Adsorption Kinetics in Martian Clay

1. Adsorption Kineticsin Martian Clay By: David Kennington Mentor: Vincent Chevrier

2. Background • Daily variations of humidity in martian atmosphere • Clays provide large reservoir for water • Specific Surface Area: 67,505m2kg-1 • Background • Goal • Equipment • Experiment • LangmuirTheory • BETTheory • Data • Conclusionand FutureWork Picture: Clay represented by green Credit:NASA/JPL/JHUAPL/Brown University

3. Goal • Are clays responsible for large water variations in atmosphere? • What conditions is water ice stable on mars? depth, temperature, location? • Calculate coefficient of adsorption ka and coefficient of desoprtion kd • To find this I measured the amount of water adsorbed by substitute Martian clay over time in mars like conditions • Background • Goal • Equipment • Experiment • LangmuirTheory • BETTheory • Data • Conclusionand FutureWork

4. Equipment • Andromeda Chamber: creates Martian atmosphere, 95% CO2, -20° C to 4° C, 7 millibars. • The clay used in my experiments is a montmorillonite from Panther Creek, Colorado with a grain size less than 63 μm • Background • Goal • Equipment • Experiment • LangmuirTheory • BETTheory • Data • Conclusionand FutureWork Picture 1: (a) The Andromeda Chamber (b) Clay Sample Picture 2: Me working on the chamber Picture 3: A 10 gram sample of the substitute Martian clay used during my runs

5. Experiment • Cooked clay at 100° C with back pressure 33 millibars for 24 hours • Froze clay for 24 hours • Relative Humidity set from 5% to 80% • Sample ran for 4 hours to allow it to reach steady state • Mass of sample measured to measure the amount of water adsorbed • Clay is then cooked again after run to measure amount of water adsorbed • Background • Goal • Equipment • Experiment • LangmuirTheory • BETTheory • Data • Conclusionand FutureWork

6. Langmuir Theory • Langmuir’s Equations: • Only for a single monolayer of atoms on the clay. Isotherm breaks down with multiple layers of atoms • Can calculate specific surface area from linear fit • As = 74,520 m2 kg-1 • Background • Goal • Equipment • Experiment • LangmuirTheory • BETTheory • Data • Conclusionand FutureWork Graph: The linear fit for the monolayer is the langmuir isotherm. The pink dots is where langmuir’s isotherm breaks down

7. BET Theory • BET: Brunauer, Emmett, and TellerBET Equation: • V = volume of water adsorbed per kg • Vm = volume of one monolayer per kg • Can calculate Specific surface area with Vm and monolayer thickness. As = 60,490 m2 kg-1 • Background • Goal • Equipment • Experiment • LangmuirTheory • BETTheory • Data • Conclusionand FutureWork

8. Data • Using software (Orgin), a fit the curve is made using this equation • Theta is the area of clay covered by water over total area of clay • Orgin calculates ka and kd using the graph, the specific surface area, and the thickness of a monolayer (3e-10 m) • Background • Goal • Equipment • Experiment • LangmuirTheory • BETTheory • Data • Conclusionand FutureWork

9. Data • The theoretical equation for ka is • Background • Goal • Equipment • Experiment • LangmuirTheory • BETTheory • Data • Conclusionand FutureWork

10. Data • The theoretical equation for kd is • Background • Goal • Equipment • Experiment • LangmuirTheory • BETTheory • Data • Conclusionand FutureWork

11. Conclusion and Future Work • The data shows decent correlation to the theory so far. • Recalculating data with adjusted monolayers did not improve data • The data we have now is not efficient to make an ideal model of the adsorption kinetics in the Martian clay • More data points are needed, and longer runs are needed • Background • Goal • Equipment • Experiment • LangmuirTheory • BETTheory • Data • Conclusionand FutureWork