Distribution of hydrophilic, amphiphilic and hydrophobic ions at a liquid/liquid interface: a molecular dynamics investigation

Abstract

We report molecular dynamics studies on the interfacial distribution of ionic species of different size, shape and topology at a water/chloroform interface: hydrophilic K + Cl − , K + SCN − and K + Pic − ions, amphiphilic ammonium NTMA + cations and farnesylphosphate FPH − anions, tetrahedral hydrophobic AsPh 4 + and BPh 4 − ions, with different counterions. Contrasted distributions are observed. The K + Cl − and K + SCN − ions sit almost exclusively in the water phase, but SCN − is less 'repelled' than Cl − by the interface. The Pic − anions are partly adsorbed at the interface and dissolved in the water phase where they display remarkable p-stacking interactions. Amphiphilic NTMA + cations or FPH − anions adsorb and dilute at the interface. Less expected is the high surface activity of symmetrical AsPh 4 + and BPh 4 − ions, with marked counterion effects. The two ions fully adsorb at the interface in the AsPh 4 + BPh 4 − salt, while in the Na + BPh 4 − or AsPh 4 + Cl − salts, they display an equilibrium between the organic phase and the interface. Crossed comparisons between the different solutions reveal the important role of counterions on the distribution of a given ionic species. These results are discussed in relation to experimental data.

Interfacial Activity of the Diprotonated 222 Cryptand at the Water/"Oil" Interface Revealed by Molecular Dynamics Simulations

“…The reason has to be found in the accumulation of positively charged 222 2 + species at the interface, creating a positive potential which attracts the anions. The Cldistribution in our systems contrasts with the one observed with the more hydrophilic K+Cl-or NMetcl-systems, where all ions are "repelled" by the interface [32]. There are, however, subtle differences between the distributions of Cl-vs. r counterions simulated here (compared systems E to G and J to H).…”

“…More surprising is the calculated surface activity of la~ge spherical s+ and s-model ions, whose radius (5 A) is close to the gyration radius of 222 [49,50]. According to simulations, smaller anions like ClO,i or quaternary ammonium NRt cations are also surface active [32], leading to a decrease in INTERFACIAL ACTIVITY OF 222 2 + 141 surface tension [51][52][53][54]. Even tetracharged tetrahedral ammonium hosts and their anion complexes are surface active, as shown by MD simulations [55], in keeping with the experimental observations of their transport properties through hydrophobic membranes [56].…”

“…For instance, according to MD simulations [32,47] and surface spectroscopy studies [ 48], tetrahedral ions such as AsPht and BPh4 adsorb at the water I air and water I chloroform interfaces I respectively. More surprising is the calculated surface activity of la~ge spherical s+ and s-model ions, whose radius (5 A) is close to the gyration radius of 222 [49,50].…”

Macrotricyclic quaternary tetraammonium receptors: Halide anion recognition and interfacial activity at an aqueous interface. A molecular dynamics investigation

“…On the other hand, we recently found that large quasi‐“spherical” ions, like mono‐ or divalent cryptates,13 AsPh, BPh14–16 or dicarbollide anions17 adsorb at liquid–liquid interfaces (LLI), and it is interesting to test computationally whether the tetrahedral tetraammonium complexes behave similarly. These investigations extent our work on ions and complexes at LLIs 18–21. Interestingly, it was reported that L 4+ derivatives with R = CH 2 ϕ form two‐dimensional assemblies at the interface between the NaCl(001) surface and water where they complex chloride anions 10.…”

Uranyl Extraction by TBP from a Nitric Aqueous Solution to SC-CO2: Molecular Dynamics Simulations of Phase Demixing and Interfacial Systems

“…(28) On the other hand, hydrophilic metallic ions are "repelled" by the interface, preventing their capture by water insoluble extractants. (29) It is thus important to understand at the molecular level what happens at the liquid boundaries (30) and we report new microscopic insights into this question obtained from molecular dynamics (MD) simulations with explicit representation of the solvents and solutes.…”