Adsorption isotherm of the N 2 –CO equimolar mixture in clathrate at 50K – Implications for the composition of solid bodies of the outer Solar System S. Lectez (1), J.-M. Simon (1), O. Mousis (2,3), J.M. Salazar (1) and S. Picaud (2) (1) Laboratoire Interdisciplinaire Carnot de Bourgogne, UMR 6303 CNRS-Université de Bourgogne, Dijon, France, (2) Laboratoire UTINAM, UMR 6213 CNRS-Université de Franche Comté, Besançon, France (olivier@obs-besancon.fr), (3) Center for Radiophysics and Space Research, Cornell University, USA Abstract Adsorption isotherms of CO, N 2 and their equimo- lar mixture are presented in the case of structure I clathrate. They have been computed via Monte- Carlo simulations in the Gibbs ensemble for fu- gacities at 50K and in the 10 -11 –4.10 -3 Pa pres- sure range, corresponding to the typical thermo- dynamic conditions at which clathrates form in the protosolar nebula. The results show that pure CO molecules are adsorbed in clathrate at fugaci- ties twenty times lower than those of pure N 2 . In the case of equimolar mixtures, CO is much pref- erentially adsorbed in clathrate. These computa- tions imply that, at nebular conditions, CO has a much higher propensity than N 2 to be trapped in clathrates. Planetesimals agglomerated exclu- sively from clathrates in the outer part of the pro- tosolar nebula should then be highly devoid in N 2 . 1. Introduction The important deficiency of clathrate equilibrium data at low temperatures implies the use of the- oretical approaches to characterize the clathrates that could exist in conditions consistent with those encountered in planetary environments. However, the usual thermodynamic approaches such as the usual van der Waals & Platteeuw method are based on simplified intermolecular potentials cali- brated on equilibrium measurements performed at higher temperatures. As a consequence, the ca- pabilities of these simple potentials for predicting the composition of clathrates in the outer Solar System could be questionable. Here we use grand canonical Monte Carlo sim- ulations based on more sophisticated interatomic potentials to investigate the trapping in clathrate of a gas mixture made of CO and N 2 at ther- modynamic conditions (gas temperature of 50K and pressure ranging between 10 -11 and 4.10 -3 Pa) covering those met in the protosolar nebula when this crystalline structure forms. To do so, we directly compute the absorption isotherms of CO, N 2 and their equimolar mixture in the case of structure I clathrate. Despite the fact that these two molecules are among the most impor- tant species taking part to the composition of the outer solar system, the number of data regarding the equilibrium compositions of their clathrates is scarce, if non-existant. 2. Modeling approach Monte-carlo simulations in the Gibbs ensemble [1] have been performed for fugacities ranging from 10 -11 to 4.10 -3 Pa at a fixed temperature of 50K. Eight unit cells (2*2*2) of clathrate hydrate of cubic structure I (SI) have been simulated. The molecules of water were modeled using the well- known TIP4P/2005 model [2] and were allowed to translate and rotate during the simulation. Mod- els for N 2 and CO molecules were taken from the literature, respectively from Potoff et al. [4] and Piper et al. [3]. The dimension of the cubic sim- ulation box was fixed during the simulation and equal 24.06 Å. 10 millions MC steps were done in- cluding insertion, delation, translation and rota- tion of the molecules. For mixtures the fugacities of the two components were identical, taken into account that they are much lower than 1 Pa, the fugacity equals the partial pressure which implies that we simulated equimolar gases. EPSC Abstracts Vol. 9, EPSC2014-533, 2014 European Planetary Science Congress 2014 c  Author(s) 2014 E P S C European Planetary Science Congress