Adsorption isotherm of the N2-CO equimolar mixture in clathrate at 50K - Implications for the composition of solid bodies of the outer Solar System

Adsorption isotherms of CO, N2 and their equimolar mixture are presented in the case of structure I clathrate. They have been computed via Monte- Carlo simulations in the Gibbs ensemble for fugacities at 50K and in the 10-11-4.10-3 Pa pressure range, corresponding to the typical thermodynamic conditions at which clathrates form in the protosolar nebula. The results show that pure CO molecules are adsorbed in clathrate at fugacities twenty times lower than those of pure N2. In the case of equimolar mixtures, CO is much preferentially adsorbed in clathrate. These computations imply that, at nebular conditions, CO has a much higher propensity than N2 to be trapped in clathrates. Planetesimals agglomerated exclusively from clathrates in the outer part of the protosolar nebula should then be highly devoid in N2.