Formation Of Contact Ion Pairs Research Articles

Signatures of Ion Pairing and Aggregation in the Vibrational Spectroscopy of Super-Concentrated Aqueous Lithium Bistriflimide Solutions

Aqueous electrolytes in the super-concentrated regime, where water and ions are present in a similar concentration, have recently drawn considerable interest for their potential application in electrochemical systems, such as Li+ batteries. In this regime, the classical theories for the structure and properties of electrolyte solutions break down, and even such fundamental properties as the solution morphology on the molecular length scale can be unclear, with specific ion effects often driving the properties. Here, we study the role of ion–ion interactions in aqueous lithium bistriflimide (LiTFSI) solutions as a function of concentration, ranging from the dilute to the super-concentrated regimes. We use vibrational spectroscopy of the asymmetric −SO2 stretch modes of the TFSI– anion, specifically the linear absorption, two-dimensional (2D) IR, and polarization-dependent 2D IR and IR transient absorption spectroscopies, as a method for probing for both the cation–anion interactions and anion–anion interactions in these solutions and demonstrate specific signatures of the formation of contact ion pairs and higher-order ion aggregates. These results are validated by comparison with molecular dynamics simulations of several concentrations of LiTFSI in water, and the spectral observables are interpreted with the analysis of the normal modes of small clusters with geometries consistent with solution-phase environments derived from these simulations. Using these techniques, we find evidence that contact ion pairs, both cation–anion and anion–anion pairs, begin to have substantial population at concentrations around 2 M (∼3 m) and that an extended, highly interconnected ion-rich network forms in the super-concentrated regime.

Electrostatics, Hydrogen Bonding, and Molecular Structure at Polycation and Peptide:Lipid Membrane Interfaces.

Polycation and peptide-modified surfaces represent opportunities for developing potentially novel biocidal materials in a growing effort to combat bacterial resistance to traditional bactericides. It is well-known that the positive charge of these compounds is crucial to their function in biofouling prevention and as antimicrobials; however, methods for quantifying the number of positive charges on surface-bound polycations and peptides are necessary to predict, control, and optimize the design and therefore the utility of these compounds. This Spotlight on Applications reports on such an approach that combines second harmonic generation (SHG) spectroscopy, quartz crystal microbalance with dissipation monitoring (QCM-D), and atomistic simulations to obtain mechanistic insight into polycation-membrane interactions using supported lipid bilayers (SLBs) as our model system. We find that at high surface coverage, the large polycations we surveyed feature a considerably smaller percentage of ionization when compared to the smaller polycations and peptides. At these high charge densities, we suspect a pKa shift of the charged groups to lower charge-charge repulsion as well as the formation of a looplike conformation such that less monomeric units form contact-ion pairs with the bilayer. Our sum frequency generation (SFG) spectroscopy results complement our understanding of the polycation-membrane interaction. At a high density of the polycation poly(allylamine hydrochloride) (PAH), second-order spectral line shapes are consistent with the expulsion of interfacial water molecules possibly due to contact-ion pair formation between PAH and the lipid bilayer. This finding could be essential for understanding the underlying first steps of cell lysis and penetration by polycations and should be explored further.

Highly concentrated carbonate electrolyte for Li-ion batteries with lithium metal and graphite anodes

Highly concentrated lithium bis(fluorosulfonyl)imide (LiFSI) salt dissolved in carbonate solvent is employed as a high-performance and robust organic electrolyte for Li-ion batteries. The influences of Li salt type, concentration, and solvent type (such as diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethylene carbonate (EC)) on the electrochemical properties of Li metal and graphite anodes are systematically assessed. A superior electrolyte composition of 5.5 M LiFSI-DMC/EC is achieved, enhancing the anti-flammability, coulombic efficiency, and high rate capability. The optimal efficiency values of Li electrodeposition/stripping utilizing 5.5 M LiFSI-DMC/EC are 97.0% and 94.5% at 0.4 and 6 mA cm−2, respectively. Such an enhanced performance is due to the formation of a three-dimensional ion network, composed of contact ion pairs (CIPs) and ion aggregates (AGGs) in the highly concentrated LiFSI electrolyte, which effectively decreases the number of free solvent molecules and inhibits the formation of undesired dendritic Li structures. Raman spectroscopy is employed to confirm the formation of CIP and AGG compounds within the electrolyte. The electrochemical data of the 5.5 M LiFSI-DMC/EC electrolyte cell demonstrates a remarkable improvement in the specific capacity and rate capability of a graphite anode.

Electrical conductivity, ion pairing, and ion self-diffusion in aqueous NaCl solutions at elevated temperatures and pressures.

We have performed classical molecular dynamics (MD) simulations of aqueous sodium chloride (NaCl) solutions from 298 to 674 K at 200 bars to understand the influence of ion pairing and ion self-diffusion on electrical conductivity in high-temperature/high-pressure salt solutions. Conductivity data obtained from the MD simulation highlight an apparent anomaly, namely, a conductivity maximum as temperature increases along an isobar, which has been also observed in experimental studies. By examining both velocity autocorrelation and cross-correlation terms of the Green-Kubo integral, we quantitatively demonstrate that the conductivity anomaly arises mainly from a competition between the single-ion self-diffusion and the contact ion pair formation. The velocity autocorrelation function in conjunction with structural analysis suggests that diffusive motion of ions is suppressed at high temperatures due to the persistence of an inner hydration shell. The contribution of velocity cross-correlation functions between oppositely charged ions becomes significant at the onset of the conductivity decrease. Structural analysis based on Voronoi tessellation and pair correlation functions indicates that the fraction of contact ion pairs increases as temperature increases. Spatial decomposition of the electrical conductivity also indicates that the formation of contact ion pairs significantly decreases the electrical conductivity compared to Nernst-Einstein conductivity, but the contribution of distant opposite charges cannot be ignored except at the highest temperature due to unscreened long-range interactions.

Aggregation Patterns in Low- and High-Charge Anions Define Opposite Solubility Trends.

Molecular dynamics simulations in aqueous solution reveal the existence of two distinct patterns of aggregation in low and high charge density Lindqvist-type polyoxometalates (POMs). Our results indicate the presence of contact and solvent-shared ion pairs and specific and preferential interactions of alkalis with POMs. Highly charged POMs are capable of breaking apart the Li+ and Cs+ solvation shell, thus enhancing the formation of long-lived alkali-POM contact ion pairs, where alkalis act as an electrostatic "glue" forming large oligomers. Stronger ion pair interactions for Li+ than for Cs+ promote lower solubility for Li+ than for Cs+, evoking anomalous solubility trends. Lower charge density POMs are not capable of disrupting the Li+ solvation shell and only solvent-shared ion pairs are formed, whereas for Cs+, contact ion pairs exist. The large number of oxygen atoms in the POM surface enhances the hydrogen bonds between POM and water, thus promoting aggregation. In this case, aggregation follows normal solubility trends. Thus, aggregation depends on the strength of ion pair interactions, the capacity of POM to disrupt alkali's solvation shell, and the contact surface area between the solvent and POM.

Dielectric Decrement for Aqueous NaCl Solutions: Effect of Ionic Charge Scaling in Nonpolarizable Water Force Fields.

We investigate the dielectric constant and the dielectric decrement of aqueous NaCl solutions by means of molecular dynamic simulations. We thereby compare the performance of four different force fields and focus on disentangling the origin of the dielectric decrement and the influence of scaled ionic charges, as often used in nonpolarizable force fields to account for the missing dynamic polarizability in the shielding of electrostatic ion interactions. Three of the force fields showed excessive contact ion pair formation, which correlates with a reduced dielectric decrement. In spite of the fact that the scaling of charges only weakly influenced the average polarization of water molecules around an ion, the rescaling of ionic charges did influence the dielectric decrement, and a close-to-linear relation of the slope of the dielectric constant as a function of concentration with the ionic charge was found.

Enhanced Formation of Solvent-Shared Ion Pairs in Aqueous Calcium Perchlorate Solution toward Saturated Concentration or Deep Supercooling Temperature and Its Effects on the Water Structure.

As a candidate of Martian salts, calcium perchlorate [Ca(ClO4)2] has the potential to stabilize liquid water on the Martian surface because of its hygroscopicity and low freezing temperature when forming aqueous solution. These two properties of electrolytes in general have been suggested to result from the specific cation-anion-water interaction (ion pairing) that interrupts the structure of solvent water. To investigate how this concentration-dependent and temperature-dependent ion pairing process in aqueous Ca(ClO4)2 solution leads to its high hygroscopic property and the extreme low eutectic temperature, we have conducted two sets of experiments. First, the effects of concentrationon aqueous calcium perchlorate from 3 to 7.86 m on ion pairing were investigated using Raman spectroscopy. Deconvolution of the Raman symmetric stretching band (ν1) of ClO4- showed the enhanced formation of solvent-shared ion pairs upon increasing salt concentration at room temperature. We have confirmed that the low tendency of forming contact ion pairs in concentrated solution contributes to the high hygroscopicity of the salt. Second, the near eutectic samples were studied as a function of temperature by both combined differential scanning calorimetry-Raman spectroscopic experiments and in situ X-ray diffraction. The number of solvent-shared ion pairs was found to increase with decreasing temperature when cooled below the temperature of maximum density of the solution,driven by a change in water toward an ice-like structure in the supercooled regime. The massive presence of solvent-shared ion pairs in turn limits the development of the long-range order in the tetrahedral networks of water molecules, which is responsible for the extremely low eutectic point and deep supercooling effects observed in the Ca(ClO4)2-H2O system.

Oligomer Electrolytes for Light-Emitting Electrochemical Cells: Influence of the End Groups on Ion Coordination, Ion Binding, and Turn-on Kinetics.

The electrolyte is an essential constituent of the light-emitting electrochemical cell (LEC), since its operating mechanism is dependent on the redistribution of mobile ions in the active layer. Recent developments of new ion transporters have yielded high-performance devices, but knowledge about the interactions between the ionic species and the ion transporters and the influence of these interactions on the LEC performance is lacking. We therefore present a combined computational and experimental effort that demonstrates that the selection of the end group in a star-branched oligomeric ion transporter based on trimethylolpropane ethoxylate has a paramount influence on the ionic interactions in the electrolyte and thereby also on the performance of the corresponding LECs. With hydroxyl end groups, the cation from the salt is strongly coordinated to the ion transporter, which leads to suppression of ion pairing, but the penalty is a hindered ion release and a slow turn-on for the LEC devices. With methoxy end groups, an intermediate coordination strength is seen together with the formation of contact ion pairs, but the LEC performance is very good with fast turn-on. Using a series of ion transporters with alkyl carbonate end groups, the ion transporter:cation coordination strength is lowered further, but the turn-on kinetics are slower than what is seen for devices comprising the methoxy end-capped ion transporter.

Mono- Vs. Di-Valent Solvated Cations Studied By Raman Spectroscopy

Raman spectroscopy is a powerful tool to study molecular level interactions in e.g. battery electrolytes. As cations and solvents interact, this affects the ion-pair formation and the nature of the solvation shells, which both are key factors affecting solution equilibria and ion transport. While Raman spectroscopy has been extensively used to study lithium-ion battery (LIB) electrolytes, studies on alternative battery technology electrolytes are scarce. Most concern sodium-ion batteries (SIB) and the interpretations are eased by the chemical similarity of Li and Na — both being monovalent. For divalent battery technologies such as Ca and Mg the differences in cation solvation can be expected to be larger, due to the fundamental differences between mono- and divalent ions, and prevents the full application of LIB electrolyte know-how and hence calls for further studies. The recently unveiled calcium battery technology [1] is today limited in scope of electrolytes, basically two compositions dominate [1, 2]. The lack of variety and understanding considerably slows down the Ca battery development, why more in depth analysis of the Ca2+ solvation is called for, to highlight issues hindering fast cation transport within the electrolytes. Here monovalent electrolytes composed of NaPF6 or NaTFSI dissolved in mixtures of ethylene carbonate (EC) and dimethyl carbonate (DMC), i.e. EC:DMC (1:1) were investigated for concentrations ranging from 0.3–1.0 M. They were selected based on the similarities with common LIB electrolytes either commercialized or extensively studied at the laboratory scale [3]. Divalent electrolytes composed of Ca(TFSI)2 and Ca(BF4)2 dissolved in a mixture of ethylene carbonate (EC) and propylene carbonate (PC), i.e. EC:PC (1:1) were chosen for their compatibility with available Ca electrodes [4, 5]. Both Raman spectroscopy and density functional theory calculations were employed to understand quantitatively how the speciation and solvation differ between these monovalent and divalent electrolytes, and ultimately how it could affect ion transport within both electrolytes and electrodes. In addition to the ion conductivities also the viscosities were measured to enable us to apply the fractional Walden rule [6] and qualitatively estimate the ion-pairing as function of both temperature and salt concentration. We observe that the solvation shell configurations change with the salt concentration for both multivalent and divalent formulations; the sizes of the divalent cationic complexes are nearly twice as large as the monovalent but the solvent coordination order is similar; EC tends to predominantly coordinate the cations, followed by PC or DMC. Finally, the Walden plots indicate that formation of contact ion-pairs (CIP) and/or aggregates (AGG) may be more likely for the BF4 based electrolytes, independent of the nature of the cation.

Effects of Surface Trapping and Contact Ion Pairing on Ion Transport in Nanopores

Ion transport in highly-confined space is important to various applications, such as biosensing and seawater desalination with nanopores. All-atom molecular dynamics simulations are conducted to investigate the transport of Na$^+$ and Cl$^-$ ions through nanopores with the diameter below 6 nm. It is found that the formation of the contact ion pair plays a critical role in reducing the ion mobility inside a nanopore without surface charges. The mobility for both cations and anions decreases with the reduced pore size because it is easier to form the contact ion pairs inside the neutral nanopore with smaller diameter. Inside a charged nanopores, besides the contact ion pair formation, the surface charges also play a significant role in reducing the counterion mobility through surface trapping. It is uncovered that the mobility of Na$^+$ ions increases first and then decreases with the surface charge density, while Cl$^-$ ions have the opposite trend. A modified first-passage time model is proposed to take into account the ion pair formation and the trapped ions inside a nanopore, which provides a clear picture in describing ion transport through a nanopore.

Influence of Kosmotropes and Chaotropes on the Krafft Temperature and Critical Micelle Concentration of Tetradecyltrimethylammonium Bromide in Aqueous Solution

The study aims to show the effect of some Hofmeister series anions on the Krafft temperature (TK) and the critical micelle concentration (CMC) of tetradecyltrimethylammonium bromide (TTAB) in aqueous solution. The TK of TTAB decreases in the presence of strong kosmotropes and moderate chaotropes but increases in the presence of strong chaotropes. Strong kosmotropes and moderate chaotropes remain hydrated in the bulk phase of the aqueous solution and promote dispersion of the surfactant. On the other hand, strong chaotropes form contact ion pairs with the cationic part of TTAB and exhibit salting out effects showing an increase in TK. The CMC of TTAB decreases in the presence of all ions irrespective of their chaotropic and kosmotropic nature. This can be ascribed to screening of the micelle surface charge by the added counterions. The standard Gibbs energy ( $$ \Delta G_{\text{m}}^{\text{o}} $$ ) of micellization was found to be negative, indicating spontaneity of the process. Solubilization of a water-insoluble dye, Sudan II in TTAB micelles, was quantified by the molar solubilization ratio (MSR). The MSR value was found to be higher in the presence of NaCl compared to that in pure water indicating that the added NaCl facilitated solubilization of the dye.

Photoinduced Proton Transfer of GFP-Inspired Fluorescent Superphotoacids: Principles and Design.

Proton transfer remains one of the most fundamental processes in chemistry and biology. Superphotoacids provide an excellent platform to delineate the excited-state proton transfer (ESPT) mechanism on ultrafast time scales and enable one to precisely control photoacidity and other pertinent functionalities such as fluorescence. We modified the GFP core ( p-HBDI chromophore) into two series of highly fluorescent photoacids by fluorinating the phenolic ring and conformationally locking the backbone (i.e., biomimetics). The trifluorinated derivatives, M3F and P3F, represent two of the strongest superphotoacids with p Ka* values of -5.0 and -5.5, respectively, and they can efficiently transfer a proton to organic solvents like methanol. Tunable femtosecond stimulated Raman spectroscopy (FSRS) and femtosecond transient absorption (fs-TA) were employed to dissect the ESPT of M3F and P3F in methanol, particularly with structural dynamics information. By virtue of resonantly enhanced FSRS signal and global analysis of fs-TA spectra, we revealed an inhomogeneous ESPT mechanism consisting of three parallel routes following the initial small-scale proton motion and contact ion-pair formation within ∼300 fs: The first route consists of ultrafast protolytic dissociation facilitated by the pre-existing, largely optimized H-bonding chain; the second route is limited by solvent reorientation that establishes a suitable H-bonding wire for proton separation; the third route is controlled by rotational diffusion that requires rotation of the anisotropically reactive photoacid in a bulky solvent with a complex H-bonding structure over larger distances. Furthermore, we provided new design principles of enhancing photoacidity in a synergistic manner: incorporating electron-withdrawing groups into proximal (often as "donor") and distal (often as "acceptor") ring moieties of the dissociative hydroxyl group to lower the ground-state p Ka and increase the Δp Ka, respectively.

The behavior of ions in water is controlled by their water affinity.

The strong, long-range electrostatic forces described by Coulomb's law disappear for ions in water, and the behavior of these ions is instead controlled by their water affinity - a weak, short-range force which arises from their charge density. This was established experimentally in the mid-1980s by size-exclusion chromatography on carefully calibrated Sephadex® G-10 (which measures the effective volume and thus the water affinity of an ion) and by neutron diffraction with isotopic substitution (which measures the density and orientation of water molecules near the diffracting ion and thus its water affinity). These conclusions have been confirmed more recently by molecular dynamics simulations, which explicitly model each individual water molecule. This surprising change in force regime occurs because the oppositely charged ions in aqueous salt solutions exist functionally as ion pairs (separated by 0, 1 or 2 water molecules) as has now been shown by dielectric relaxation spectroscopy; this cancels out the strong long-range electrostatic forces and allows the weak, short-range water affinity effects to come to the fore. This microscopic structure of aqueous salt solutions is not captured by models utilizing a macroscopic dielectric constant. Additionally, the Law of Matching Water Affinity, first described in 1997 and 2004, establishes that contact ion pair formation is controlled by water affinity and is a major determinant of the solubility of charged species since only a net neutral species can change phases.

Structures and dynamic properties of the LiPF6 electrolytic solution under electric fields - a theoretical study.

Fluorinated carbonates have attracted increasing attention in high-voltage lithium ion battery applications. Under free and electric fields, their structure-related solvent dynamic properties such as charge and discharge rate, however, are rarely reported. Herein, solutions including linear fluorinated carbonates have been simulated using joint MD/DFT calculations. For the first time, the dielectric constants of the four pure fluorinated carbonates have been predicted to be ca. 5.4 (4: ethyl-2,2,2-trifluoroethyl carbonate)-12.1 (7: di-1,1,2,2,2-pentafluoroethyl carbonate), which are dependent on the number of fluorine atoms. Minor difference in the solvation free energies and few contact ion associations were observed in the ETFEC and DTFEC solvents. Their comparable viscosity values were found through calculations of ion diffusivity and conductivity. In the electric fields, the Li+ mobilities in the two solutions have similar values, showing that the dynamic properties of the electrolytic solutions are almost independent of the number of fluorine atoms on these carbonates. There emerges an oriented and ordered arrangement of the solvent molecules and thus the largely decreased dielectric constants under the electric fields, even though the field strength is very low. These interesting phenomena should be relevant in the formation of contact and aggregate ion pairs, leading to unexpected reduction of the charge-discharge rate-related ion mobility in mixed solvents.

Hydration and ion association of aqueous choline chloride and chlorocholine chloride.

The choline ion (Ch+) is ubiquitous in nature and also its synthetic homologue, chlorocholine (ClCh+), is widely used. Nevertheless, surprisingly little information on the hydration and counter-ion binding of these cations can be found in the literature. In this contribution we report effective hydration numbers, determined by dielectric relaxation spectroscopy, and ion-pair association constants with Cl-, determined by dilute-solution conductivity measurements. In combination with RISM calculations the obtained data suggest that for Ch+ water is bound to the hydroxy group via hydrogen bonds whereas for ClCh+ a rather stiff clathrate-like shell around the chlorine atom seems to be formed. With Cl- both cations form contact ion pairs with association constants of only ∼2 to 3 M-1.

Interactions of Some Hofmeister Cations with Sodium Dodecyl Sulfate in Aqueous Solution

AbstractIn this work, we have investigated the influence of some alkali metal ions on the Krafft temperature (TK) and critical micelle concentration (CMC) of a classical ionic surfactant, sodium dodecyl sulfate (SDS), over a wide range of temperature. The alkali metal cations such as Li+, Na+, Cs+, and K+ are found to affect the solubility and hence the TK of the surfactant. It was observed that kosmotropic Li+ lowers the TK of the surfactant. Due to the common ion effect, the solubility of SDS decreases in the presence of Na+, resulting in an increase in the TK. On the other hand, chaotropic K+ and Cs+, capable of forming contact ion pairs with the chaotropic dodecyl sulfate ion, lower the solubility and hence elevate the TK. In terms of decreasing the TK, the ions follow the trend: Li+ > Na+ > Cs+ > K+ except for 0.0025 M CsCl. The added cations screen the charge of the micelle surface and facilitate closer packing of the surfactant with a consequent decrease in the CMC. In terms of the effectiveness in lowering the CMC, the ions follow the order: Cs+ > K+ > Na+ > Li+. In the presence of added electrolytes, the γCMC values are found to be lower than the corresponding values in pure water. The thermodynamic parameters (Gibbs free energy, enthalpy, and entropy changes) of micellization were calculated to gain insights into the mechanism of the process.

Structure of alkaline aqueous NaBH4 solutions by X-ray scattering and empirical potential structure refinement

Structure of alkaline aqueous NaBH4 solutions at NaBH4 concentrations of 1.0 to 9.1 mol·dm−3 (almost saturated) as well as a 1.3 mol·dm−3 NaOH solution was studied by X-ray diffraction and Empirical Potential Structure Refinement (EPSR) modelling. In the 1.0 mol dm−3 alkaline NaBH4 solution, Na+ is surrounded by around six water molecules with octahedral geometry. With an increase in NaBH4 concentration, the hydration number of Na+ decreases from 4.8 ± 1.1 at 2.25 mol·dm−3 solution to 2.9 ± 1.3 at 9.1 mol·dm−3 with an almost constant Na+-O (W: H2O) distance of 2.34 Å. The decrease in hydration number for Na+ is compensated by the formation of ion pairs between Na+ and BH4− to ensure an octahedral hydrated geometry for Na+. About 6.0 ± 1.6 water molecules are likely to bond to BH4−via tetrahedral edges or tetrahedral corners without a very specific hydration geometry, giving rise to each hydrogen atom of BH4− bound to 2.3 ± 1.0 water molecules through dihydrogen bonds. The hydration number around BH4− decreases with increasing concentration. BH4− tends to form contact ion pairs with Na+ at the Na-B distances of 3.24 Å and 2.82 Å in tetrahedral-corner-shared and tetrahedral-edge-shared fashions, respectively; the number of Na-B interactions increases from 0.3 ± 0.5 in 1.0 mol·dm−3 (almost negligible) to 2.1 ± 0.9 in 9.0 mol·dm−3. Tetrahedral-edge-shared bidentate ion pair is likely to be the preferred and dominant ion cluster over the wide concentration range. Ion association between the Na+ and BH4− was crosschecked by density function theory (DFT) calculations on [NaBH4(H2O)6] clusters.

Solvent and Salt Effect on Lithium Ion Solvation and Contact Ion Pair Formation in Organic Carbonates: A Quantum Chemical Perspective

Quantum chemical calculations have been employed to investigate the solvation of lithium cations in ethylene carbonate/propylene carbonate and propylene carbonate/dimethyl carbonate mixed electrolytes. The impact of the presence of the counteranion on the solvation of Li+ in pure propylene carbonate and dimethyl carbonate was also studied. The calculations revealed small free-energy changes for the transitions between different preferred structures in mixed solvents. This implies that transitions between distinct local arrangements can take place in the mixtures. The addition of dimethyl carbonate causes a significant increase of the dipole moment of solvation clusters, indicating important molecular-scale modifications when dimethyl carbonate is used as a co-solvent. The presence of an anion in the solvation shell of Li+ modifies the intermolecular structure comprising four carbonate molecules in dilute solutions, allowing only two carbonate molecules to coordinate to Li+. The bidentate complexation of Li+ with the anion’s electron donor atoms, however, maintains the local tetrahedral structure on interatomic length scales. The neutralization of the solvation shell of Li+ due to contact ion pair formation and the consequent implications on the underlying mechanisms provide a rational explanation for the ionic conductivity drop of electrolyte solutions at high salt concentrations.

Photoinduced electronic interactions between acridine derivatives and small gold nanoparticles: A spectroscopic insight

The central tricyclic moiety of acridine derivatives has fascinating redox properties. We take advantage of the electronic properties of two such acridine derivatives, 9-amino acridine (9AA-HCl) and acridine yellow, recognized so far as potential antimicrobial agents, since these drugs can act both as potential electron donor and acceptor, depending upon the reduction potential of its adjacent chemical entity. Precise timescale of formation of contact ion pair (CIP) and solvent separated ion pair in solution involving donors and acceptors has been much encountered and debated in literature till date. In our present communication, we like to decipher the mechanism and timescale of individual steps of photoinduced electron transfer (PET) between these drugs and gold nanoparticles (AuNPs) with dimension of 3-5 nm, which is important and presently our subject of interest since AuNPs could be used for shuttling of electrons between different donor and acceptor moieties. Steady-state and time-resolved absorption and fluorescence studies evidently reveal the signature of PET both in singlet and triplet states. Using ultrafast time-resolved fluorescence techniques we have tried to quantify precisely the time required for formation of CIP which is within fs-ps time regime. Moreover, 9AA-HCl, being a planar molecule, dimerises at excited state in presence of high concentration of AuNPs, due to its immobilisation over their surface. The occurrence of excited state proton transfer in triplet state for 9AA-HCl-AuNPs conjugate is depicted by laser flash photolysis experiments. The presence of all such charge transfer channels, provokes us to delineate the intricacies of the mechanisms involved which would help further to evaluate the possibility of making devices for energy storage using these drugs.

Contact ion-pair structure in concentrated cesium chloride aqueous solutions: An extended X-ray absorption fine structure study

The hydration and ion pair structure of Cl− ion in aqueous cesium chloride solutions at 0.5, 6 and 11 mol/kg (H2O) has been studied using Extended X-ray Absorption Fine Structure (EXAFS) at the Cl K-edge. The co-refinement of multiple EXAFS datasets shows that the hydration structure of the free Cl− ion is consistent with previously reported studies. In concentrated solution the Cl− ion is strongly associated with the cation forming contact ion pairs with 1.6 and 2.8 Cs ions in 6 and 11 mol/kg aqueous CsCl, respectively. Together with ion-pairing structure of RbCl and RbBr, reported in our previous work, these results for the Cs-Cl ion-pair structure provide a more general view of ion-pairing in concentrated aqueous alkali halide solutions. The reported Cs-Cl ion-pair structure provides a basis for the development and testing of improved interaction potentials leading to more realistic structures from molecular dynamics (MD) simulations.