Investigation of the local composition enhancement and related dynamics in supercritical CO2-cosolvent mixtures via computer simulation: The case of ethanol in CO2

Abstract

The supercritical mixture ethanol-carbon dioxide (EtOH-CO2) with mole fraction of ethanol X(EtOH) congruent with 0.1 was investigated at 348 K, by employing the molecular dynamics simulation technique in the canonical ensemble. The local intermolecular structure of the fluid was studied in terms of the calculated appropriate pair radial distribution functions. The estimated average local coordination numbers and mole fractions around the species in the mixture reveal the existence of local composition enhancement of ethanol around the ethanol molecules. This finding indicates the nonideal mixing behavior of the mixture due to the existence of aggregation between the ethanol molecules. Furthermore, the local environment redistribution dynamics have been explored by analyzing the time correlation functions (TCFs) of the total local coordination number (solvent, cosolvent) around the cosolvent molecules in appropriate parts. The analysis of these total TCFs in the auto-(solvent-solvent, cosolvent-cosolvent) and cross-(solvent-cosolvent, cosolvent-solvent) TCFs has shown that the time dependent redistribution process of the first solvation shell of ethanol is mainly determined by the redistribution of the CO2 solvent molecules. These results might be explained on the basis of the CO2-CO2 and EtOH-CO2 intermolecular forces, which are sufficiently weaker in comparison to the EtOH-EtOH hydrogen bonding interactions, creating in this way a significantly faster redistribution of the CO2 molecules in comparison with EtOH. Finally, the self-diffusion coefficients and the single reorientational dynamics of both the cosolvent and solvent species in the mixture have been predicted and discussed in relationship with the local environment around the species, which in the case of the EtOH molecules seem to be strongly affected.

Hydrogen bond, electron donor-acceptor dimer, and residence dynamics in supercritical CO2-ethanol mixtures and the effect of hydrogen bonding on single reorientational and translational dynamics: A molecular dynamics simulation study

“…The simulation was performed at constant temperature and volume, corresponding to an experimentally reported sc thermodynamic state point ͑T=348 K, = 0.611 g / cm 3 ͒. 31 According to previous experimental studies the critical temperature of the mixture at this particular composition has estimated to be about 326 K, 31,36 therefore the investigated fluid mixture is at sc conditions. The intermolecular interactions are represented as pairwise additive with site-site Lennard-Jones ͑LJ͒ and Coulombic interactions.…”

“…Although a small number of experimental and theoretical studies devoted to the properties of sc CO 2 -methanol mixed solvents ͑with X methanol ϳ 0.01-0.11͒ have been reported up to now, [18][19][20][21][22][23][24][25][26][27][28][29] studies on the properties of scCO 2 -ethanol ͑EtOH͒ systems are quite rare and most of them correspond mainly to dilute mixtures. 24,[30][31][32][33][34][35][36][37] In a quite recent study, two of the authors presented a molecular dynamics ͑MD͒ simulation study on the local intermolecular structure and related dynamics for the sc CO 2 -EtOH binary mixture at a mole fraction X ethanol ϳ 0.1. 36 The results obtained in that study have revealed a nonideal mixing behavior of the mixture and the existence of a somewhat restricted aggregation between the ethanol molecules.…”

“…24,[30][31][32][33][34][35][36][37] In a quite recent study, two of the authors presented a molecular dynamics ͑MD͒ simulation study on the local intermolecular structure and related dynamics for the sc CO 2 -EtOH binary mixture at a mole fraction X ethanol ϳ 0.1. 36 The results obtained in that study have revealed a nonideal mixing behavior of the mixture and the existence of a somewhat restricted aggregation between the ethanol molecules. This effect has been also observed in the case of sc CO 2 -MeOH mixture and reported by us in the framework of a recent MD study of the system.…”

An AOT reverse micelle in a medium of supercritical carbon dioxide

“…At present, several models of carbon dioxide have been described in the literature [14][15][16][17][18][19][20][21][22][23][24]. The three following main principles of describing molecules may be distinguished among them:…”

“…Within the framework of this approximation, three partial charges correspond to the force centers of the atoms in CO 2 . The EPM2 three point model [15][16][17][18] is most commonly used among them.…”