Rates of heterogeneous nucleation can be greatly increased not only through control of the chemistry of a surface, but also of its topography. Following previous work in which we showed that scratching a mica surface significantly enhances crystal nucleation from vapor, we here use a new experimental approach to understand better the effect of topography on crystal nucleation. The compounds carbon tetrabromide, camphor, norbornane, and hexachloroethane were deposited from vapor onto mica sheets containing various surface defects, and their nucleation was studied using optical microscopy. Following subsequent evaporation of the crystals, examination of the sites where these had nucleated with a scanning electron microscope enabled the nature of each material's preferred nucleation sites to be determined, and all four compounds appeared to exhibit a strong preference for sites characterized by delamination of the layered mica structure. Indeed, comparison of the four compounds on the same mica substrates showed that they all favored the same nucleation sites. These observations are attributed to the presence of an acute wedge geometry at the delamination lines, which can provide a thermodynamic reduction in the free energy barrier to nucleation directly from vapor; alternatively, the results are also consistent with a two-step nucleation mechanism via a liquid capillary condensate.
Our understanding of crystal nucleation is a limiting factor in many fields, not least in the atmospheric sciences. It was recently found that feldspar, a component of airborne desert dust, plays a dominant role in triggering ice formation in clouds, but the origin of this effect was unclear. By investigating the structure/property relationships of a wide range of feldspars, we demonstrate that alkali feldspars with certain microtextures, related to phase separation into Na and K-rich regions, show exceptional ice-nucleating abilities in supercooled water. We found no correlation between ice-nucleating efficiency and the crystal structures or the chemical compositions of these active feldspars, which suggests that specific topographical features associated with these microtextures are key in the activity of these feldspars. That topography likely acts to promote ice nucleation, improves our understanding of ice formation in clouds, and may also enable the design and manufacture of bespoke nucleating materials for uses such as cloud seeding and cryopreservation.
Phase separation induced by the close proximity of two surfaces occurs in thin films of incompletely miscible liquids. This phenomenon is thermodynamically analogous to capillary condensation of liquid from vapor and leads to a discontinuity in the force between two surfaces across the liquid. We present measurements of the distance at which phase separation takes place in nonpolar liquids containing water at activities from 0.7 to 1 between two kinds of chemically different mica surfaces. The generality of the effect is established by similar results obtained with other sparingly soluble solutes. The importance of surface adsorption and kinetic effects is discussed and comparisons are made with a modified Kelvin equation. The results are relevant to recent theoretical and simulation studies of the phase behavior of liquids in narrow pores and thin films.
We have measured the adsorption isotherms of water on a single surface of freshly cleaved mica with K+ on the surface, and on mica where the K+ has been exchanged for H+. Using a very sensitive interferometric technique, we have found a significant difference between the two isotherms at submonolayer coverage, for relative vapor pressures p/p0 < 0.5. The K+-mica isotherm shows a pronounced convexity, suggesting distinct adsorption sites, whereas the H+-mica isotherm is flatter. The two isotherms converge above monolayer coverage. The results give a graphic demonstration of the importance of nanoscale surface heterogeneities for vapor adsorption at submonolayer coverage.
Significance Calcium carbonate is a widespread compound whose two common crystalline forms, calcite and aragonite, are important biominerals. Although aragonite is only marginally less stable than calcite under ambient conditions, it usually only crystallizes from solution at high temperatures or in the presence of magnesium ions. However, organisms readily form both calcite and aragonite biominerals, a capacity usually attributed to the action of specific organic macromolecules. By investigating calcium carbonate precipitation in submicron pores we here show that aragonite is promoted in confinement and that pure aragonite crystallizes in nanoscale pores in the absence of any additives. This is of great significance to biomineralization processes, which invariably occur in small volumes, and suggests that organisms may exploit confinement effects to control polymorph.
When two surfaces in a condensable vapor are brought together, the vapor will capillary condense in the narrow gap between the surfaces. The surface forces apparatus, (SFA) has been used to study this condensation process with mica surfaces in ethanol vapor close to saturation. In particular, the critical surface separation at which the condensed bridge forms has been quantified. For thin adsorbed films (≤2 nm), the results are not consistent with a model of a liquid bridge formed by adsorbed films thickening under the influence of van der Waals forces. Instead, nucleation from vapor in the gap between the surfaces is possibly contributing to the formation of the bridge. The short-range interaction of mica surfaces in near-saturated ethanol vapor is also presented. The contact adhesion is much smaller than in nonpolar liquids due to the shielding of the ionic components of the adhesion. The solvation force is found to be similar to that in nonpolar liquids, except that the innermost minima are deeper. This is attributed to the amphiphilic nature of the ethanol molecule.
There is currently considerable interest in two-step models of crystal nucleation, which have been implicated in a number of systems including proteins, colloids and small organic molecules. Classical nucleation theory (CNT) postulates the formation of an ordered crystalline nucleus directly from dilute vapour or solution. By contrast, the new models explain how crystallisation via a more concentrated but still fluid (disordered) phase can lead to a significant enhancement of nucleation rates. In this article, we extend recent work showing that crystal deposition from vapour can also be greatly accelerated by the operation of a two-step mechanism. The process relies on a very acute, annular wedge, in which restricted amounts of liquid condense below the bulk melting point Tm. Crystals then nucleate in the liquid condensates at sufficient temperature depressions ΔT (typically ≥30 K) below Tm, followed by rapid growth of these crystals from the saturated vapour. By using a range of model substances (neopentanol, norbornane, hexamethylcyclotrisiloxane, hexachloroethane, menthol, cyclooctane and pinacol) we show that this is a viable mechanism for substances with reasonably high absolute vapour pressures (>ca. 1 mm Hg). The lack of appreciable crystal deposition with substances of significantly lower vapour pressures (
Classical nucleation theory (CNT) has been extensively employed to interpret crystal nucleation phenomena and postulates the formation of an ordered crystalline nucleus directly from vapor or solution. Here, we provide the first experimental demonstration of a two-step mechanism that facilitates deposition of crystals on solid surfaces from vapor. Crucially, this occurs from saturated vapor without the need for supersaturation, conditions that, according to CNT, cannot lead to direct deposition of crystals from vapor. Instead, the process relies on condensation of supercooled liquid in surface cavities below the melting point. Crystals then nucleate in this liquid, leading to rapid deposition of more solid. Such a mechanism has been postulated for atmospheric nucleation of ice on aerosol particles and may have analogies in the crystallization of biominerals via amorphous precursor phases.
Rates of homogeneous nucleation of ice in micrometre-sized water droplets are reported. Measurements were made using a new system in which droplets were supported on a hydrophobic substrate and their phase was monitored using optical microscopy as they were cooled at a controlled rate. Our nucleation rates are in agreement, given the quoted uncertainties, with the most recent literature data. However, the level of uncertainty in the rate of homogeneous freezing remains unacceptable given the importance of homogeneous nucleation to cloud formation in the Earth's atmosphere. We go on to use the most recent thermodynamic data for cubic ice (the metastable phase thought to nucleate from supercooled water) to estimate the interfacial energy of the cubic ice-supercooled water interface. We estimate a value of 20.8 +/- 1.2 mJ m(-2) in the temperature range 234.9-236.7 K.
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