Carbon molecules in space: a thermal equation of state study of solid hexamethylenetetramine
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Date
2020-02-04
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Publisher
Australian Institute of Physics
Abstract
Properties such as compressibility, thermo-elasticity and the energy landscape remain unknown for many organic compounds under conditions encountered on extraterrestrial planets and moons and in space. In this study, a thermal Equation of State (EoS) for the crystalline solid hexamethylenetetramine was determined by neutron powder diffraction in the temperature and pressure ranges of 113-480 K and 0-5 GPa, respectively. The material was chosen as a molecular model for its high symmetry and its property of remaining in the same phase throughout the experimental conditions selected to simulate the planetary environments. Equations of States (EoSs) show how the thermodynamic variables of temperature (T), pressure (P) and volume (V) are inter-related. The ideal gas law, PV = nRT, is an example of an EoS which is used as a simple but effective model to explain the properties of gases. More complex EoSs, where the assumption of ideality is relaxed, can be applied to solids in order to describe how the geometry and energy transform when they experience dramatic changes in their environment. Such information acquires enormous importance in planetary materials science, where scientists are trying to understand the fate of carbon, the fourth most abundant element in our galaxy, in the context of the origin of life and planetary environments. Despite the large heterogeneity of galactic and interstellar regions, the organic chemistry of the universe seems to follow common pathways. Molecules of high astrobiological and astrophysical relevance such as amino acids, polyaromatic hydrocarbons, and N-heterocycles have been identified across the solar system, but how they behave under such varied conditions is a question yet to be answered. Key to our approach was the determination of how the internal energy (U), entropy (S) and the Gibbs free energy (G) vary with pressure not only computationally, but also, and for the first time, experimentally. A new method has been developed, able to transform directly variable-PT crystallographic data into thermodynamic information. Although it is quite common to model thermal expansion at ambient pressure with a VTEoS, and compression at ambient temperature using a PV-EoS, determinations of PVT-EoSs are much less common, particularly for organic materials. This paucity of PTV-EoSs reflects the difficulty of varying pressure and temperature simultaneously in crystallographic experiments, especially at reduced temperatures. The task was addressed in this study by the variable-temperature insert for the Paris-Edinburgh press available on the PEARL instrument at the ISIS Neutron Spallation Source (UK). The results were successfully combined with periodic DFT (Figure 1) and other semiempirical calculations, where pressure and temperature can be included at little time cost, enabling the stability profile of the material to be understood, right down to the level of individual intermolecular interactions.
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Keywords
Accelerator neutron source facilities, Amines, Atmospheres, Chemistry, Coherent scattering, Diffraction, Elements, Energy, Equations, Evolution, Nonmetals, Organic compounds, Physical properties, Pressure range, Thermodynamic properties
Citation
Novelli, G., McIntyre, G. J., Maynard-Casely, H. E., Marshall, W. G., Kamenev, K. V., & Parsons, S. (2020). Carbon molecules in space: a thermal equation of state study of solid hexamethylenetetramine. Paper presented to the 44th Condensed Matter and Materials Meeting, Holiday Inn, Rotorua, New Zealand, 4-7 February 2020, (pp. 41). Retrieved from: https://physics.org.au/wp-content/uploads/cmm/2020/CMM20_ConferenceHandbook(04Feb2020).pdf