ja5b01105_si_001.pdf (2.84 MB)
Fluorinated Carbide-Derived Carbon: More Hydrophilic, Yet Apparently More Hydrophobic
journal contribution
posted on 2015-05-13, 00:00 authored by Amir H. Farmahini, David
S. Sholl, Suresh K. BhatiaWe explore the effect of fluorine
doping on hydrophobicity of nanoporous
silicon carbide-derived carbon (SiCDC), and investigate the underlying
barriers for adsorption and diffusion of water vapor and CO2 in the fluorinated and nonfluorinated structures. We develop atomistic
models of fluorine-doped SiCDC at three different levels of fluorination,
based on a hybrid reverse Monte Carlo constructed model of SiCDC,
and develop a novel first-principles force field for the simulation
of adsorption and transport of water and CO2 in the fluorine-doped
carbon materials. We demonstrate an apparent dual effect of fluorination,
showing that while fluorination generates more hydrophilic carbon
surfaces, they actually act as more hydrophobic structures due to
enhanced energy barriers in the disordered network of microporous
carbon. While an increase in adsorption energy and in water uptake
is seen for fluorine-doped carbon, large internal free energy barriers
as well as the results of MD simulations demonstrate that the increased
adsorption is kinetically limited and not experimentally observable
on practical time scales. We show that an increase in apparent hydrophobicity
due to fluorination is mediated by larger free energy barriers arising
from stronger binding of fluid molecules inside the pore network,
as opposed to repulsion or steric hindrance to the diffusion of molecules
through narrow pore entries. For carbon dioxide, adsorption enthalpies
and activation energy barriers are both decreased on fluorination,
indicating weakened solid–fluid binding energies in the fluorinated
systems.