Importance of counter-ions in alkali and alkaline-earth cation extraction by 18-crown-6: molecular dynamics studies at the water/sc-CO2 interfaceElectronic supplementary information (ESI) available: Table S1: Energy component analysis for the KX and KX2 salts and the corresponding complexes with 18C

We present a molecular dynamics study of concentrated solutions of K+ and Sr2+ cations and their complexes with 18C6 at the water/supercritical-CO2 interface, and compare picrate “Pic−” vs. perfluorooctanoate “PFO−” counter-ions at 305 and 350 K. The results allow to understand why K+ is poorly extracted in conditions where Sr2+ is transferred to the supercritical fluid. In all cases, the solutes are found to concentrate near the interface, with interesting differences, depending on the cation, counter-ion and extractant. With the two anions, the SrX2 salts are surface active, as a result of cation–anion pairing, while the distribution of the K+ ions depends on X−. The inclusive K+⊂18C6 complexes partially decomplex, and decomplexation is more effective at 350 K than at 305 K. The most remarkable result is the extraction of Sr2+⊂18C6,2PFO− complexes in which the anions axially co-complex the cation, while retaining CO2-philic interactions via their perfluorinated chain. This Sr2+ extraction is confirmed by several computational tests which point to the importance of concentration effects and agree with experiment. It contrasts with the formation of an interfacial film with Sr2+⊂18C6,2Pic− complexes, assembled via bridging Pic−⋯Sr2+⋯Pic− and stacking Pic−⋯Pic− interactions.

[1]  G. Wipff,et al.  Ethers, crown ethers and 18-crown-6 K+ complexes at a water/SC-CO2 interface: a molecular dynamics study , 2003 .

[2]  K. Johnston,et al.  Carbon dioxide-in-water microemulsions. , 2003, Journal of the American Chemical Society.

[3]  G. Wipff,et al.  Interfacial activity of nitric acid at the water–supercritical CO2 interface: a molecular dynamics investigation , 2002 .

[4]  M. Baaden,et al.  Molecular Dynamics Study of the Uranyl Extraction by Tri-n-butylphosphate (TBP): Demixing of Water/“Oil”/TBP Solutions with a Comparison of Supercritical CO2 and Chloroform , 2002 .

[5]  P. Cummings,et al.  Molecular Simulation of a Dichain Surfactant/Water/Carbon Dioxide System. 1. Structural Properties of Aggregates , 2001 .

[6]  S. Tachimori,et al.  The effect of hydrophobic picrate on the extraction and separation of lanthanides(III) with carbamoylmethylene phosphine oxide , 2001 .

[7]  G. Wipff,et al.  Importance of interfacial phenomena in assisted ion extraction by supercritical CO2: a molecular dynamics investigation , 2001 .

[8]  M. Baaden,et al.  Interfacial Features of Assisted Liquid-Liquid Extraction of Uranyl and Cesium Salts: A Molecular Dynamics Investigation , 2000 .

[9]  C. Erkey.  Supercritical carbon dioxide extraction of metals from aqueous solutions: a review , 2000 .

[10]  G. Wipff,et al.  Distribution of hydrophilic, amphiphilic and hydrophobic ions at a liquid/liquid interface: a molecular dynamics investigation , 2000 .

[11]  G. Wipff,et al.  About the TATB assumption: effect of charge reversal on transfer of large spherical ions from aqueous to non-aqueous solvents and on their interfacial behaviour , 2000 .

[12]  R. Smith,et al.  Quantitative extraction of aqueous alkali metal ions using supercritical carbon dioxide and polyethylene glycol ligands , 2000 .

[13]  P. Mohapatra,et al.  Extraction of actinides and fission products from salt solutions using polyethylene glycols (PEGs) , 1999 .

[14]  P. Cummings,et al.  Self-Assembly of Reverse Micelles in Water/Surfactant/Carbon Dioxide Systems by Molecular Simulation , 1999 .

[15]  C. Barnes,et al.  Solvent extraction of strontium nitrate by a crown ether using room-temperature ionic liquids† , 1999 .

[16]  R. Smith,et al.  Perfluorocarboxylic acid counter ion enhanced extraction of aqueous alkali metal ions with supercritical carbon dioxide , 1999 .

[17]  Robin D. Rogers,et al.  PARTITIONING OF AROMATIC MOLECULES IN AQUEOUS BIPHASIC SYSTEMS , 1999 .

[18]  G. Wipff,et al.  About the TATB hypothesis: solvation of the Asφ4+ and Bφ4- ions and their tetrahedral and spherical analogues in aqueous/nonaqueous solvents and at a water–chloroform interface , 1999 .

[19]  L. Troxler,et al.  Simulations of Liquid-Liquid Interfaces: A Key Border in Supramolecular Chemistry , 1999 .

[20]  C. Wai,et al.  Selective extraction of strontium with supercritical fluid carbon dioxide. , 1999, Chemical communications.

[21]  A. Engler,et al.  Demixing of Binary Water−Chloroform Mixtures Containing Ionophoric Solutes and Ion Recognition at a Liquid−Liquid Interface: A Molecular Dynamics Study , 1998 .

[22]  T. Bruno,et al.  Solubility of p-t-Butylcalixarenes in Supercritical Carbon dioxide: A Comparison of Static and Dynamic Measurement Techniques , 1998 .

[23]  L. Troxler,et al.  DO PICRATE ANIONS ATTRACT EACH OTHER IN SOLUTION? MOLECULAR DYNAMICS SIMULATIONS IN WATER AND IN ACETONITRILE SOLUTIONS , 1998 .

[24]  L. Troxler,et al.  Interfacial Behavior of Ionophoric Systems: Molecular Dynamics Studies on 18-Crown-6 and Its Complexes at the Water-Chloroform Interface , 1998 .

[25]  S. Aggarwal,et al.  Thermodynamics of Extraction of Plutonium(VI) in PolyEthylene Glycol (PEG) based Aqueous Biphasic System (ABS) using 18-Crown-6 and 15-Crown-5 , 1998 .

[26]  S. Aggarwal,et al.  Thermodynamics of Extraction of Plutonium(IV) and Americium(III) in PolyEthylene Glycol (PEG) and (NH4)2SO2 based Aqueous Biphasic System (ABS) using 18-Crown-6 , 1998 .

[27]  L. Troxler,et al.  Migration of Ionophores and Salts through a Water−Chloroform Liquid−Liquid Interface: Molecular Dynamics−Potential of Mean Force Investigations , 1998 .

[28]  A. Jaber,et al.  Liquid-liquid extraction of some lanthanide metal ions by polyoxyalkylene systems. , 1997, Talanta.

[29]  Robin D. Rogers,et al.  Metal ion separations in polyethylene glycol-based aqueous biphasic systems: correlation of partitioning behavior with available thermodynamic hydration data , 1996 .

[30]  G. Wipff,et al.  Liquid-Liquid Extraction of Alkali Cations by Calix[4]Crown Ionophores: Conformation and Solvent Dependent Na+/Cs+ Binding Selectivity. A MD FEP Study in Pure Chloroform and MD Simulations at the Water/Chloroform Interface , 1996 .

[31]  A. Varnek,et al.  MD SIMULATIONS ON IONOPHORES AT A WATER-CHLOROFORM INTERFACE. PART I. CALIX4ARENES UNCOMPLEXED AND THE 222 CRYPTAND , 1996 .

[32]  M. A. Hughes,et al.  Raman microscopic studies of the liquid-liquid interface between an organic layer and an aqueous solution containing metal ions , 1996 .

[33]  R. Bartsch,et al.  ANION EFFECT ON SELECTIVITY IN SOLVENT EXTRACTION OF ALKALI METAL SALTS BY CROWN ETHERS , 1995 .

[34]  L. E. Bowman,et al.  Water solubility measurements in supercritical fluids and high-pressure liquids using near-infrared spectroscopy , 1995 .

[35]  Wilfred F. van Gunsteren,et al.  A generalized reaction field method for molecular dynamics simulations , 1995 .

[36]  K. Johnston,et al.  Water-in-Carbon Dioxide Microemulsions with a Fluorocarbon-Hydrocarbon Hybrid Surfactant , 1994 .

[37]  D. Reinhoudt,et al.  Carrier-mediated transport through liquid membranes , 1994 .

[38]  T. Darden,et al.  Particle mesh Ewald: An N⋅log(N) method for Ewald sums in large systems , 1993 .

[39]  R. Bartsch,et al.  Anion effect on selectivity in crown ether extraction of alkali metal cations , 1993 .

[40]  H. Watarai.  What's happening at the liquid—liquid interface in solvent extraction chemistry? , 1993 .

[41]  J. Åqvist,et al.  Ion-water interaction potentials derived from free energy perturbation simulations , 1990 .

[42]  W. McDowell.  Crown Ethers as Solvent Extraction Reagents: Where do We Stand? , 1988 .

[43]  Peter A. Kollman,et al.  Many-body potential for molecular interactions , 1988 .

[44]  J. Lehn,et al.  Monte Carlo study of the conformation-dependent hydration of the 18-crown-6 macrocycle , 1985 .

[45]  H. Berendsen,et al.  Molecular dynamics with coupling to an external bath , 1984 .

[46]  W. L. Jorgensen,et al.  Comparison of simple potential functions for simulating liquid water , 1983 .

[47]  Ian R. McDonald,et al.  Interaction site models for carbon dioxide , 1981 .

[48]  J. D. Lamb,et al.  Effects of salt concentration and anion on the rate of carrier facilitated transport of metal cations through bulk liquid membranes containing crown ethers , 1980 .

[49]  D. Reinhoudt,et al.  Stability and reactivity of crown-ether complexes , 1980 .