From methanol to 1-nonanol: chain length effects on the dynamic properties in linear alcohols

Neat 1-alcohols, from methanol to 1-nonanol, are studied using molecular dynamics simulations with the following goal: ascertaining the effect of the alkyl chain length on their dynamic properties. The alcohols were investigated in terms of their diffusion, rotational reorientation, hydrogen bonding dynamics and power spectra, through three commonly used force fields: OPLS united-atom, TraPPE united-atom and CHARMM all-atom. The increase in alkyl chain length itself doesn't change the structural properties of the hydrogen bond network, but the dynamics of it, accompanied by a slowdown of both translational and rotational dynamics of these compounds. The Stokes-Einstein and Debye-Stokes-Einstein analyses further reveal a progressive decoupling of translation and rotation that grows with chain length, which is the most pronounced in the all-atom model. While all three force fields reproduce the qualitative chain-length trends, there are quantitative differences in diffusion coefficients, rotational correlation time and intermittent hydrogen bond lifetimes. The calculated dynamic properties are discussed in the context of available experimental data, highlighting the strengths and weaknesses of each force field in modelling linear alcohols.