Debye process in Ibuprofen glass-forming liquid: insights from molecular dynamics simulation

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Materials Science

Background:

• Ibuprofen is a glass-forming liquid exhibiting peculiar low-amplitude Debye-type relaxation.
• Dielectric relaxation spectroscopy (DRS) has experimentally observed this phenomenon.

Purpose of the Study:

• Investigate the dynamical properties of racemic ibuprofen glass-forming liquid using molecular dynamics simulations.
• Elucidate the origin of the experimentally observed low-amplitude Debye-type relaxation.

Main Methods:

• Molecular dynamics (MD) simulations were performed at temperatures ranging from 360 to 500 K.
• Calculated single and total dipolar autocorrelation functions to analyze molecular dynamics.
• Investigated the influence of intermolecular hydrogen-bonded structures (linear/cyclic dimers and trimers) on molecular dynamics.

Main Results:

• The total dipole correlation is primarily governed by single-molecule correlations, originating from antiparallel dipole arrangements.
• Debye-type relaxation is dominated by the internal cis-trans conversion of the O=C-O-H group, coupled with changes in hydrogen-bonded structures.
• Cyclic dimers hinder cis-trans conversion, while linear dimers/trimers promote it, stabilizing the trans isomer. This explains the low dielectric strength observed experimentally.

Conclusions:

• The low amplitude of Debye-type relaxation in ibuprofen is attributed to the slow cis-trans conversion of the O=C-O-H group and the specific roles of hydrogen-bonded structures.
• The dynamics suggest that molecular rotation is significantly faster than cis-trans isomerization, leading to averaged local environments and simple Debye relaxation.
• The fraction of ibuprofen molecules in the trans conformation directly influences the observed dielectric strength.