Self-diffusion of aromatic compounds in the carbon tetrachloride – acetone binary system. NMR experiment and MD simulation

Abstract

In this work, we studied the influence of the structure of molecules of aromatic compounds (methylparaben (MP), propylparaben (PP), syringic acid (SA), gallic acid (GA), protocatechualdehide (PAL), p-coumaric acid (p-CA), and caffeic acid (CA)) on their molecular association and self-diffusion in the binary carbon tetrachloride – acetone solvent. The self-diffusion coefficients of PAL, SA, and GA in the binary carbon tetrachloride – acetone‑d6 solvent were measured by the pulsed gradient spin-echo nuclear magnetic resonance (PGSE NMR) method at temperatures of 288, 298, and 308 K. The data obtained at 298 K were also compared with the data on the self-diffusion coefficients of MP, PP, CFA, and p-CA in the binary solvent available in the literature. According to the presented data, the self-diffusion coefficients of all the substances under consideration become greater with an increase in the acetone‑d6 concentration and decrease in the MP > PAL > PP > p-CA > SA > CA > GA series of compounds. Quantitative assessment of the degree of influence of molecular association on solute self-diffusion was carried out within the framework of a method based on the ratio of the self-diffusion coefficient of a solute to the self-diffusion coefficient of its monomers. The study shows that the degree of influence of molecular association of the solute on its self-diffusion is determined by the number and type of active functional groups in the structure of the solute molecules (MP < PP < PAL < SA < p-CA < CA < GA), but does not depend on the composition of the binary solvent. According to the results of molecular dynamics (MD) simulation, this effect correlates with the increase in the degree of self-association of solutes and decrease in the degree of solute – acetone heteroassociation observed when the carbon tetrachloride concentration becomes higher. This effect is also responsible for the similar concentration dependencies of the self-diffusion coefficients of active and inert compounds.