridging liquid/n-decane interface.     

Dynamics of colloidal particles in ice

We use x-ray photon correlation spectroscopy (XPCS) to probe the dynamics of colloidal particles in polycrystalline ice. During freezing, the dendritic ice morphology and rejection of particles from the ice created regions of high particle density, where some of the colloids were forced into contact and formed disordered aggregates. The particles in these high density regions underwent ballistic motion, with a characteristic velocity that increased with temperature. This ballistic motion is coupled with both stretched and compressed exponential decays of the intensity autocorrelation function. We suggest that this behavior could result from ice grain boundary migration.     

Chemical kinetics of reactions in the unfrozen solution of ice

Some reactions are accelerated in ice compared to aqueous solution at higher temperatures. Accelerated reactions in ice take place mainly due to the freeze-concentration effect of solutes in an unfrozen solution at temperatures higher than the eutectic point of the solution. Pincock was the first to report an acceleration model for reactions in ice,1 which successfully simulated experimental results. We propose here a modified version of the model for reactions in ice. The new model includes the total molar change involved in reactions in ice. Furthermore, we explain why many reactions are not accelerated in ice. The acceleration of reactions can be observed in the cases of (i) second- or higher-order reactions, (ii) low concentrations, and (iii) reactions with a small activation energy. Reactions with a buffer solution or additives in order to adjust ion strength, zero- or first-order reactions, or reactions containing high reactant concentrations are not accelerated by freezing. We conclude that the acceleration of reactions in the unfrozen solution of ice is not an abnormal phenomenon.     

Freeze-thaw behaviour of aqueous glucose solutions--the crystallisation of cubic ice

We report on the polymorphic transitions of ice in aqueous solutions of glucose during freezing and thawing over a temperature range of 298-153 K. Emphasis is placed on the sub-glass temperature range where the systems consist of cubic ice (ice-1c) crystals embedded in a freeze concentrated, vitrified glucose solution. The systems were studied by a combination of thermal, cryomicroscopic and X-ray diffraction techniques. At the glass transition (230 K) the solution phase contained 80 mol% of unfrozen water which, on further cooling, was shown to crystallise as cubic ice (ice-1c), nucleated in the vitrified matrix. The thermal stability of the ice-1c formed was studied by annealing and isothermal changes in the diffraction patterns with time. The polymorphic transition 1c --> 1h could be fitted to first order kinetics. Contrary to currently held belief, this study has provided evidence that ice-1c can be formed directly in the bulk water phase of a vitrified solution.     

A facet of recent ice sciences

Of prime interest in the numerous studies on water, an important substance to mankind and all other living systems, may be its chemical, physical, biological or geological characteristics but underlying all these is a basic structural problem. One of the important questions that still remains unanswered in this field is: why ordinary ice keep its proton-disordered state down to the lowest temperature. We found that the slowing down of water re-orientational motion at low temperatures leads to freezing of the disordered state in the ice crystal at around 110 K. This was the origin of the deviation of the crystal from the third law of thermodynamics. Doping by a particular kind of impurity recovered the mobility of the molecule to exhibit a long-awaited ordering transition at 72 K. The dopant dramatically accelerated the motion of water molecules to change the crystal from a non-equilibrium frozen-in disordered state to the equilibrium one within our experimental time. New steps in ice sciences and procedures used in these experiments are reviewed briefly. The structure and some properties of the low-temperature ordered phase, designated as ice XI, are described.     

Metadynamics simulations of ice nucleation and growth

The metadynamics method for accelerating rate events in molecular simulations is applied to the problem of ice freezing. We demonstrate homogeneous nucleation and growth of ice at 180 K in the isothermal-isobaric ensemble without the presence of external fields or surfaces. This result represents the first report of continuous and dynamic ice nucleation in a system of freely evolving density. Simulations are conducted using a variety of periodic simulation domains. In all cases the cubic polymorph ice I(c) is grown. The influence of boundary effects on estimates of the nucleation free energy barrier are discussed in relation to differences between this and earlier work.     

Electrolyte screening effect on the photoprotolytic cycle of excited photoacid in ice

Time-resolved emission was used to measure the photoprotolytic cycle of an excited photoacid as a function of temperature, both in liquid water and in ice, in the presence of an inert salt. The inert salt affects the geminate recombination between the transferred proton with the conjugate base of the photoacid. We used the Debye-Hückel theory to express the screening of the Coulomb electrical potential by the inert salt. We find that in the liquid phase the measured screening effect is small and the Debye-Hückel expression slightly overestimates the experimental effect. In ice, the screening effect is rather large and the Debye-Hückel expression under estimates the measured effect. We explain the large screening in ice by the "salting-out" effect in ice that tends to concentrate the impurities to confined volumes to minimize the ice crystal energy.     

Determination of ice content in hardened concrete by low-temperature calorimetry

Low-temperature calorimetry has been used to determine the ice content in concrete at different temperatures when exposed to low-temperature environments. However, the analysis of the ice content from the measured data of heat flow is not straightforward. In this study, two important factors influencing the ice content calculation are discussed. The importance of the baseline determination for the calculation of the ice content is realized. Two different methods of generating the baseline are discussed. First, the ‘J-baseline’ is discussed which is a recently proposed extrapolation method based on the accumulated heat curves measured in the freezing and the melting process. Second, the ‘C-baseline’ is discussed in which a calculated baseline is used where the heat capacity of both water and ice and the phase changing behaviour under different testing temperatures are considered. It turns out that both the ‘J-baseline’ method and the ‘C-baseline’ method can be used to calculate the approximate baseline. The heat of fusion of the water confined in small pores is another important parameter to be considered in ice content calculation. This property must be carefully analyzed in order to accurately calculate the ice contents at different temperatures in the freezing and melting process. It should be noted that there is no general agreement on how to obtain the important temperature dependence of the heat of fusion of water confined in small pores. By performing comparison studies, the present study shows the influence of the different values of the heat of fusion commonly adopted on the calculated ice content for the studied concrete samples. The importance and necessity to use an accurate value of the heat of fusion is emphasized. Based on the calculation of the baseline proposed in this work and by carefully selecting the values for the heat of fusion, the ice content in a hardened concrete sample is expected to be estimated with an acceptable accuracy.     

Initiation of the ice phase by marine biogenic surfaces in supersaturated gas and supercooled aqueous phases

Biogenic particles have the potential to affect the formation of ice crystals in the atmosphere with subsequent consequences for the hydrological cycle and climate. We present laboratory observations of heterogeneous ice nucleation in immersion and deposition modes under atmospherically relevant conditions initiated by Nannochloris atomus and Emiliania huxleyi, marine phytoplankton with structurally and chemically distinct cell walls. Temperatures at which freezing, melting, and water uptake occur are observed using optical microscopy. The intact and fragmented unarmoured cells of N. atomus in aqueous NaCl droplets enhance ice nucleation by 10-20 K over the homogeneous freezing limit and can be described by a modified water activity based ice nucleation approach. E. huxleyi cells covered by calcite plates do not enhance droplet freezing temperatures. Both species nucleate ice in the deposition mode at an ice saturation ratio, S(ice), as low as ~1.2 and below 240 K, however, for each, different nucleation modes occur at warmer temperatures. These observations show that markedly different biogenic surfaces have both comparable and contrasting effects on ice nucleation behaviour depending on the presence of the aqueous phase and the extent of supercooling and water vapour supersaturation. We derive heterogeneous ice nucleation rate coefficients, J(het), and cumulative ice nuclei spectra, K, for quantification and analysis using time-dependent and time-independent approaches, respectively. Contact angles, α, derived from J(het)via immersion freezing depend on T, a(w), and S(ice). For deposition freezing, α can be described as a function of S(ice) only. The different approaches yield different predictions of atmospheric ice crystal numbers primarily due to the time evolution allowed for the time-dependent approach with implications for the evolution of mixed-phase and ice clouds.     

Anisotropy in the crystal growth of hexagonal ice, I(h)

Growth of ice crystals has attracted attention because ice and water are ubiquitous in the environment and play critical roles in natural processes. Hexagonal ice, I(h), is the most common form of ice among 15 known crystalline phases of ice. In this work we report the results of an extensive and systematic molecular dynamics study of the temperature dependence of the crystal growth on the three primary crystal faces of hexagonal ice, the basal {0001} face, the prism {1010} face, and the secondary prism {1120} face, utilizing the TIP4P-2005 water model. New insights into the nature of its anisotropic growth are uncovered. It is demonstrated that the ice growth is indeed anisotropic; the growth and melting of the basal face are the slowest of the three faces, its maximum growth rates being 31% and 43% slower, respectively, than those of the prism and the secondary prism faces. It is also shown that application of periodic boundary conditions can lead to varying size effect for different orientations of an ice crystal caused by the anisotropic physical properties of the crystal, and results in measurably different thermodynamic melting temperatures in three systems of similar, yet moderate, size. Evidence obtained here provides the grounds on which to clarify the current understanding of ice growth on the secondary prism face of ice. We also revisit the effect of the integration time step on the crystal growth of ice in a more thorough and systematic way. Careful evaluation demonstrates that increasing the integration time step size measurably affects the free energy of the bulk phases and shifts the temperature dependence of the growth rate curve to lower temperatures by approximately 1 K when the step is changed from 1 fs to 2 fs, and by 3 K when 3 fs steps are used. A thorough investigation of the numerical aspects of the simulations exposes important consequences of the simulation parameter choices upon the delicate dynamic balance that is involved in ice crystal growth.     

Texture change of ice on anomalously preserved methane clathrate hydrate

We used a confocal scanning microscope to observe growth and texture change of ice due to the dissociation of methane gas clathrate hydrate (CH(4) hydrate). The experiments were done under CH(4) gas atmospheric pressure and isothermal conditions between 170 and 268 K. Above 193 K, the dissociation of CH(4) hydrate resulted in many small ice particles that covered the hydrate surface. These ice particles had roughly the same shape and density between 193 and 210 K. In contrast, above 230 K the ice particles developed into a sheet of ice that covered the hydrate surface. Moreover, the measured release of CH(4) gas decreased when the sheet of ice formed at the surface of the hydrate. These findings can explain the anomalous preservation behavior of CH(4) hydrate; that is, the known increase of storage stability of CH(4) hydrate above 240 K is likely related to the formation of the ice that we observed in the experiments.     

Burial of gas-phase HNO(3) by growing ice surfaces under tropospheric conditions

The uptake of gas-phase nitric acid by ice surfaces undergoing growth by vapor deposition has been performed for the first time under conditions of the free troposphere. The investigation was performed using a coated-wall flow tube coupled to a chemical ionization mass spectrometer, at nitric acid partial pressures between 10(-7) and 10(-6) hPa, at 214, 229 and 239 K. Ice surfaces were prepared as smooth ice films from ultra-pure water. During the experiments an excess flow of water vapor was added to the carrier gas flow and the existing ice surfaces grew by depositing water vapor. The average growth rates ranged from 0.7-5 microm min(-1), values similar to those which prevail in some portions of the atmosphere. With growing ice the long term uptake of nitric acid is significantly enhanced compared to an experiment performed at equilibrium, i.e. at 100% relative humidity (RH) with respect to ice. The fraction of HNO(3) that is deposited onto the growing ice surface is independent of the growth rate and may be driven by the solubility of the nitric acid in the growing ice film rather than by condensation kinetics alone.     

Heterogeneous ice nucleation in aqueous solutions: the role of water activity

Heterogeneous ice nucleation experiments have been performed with four different ice nuclei (IN), namely nonadecanol, silica, silver iodide and Arizona test dust. All IN are either immersed in the droplets or located at the droplets surface. The IN were exposed to various aqueous solutions, which consist of (NH4)2SO4, H2SO4, MgCl2, NaCl, LiCl, Ca(NO3)2, K2CO3, CH3COONa, ethylene glycol, glycerol, malonic acid, PEG300 or a NaCl/malonic acid mixture. Freezing was studied using a differential scanning calorimeter and a cold finger cell. The results show that the heterogeneous ice freezing temperatures decrease with increasing solute concentration; however, the magnitude of this effect is solute dependent. In contrast, when the results are analyzed in terms of the solution water activity a very consistent behavior emerges: heterogeneous ice nucleation temperatures for all four IN converge each onto a single line, irrespective of the nature of the solute. We find that a constant offset with respect to the ice melting point curve, Deltaaw,het, can describe the observed freezing temperatures for each IN. Such a behavior is well-known for homogeneous ice nucleation from supercooled liquid droplets and has led to the development of water-activity-based ice nucleation theory. The large variety of investigated solutes together with different general types of ice nuclei studied (monolayers, ionic crystals, covalently bound network-forming compounds, and a mixture of chemically different crystallites) underlines the general applicability of water-activity-based ice nucleation theory also for heterogeneous ice nucleation in the immersion mode. Finally, the ice nucleation efficiencies of the various IN, as well as the atmospheric implication of the developed parametrization are discussed.     

Powder X-ray diffraction observations of ice crystals formed from disaccharide solutions

Powder X-ray diffraction (PXRD) measurements on rapid freezing samples of disaccharide (trehalose, sucrose, and maltose) solutions indicated that the crystalline phases in the sample were both hexagonal and cubic ice. The cubic ice existed at a higher ratio in the higher disaccharide concentration samples. The temperature ramping experiments revealed that the cubic ice was stable below 233 K, which was obviously higher than the temperature expected for a pure water system. The diffraction peak width of the hexagonal ice crystal was independent in the disaccharide concentrations. This indicated that the crystallite size of the hexagonal ice was more than several hundreds of nanometre, which coincided with the ice particle size previously observed in the freeze-fractured replica samples. The comparison of the present PXRD data with the replica observations by transmission electron microscope in an earlier study allows us to conclude that the cubic ice was formed at the grain boundary between the hexagonal ice and the coexisted non-crystalline disaccharide phase.     

Isoconfigurational molecular dynamics study of the kinetics of ice crystal growth

Spontaneous self-assembling, such as formation of molecular crystals, is a fascinating topic for investigation. Ability to initiate and control such transformations promises numerous benefits, but our knowledge of underlying mechanisms of such processes is rather limited. The process of freezing of water is an excellent testing ground for such studies. In this paper we report the results of a systematic molecular dynamics study of ice growth at three different temperatures below the melting point initiated from a number of initial interface structures within the isoconfigurational ensemble. It is shown that a specific structure at a growing ice-water interface is able to affect the growth process over a time scale of 1-2 ns. This structural effect can be characterized in terms of relative growth propensities. On the basis of the differences in the shape between isoconfigurational rate distributions and the rate distribution typical of the specific temperature several different kinds of relative growth propensities have been identified. The initial interfacial configurations employed in this work have been assigned using the proposed classification and possible mechanisms of propensity realization have been suggested for selected cases. Results reported in this paper clearly indicate that local structure effects can have significant impact on tendency for a particular ice surface to grow (or melt). The structural effect on ordering propensities is, most probably, a more universal behaviour and might be expected to be seen in other similar problems such as, for example, protein folding.     

Kinetic aspects of the thermostatted growth of ice from supercooled water in simulations

In experiments, the growth rate of ice from supercooled water is seen to increase with the degree of supercooling, that is, the lower the temperature, the faster the crystallization takes place. In molecular dynamics simulations of the freezing process, however, the temperature is usually kept constant by means of a thermostat that artificially removes the heat released during the crystallization by scaling the velocities of the particles. This direct removal of energy from the system replaces a more realistic heat-conduction mechanism and is believed to be responsible for the curious observation that the thermostatted ice growth proceeds fastest near the melting point and more slowly at lower temperatures, which is exactly opposite to the experimental findings [M. A. Carignano, P. B. Shepson, and I. Szleifer, Mol. Phys. 103, 2957 (2005)]. This trend is explained by the diffusion and the reorientation of molecules in the liquid becoming the rate-determining steps for the crystal growth, both of which are slower at low temperatures. Yet, for a different set of simulations, a kinetic behavior analogous to the experimental finding has been reported [H. Nada and Y. Furukawa, J. Crystal Growth 283, 242 (2005)]. To clarify this apparent contradiction, we perform relatively long simulations of the TIP4P/Ice model in an extended range of temperatures. The temperature dependence of the thermostatted ice growth is seen to be more complex than was previously reported: The crystallization process is very slow close to the melting point at 270 K, where the thermodynamic driving force for the phase transition is weak. On lowering the temperature, the growth rate initially increases, but displays a maximum near 260 K. At even lower temperatures, the freezing process slows down again due to the reduced diffusivity in the liquid. The velocity of the thermostatted melting process, in contrast, shows a monotonic increase upon raising the temperature beyond the normal melting point. In this case, the effects of the increasing thermodynamic driving force and the faster diffusion at higher temperatures reinforce each other. In the context of this study, we also report data for the diffusion coefficient as a function of temperature for the water models TIP4P/Ice and TIP4P/2005.