Solvent effects in the thermal decomposition reaction of ammonium carbamate: A computational molecular dynamics study of the relative solubilities of CO2 and NH3 in water, ethylene glycol, and their mixtures

• We study solubilities of CO2 and NH3 via classical molecular dynamics simulations.
• The solvents considered are: H2O, ethylene glycol (EG), and their mixtures.
• Both CO2 and NH3 are more soluble in EG than in H2O.
• NH3 is more soluble than CO2 in either H2O or EG.
• The solubility in H2O of both CO2 and NH3 decreases with increasing temperature.
• The solubility in H2O of both CO2 and NH3 decreases at higher gas concentration.
• Addition of H2O to EG decreases the solubility of CO2 and NH3.

The endothermic decomposition of ammonium carbamate has been proposed as a novel heat sink mechanism for aircraft thermal management (Johnson et al., 2012). The products of the reversible decomposition are carbon dioxide and ammonia which need to be efficiently removed from the carrier fluid in order to better control the reaction and thus the heat transfer. Molecular dynamics simulations can provide insight into the transport properties of carbon dioxide and ammonia in the carrier fluid. In this work, extensive classical non-reactive molecular dynamics simulations were carried out to explore the solvent effects in the thermal decomposition reaction of ammonium carbamate by studying the temperature and concentration dependence of relative solubility and diffusivity of carbon dioxide and ammonia in water, ethylene glycol, and their mixtures at standard temperature and pressure and at the elevated temperature of the thermal decomposition reaction of ammonium carbamate. This comparative study shows that ammonia is more soluble than carbon dioxide in either water or ethylene glycol and that both carbon dioxide and ammonia are more soluble in ethylene glycol than in water. Our simulations of water–ethylene glycol mixtures show that increasing the molar fraction of ethylene glycol leads to increased solubility of carbon dioxide and ammonia in the mixture. Even though this is a non-reactive study, it is able to capture the general solubility trends. Accounting for the reactions of carbon dioxide and ammonia in the solution would further amplify the observed trends by amplifying the retaining of CO2 and NH3 in the solution. We present a low-cost computational procedure for relative solubility evaluation that can be used in a broader engineering design context.