Molecular dynamics simulations of the structure and transport properties of tetra-butylphosphonium amino acid ionic liquids

Systematic molecular dynamics simulations are used to study the structure, dynamics and transport properties of the ionic liquids composed of the tetra-butylphosphonium ([TBP](+), or [P(C(4)H(9))(4)](+)) cation with six amino acid ([AA](-)) anions. The structural features of these ionic liquids were characterized by calculating the partial site-site radial distribution functions, g(r), and computing the dihedral angle distribution of n-butyl side chains in the [TBP](+) cations. The dynamics of the ionic liquids are described by studying the velocity autocorrelation function (VACF) and the mean-square displacement (MSD) for the centers of mass of the ions at different temperatures. The ionic diffusion coefficients and the electrical conductivities were evaluated from both the Einstein and Green-Kubo methods. The cross-correlation terms in the electric-current autocorrelation functions, which are an indication of the ion pair correlations, are investigated. The cationic transference numbers were also estimated to study the contributions of the anions and cations to the transport of charge in these ionic liquids. We determined the role of the amino acid anion structures on the dynamical behavior and the transport coefficients of this family of ionic liquids. In general, the MSD and self-diffusion coefficients of the relatively heavier non-planar [TBP](+) cations are smaller than those of the lighter amino acid anions. Introducing polar functional groups (acid or amide) in the side chain of [AA](-) decreases the diffusion coefficient and electrical conductivity of AAILs. The major factors for determining the magnitude of the transport coefficients are the chemical functionality and the length of the alkyl side chain of the [AA](-) anion of these [TBP][AA] ionic liquids.     

Molecular dynamics simulations of triflic acid and triflate ion/water mixtures: a proton conducting electrolytic component in fuel cells

Triflic acid is a functional group of perflourosulfonated polymer electrolyte membranes where the sulfonate group is responsible for proton conduction. However, even at extremely low hydration, triflic acid exists as a triflate ion. In this work, we have developed a force-field for triflic acid and triflate ion by deriving force-field parameters using ab initio calculations and incorporated these parameters with the Optimized Potentials for Liquid Simulations - All Atom (OPLS-AA) force-field. We have employed classical molecular dynamics (MD) simulations with the developed force field to characterize structural and dynamical properties of triflic acid (270-450 K) and triflate ion/water mixtures (300 K). The radial distribution functions (RDFs) show the hydrophobic nature of CF(3) group and presence of strong hydrogen bonding in triflic acid and temperature has an insignificant effect. Results from our MD simulations show that the diffusion of triflic acid increases with temperature. The RDFs from triflate ion/water mixtures shows that increasing hydration causes water molecules to orient around the SO(3)(-) group of triflate ions, solvate the hydronium ions, and other water molecules. The diffusion of triflate ions, hydronium ion, and water molecules shows an increase with hydration. At λ = 1, the diffusion of triflate ion is 30 times lower than the diffusion of triflic acid due to the formation of stable triflate ion-hydronium ion complex. With increasing hydration, water molecules break the stability of triflate ion-hydronium ion complex leading to enhanced diffusion. The RDFs and diffusion coefficients of triflate ions, hydronium ions and water molecules resemble qualitatively the previous findings using per-fluorosulfonated membranes.     

How alkali metal ion binding alters the conformation preferences of gramicidin A: a molecular dynamics and ion mobility study

Here, we present a systematic study combing electrospray ionization-ion mobility experiments and an enhanced sampling molecular dynamics, specifically integrated tempering sampling molecular dynamics simulations (ITS-MDS), to explore the conformations of alkali metal ion (Na, K, and Cs) adducts of gramicidin A (GA) in vacuo. Folding simulation is performed to obtain inherent conformational preferences of neutral GA to provide insights about how the binding of metal ions influences the intrinsic conformations of GA. The comparison between conformations of neutral GA and alkali metal ion adducts reveals a high degree of structural similarity, especially between neutral GA and [GA + Na](+); however, the structural similarities decrease as ionic radius of the metal increases. Collision cross section (CCS) profiles for [GA + Na](+) and [GA + Cs](+) ions obtained from by ITS-MDS compare favorably with the experimental CCS, but there are significant differences from CCS profiles for [GA + K](+) ions. Such discrepancies between the calculated and measured CCS profiles for [GA + K](+) are discussed in terms of limitations in the simulation force field as well as possible size-dependent coordination of the [GA + K](+) ion complex.     

Interaction of water with the model ionic liquid [bmim][BF4]: molecular dynamics simulations and comparison with NMR data

We report on molecular dynamics simulations of the ionic liquid [bmim][BF 4] and its mixtures with water, from zero up to 0.5 mol fraction of water. All of the simulations are carried out with two published force fields. The results are compared with each other and with published as well as new NMR data on the same mixtures, whenever possible. We perform extensive analyses of structural quantities, such as pair correlation functions, nearest-neighbor analysis and size distribution of the water clusters formed at higher concentrations. We show that the water clusters are formed almost exclusively by linear chains of hydrogen-bonded molecules. There is a nanoscale structuring of the mixtures but no macroscopic phase separation among the components, in agreement with experiment. Roughly, we identify two solvation regimes. At low water content, the ions are selectively coordinated by individual water molecules, but their ionic network is largely unperturbed. At high water content, the ionic network is somewhat disrupted or swollen in a nonspecific way by the water clusters.     

Mass spectral studies of meso-dialkyl, alkyl aryl and cycloalkyl calix(4)pyrroles under positive and negative ion electrospray ionization conditions

A series of meso-dialkyl, alkyl aryl and cycloalkyl calix(4)pyrroles (1-15) are studied under positive and negative ion electrospray ionization (ESI) conditions. The positive ion spectra show abundant [M + H](+) and [M + Na](+) ions and the negative ion spectra show the [M + Cl](-) (the Cl(-) ions from the solvent) and [M - H](-) ions. The collision induced dissociation (CID) spectra of [M + H](+), [M + Na](+), [M + Cl](-) and [M - H](-) ions are studied to understand their dissociation pathway and compared to that reported for M(+) under electron ionization (EI) conditions. The beta-cleavage process that was diagnostic to M(+) is absent in all the CID spectra of the ions studied under ESI. Dissociation of all the studied ions resulted in the fragment ions formed by sequential elimination of pyrrole (A) and/or dialkyl/alkyl aryl/cycloalkyl (B) groups involving hydrogen migration to pyrrole ring at each cleavage of A--B bond, which clearly reveals the arrangement of A and B groups in the calix(4)pyrroles. The source of hydrogen that migrates to pyrrole ring during A--B bond cleavage is investigated by the experiments on deuterated compounds and [M + D](+) ions; and confirmed that the hydrogen attached to pyrrole nitrogen, hydrogen on alpha-carbon of alkyl group and the H(+)/Na(+) ion that added during ESI process to generate [M + H](+)/[M + Na](+) ions involve in the migration. The yields of [M + Na](+) ions are found to be different for the isomeric meso-cycloalkyl compounds (cycloheptyl, and 2-, 3- and 4-methyl cyclohexyl) and for normal and N-confused calix(4)pyrroles. The isomeric methyl and 3-hydroxy/4-hydroxy phenyl calix(4)pyrroles show specific fragmentation pattern during the dissociation of their [M - H](-) ions.     

Molecular dynamics simulation of polymer electrolytes based on poly(ethylene oxide) and ionic liquids. II. Dynamical properties

Dynamical properties of polymer electrolytes based on poly(ethylene oxide) (PEO) and ionic liquids of 1-alkyl-3-methylimidazolium cations were calculated by molecular dynamics simulations with previously proposed models [L. T. Costa and M. C. Ribeiro, J. Chem. Phys. 124, 184902 (2006)]. The effect of changing the ionic liquid concentration, temperature, and the 1-alkyl-chain lengths, [1,3-dimethylimidazolium]PF(6) and [1-butyl-3-methylimidazolium]PF(6) ([dmim]PF(6) and [bmim]PF(6)), was investigated. Cation diffusion coefficient is higher than those of anion and oxygen atoms of PEO chains. Ionic mobility in PEO[bmim]PF(6) is higher than in PEO[dmim]PF(6), so that the ionic conductivity kappa of the former is approximately ten times larger than the latter. The ratio between kappa and its estimate from the Nernst-Einstein equation kappa/kappa(NE), which is inversely proportional to the strength of ion pairs, is higher in ionic liquid polymer electrolytes than in polymer electrolytes based on inorganic salts with Li(+) cations. Calculated time correlation functions corroborate previous evidence from the analysis of equilibrium structure that the ion pairs in ionic liquid polymer electrolytes are relatively weak. Structural relaxation at distinct spatial scales is revealed by the calculation of the intermediate scattering function at different wavevectors. These data are reproduced with stretched exponential functions, so that temperature and wavevector dependences of best fit parameters can be compared with corresponding results for polymer electrolytes containing simpler ions.     

A critical evaluation of high performance liquid chromatography-electrospray ionisation-mass spectrometry and capillary electrophoresis-electrospray-mass spectrometry for the detection and determination of small molecules of significance in clinical and forensic science

A critical review of applications for the period 2000-2003, taken from the Web of Knowledge database, of the techniques high performance liquid chromatography-electrospray ionisation-mass spectrometry (HPLC-ESI-MS) and capillary electrophoresis-electrospray ionisation-mass spectrometry (CE-ESI-MS) to the detection and determination of small molecules of significance in clinical and forensic science is presented. The molecules of mass less than 500 Da are chosen according to selected structural classes in which they give ESI signals primarily as [M+H](+) ions although other ions such as [M-H](-), [M+Na](+) and [M+NH(4)](+) are also reported. The structural classes are drugs with amine-containing side chains, drugs with N-containing saturated ring structures, 1,4-benzodiazepines, carbohydrates, benzimidazoles, other heterocycles, sulphonylureas, anthracyclines, sulphonamides, penicillins, cephalosporins, tetracyclines, nitrocatechols, steroids, flavonoids, oxazaphosphorines, cannabinols, and miscellaneous molecules. Details are given on the fragmentations, where available, that these ionic species exhibit in-source and in ion-trap, triple quadrupole and time-of-flight mass spectrometers. The review then gives a critical evaluation of these recent HPLC-ESI-MS and CE-ESI-MS analytical methods for the detection and determination of small molecules of clinical and forensic significance. Analytical information on, for example, sample concentration techniques, HPLC and CE separation conditions, recoveries from biological media and limits of detection (LODs) are provided.     

Ion mobilities and microscopic dynamics in liquid (Li,K)Cl

The dynamical properties of ionic melts formed from mixtures of LiCl and KCl have been studied across the full composition range in computer simulations of sufficient length to enable reliable values for such collective transport coefficients as the viscosity, conductivity, and internal mobilities to be determined reliably. Interest centers on the nontrivial concentration dependence exhibited by these transport coefficients, which agrees well with that observed experimentally, and in relating this to the strength of the association between an ion and its first coordination shell. The relationships between the various transport coefficients, such as those between the diffusion coefficient and the viscosity (Stokes-Einstein) and the conductivity (Nernst-Einstein) also exhibit composition dependences that reflect this association. The connection between the internal mobility and two measures of the coordination shell dynamics (the cage relaxation time and the self-exchange velocity) is explored; it is shown that the self-exchange velocity follows the composition and temperature dependence of the internal mobility very well. Finally, it is shown that allowing for anion polarization in the interaction model increases the mobility of all species without changing the structure of the melt discernibly, with the largest effect being found for the Li(+) ion.     

Speciation of copper(II) complexes in an ionic liquid based on choline chloride and in choline chloride/water mixtures

A deep-eutectic solvent with the properties of an ionic liquid is formed when choline chloride is mixed with copper(II) chloride dihydrate in a 1:2 molar ratio. EXAFS and UV-vis-near-IR optical absorption spectroscopy have been used to compare the coordination sphere of the cupric ion in this ionic liquid with that of the cupric ion in solutions of 0.1 M of CuCl(2)·2H(2)O in solvents with varying molar ratios of choline chloride and water. The EXAFS data show that species with three chloride ions and one water molecule coordinated to the cupric ion as well as species with two chloride molecules and two water molecules coordinated to the cupric ion are present in the ionic liquid. On the other hand, a fully hydrated copper(II) ion is formed in an aqueous solution free of choline chloride, and the tetrachlorocuprate(II) complex forms in aqueous choline chloride solutions with more than 50 wt % of choline chloride. In solutions with between 0 and 50 wt % of choline chloride, mixed chloro-aquo complexes occur. Upon standing at room temperature, crystals of CuCl(2)·2H(2)O and of Cu(choline)Cl(3) formed in the ionic liquid. Cu(choline)Cl(3) is the first example of a choline cation coordinating to a transition-metal ion. Crystals of [choline](3)[CuCl(4)][Cl] and of [choline](4)[Cu(4)Cl(10)O] were also synthesized from molecular or ionic liquid solvents, and their crystal structures were determined.     

Electrospray ionization mass spectrometry of ginsenosides

Ginsenosides R(b1), R(b2), R(c), R(d), R(e), R(f), R(g1), R(g2) and F(11) were studied systematically by electrospray ionization mass spectrometry in positive- and negative-ion modes with a mobile-phase additive, ammonium acetate. In general, ion sensitivities for the ginsenosides were greater in the negative-ion mode, but more structural information on the ginsenosides was obtained in the positive-ion mode. [M + H](+), [M + NH(4)](+), [M + Na](+) and [M + K](+) ions were observed for all of the ginsenosides studied, with the exception of R(f) and F(11), for which [M + NH(4)](+) ions were not observed. The signal intensities of [M + H](+), [M + NH(4)](+), [M + Na](+) and [M + K](+) ions varied with the cone voltage. The highest signal intensities for [M + H](+) and [M + NH(4)](+) ions were obtained at low cone voltage (15-30 V), whereas those for [M + Na](+) and [M + K](+) ions were obtained at relatively high cone voltage (70-90 V). Collision-induced dissociation yielded characteristic positively charged fragment ions at m/z 407, 425 and 443 for (20S)-protopanaxadiol, m/z 405, 423 and 441 for (20S)-protopanaxatriol and m/z 421, 439, 457 and 475 for (24R)-pseudoginsenoside F(11). Ginsenoside types were identified by these characteristic ions and the charged saccharide groups. Glycosidic bond cleavage and elimination of H(2)O were the two major fragmentation pathways observed in the product ion mass spectra of [M + H](+) and [M + NH(4)](+). In the product ion mass spectra of [M - H](-), the major fragmentation route observed was glycosidic bond cleavage. Adduct ions [M + 2AcO + Na](-), [M + AcO](-), [M - CH(2)O + AcO](-), [M + 2AcO](2-), [M - H + AcO](2-) and [M - 2H](2-) were observed at low cone voltage (15-30 V) only.     

Even-electron ions: a systematic study of the neutral species lost in the dissociation of quasi-molecular ions

The collision-induced dissociations of the even-electron [M + H](+) and/or [M - H](-) ions of 121 model compounds (mainly small aromatic compounds with one to three functional groups) ionized by electrospray ionization (ESI) or atmospheric pressure chemical ionization (APCI) have been studied using an ion trap instrument, and the results are compared with the literature data. While some functional groups (such as COOH, COOCH(3), SO(3)H in the negative ion mode, or NO(2) in both the positive and negative ion modes) generally promote the loss of neutrals that are characteristic as well as specific, other functional groups (such as COOH in the positive ion mode) give rise to the loss of neutrals that are characteristic, but not specific. Finally, functional groups such as OH and NH(2) in aromatic compounds do not lead to the loss of a neutral that reflects the presence of these substituents. In general, the dissociation of [M + H](+) and [M - H](-) ions generated from aliphatic compounds or compounds containing an aliphatic moiety obeys the even-electron rule (loss of a molecule), but deviations from this rule (loss of a radical) are sometimes observed for aromatic compounds, in particular for nitroaromatic compounds. Thermochemical data and ab initio calculations at the CBS-QB3 level of theory provide an explanation for these exceptions. When comparing the dissociation behaviour of the even-electron [M + H](+) and/or [M - H](-) ions (generated by ESI or APCI) with that of the corresponding odd-electron [M](+) ions (generated by electron ionization, EI), three cases may be distinguished: (1) the dissociation of the two ionic species differs completely; (2) the dissociation involves the loss of a common neutral, yielding product ions differing in mass by one Da, or (3) the dissociations lead to a common product ion.     

Structural determination of the novel fragmentation routes of zwitteronic morphine opiate antagonists naloxonazine and naloxone hydrochlorides using electrospray ionization tandem mass spectrometry

Electrospray ionization quadrupole time-of-flight (ESI-QqToF) mass spectra of the zwitteronic salts naloxonazine dihydrochloride 1 and naloxone hydrochloride 2, a common series of morphine opiate receptor antagonists, were recorded using different declustering potentials. The singly charged ion [M+H-2HCl](+) at m/z 651.3170 and the doubly charged ion [M+2H-2HCl](2+) at m/z 326.1700 were noted for naloxonazine dihydrochloride 1; and the singly charged ion [M+H-HCl](+) at m/z 328.1541 was observed for naloxone hydrochloride 2. Low-energy collision-induced dissociation tandem mass spectrometry (CID-MS/MS) experiments established the fragmentation routes of these compounds. In addition to the characteristic diagnostic product ions obtained, we noticed the formation of a series of radical product ions for the zwitteronic compounds 1 and 2, and also the formation of a distonic ion product formed from the singly charged ion [M+H-HCl](+) of naloxone hydrochloride 2. Confirmation of the various established fragmentation routes was effected by conducting a series of ESI-CID-QqTof-MS/MS product ion scans, which were initiated by CID in the atmospheric pressure/vacuum interface using a higher declustering potential. Deuterium labeling was also performed on the zwitteronic salts 1 and 2, in which the hydrogen atoms of the OH and NH groups were exchanged with deuterium atoms. Low-energy CID-QqTof-MS/MS product ion scans of the singly charged and doubly charged deuteriated molecules confirmed the initial fragmentation patterns proposed for the protonated molecules. Precursor ion scan analyses were also performed with a conventional quadrupole-hexapole-quadrupole tandem mass spectrometer and allowed the confirmation of the genesis of some diagnostic ions.     

Structure and dynamics at the surface of a concentrated aqueous solution of CsF

A molecular dynamics simulation of a liquid layer of a concentrated CsF solution in water has been performed in order to compare the results with those obtained in an experimental study of our group. The main result of the experiment was the existence of a monolayer of nearly pure water constituting the surface and a homogeneous mixture constituting the bulk of the system. The simulation reveals the same phenomena which can be explained by the circumstance that the ions near the surface mostly keep their first solvation shell intact. The water molecules belonging to these shells and being placed on the vapor side constitute this monolayer. The density profiles of the ions indicate that the Cs ions penetrate further into the surface than the F ions. The orientational structure of the first shell of water molecules around an ion is the same for ions in the surface and ions in the bulk in contrast to the dynamics which is altered. The spectra of the librational motion are shifted to lower frequencies. In addition to that the spectra belonging to libration which involves motion of the dipole moment develop a peak in the low frequency range irrespective of whether the water molecules are bonded to Cs or to F ions. This can be correlated with an overall preferred orientation of the water molecules in the surface which is most pronounced for the dipole moment. The calculation of the diffusion coefficients shows that the top surface layer of nearly pure water is a region of enhanced and extremely anisotropic mobility. The mean residence time of water molecules in the surface in the first shell of an ion is reduced according to the enhanced mobility.     

Ethylenediamine as a liquid chemical reagent to probe hydrogen bonding and host-guest interactions with crown ethers in an ion trap tandem mass spectrometer

Ethylenediamine (EDA) was used as a novel liquid chemical reagent to probe hydrogen bonding and host-guest interactions with crown ether derivatives in an ion trap mass spectrometer (ITMS). Selective ion/molecule reaction product ions were generated by reactions of EDA with oxygenated and aza-crown ethers. For the oxygenated crown ethers, glycols and dimethylglycols, ion/molecule reactions led to the formation of the protonated molecules ([M+H](+)) and adduct ions including [M+30](+), [M+44](+) and [M+61](+). The aza-crown ethers produced [M+H](+), [M+13](+) and [M+27](+) ions. Collisionally activated dissociation (CAD) experiments were applied to probe the binding strength of these ion/molecule reaction products. CAD results indicated that all these hydrogen-bonding complexes are weakly bound except for the [M+44](+) ion of 18-crown-6, since all the complexes dissociate to the protonated polyether and/or protonated EDA. Fragmentation of the [M+H](+) ions under CAD conditions indicates the extensive covalent bond cleavage of the protonated crown ether skeleton.     

Observation of Na(+)-bound oligomers of quercetin in the gas phase

While developing a liquid chromatography/tandem mass spectrometry method for the analysis of the flavonoid quercitin, it was observed that quercetin (3,3',4',5,7-pentahydroxyflavone) exhibited clustering in both the positive and negative ion mode. Two series of positive ion clusters were observed; the first series corresponds to singly charged [2M + Na](+) at m/z 627.2 to [13M + Na](+) at m/z 3947.5, while the second series corresponds to doubly charged [7M + 2Na](2+) at m/z 1080.4 to [25M + 2Na](2+) at m/z 3798.5. In the negative ion mode, the behavior of quercetin parallels that of apigenin (4',5,7-trihydroxyflavone) in that [M + NO(3)](-), [2M + NO(3)](-), and [3M + NO(3)](-) were observed at m/z 364.1, 666.0, and 968.9, respectively; in addition, quercitin clusters with chloride ions ([2M + Cl](-) at m/z 638.9 and [3M + Cl](-) at m/z 940. 9) were observed. The results of tandem mass spectrometric examination of several cluster ions are reported.     

Association of ionic liquids in solution: a combined dielectric and conductivity study of [bmim][Cl] in water and in acetonitrile

Ion association of the ionic liquid [bmim][Cl] in acetonitrile and in water was studied by dielectric spectroscopy for salt concentrations c ≤ 1.3 M at 298.15 K and by measurement of molar electrical conductivities, Λ, of dilute solutions (c ≤ 0.006 M) in the temperature range 273.15 ? T/K ≤ 313.15. Whilst acetonitrile solutions of [bmim][Cl] exhibit moderate ion pairing, with an association constant of K°(A) ≈ 60 M(-1) and increasing with temperature, [bmim][Cl] is only weakly associated in water (K°(A) ≈ 6 M(-1)) and ion pairing decreases with rising temperature. Only contact ion pairs were detected in both solvents. Standard-state enthalpy, entropy and heat capacity changes of ion association were derived, as well as the activation enthalpy of charge transport and the limiting conductivity of the cation, λ(∞)?([bmim](+)). These data, in conjunction with effective solvation numbers obtained from the dielectric spectra, suggest that the solvation of [bmim](+) is much weaker in water than in acetonitrile.     

Insights into Tris‐(2‐Hydroxylethyl)methylammonium Methylsulfate Aqueous Solutions

We report herein a combined experimental–computational study on tris‐(2‐hydroxylethyl)methylammonium methylsulfate in water solutions, as a representative ionic liquid of the aqueous‐solution behavior of hydroxylammonium‐based ionic liquids. Relevant thermophysical properties were measured as a function of mixture composition and temperature. Classical molecular dynamics simulations were performed to infer microscopic structural features. The reported results for ionic liquid in water‐rich solutions show that it behaves as isolated non‐interacting ions solvated by water molecules, through well‐defined solvation shells, exerting a disrupting effect on the water hydrogen bonding network. Nevertheless, as ionic liquid concentration increase, interionic association increases, even for diluted water solutions, evolving from the typical behavior of strong electrolytes in solution toward large interacting structures. For ionic‐liquid‐rich mixtures, water exerts a minor disrupting effect on the fluid’s structuring because it occupies regions around each ion (developing water–ion hydrogen bonds) but without significantly weakening anion–cation interactions.     

Structure and dynamics of N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide ionic liquid from molecular dynamics simulations

Molecular dynamics (MD) simulations were performed on N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide (mppy+TFSI-) from 303 to 393 K to improve understanding of the structure and ion transport of this ionic liquid. The density, ion self-diffusion coefficients, conductivity, and viscosity of mppy+TFSI- predicted from MD simulations are in good agreement with experimental measurements. The time-dependent shear modulus of the ionic liquids was calculated and compared with that for nonionic liquids. On average each mppy+ cation was found to be coordinated by four TFSI- anions. The angular distributions of N(TFSI-)-N(mppy+)-N(TFSI-) and N(mppy+)-N(TFSI-)-N(mppy+) exhibit a maximum at 80-90 degrees and a second maximum at 180 degrees . Correlation of ion motion was found to lower ionic conductivity by approximately one-third from the expected value based upon ion self-diffusion coefficients. Rotational motion of the cation and anion are anisotropic with the degree of anisotropy increasing with decreasing temperature. Electrostatic interactions are responsible for slowing down the dynamics of the ionic liquid by more than an order of magnitude and a dramatic decrease of the time-dependent shear modulus.     

A molecular dynamics study and molecular level explanation of pressure dependence of ionic conductivity of potassium chloride in water

Experimental ionic conductivity of different alkali ions in water shows markedly different dependences on pressure. Existing theories such as that of Hubbard-Onsager are unable to explain these dependences on pressure of the ionic conductivity for all ions. We report molecular dynamics investigation of potassium chloride solution at low dilution in water at several pressures between 1 bar and 2 kbar. Two different potential models have been employed. One of the models successfully reproduces the experimentally observed trend in ionic conductivity of K(+) ions in water over the 0.001-2 kbar range. We also propose a theoretical explanation, albeit at a qualitative level, to account for the dependence of ionic conductivity on pressure in terms of the previously studied Levitation Effect. It also provides a microscopic picture in terms of the pore network in liquid water.