Entropy deepens loading chemical potentials of small alcohols by narrow carbon nanotubes

Abstract

Small alcohol confinement within narrow carbon nanotubes has been extensively and systematically studied via rigorous free-energy calculations. Employing molecular dynamics simulations, thermodynamic integration and thermodynamic cycling, the loading process of methanol and ethanol from aqueous solution into (6,6), (7,7) and (8,8) single-walled carbon nanotubes was computed and decomposed into its entropic and energetic terms. For all tubes and alcohols, loading is favoured from infinite dilution in water; for the same alcohol, wider tubes allow for the formation of a collective dipole which is cooperative in terms of electrostatics and reduce the rotational freedom of the loaded particles; narrow tubes only permit the formation of dipole-dipole dimers instead, with a (rotational) entropic gain that compensates for the loss of long-range dipole-dipole interactions. The latter renders deeper loading chemical potentials for narrower tubes when partitioning small alcohols from aqueous solution and it is a clear example of an entropy-energy compensation phenomenon.