High pressure-temperature behavior of long-chain alkanes

Carbonaceous chondrites are undifferentiated meteorites that provide valuable insights into the Earth’s pristine chemistry and host a variety of hydrocarbons including polyaromatic hydrocarbons (PAH) and alkanes. It is speculated that these meteorites might have delivered life-essential organic matter to the Earth. Therefore, it is important to understand the survivability of these hydrocarbons inside the meteorites at extreme pressures and temperatures on entry to the Earth’s atmosphere. However, the P-T stability of the long-chain alkanes is not well known. Thus, we have undertaken an in-depth investigation of tricosane-C₂₃H₄₈. Our preliminary study on the high-pressure behavior of C₂₃H₄₈ indicates that upon compression it transforms from a ‘linear’ to a ‘bent’ configuration (Basu et al., 2020). To investigate the combined effect of alkane chain-length and P-T, we explored octadecane-C₁₈H₃₈. Upon compression, we observe a similar transition from a linear to bent configuration, albeit at a lower pressure. On laser-heating in a diamond cell, we note dissociation of C₁₈H₃₈ into smaller alkane chains, elemental carbon (C), and molecular hydrogen (H₂), with the formation of binary van der Waals mixtures.
Reference: Basu, A., et al., (2020), Journal of Applied Physics, 127, 105901.