453660 Resolving Experimentally-Inferred Solvophobic Interactions in the Overlap Region

Monday, November 14, 2016
Grand Ballroom B (Hilton San Francisco Union Square)
J. Wesley Barnett1, Amna Bhutta2, Sarah Bierbrier1, Natalia da Silva Moura1 and Hank Ashbaugh1, (1)Chemical and Biomolecular Engineering, Tulane University, New Orleans, LA, (2)Chemical Engineering, Rutgers University, Piscataway, NJ

Solvent-mediated interactions between non-polar atoms is a principal driving force in molecular self-assembly. Experimental measurement of two atoms’ solvent-mediated interactions is difficult; such a measurement when two atoms are overlapping is impossible. In the 1970’s, however, Ben-Naim derived useful experimental estimates of the solvent-mediated contribution of the potential-of-mean-force (PMF) for two methanes at distances within the overlap region. By approximating two methanes separated by 1.54 Å―the bond length of an ethane―as a single ethane, he could calculate the free energy of solvation of both ethane and methane, and therefore calculate the potential of mean force at 1.54 Å as: μexethane – 2μexmethane.

In this study we use simulations to revisit Ben-Naim’s results. By using previous work on interpolating and calculating the solvent-mediated interactions of methanes in the overlap region, we are able to quantitatively compare simulation results with experimental estimates. We compare methane interaction free energies, entropies, and enthalpies―in water and in ethanol―with Ben Naim’s estimates. By using accurate force field models―TraPPE-UA and TIP4P2005―we are able to reproduce Ben-Naim’s experimental estimates. We observe, however, that such results understimate the solvent-mediated contribution to the PMF in the overlap region. When we perform a free-energy perturbation on the ethane, transforming the methyl groups into methanes the results become consistent with previous simulation work on solvent-mediated contributions in the overlap region.

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