Ethers, crown ethers and 18-crown-6 K⁺complexes at a water/SC-CO₂interface: a molecular dynamics study

Abstract

We present a molecular dynamics study of 18C6, 15C5 crown ethers and their acyclic polyether analogues at the water/supercritical-CO 2 interface. The aqueous and CO 2 components form distinct phases, separated by an interface, where in all systems, the ethers are found to concentrate. Simulations of the inclusive K + &18C6 Pic À complexes led to decomplexation of K + and accumulation of the crown ethers on the CO 2 side of the interface, while Pic À anions stack in the aqueous phase. When the K + cations are constrained to form inclusive complexes with 18C6, all complexes concentrate at the interface, be the counterions free to move, or constrained to coordinate to K + . In the presence of nitric acid, modeled by equimolar mixtures of HNO 3 , NO 3À and H 3 O + forms, the ethers remain surface active, as does the neutral form of the acid. These simulations demonstrate the importance of interfacial phenomena in assisted ion extraction to supercritical-CO 2 .

Cation extraction by 18-crown-6 to a room-temperature ionic liquid: The effect of solvent humidity investigated by molecular dynamics simulations

“…8 and 9. For the two systems, and in contrast to what was observed at the aqueous interface with chloroform, 75 or with SC-CO 2 , 76 there is important solvent mixing, leading to an ill-defined interfacial domain. There are about 30 PF 6…”

Importance of counter-ions in alkali and alkaline-earth cation extraction by 18-crown-6: molecular dynamics studies at the water/sc-CO2interface

“…This is consistent with our simulation results on this system, in which some complexes are extracted to CO 2 , while with Pic À as counter-ions, no such extraction proceeds (see also ref. 22). Some differences can be noted, however, concerning the microscopic nature of the extracted complexes, which involves more versatile structures including dragged water molecules.…”

“…The simulations revealed marked similarities between water-'' oil '' and water-CO 2 interfaces, onto which extractant molecules and their complexes adsorb. [19][20][21] As concerns crown ethers, 18C6 molecules were found to concentrate at the interface between water and '' oil '' ('' oil '' modeled by liquid chloroform at 300 K, 10 or by sc-CO 2 at 350 K 22 ), while most K + &18C6,Pic À '' complexes '' decomplexed. The present paper reports new investigations, with a comparison of K + vs. Sr 2+ cation extraction by 18C6 to sc-CO 2 at two temperatures, with two types of counter-ions: Pic À anions used for classical extraction studies, and perfluorinated octanoate anions (F 15 C 7 COO À , hereafter denoted PFO À ) used for extraction to sc-CO 2 (Fig.…”

“…Note that simulations on K + &18C6,Pic À have been reported at 350 K with 12 '' complexes '' in the simulation box. 22 They are considered here in order to allow for consistent comparisons with the other systems. In the following, the inclusion symbol & is y Electronic supplementary information (ESI) available: Table S1: Energy component analysis for the KX and KX 2 salts and the corresponding complexes with 18C6.…”

The hydration structure of 18-crown-6/K+ complex as studied by Monte Carlo simulation using ab initio fitted potential

“…The 18-crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane) molecule posseses a special capability to form a complex with the alkali metal ions, especially with K + , and has been proposed as a transporter for the ions between two solvent phases [1][2][3]. Solvation of the crown molecule is a central issue in this field because it can reveal the transport mechanism which may lead to design of more efficient agents.…”