Mixing of the Immiscible: Hydrocarbons in Water-Ice near the Ice Crystallization Temperature

Abstract

Structural changes in hydrocarbon-doped water-ice during amorphous to crystalline phase conversion are investigated using polycyclic aromatic hydrocarbons (PAHs) as probes. We show that aggregation of impurity molecules occurs due to the amorphous-crystalline transition in ice, especially when they are hydrophobic molecules such as PAHs. Using ultraviolet-visible (UV-vis), Fourier-transform Infrared (FTIR), and laser-induced-fluorescence (LIF) spectroscopic techniques, we show that, although ice infrared absorption features change from a broad structureless band corresponding to amorphous ice to a sharp structured crystalline ice bands, simultaneously, sharper isolated PAH UV absorption features measured in the amorphous ice host turn broad upon ice crystallization. A simultaneous decrease in the monomer fluorescence and increase in the excimer emission band is observed, a clear indication for the formation of PAH molecular aggregates when amorphous ice is converted to crystalline ice at higher temperatures. Similar to the irreversible amorphous-crystalline phase transitions, the UV, fluorescence, and excimer emissions indicate that PAHs undergo irreversible aggregation. Our studies suggest that organic impurities exist as aggregates rather than monomers trapped in crystalline water-ice when cycled through temperatures that convert amorphous ice to crystalline ice, rendering a better insight into phenomena such as the formation of cometary crust. This aggregate formation also may significantly change the secondary reaction pathways and rates in impurity-doped ices in the lab, on Earth, in the solar system, and in the interstellar medium.

Influence of ice structure on the soft UV photochemistry of PAHs embedded in solid water

“…The very presence of PAH molecules at fractions of a few percent within the ice sample tends to lead to a less structured, more amorphous solid. This would seem to suggest that the presence of the PAH as a trace impurity in the ice stabilises the metastable phase, in line with the results of Lignell & Gudipati (2015) who observed that crystallisation of a mixed pyrene-ASW ice at 140 K occurs significantly more slowly than for a pure water ice. Wavelength/ μm As well as hydroxy-PAH reaction products (and their subsequent dehydrogenation products), UV irradiation of PAHs and PAH-water ice mixtures gives rise to photofragmentation of the PAHs (Cook et al 2015;De Barros et al 2017).…”

“…The differences between PAH interaction with LDA and with Ih can be clearly traced to PAH-surface bonding, depending on the orientation of water molecules at the ice surface. It has long been known that during the thermal processing of amorphous ice, small molecules present in trace amounts in the ice diffuse during its restructuring, leading to segregation and desorption processes (Collings et al 2004); PAHs have been shown to aggregate during ice heating because their adsorption energy is too high for desorption to occur (Lignell & Gudipati 2015;Michoulier et al 2018a). Recent studies on reactivity in interstellar ice analogues have highlighted the important role played by the structure of ice on diffusion of reactants and production rates of interstellar complex organic molecules (iCOMs, Ghesquière et al (2018)).…”

Europa’s surface water ice crystallinity: Discrepancy between observations and thermophysical and particle flux modeling

“…Since optical constants of both crystalline and amorphous water ice at Europa temperatures in the NIR have only been published by a couple of groups in the past ∼ 20 years (Mastrapa et al, 2008;Grundy and Schmitt, 1998), we include an additional transmission dataset of crystalline and amorphous water ice from the ISL at JPL. The ISL reproduces surface environmental conditions (e.g., particle radiation bombardment, temperature, and composition) of the surfaces of various Solar System objects, such as comets and Europa (e.g., Barnett et al, 2012;Lignell and Gudipati, 2015;Gudipati et al, 2017). NIR spectra (1.25 − 4 µm) of pure water ice were acquired in 2010 by both depositing water ice from vapor at constant temperatures ranging from 18 to 140 K, and by ceasing deposition while varying the temperature.…”

Studying Ice with Environmental Scanning Electron Microscopy

“…These impurities are stored, transformed, and eventually released from the crystallized water. The information about the location and speciation of this contamination under varying environmental conditions is critical for assessing their reactivity and fate [ 95 , 96 , 97 ]. The most important impurities in polar regions are ions originating from sea salt, such as Na + , Cl − , and Br − , and have been observed in snow, ice cores, and aerosols forming a large chemical reservoir [ 98 , 99 , 100 , 101 ].…”