2011 Annual Science Report
NASA Jet Propulsion Laboratory - Titan Reporting | SEP 2010 – AUG 2011
Task 3.3.3 Solubility in Lakes
Atomistic simulations are being used to study the chemical environment of Titan’s hydrocarbon lakes.
Graduate student Joshua Kretchmer, in conjunction with Collabortor Thomas Miller, is using atomistic simulations to study the chemical environment of Titan’s hydrocarbon lakes. Given the dearth of polar molecules in these lakes, the solution-phase energetics and dynamics are governed by weak interactions that are usually overshadowed in aqueous environments. To gain insight into the nature of these interactions, enhanced sampling techniques are being used to study the pair-wise free energy of association, solvation free energies, and radial distribution functions of small non-polar solutes in the bulk solution of the hydrocarbon lakes. Specific focus is on solute molecules that are most prevalent in the Titan atmosphere. Investigation of the solvation free energies of these solutes provides quantitative information regarding their propensity to dissolve in the hydrocarbon lakes, and the radial distribution functions provide direct insight into the aggregation and potential reactivity of these molecules.
Surface properties of the hydrocarbons lakes are also of central interest. Simulations of methane at liquid-vapor phase coexistence and the Titan surface temperature are thus also being performed. The interfacial liquid structure of methane is being quantified, including the density and local orientational ordering of the molecules. Furthermore, the density, interfacial depth profile, and solvation structure of dissolved molecules are being studied to elucidate the possible range of chemical environments at the lake surfaces.
Finally, to explore the effect of solute and solvent polarizability in the non-polar lakes, molecular dynamics simulations that employ polarizable force fields (adiabatic shell model) are being used to model the aggregation and self-assembly of polarizable solutes in the hydrocarbon lakes. Extensive simulations are being run on mixtures of polarizable and non-polarizable argon atoms under liquid conditions. By tuning both the relative concentration of polarizable molecules and the degree of polarizability, the conditions that allow for self-assembly can be studied. Following the study of polarizable atoms, more complicated molecules with polarizable heads and non-polarizable tails are also being studied to determine the possibility of micelle formation. The conditions necessary for non-polar aggregation in the simulations are being compared to the conditions present in the hydrocarbon lakes to draw conclusions as to the existence of such aggregates in the hydrocarbon lakes.