From methanol to 1-nonanol: Chain length effects on dielectric relaxation and dipolar correlations in linear alcohols

The dielectric properties of a series of alcohols, from methanol to 1nonanol, were studied using molecular dynamics simulations. Three commonly used force elds, the united atom OPLS and TraPPE and the all atom CHARMM, were employed to calculate the static dielectric constant, Kirkwood correlation factor, collective and single molecule dipole relaxation and the frequency domain dielectric spectra. All three force elds underestimate the static dielectric constant, with CHARMM and OPLS doing so by a near constant factor across the series, and TraPPE showing strong chain length dependence. The collective and single molecule dipole relaxation times grow signicantly with the increase in the alkyl chain length, most prominently for CHARMM and more modestly for the two united-atom force elds. A dynamic Kirkwood factor, dened as the ratio of collective to single molecule dipole relaxation times, is introduced to connect the static and dynamic picture. This dynamic Kirkwood factor is shown to be systematically larger than its static counterpart and its growth with chain length in the CHARMM model mirrors the transition of the alkyl tail from a dynamic spectator to a dynamic governor. The results are discussed alongside available experimental data, highlighting the strengths and weaknesses of each force eld in modeling the dielectric behavior of linear alcohols.