Hemant K. Kashyap

What is the Origin of the Prepeak in the X-ray Scattering of Imidazolium-Based Room-Temperature Ionic Liquids?

The observation of a first sharp diffraction peak (FSDP) at low frequency in the X-ray and neutron scattering spectra of different imidazolium-based room-temperature ionic liquids (RTILs) (the so-called prepeak) has often been experimentally interpreted as indicative of mesoscopic organization leading to nanoscale segregation and the formation of domains of different morphologies. This interpretation that has permeated the analysis of many recently published articles deserves an in depth theoretical analysis. In this article, we use several different computational techniques to thoroughly dissect the atomistic components giving rise to the low-frequency FSDP as well as other features in the structure function (S(q)). By understanding how S(q) changes as imidazolium-based ionic systems undergo solid-liquid phase transition, and by artificially perturbing the liquid structure in a way that directly couples to the intensity of the FSDP, we are able to identify in a rigorous way its geometric origin. Similar to the solid phase, the liquid phase is characterized by two typical length scales between polar groups. The shorter length scale gives rise to a shoulder peak in S(q) at about 0.9 Å(-1) whereas the longer one gives rise to the prepeak.

Temperature-dependent structure of methyltributylammonium bis(trifluoromethylsulfonyl)amide: X ray scattering and simulations

We report the combined results of computational and x ray scattering studies of amorphous methyltributylammonium bis(trifluoromethylsulfonyl)amide as a function of temperature. These studies included the temperature range for the normal isotropic liquid, a deeply supercooled liquid and the glass. The low q peaks in the range from 0.3 to 1.5 A(-1) in the structure function of this liquid can be properly accounted for by correlations between first and second nearest neighbors. The lowest q peak can be assigned to real space correlations between ions of the same charge, while the second peak arises mostly from nearest neighbors of opposite charge. Peaks at larger q values are mostly intramolecular in nature. While our simulated structure functions provide an excellent match to our experimental results and our experimental findings agree with previous studies reported for this liquid, the prior interpretation of the experimental data in terms of an interdigitated smectic A phase is not supported by our simulations. In this work, we introduce a set of general theoretical partitions of real and reciprocal space correlations that allow for unambiguous analysis of all intra- and interionic contributions to the structure function and coherent scattering intensity. We find that the intermolecular contributions to the x ray scattering intensity are dominated by the anions and cross terms between cations and anions for this ionic liquid.

Temperature-dependent structure of ionic liquids: X-ray scattering and simulations

In this article we determine the temperature-dependent structure of the tetradecyltrihexylphosphonium bis(trifluoromethylsulfonyl)amide ionic liquid using a combination of X-ray scattering and molecular dynamics simulations. As in many other room-temperature ionic liquids three characteristic intermolecular peaks can be detected in the structure function S(q). A prepeak or first sharp diffraction peak is observed at about q = 0.42 A(-1). Long range anion-anion correlations are the most important contributors to this peak. In all systems we have studied to date, this prepeak is a signature of solvation asymmetry. The peak in S(q) near q = 0.75 A(-1) is the signature of ionic alternation and arises from the charge ordered separation of ions of the same charge. The most intense diffraction peak near q = 1.37 A(-1) arises from short-range separation between ions of opposite charge combined with a significant contribution from cationic carbon-carbon interactions, indicating that cationic hydrophobic tails have significant contacts.

How Does the Ionic Liquid Organizational Landscape Change when Nonpolar Cationic Alkyl Groups Are Replaced by Polar Isoelectronic Diethers

X-ray scattering experiments and molecular dynamics simulations have been performed to investigate the structure of four room temperature ionic liquids (ILs) comprising the bis(trifluoromethylsulfonyl)amide (NTf(2)(-)) anion paired with the triethyloctylammonium (N(2228)(+)) and triethyloctylphosphonium (P(2228)(+)) cations and their isoelectronic diether analogs, the (2-ethoxyethoxy)ethyltriethylammonium (N(222(2O2O2))(+)) and (2-ethoxyethoxy)ethyltriethylphosphonium (P(222(2O2O2))(+)) cations. Agreement between simulations and experiments is good and permits a clear interpretation of the important topological differences between these systems. The first sharp diffraction peak (or prepeak) in the structure function S(q) that is present in the case of the liquids containing the alkyl-substituted cations is absent in the case of the diether substituted analogs. Using different theoretical partitioning schemes for the X-ray structure function, we show that the prepeak present in the alkyl-substituted ILs arises from polarity alternations between charged groups and nonpolar alkyl tails. In the case of the diether substituted ILs, we find considerable curling of tails. Anions can be found with high probability in two different environments: close to the cationic nitrogen (phosphorus) and also close to the two ether groups. For the two diether systems, anions are found in locations from which they are excluded in the alkyl-substituted systems. This removes the longer range (polar/nonpolar) pattern of alternation that gives rise to the prepeak in alkyl-substituted systems.

Structure of 1-Alkyl-1-methylpyrrolidinium Bis(trifluoromethylsulfonyl)amide Ionic Liquids with Linear, Branched, and Cyclic Alkyl Groups

X-ray scattering and molecular dynamics simulations have been carried out to investigate structural differences and similarities in the condensed phase between pyrrolidinium-based ionic liquids paired with the bis(trifluoromethylsulfonyl)amide (NTf2(-)) anion where the cationic tail is linear, branched, or cyclic. This is important in light of the charge and polarity type alternations that have recently been shown to be present in the case of liquids with cations of moderately long linear tails. For this study, we have chosen to use the 1-alkyl-1-methylpyrrolidinium, Pyrr(1,n(+)) with n = 5 or 7, as systems with linear tails, 1-(2-ethylhexyl)-1-methylpyrrolidinium, Pyrr(1,EtHx(+)), as a system with a branched tail, and 1-(cyclohexylmethyl)-1-methylpyrrolidinium, Pyrr(1,ChxMe(+)), as a system with a cyclic tail. We put these results into context by comparing these data with recently published results for the Pyrr(1,n(+))/NTf2(-) ionic liquids with n = 4, 6, 8, and 10.1,2 General methods for interpreting the structure function S(q) in terms of q-dependent natural partitionings are described. This allows for an in-depth analysis of the scattering data based on molecular dynamics (MD) trajectories that highlight the effect of modifying the cationic tail.

Anions, the Reporters of Structure in Ionic Liquids

In this work we compare the role that different anions play in the structure function S(q) for a set of liquids with the same cation. It is well established that because of their amphiphilic nature and their often larger size, cations play a fundamental role in the structural landscape of ionic liquids. On the other hand, it is often atoms in the anions that display the largest X-ray form factors and therefore play a very significant role as reporters of structure in small- and wide-angle X-ray scattering (SAXS/WAXS)-type experiments. For a set of liquids with similar topological landscape, how does S(q) change when the anionic scattering is deemphasized? Also, how do we computationally recover the typical length scale of important and perhaps universal ionic liquid structural features such as charge alternation when these are experimentally inaccessible from S(q) because of interference cancellations? We answer these questions by studying three different tetrapentylammonium-based liquids with the I(-), PF6(-) and N(CN)2(-) anions.

How Is Charge Transport Different in Ionic Liquids and Electrolyte Solutions

In this article we show that, analyzed in a barycentric reference frame, the deviation in conductivity measured directly from impedance experiments with respect to that estimated indirectly from NMR diffusion experiments has different origins in electrolyte solutions and pure salts. In the case of electrolyte solutions, the momentum conservation law is satisfied by solvent + ions. Instead, in a molten salt or ionic liquid momentum conservation must be satisfied solely by the ions. This has significant implications. While positively correlated motion of ions of opposite charge is a well justified explanation for the reduction in impedance conductivity in the case of electrolyte solutions, it is not so in the case of ionic liquids and molten salts. This work presents a set of equations that in the case of ionic liquids and molten salts can be used to obtain from direct measurements of impedance and NMR the distinct part of the diffusion coefficient matrix in the barycentric reference frame. In other words, by using experimentally measurable quantities, these equations allow us to access the motional coupling between ions for which there is no single direct experimental measurement technique. While equations of this type have been proposed before, the ones presented here can be easily derived from the momentum conservation law and linear response theory. Our results indicate that the decrease in the impedance conductivity with respect to NMR conductivity in ionic liquids and molten salts is due to anticorrelated motion of ions of same charge. This scenario is different in electrolyte solutions, where the positively correlated motion of ions of opposite charge makes a significant contribution to the decrease in the impedance conductivity. In contrast, in a system comprising a single binary salt (a room temperature ionic liquid or a molten salt), the cation-anion distinct diffusion coefficient is negative definite and opposes the contribution from the cation-cation and anion-anion distinct diffusion coefficients. This property of the cation-anion distinct diffusion coefficient in systems comprising just two ion-constituents holds true not just in the barycentric reference frame but also in any of the internal reference frames of nonequilibrium thermodynamics.

Stokes Shift Dynamics in Ionic Liquids: Temperature Dependence

A recently developed molecular theory is used to investigate the temperature dependence of the dynamic fluorescence Stokes shift of a dipolar solute in seven imidazolium ionic liquids. The temperature range considered is 278.15-338.15 K, for which experimental dielectric relaxation data (frequency range of 0.2 ≤ ν/GHz ≤ 89) are available. The theory used here explores and substantiates the relation between fluorescence spectral dynamics and dielectric relaxation in ionic liquids. The slope of the temperature-dependent change in the calculated total dynamic Stokes shift is predicted to follow an inverse-linear correlation with that (slope) of the experimentally measured temperature dependence of the static dielectric constant of these liquids. This explains the experimentally observed decrease of polarity parameter, E(T)(30), with temperature for several different ionic liquids. A significant part of the stabilization energy of a dissolved excited dipolar solute is found to arise from the reorientational dynamics of the dipolar ions (mainly imidazolium cations) of these liquids. The separated solute-solvent dipole-ion interaction contribution to the shift exhibits a stronger temperature dependence than the dipole-dipole interaction component. Calculations predict bimodal Stokes shift dynamics for all of these liquids with a fast initial component arising from rapid angular adjustment of the dipolar ions. The slow, stretched-exponential component shows a systematic temperature dependence and is linked to an environment rearrangement through the center-of-mass motion of the ions. Subsequently, calculated solvation activation energies are found to be closely related to those observed in the corresponding conductivity and viscosity measurements for these ionic liquids.

Communication: Anomalous temperature dependence of the intermediate range order in phosphonium ionic liquids.

In a recent article by the Castner and Margulis groups [Faraday Discuss. 154, 133 (2012)], we described in detail the structure of the tetradecyltrihexylphosphonium bis(trifluoromethylsulfonyl)-amide ionic liquid as a function of temperature using X-ray scattering, and theoretical partitions of the computationally derived structure function. Interestingly, and as opposed to the case in most other ionic-liquids, the first sharp diffraction peak or prepeak appears to increase in intensity as temperature is increased. This phenomenon is counter intuitive as one would expect that intermediate range order fades as temperature increases. This Communication shows that a loss of hydrophobic tail organization at higher temperatures is counterbalanced by better organization of polar components giving rise to the increase in intensity of the prepeak.

How the Structure of Pyrrolidinium Ionic Liquids Is Susceptible to High Pressure

The structural landscape of room-temperature ionic liquids (RTILs) with longer cationic alkyl tail(s) exhibits polarity ordering (PO) along with charge ordering (CO). In polarity ordering, which is also referred to as intermediate-range ordering, polar groups are separated by segregated domains of apolar groups and vice versa. Charge ordering resembles alternation of positive-negative charge groups. In this work, how these two characteristic orderings respond to applied external pressure has been investigated via molecular dynamics simulations. The present study complements the recent experimental studies of Yoshimura et al. (J. Phys. Chem. B 2015, 119, 8146-8153) and computational studies of Russina et al. (Phys. Chem. Chem. Phys. 2015, 17, 29496-29500) wherein the authors described in detail the effects of pressure on the structural and conformational changes in imidazolium based ionic liquids. Our simulations predict that for 1-alkyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide, Pyrr1,n(+)/NTf2(-) with n = 8 and 10, the PO and CO fade when the external pressure increases from ambient pressure to 10000 bar. We observe that the apolar tail group as well as the polar group correlations are susceptible to the applied pressure. The decrease of polar-polar and apolar-apolar correlations at higher pressure is accompanied by the enhancement in the polar-apolar correlations and increased stability/probability of gauche conformations along the cationic tails.