Date of Graduation

8-2025

Document Type

Thesis

Degree Level

Graduate

Department

Materials Science & Engineering

Advisor/Mentor

Wang, Feng

Committee Member

Kohanek, Julia

Second Committee Member

Ware, Morgan

Third Committee Member

Chevrier, Vincent

Keywords

Acetonitrile; Glassy Materials; Molecular Dynamics; Titan; Titan's Atmosphere

Abstract

Titan is one of the most interesting satellites in the solar system. It is the only other place in the solar system containing stable deposits of liquid and a hydrological cycle other than earth. The abundant existence of N2 and CH4 in Titan’s atmosphere makes nitrile formation highly likely both in the atmosphere and surface. Infrared spectrometer data from Voyager 1 expedition revealed the presence of acetonitrile (CH3CN) in the atmosphere along with several other nitriles such as hydrogen cyanide (HCN) and cyanoacetylene (HC3N). According to theoretical models nitrile formation is caused by photodisscociation of nitrogen in the tholin haze of Titan. The average surface surface temperature and pressure of Titan is ∼94 K and ∼1.5 atm respectively, with temperature varying with height in the range between about 80 K – 200 K. The experimental estimate of the glass transition temperature of CH3CN was found to be 93 K by binary solution analysis carried out by Angell et al.[1] Hence under Titan-like conditions. It is important to study the structural, thermodynamic and dynamic properties of CH3CN to understand the behavior relevance of amorphous and crystalline states of CH3CN in Titan. In this work, the supercooled, glassy and crystalline states and nucleation kinetics of CH3CN was studied using molecular dynamics simulation methods. From the specific volume vs temperature plot the glass transition was observed to occur below 130 K, which slightly overestimates the experimental value 93 K. The Vogel- Fulcher-Tammann equation fit of the temperature dependence of the density relaxation time, dipole-dipole correlation time and diffusion coefficient follows non-Arrhenius behavior and suggests that CH3CN forms fragile glass. In TraPPE-UA force field model, acetonitrile crystallizes spontaneously at 135 K in the experimentally known and high temperature α- phase. Slow heating of the crystal gives an estimate melting temperature of ∼220 K, which is close to the experimental estimates. The local bond orientational order parameter further confirms this estimate manifesting a sudden drop near the melting temperature.

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