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Water-Carbon Interactions: Potential Energy Calibration Using Experimental Data

T. Werder, J.H. Walther, R.L. Jaffe and P. Koumoutsakos
NASA Ames Research Center, US

Keywords: graphite, water, contact angle, potential, molecular dynamics

Functionalized carbon nanotubes (CNT) are being studied for use as nanoscale biosensors and water or proton channels. Several molecular dynamics (MD) studies of water in carbon nanotubes have recently been performed to gain further insight into the latter, but in these MD studies a wide range intermolecular potentials for water-carbon interactions was employed. In general, experimental data are not available for calibrating the strength of the water-carbon interaction, so it is difficult to assess the accuracy of these potentials. In the present study we use MD simulations to compute the static contact angle (q) for nanometer-size water droplets on graphite and compare the results with available experimental data. For the case of a 2000-molecule water droplet on graphite, we demonstrate that q is sensitive to changes in the carbon-water interaction strength. This allows us to use the experimental value for the contact angle of water on graphite (q =86°) to calibrate the water-carbon potential. Using a series of water-carbon potentials that range from hydrophobic to complete wetting behavior in our MD simulations, we determine the relationship between the binding energy (D E ) of a water monomer on graphite and q for the droplet. The data show complete wetting for D E < -13.1 kJ/mol and a linear relationship between D E and q for other values. From these data we have obtained a new potential to compute the binding energy of water on graphite (D E = -7.3 kJ/mol) and to characterize the water-graphite/carbon nanotube interface.

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