5 research outputs found
Thermal desorption of HāO ice: from nanoscale films to the bulk
The desorption properties of H2O films are investigated across a wide range of film thicknesses from 53 nanometres (nm) to 101 micrometres (Ī¼m) using a quartz-crystal microbalance (QCM) and temperature-programmed desorption. Three desorption stages are observed belonging to amorphous solid water (ASW), stacking disordered ice I (ice Isd), and hexagonal ice I (ice Ih). The desorption of ASW is only detectable for theĀ ā„10Ā Ī¼m films and is separated from the ice I desorption by 10ā15Ā K with an associated desorption energy of ā¼64Ā kJ molā1. The desorption energy of the 53-nm film was found to be near 50Ā kJ molā1 as also noted in the literature, but with increasing film thickness, the desorption energy of ice I rises, reaching a plateau around 65ā70Ā kJ molā1. The reason for the increased desorption energy is suggested to be due to molecules unable to desorb due to the thick covering layer of H2O and possibly re-adsorption events. Before complete desorption of ice I at around 220Ā K for the 101Ā Ī¼m film, a two-stage ice I desorption is observed with the QCM for theĀ ā„10Ā Ī¼m films near 200Ā K. This event corresponds to the desorption of ice Isd as corroborated by X-ray diffraction patterns collected upon heating from 92 to 260Ā K at ambient pressure. Cubic ice is not observed as is commonly stated in the literature as resulting from the crystallization of ASW. Therefore, ice Isd is the correct terminology for the initial crystallization product of ASW
Hydrophobic Hydration of the Hydrocarbon Adamantane in Amorphous Ice
Hydrophobic molecules are by definition difficult to hydrate. Previous studies in the area of hydrophobic hydration have therefore often relied on using amphiphilic molecules where the hydrophilic part of a molecule enabled the solubility in liquid water. Here we show that the hydrophobic adamantane (C10H16) molecule can be fully hydrated through vapour codeposition with water onto a cryogenic substrate at 80 K resulting in the matrix isolation of adamantane in amorphous ice. Using neutron diffraction in combination with the isotopic substitution method and the empirical potential structure refinement technique, we find that the first hydration shell of adamantane is well structured consisting of a hydrogen-bonded cage of 28 water molecules that is also found in cubic structure II clathrate hydrates. The four hexagonal faces of the 51264 cage are situated above the four methine (CH) groups of adamantane whereas the methylene (CH2) groups are positioned below the edges of two adjoining pentagonal faces. The oxygen atoms of the 28 water molecules can be categorised on the basis of symmetry equivalences as twelve A, twelve B and four C oxygens. The water molecules of the first hydration shell display orientations consistent with those expected for a clathrate-hydrate-type cage, but also unfavourable ones with respect to the hydrogen bonding between the water molecules. Annealing the samples at 140 K, which is just below the crystallisation temperature of the matrix, removes the unfavourable orientations and leads to a slight increase of the structural order of the first hydration shell. The very closest water molecules display a tendency for their dipole moments to point towards the adamantane which is attributed to steric effects. Other than this, no significant polarisation effects are observed which is consistent with weak interactions between adamantane and the amorphous ice matrix. FT-IR spectroscopy shows that the incorporation of adamantane into amorphous ice leads to a weakening of the hydrogen bonds. In summary, the matrix-isolation of the highly symmetric adamantane in amorphous ice provides an interesting test case for hydrophobic hydration. Studying the structure and spectroscopic properties of water at the interface with hydrophobic hydrocarbons is also relevant for astrophysical environments, such as comets or the interstellar medium, where amorphous ice and hydrocarbons have been shown to coexist in large quantities
Amorphous mixtures of small hydrocarbons and ice
Water is a highly relevant molecule for a range of critical processes including clathrate
hydrates, icy comets and hydration shells of hydrophobic moieties. Carbon, a ābuilding
block of lifeā, often likes to coexist with water. With respect to waterās and carbonās
chemical and physical properties, they are very different from one another. This project
explores and establishes important processes happening at the interface between these
species. This research initially focuses on the thermal desorption properties of amorphous
mixtures and layered films of H2O and toluene at various ratios by incorporating the
research on thin films with a film thickness of ~55 Langmuir and by comparing to the
thick films synthesised with film thickness ranging from ~72 to 172 Āµm to observe
changes in the desorption behaviour. It also highlights the influence of four other
hydrocarbons, tetramethylbutane (TMB), dimethylbutane (DMB),
tetramethylcyclopropane (TMCP) and cyclohexane (CYH) at various ratios and their
impact on the properties of H2O. A range of analytical techniques have been considered
to investigate the behaviour of amorphous hydrocarbon/H2O mixtures at temperatures
ranging from ~100 to 260 K including differential scanning calorimetry, powder X-ray
diffraction, small-angle X-ray scattering and Fourier-Transform infrared spectroscopy.
From these analytical techniques, they have shown that a small amount of hydrocarbon
can have a substantial impact on the crystallisation temperature from 162 K for pure ASW
to 175 K for the cyclohexane/H2O 1:34 ratio. Each hydrocarbon/H2O mixture also shows
a different stacking disordered ice XRD pattern once the sample has crystallised which is
indicated from the profile of the peaks. From the FT-IR spectra, structural changes can
be observed upon heating and differences in the hydrogen bonding of the as-made
samples. Finally, the designing and implementation of an incubator setup is carried out
to investigate the kinetics of clathrate hydrate formation in which it was found that new
clathrate hydrates have been synthesised. The impact that these results would have, would
serve the wider community ranging from chemistry to astrophysics to atmospheric
sciences. It would also be highly relevant for understanding the properties of H2O in nanoconfinements as well as for optimising computational models considering H2O
Hydrophobic Hydration of the Hydrocarbon Adamantane in Amorphous Ice
Hydrophobic molecules are by definition difficult to hydrate. Previous studies in the area of hydrophobic hydration have therefore often relied on using amphiphilic molecules where the hydrophilic part of a molecule enabled the solubility in liquid water. Here we show that the hydrophobic adamantane (C10H16) molecule can be fully hydrated through vapour codeposition with water onto a cryogenic substrate at 80 K resulting in the matrix isolation of adamantane in amorphous ice. Using neutron diffraction in combination with the isotopic substitution method and the empirical potential structure refinement technique, we find that the first hydration shell of adamantane is well structured consisting of a hydrogen-bonded cage of 28 water molecules that is also found in cubic structure II clathrate hydrates. The four hexagonal faces of the 51264 cage are situated above the four methine (CH) groups of adamantane whereas the methylene (CH2) groups are positioned below the edges of two adjoining pentagonal faces. The oxygen atoms of the 28 water molecules can be categorised on the basis of symmetry equivalences as twelve A, twelve B and four C oxygens. The water molecules of the first hydration shell display orientations consistent with those expected for a clathrate-hydrate-type cage, but also unfavourable ones with respect to the hydrogen bonding between the water molecules. Annealing the samples at 140 K, which is just below the crystallisation temperature of the matrix, removes the unfavourable orientations and leads to a slight increase of the structural order of the first hydration shell. The very closest water molecules display a tendency for their dipole moments to point towards the adamantane which is attributed to steric effects. Other than this, no significant polarisation effects are observed which is consistent with weak interactions between adamantane and the amorphous ice matrix. FT-IR spectroscopy shows that the incorporation of adamantane into amorphous ice leads to a weakening of the hydrogen bonds. In summary, the matrix-isolation of the highly symmetric adamantane in amorphous ice provides an interesting test case for hydrophobic hydration. Studying the structure and spectroscopic properties of water at the interface with hydrophobic hydrocarbons is also relevant for astrophysical environments, such as comets or the interstellar medium, where amorphous ice and hydrocarbons have been shown to coexist in large quantities