Temperature Dependence of the Dielectric Permittivity of Acetic Acid, Propionic Acid and Their Methyl Esters: A Molecular Dynamics Simulation Study

Abstract

For most liquids, the static relative dielectric permittivity is a decreasing function of temperature, because enhanced thermal motion reduces the ability of the molecular dipoles to orient under the effect of an external electric field. Monocarboxylic fatty acids ranging from acetic to octanoic acid represent an exception to this general rule. Close to room temperature, their dielectric permittivity increases slightly with increasing temperature. Herein, the causes for this anomaly are investigated based on molecular dynamics simulations of acetic and propionic acids at different temperatures in the interval 283-363 K, using the GROMOS 53A6(OXY) force field. The corresponding methyl esters are also considered for comparison. The dielectric permittivity is calculated using either the box-dipole fluctuation (BDF) or the external electric field (EEF) methods. The normal and anomalous temperature dependences of the permittivity for the esters and acids, respectively, are reproduced. Furthermore, in the EEF approach, the response of the acids to an applied field of increasing strength is found to present two successive linear regimes before reaching saturation. The low-field permittivity ε, comparable to that obtained using the BDF approach, increases with increasing temperature. The higher-field permittivity ε' is slightly larger, and decreases with increasing temperature. Further analyses of the simulations in terms of radial distribution functions, hydrogen-bonded structures, and diffusion properties suggest that increasing the temperature or the applied field strength both promote a relative population shift from cyclic (mainly dimeric) to extended (chain-like) hydrogen-bonded structures. The lower effective dipole moment associated with the former structures compared to the latter ones provides an explanation for the peculiar dielectric properties of the two acids compared to their methyl esters.

Effect of proximity to ionic liquid-solvent demixing on electrical double layers

“…where β = 1/(k B T ), u is the electrostatic potential in k B T /e units with k B denoting the Boltzmann constant; c = ρ + − ρ − is the charge density per elementary charge e and λ B = βe 2 /ǫ is the Bjerrum length, where ǫ is the dielectric constant. It is well known that ǫ depends on temperature, particularly for polar solvents [33][34][35]. Nevertheless, we assume ǫ to be temperature-independent, and note that its temperature variation should not affect the results qualitatively, as pointed out in [30].…”

Reoptimized interaction parameters for the peptide‐backbone model compound N‐methylacetamide in the GROMOS force field: Influence on the folding properties of two beta‐peptides in methanol

“…The static relative dielectric permittivity ε liq was calculated at 1 bar and 333 K using the external electric field (EEF) method. [25, 26] In this approach, a homogeneous and time‐independent external electric field is applied in the z ‐direction of the computational box, and the polarization response P z is measured at different field strengths $E_z^{{\rm ext}}$ . The dielectric permittivity is then calculated based on the linear region of the corresponding graph (i.e., before saturation occurs), using the relation where $E_z^{{\rm ext}^{\prime} } = (4\pi \varepsilon _0 )^{ - 1} E_z^{{\rm ext}}$ .…”

Vibrational Resonant Inelastic X-Ray Scattering Inliquid Acetic Acid: A Ruler for Molecular Chain Lengths

“…Carboxylic acids have received a lot of experimental and theoretical interest over more than a century. There is now a consensus that the predominant structures of ACA in the gas phase are the monomer and the symmetric cyclic dimer 11 . In the solid state, the crystalline unit cell consists of non-symmetric dimers which belong to a chain structure 11 .…”

“…There is now a consensus that the predominant structures of ACA in the gas phase are the monomer and the symmetric cyclic dimer 11 . In the solid state, the crystalline unit cell consists of non-symmetric dimers which belong to a chain structure 11 . The local structure of liquid ACA is more complex and is still heavily debated (see Ref.…”