Solubility of Caffeine in Supercritical CO₂: A Molecular Dynamics Simulation Study

Abstract

The extraction of caffeine from green tea leaves and cocoa beans is a common industrial process for the production of decaffeinated beverages and pharmaceuticals. The choice of the solvent critically determines the yield of this extraction process. Being an environmentally benign and recyclable solvent, supercritical carbon dioxide (scCO 2 ) has emerged as the most desirable green solvent for caffeine extraction. The present study investigates the solvation properties of caffeine in scCO 2 at two different temperatures (318 and 350 K) using molecular dynamics simulations. Unlike in water, the caffeine molecules in scCO 2 do not aggregate to form clusters due to relatively stronger caffeine−CO 2 interactions. A well-structured scCO 2 solvent shell envelops each caffeine molecule as a result of strong electron−donor−acceptor (EDA) and hydrogenbonding interactions between these two species. Upon heating, although marginal site-specific changes in the distribution of nearest CO 2 around caffeine are observed, the overall distribution is retained. At a higher temperature, the caffeine−CO 2 hydrogen-bonding interactions are weakened, while their EDA interactions become relatively stronger. The results underscore the importance of the interplay of these interactions in determining stable solvent structures and solubility of caffeine in scCO 2 .

Supercritical Methanol and Ethanol Solubility Estimation by Using Molecular Dynamics Simulation

“…Wang et al [29] conducted MD simulation to examine the phase change process of dodecane droplets in a supercritical nitrogen environment and they expressed three phase transition modes for the droplets. Reddy and Saharay [36] adopted MD simulation methods in order to investigate the solvation properties of caffeine in SC-CO 2 at temperatures of 318 and 350 K.…”

The Anomalous Behavior of Thermodynamic Parameters in the Three Widom Deltas of Carbon Dioxide-Ethanol Mixture

“…In particular the CO 2 –ethanol mixture is applied in the process of cannabis extraction [ 33 ], fish oil extraction [ 34 ], caffeine extraction [ 35 ], etc. The increase of solubility that is achieved in the Widom delta (due to clustering) can significantly increase the efficiency of these processes [ 36 ]. It is also worth mentioning that another substance can be initially dissolved in ethanol, and due to the effective diffusion of SCF carbon dioxide, the substance then spreads throughout the system [ 37 ].…”

The Polar Cosolvent Effect on Caffeine Solvation in Supercritical CO2–Ethanol Mixtures: A Molecular Modeling Approach

“…On the other hand, supercritical carbon dioxide (SC CO 2 ) is an inexpensive, nonflammable, and nontoxic solvent. For these reasons, it is considered to be an attractive alternative to these conventional organic solvents. − Supercritical-fluid (SCF) extraction, a well-established separation technology, has received wide attention in recent decades and has been successfully employed in a variety of applications including food processing, extraction of active principles for pharmaceutical formulations, wastewater treatment, enhanced oil recovery operations, and as an analytical chromatographic tool. − …”

“…The low solubility of polar substances in nonpolar SCFs, such as SC CO 2 , is an obstacle to the development of viable extraction operations using supercritical CO 2 . Although SC CO 2 is quite efficient for the selective extraction of caffeine, the solubility of caffeine in this solvent is lower than that in liquid organic solvents. ,− Due to this, the use of a large amount of CO 2 during the extraction process is required, causing a substantial increase in operating costs. The addition of small amounts of polar cosolvents to nonpolar SCFs increases the polarity of the supercritical mixed solvent. − In this way, the solubility of polar solutes in these mixed solvents is higher than that in pure nonpolar SCFs. − A proper choice of the cosolvent used may control the selectivity and dissolution of specific polar compounds, thus making the CO 2 –cosolvent systems extremely useful in extraction processes.…”