Supra-molecular structure and rheological aspects of liquid terminal 1,n‑diols from ethylene glycol, 1,3‑propandiol, 1,4‑butanediol to 1,5‑pentanediol

Abstract

A structural and rheological study of terminal 1,n‑diols from etane‑1,2‑diol to pentane‑1,5‑diol at 25 °C was performed using a combination of the experimental small- and wide-angle X-ray scattering (SWAXS) technique and theoretical TraPPE-UA force-field model-based molecular dynamics (MD) simulations. Our already well-established complemented-system approach was used to calculate the SWAXS intensities from the simulation data needed to validate the accuracy of the theoretical results by comparing them with the experimental SWAXS data. Next, a simulated analogue of the contrast-matching method was used to reveal the supra-molecular structural details of the studied diols, which were then further discussed in a comparison with those of the basic primary mono-ols from our previous study (Tomšič M., et al., J. Phys. Chem B, 2007). This revealed the important structural differences arising from the influence of an additional –OH group in the molecule and an increasing diol alkyl chain length. The additional –OH group enhances the H-bonding and leads to a more compact supra-molecular structure that is increasingly locally inhomogeneous with an increasing diol alkyl chain length. Furthermore, the shear-rate-dependent viscosities were calculated for the modelled 1,n‑diols at 25 °C using the non-equilibrium MD periodic perturbation method and the extrapolated zero-shear viscosities were compared with the experimental data. We directly related the effect of the revealed structural details to the viscosity of diols. The level of H-bonding in the system seems to affect the viscosity less than the type of structural details and the molecular conformations. Allowing the modelled system to occupy a higher degree of stretched molecular conformations was in turn reflected in the greatly increased viscosity of the system. Terminal 1,n‑diols exhibit much higher viscosities than the basic mono-ols due to a more rigid cross-linking between the sequentially H-bonded OH-aggregates in the system.