Solvation of salicylic acid in pure, methanol-modified and water-modified supercritical carbon dioxide: Molecular dynamics simulation

• Salicylic acid in pure, methanol- and water-modified supercritical CO2 is studied.
• Molecular dynamics simulation is performed.
• Salicylic acid forms stable solvated complexes with co-solvents.
• Solvate complexes of salicylic acid with both co-solvents are formed similarly.
• Water (unlike methanol) is self-associated in the supercritical CO2.
• In methanol-modified fluid solvation process is slower than in water-modified one.

In the present work, solvation of the salicylic acid in pure, methanol-modified and water-modified (by adding 0.035 methanol or 0.0079 water mole fraction) supercritical carbon dioxide (sc-CO2) at 318 K and 0.7 g/cm3 has been studied by computer simulation techniques. It was shown that solvation of salicylic acid in pure sc-CO2 is governed by electron donor–acceptor interactions and proceeds more slowly than in modified sc-CO2, where salicylic acid forms solvate complex with co-solvent by means of hydrogen bonding through carboxyl group. Salicylic acid hydroxyl group participates only in intramolecular hydrogen bond and does not interact with solvent molecules. The salicylic acid–co-solvent complexes are stable: the duration of their existence is much higher than lifetime of other hydrogen bonds in the fluid. The behavior of two co-solvents is different: methanol exists in the form of monomers and hydrogen-bonded dimers in the supercritical fluid, the water molecules tend to form microclusters with spatially-branched structure.

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