Exploring the capabilities and limitations of the Van Hove function to understand directional correlations in ion movements within Li-ion battery electrolytes.

Understanding microscopic directional correlations in ion movements within lithium-ion battery (LIB) electrolytes is important because these correlations directly affect the ionic conductivity. Onsager transport coefficients are widely used to understand these correlations. On the other hand, the Van Hove function (VHF) is also capable of determining correlated motions. However, identifying various types of ion correlated motions in LIB electrolytes using VHF is not well explored. Here, we have conducted molecular dynamics simulations of a representative experimental LIB electrolyte system-lithium hexafluorophosphate (LiPF6)-at different concentrations in a (9:1 wt. %) mixture of ethyl methyl carbonate and fluoroethylene carbonate in order to explore the capabilities and limitations of using VHF to understand different types of ion correlations. We conclude that analysis of VHF can qualitatively describe both the positive correlation between cation-anion at different salt concentrations and the negative correlation between cation-cation and anion-anion present in high salt concentration, but it cannot foretell which correlation is dominating at any given electrolyte concentration. This type of quantitative information can be obtained only via Onsager's approach. This could be seen as a limitation of relying solely on VHF to fully understand ion correlation in electrolyte media.

Temperature-Dependent Dielectric Relaxation Measurements of (Betaine + Urea + Water) Deep Eutectic Solvent in Hz-GHz Frequency Window: Microscopic Insights into Constituent Contributions and Relaxation Mechanisms.

A combined experimental and simulation study of dielectric relaxation (DR) of a deep eutectic solvent (DES) composed of betaine, urea, and water with the composition [Betaine:Urea:Water = 11.7:12:1 (weight ratio) and 9:18:5 (molar ratio)] was performed to explore and understand the interaction and dynamics of this system. Temperature-dependent (303 ≤ T/K ≤ 343) measurements were performed over 9 decades of frequency, combining three different measurement setups. Measured DR, comprising four distinct steps with relaxation times spreading over a few picoseconds to several nanoseconds, was found to agree well with simulations. The simulated total DR spectra, upon dissection into three self (intraspecies) and three cross (interspecies) interaction contributions, revealed that the betaine-betaine self-term dominated (∼65%) the relaxation, while the urea-urea and water-water interactions contributed only ∼7% and ∼1%, respectively. The cross-terms (betaine-urea, betaine-water, and urea-water) together accounted for <30% of the total DR. The slowest DR component with a time constant of ∼1-10 ns derived dominant contribution from betaine-betaine interactions, where betaine-water and urea-water interactions also contributed. The subnanosecond (0.1-0.6 ns) time scale originated from all interactions except betaine-water interaction. An extensive interaction of water with betaine and urea severely reduced the average number of water-water H-bonds (∼0.7) and heavily decreased the static dielectric constant of water in this DES (εs ∼ 2). Furthermore, simulated first rank collective single particle reorientational relaxations (C1(t)) and the structural H-bond fluctuation dynamics (CHB (t)) exhibited multiexponential kinetics with time scales that corresponded well with those found both in the simulated and measured DR.

Structurally Dynamic Monocyte-Liposome Hybrid Vesicles as an Anticancer Drug Delivery Vehicle: A Crucial Correlation of Microscopic Elasticity and Ultrafast Dynamics.

A biomimetic cell-based carrier system based on monocyte membranes and liposomes has been designed to create a hybrid "Monocyte-LP" which inherits the surface antigens of the monocytes along with the drug encapsulation property of the liposome. Förster resonance energy transfer (FRET) and polarization gated anisotropy measurements show the stiffness of the vesicles obtained from monocyte membranes (Mons), phosphatidylcholine membranes (LP), and Monocyte-LP to follow an increasing order of Mons > Monocyte-LP > LP. The dynamics of interface bound water molecules plays a key role in the elasticity of the vesicles, which in turn imparts higher delivery efficacy to the hybrid Monocyte-LP for a model anticancer drug doxorubicin than the other two vesicles, indicating a critical balance between flexibility and rigidity for an efficient cellular uptake. The present work provides insight on the influence of elasticity of delivery vehicles for enhanced drug delivery.

Temperature-dependent dielectric relaxation measurements of (acetamide + K/Na SCN) deep eutectic solvents: Decoding the impact of cation identity via computer simulations.

The impact of successive replacement of K+ by Na+ on the megahertz-gigahertz polarization response of 0.25[fKSCN + (1 - f)NaSCN] + 0.75CH3CONH2 deep eutectic solvents (DESs) was explored via temperature-dependent (303 ≤ T/K ≤ 343) dielectric relaxation (DR) measurements and computer simulations. Both the DR measurements (0.2 ≤ ν/GHz ≤ 50) and the simulations revealed multi-Debye relaxations accompanied by a decrease in the solution static dielectric constant (ɛs) upon the replacement of K+ by Na+. Accurate measurements of the DR response of DESs below 100 MHz were limited by the well-known one-over-frequency divergence for conducting solutions. This problem was tackled in simulations by removing the zero frequency contributions arising from the ion current to the total simulated DR response. The temperature-dependent measurements revealed a much stronger viscosity decoupling of DR times for Na+-containing DES than for the corresponding K+ system. The differential scanning calorimetry measurements indicated a higher glass transition temperature for Na+-DES (∼220 K) than K+-DES (∼200 K), implying more fragility and cooperativity for the former (Na+-DES) than the latter. The computer simulations revealed a gradual decrease in the average number of H bonds (⟨nHB⟩) per acetamide molecule and increased frustrations in the average orientational order upon the replacement of K+ by Na+. Both the measured and simulated ɛs values were found to decrease linearly with ⟨nHB⟩. Decompositions of the simulated DR spectra revealed that the cation-dependent cross interaction (dipole-ion) term contributes negligibly to ɛs and appears in the terahertz regime. Finally, the simulated collective single-particle reorientational relaxations and the structural H-bond fluctuation dynamics revealed the microscopic origin of the cation identity dependence shown by the measured DR relaxation times.

Identical Diffusion Distributions and Co-Cluster Formation Dictate Azeotrope Formation: Microscopic Evidences and Experimental Signatures.

What selects azeotropic pairs and governs the azeotropic conditions (composition and temperature) is an open and intriguing question. A combined simulation and experimental work presented here investigates this by considering ethanol-water mixtures. We find identical distributions of center-of-mass diffusion coefficients for ethanol and water molecules under the azeotropic condition (95.5 wt % ethanol +4.5 wt % water, Tazeo = 351.1K). Moreover, the particle displacements show strong interspecies correlations at Tazeo. Interestingly, simulated reorientation time distributions become identical at Tazeo but at a composition different from that at which the translational diffusion distributions overlapped. Cluster analyses indicate that solutions at Tazeo with xwater ≤ 15 wt % are more microheterogeneous than those with higher water content, although no anomaly in the composition-dependent solution structural properties was detected. Ethanol-water and ethanol-ethanol interaction energies show pronounced nonideal composition dependence, but the size of the relative fluctuations in them remained small (∼0.5kBT). Rare water-water H-bonding, predominant water-ethanol H-bonding, and a sizable population of "free" water molecules characterize the azeotropic solutions. The red edge excitation spectroscopic (REES) measurements with a dissolved anionic fluorescent dye, coumarin343 (C343), support the predicted solution microheterogeneity by showing a nonmonotonic composition dependence of the excitation energy-induced changes in the fluorescence emission spectral frequencies and bandwidths, the largest changes being under the azeotropic condition. Subsequent dynamic anisotropy measurements reveal a nonmonotonic composition dependence of C343 rotation times with a peak under the azeotropic condition. In summary, equalization of the component translational diffusion coefficients and solution microheterogeneity with regular composition dependence of the solution structure appear to characterize the ethanol-water azeotrope.

Dielectric relaxation and dielectric decrement in ionic acetamide deep eutectic solvents: Spectral decomposition and comparison with experiments.

Frequency-dependent dielectric relaxation in three deep eutectic solvents (DESs), (acetamide+LiClO4/NO3/Br), was investigated in the temperature range, 329 ≤ T/K ≤ 358, via molecular dynamics simulations. Subsequently, decomposition of the real and the imaginary components of the simulated dielectric spectra was carried out to separate the rotational (dipole-dipole), translational (ion-ion), and ro-translational (dipole-ion) contributions. The dipolar contribution, as expected, was found to dominate all the frequency-dependent dielectric spectra over the entire frequency regime, while the other two components together made tiny contributions only. The translational (ion-ion) and the cross ro-translational contributions appeared in the THz regime in contrast to the viscosity-dependent dipolar relaxations that dominated the MHz-GHz frequency window. Our simulations predicted, in agreement with experiments, anion-dependent decrement of the static dielectric constant (ɛs ∼ 20 to 30) for acetamide (ɛs ∼ 66) in these ionic DESs. Simulated dipole-correlations (Kirkwood g factor) indicated significant orientational frustrations. The frustrated orientational structure was found to be associated with the anion-dependent damage of the acetamide H-bond network. Single dipole reorientation time distributions suggested slowed down acetamide rotations but did not indicate presence of any "rotationally frozen" molecule. The dielectric decrement is, therefore, largely static in origin. This provides a new insight into the ion dependence of the dielectric behavior of these ionic DESs. A good agreement between the simulated and the experimental timescales was also noticed.

Modulatory role of copper on hIAPP aggregation and toxicity in presence of insulin.

Aggregation of the human islets amyloid polypeptide, or hIAPP, is linked to ß-cell death in type II diabetes mellitus (T2DM). Different pancreatic ß-cell environmental variables such as pH, insulin and metal ions play a key role in controlling the hIAPP aggregation. Since insulin and hIAPP are co-secreted, it is known from numerous studies that insulin suppresses hIAPP fibrillation by preventing the initial dimerization process. On the other hand, zinc and copper each have an inhibitory impact on hIAPP fibrillation, but copper promotes the production of toxic oligomers. Interestingly, the insulin oligomeric equilibrium is controlled by the concentration of zinc ions when the effect of insulin and zinc has been tested together. Lower zinc concentrations cause the equilibrium to shift towards the monomer and dimer states of insulin, which bind to monomeric hIAPP and stop it from developing into a fibril. On the other hand, the combined effects of copper and insulin have not yet been studied. In this study, we have demonstrated how the presence of copper affects hIAPP aggregation and the toxicity of the resultant conformers with or without insulin. For this purpose, we have used a set of biophysical techniques, including NMR, fluorescence, CD etc., in combination with AFM and cell cytotoxicity assay. In the presence and/or absence of insulin, copper induces hIAPP to form structurally distinct stable toxic oligomers, deterring the fibrillation process. More specifically, the oligomers generated in the presence of insulin have slightly higher toxicity than those formed in the absence of insulin. This research will increase our understanding of the combined modulatory effect of two ß-cell environmental factors on hIAPP aggregation.

Molecular Thermodynamic Origin of Substrate Promiscuity in the Enzyme Laccase: Toward a Broad-Spectrum Degrader of Dye Effluents.

Industrial dye effluents have emerged as significant health hazard. Laccases found in white rot fungi can degrade an assortment of dyes. Here, we explore the molecular thermodynamic origin of the substrate promiscuity in laccases using a combination of steady-state UV-visible absorption spectroscopy, molecular docking, and molecular dynamics (MD) simulation studies on the interaction of laccase with five dye molecules with varying charge, size, and shape. The spectroscopic studies confirm that all of these dyes can be degraded by laccase. Using MD simulations, we have demonstrated the presence of various distinct conformations of a loop in the protein active site that can accommodate the wide range of dye molecules. We have also shown that the diverse selection of dye molecules may exhibit surprisingly similar binding affinity due to cancellation of different thermodynamic factors. Our results highlight the potential of laccase as a multipurpose degrader for industrial dye effluents.

Difference in "Supercooling" Affinity between (Acetamide + Na/KSCN) Deep Eutectics: Reflections in the Simulated Anomalous Motions of the Constituents and Solution Microheterogeneity Features.

Deep depression of freezing points of ionic amide deep eutectic solvents (DESs) is known to exhibit a significant dependence on the identity of ions present in those systems and the nature of the functional group attached to the host amide. This deep depression of the freezing point is sometimes termed as "supercooling". For (acetamide + electrolyte) DESs, experiments have revealed signatures of ion-dependent spatiotemporal heterogeneity features. The focus of this work is to provide microscopic explanations of these experimentally observed macroscopic system properties in terms of particle jumps and insights about the origin of the cation dependence. For this purpose, extensive molecular dynamics simulations have been performed employing (acetamide + Na/KSCN) deep eutectics as representative ionic systems at 303, 318, 333, and 348 K. The individual translational motions of acetamide and the ions are followed, and their connections to solution heterogeneity are explored. The center-of-mass motion for Na+ has been found to be more anomalous than that for K+. This difference corroborates well with experimental reports on heterogeneous relaxations in these systems. Simulated viscosity coefficients and dynamic heterogeneity features also reflect this difference. Moreover, simulated reorientational relaxations of acetamide molecules in these ionic DESs suggest that a Na+-containing DES is more heterogeneous than the corresponding K+-containing system. Estimated void and neck distributions for acetamide molecules differ as the alkali metal ions differ. In brief, this study provides a detailed microscopic view of the cation dependence of the microheterogeneous relaxation dynamics of these DESs reported repeatedly by different experiments.

Mechanistic insight into functionally different human islet polypeptide (hIAPP) amyloid: the intrinsic role of the C-terminal structural motifs.

Targeting amyloidosis requires high-resolution insight into the underlying mechanisms of amyloid aggregation. The sequence-specific intrinsic properties of a peptide or protein largely govern the amyloidogenic propensity. Thus, it is essential to delineate the structural motifs that define the subsequent downstream amyloidogenic cascade of events. Additionally, it is important to understand the role played by extrinsic factors, such as temperature or sample agitation, in modulating the overall energy barrier that prompts divergent nucleation events. Consequently, these changes can affect the fibrillation kinetics, resulting in structurally and functionally distinct amyloidogenic conformers associated with disease pathogenesis. Here, we have focused on human Islet Polypeptide (hIAPP) amyloidogenesis for the full-length peptide along with its N- and C-terminal fragments, under different temperatures and sample agitation conditions. This helped us to gain a comprehensive understanding of the intrinsic role of specific functional epitopes in the primary structure of the peptide that regulates amyloidogenesis and subsequent cytotoxicity. Intriguingly, our study involving an array of biophysical experiments and ex vivo data suggests a direct influence of external changes on the C-terminal fibrillating sequence. Furthermore, the observations indicate a possible collaborative role of this segment in nucleating hIAPP amyloidogenesis in a physiological scenario, thus making it a potential target for future therapeutic interventions.

Dynamical Anomaly of Aqueous Amphiphilic Solutions: Connection to Solution H-Bond Fluctuation Dynamics?

We have investigated the possible connection between "dynamical anomaly" observed in time-resolved fluorescence measurements of reactive and nonreactive solute-centered relaxation dynamics in aqueous binary mixtures of different amphiphiles and the solution intra- and interspecies H-bond fluctuation dynamics. Earlier studies have connected the anomalous thermodynamic properties of binary mixtures at very low amphiphile concentrations to the structural distortion of water. This is termed as "structural anomaly." Interestingly, the abrupt changes in the composition-dependent average rates of solute relaxation dynamics occur at amphiphile mole fractions approximately twice as large as those where structural anomalies appear. We have investigated this anomalous solution dynamical aspect by considering (water + tertiary butanol) as a model system and performed molecular dynamics simulations at several tertiary butanol (TBA) concentrations covering the extremely dilute to the moderately concentrated regimes. The "dynamical anomaly" has been followed via monitoring the composition dependence of the intra- and interspecies H-bond fluctuations and reorientational relaxations of TBA and water molecules. Solution structural aspects have been followed via examining the tetrahedral order parameter, radial and spatial distribution functions, numbers of H bonds per water and TBA molecules, and the respective populations participating in H-bond formation. Our simulations reveal abrupt changes in the H-bond fluctuations and reorientational dynamics and tetrahedral order parameter at amphiphile concentrations differing approximately by a factor of 2 and corroborates well with the steady-state and the time-resolved spectroscopic measurements. This work therefore explains, following a uniform and cogent manner, both the experimentally observed structural and dynamical anomalies in microscopic terms.

Temperature-Dependent Dielectric Relaxation in Ionic Acetamide Deep Eutectics: Partial Viscosity Decoupling and Explanations from the Simulated Single-Particle Reorientation Dynamics and Hydrogen-Bond Fluctuations.

We report here temperature-dependent (293 ≤ T (K) ≤ 336) dielectric relaxation (DR) measurements of (acetamide + LiBr/NO3-/ClO4-) deep eutectic solvents (DESs) in the frequency window of 0.2 ≤ ν (GHz) ≤ 50 and explore, via molecular dynamics simulations, the relative roles for the collective single-particle reorientational relaxations and the H-bond dynamics of acetamide in the measured DR response. In addition, DR measurements of neat molten acetamide were performed. Recorded DR spectra of these DESs require multi-Debye fits and produce well-separated DR time scales that are spread over several picoseconds to ∼1 ns. Simulations suggest DR time scales derive contributions from both the collective reorientational (Cl(t)) relaxation and structural H-bond (CHB(t)) dynamics of acetamide. A good correlation between the measured and simulated activation energies further reveals a strong connection between the measured DR and the simulated Cl(t) and CHB(t). Average DR times exhibit a strong fractional viscosity dependence, suggesting substantial microheterogeneity in these media. Simulations of Cl(t) and CHB(t) reveal strong stretched exponential relaxations with a stretching exponent, 0.4 ≤ ß ≤ 0.7. The ratio between the average reorientational correlation times of first and second ranks, ⟨τ⟩l=1/⟨τ⟩l=2, deviates appreciably from Debye's l(l+1) law for homogeneous media. Importantly, a pronounced translation-rotation decoupling between the simulated reorientation and center-of-mass diffusion times was observed.

Heterogeneous Orientational Relaxations and Translation-Rotation Decoupling in (Choline Chloride + Urea) Deep Eutectic Solvents: Investigation through Molecular Dynamics Simulations and Dielectric Relaxation Measurements.

Molecular dynamics simulations and dielectric relaxation (DR) measurements in the frequency window, 0.2 ≤ ν/GHz ≤ 50, have been performed to explore the heterogeneous reorientation dynamics in [f choline chloride + (1 - f) urea] deep eutectic solvents (DESs) at f = 0.33 and 0.40 in the temperature range 293 ≤ T/K ≤ 333. The solution viscosity varies by more than an order of magnitude. DR measurements in these DESs reveal multiple relaxation timescales-τ1 ∼ 500 ps, τ2 ∼ 100 ps, τ3 ∼ 30 ps, and τ4 ∼ 5 ps. Simulated rank-dependent collective single-particle reorientational (Cl(t), l being the rank) and structural H-bond [CHB(t)] relaxations can explain the microscopic origin of all these DR timescales. The average DR times, ⟨τDR⟩, exhibit a pronounced fractional viscosity dependence, ⟨τDR⟩ ∝ (η/T)p, with p = 0.1. This experimental evidence of pronounced heterogeneous reorientation dynamics in these DESs is supported by a strong translation-rotation decoupling and a significant deviation of the average reorientational correlation times (⟨τl⟩) from Debye's l(l + 1) law. The simulated ratios between the average rotation and translation timescales for both urea and choline correctly reduce to the appropriate hydrodynamic limit at high temperatures. The stretched exponential relaxations of the simulated self-dynamic structure factors and the non-Gaussian single-particle displacement distributions further support strong temporal heterogeneity in these DESs. Dynamic susceptibilities from the simulated four-point correlations exhibit long correlated timescales. Moreover, simulated activation energies estimated from the temperature-dependent C1(t) decays and the translational diffusion coefficients from the velocity autocorrelation functions agree favorably with those from the corresponding DR and the pulsed field gradient nuclear magnetic resonance measurements.

Water in biodegradable glucose-water-urea deep eutectic solvent: modifications of structure and dynamics in a crowded environment.

Molecular dynamics simulations have been performed on a highly viscous (η ∼ 255 cP) naturally abundant deep eutectic solvent (NADES) composed of glucose, urea and water in a weight ratio of 6 : 4 : 1 at 328 K. The simulated system contains 66 glucose, 111 water and 133 urea molecules. A neat system with 256 water molecules has also been simulated. In this study, the water structure and dynamics in a crowded environment have been investigated by computing inter-species radial distribution functions (RDFs), quantitative and qualitative analyses of intra-species water H-bonds, heterogeneity timescales from the anomalous mean square displacements, and two-point and four-point density-time correlation functions. The simulated structures indicate asymmetric interactions between water and glucose molecules, and considerable water-clustering. In addition, a dramatic distortion of the orientational order has been reflected. A severe decrease in the average number of water-water H-bonds and the corresponding participation of water molecules have been detected, although the water H-bond length distribution does not differ much from that for the neat system. The participation populations of water for H-bonding with itself and the other two species have been expressed by constructing a pi-chart. Only ∼16% of the total water molecules have been found to be simultaneously H-bonded with glucose and urea molecules. A qualitative picture of water clustering has been proposed through the interpretation of the observed drastic deviation of water angle distributions. Centre-of-mass translations and structural H-bond relaxations have been found to be significantly slowed down relative to those in neat water. Evidence of hop-trap movements for DES water has been found.

Interactions and Dynamics in Aqueous Solutions of pH-Responsive Polymers: A Combined Fluorescence and Dielectric Relaxation Study.

Interaction and dynamics of aqueous solutions of pH-responsive smart polymers are investigated via steady-state, time-resolved fluorescence emission spectroscopy with the help of external local reporter coumarin 153 (C153), while MHz to GHz dielectric relaxation spectroscopic (DRS) measurement reports the intrinsic medium relaxation features. A series of pH-responsive random copolymers (DPL-DP60) comprising of a pH-responsive moiety 2-((leucinyl)oxy)ethyl methacrylate (l-Leu-HEMA) and hydrophobic methyl methacrylate (MMA) are synthesized and characterized. A balance between the pH-responsive (l-Leu-HEMA) and the hydrophobic (MMA) content dictates the phase transition pH, which is found to be ∼5-7 for these aqueous copolymer solutions (1 mg/mL). Dynamic light scattering measurements in aqueous solutions of these polymers reflect a small particle size (∼2-8 nm) at solution pH below their individual phase transition pH, while a large particle size (∼140-340 nm) forms beyond their phase transition pH. No signature of a phase transition pH-driven abrupt change in static and dynamic properties of aqueous polymer solutions has been registered from pH-dependent dielectric relaxation as well as solute (C153)-centric fluorescence measurements. A significant impact of varying the l-Leu-HEMA/MMA segment ratio on steady-state fluorescence emission and rotational anisotropy decay of the fluorophore solute (C153) has been observed. MHz to GHz DRS in aqueous solutions of these pH-responsive polymers reflects bulk water-like dielectric features.

Heterogeneous dynamics, correlated time and length scales in ionic deep eutectics: Anion and temperature dependence.

Heterogeneous relaxation dynamics often characterizes deep eutectic solvents. Extensive and molecular dynamics simulations have been carried out in the temperature range, 303 ≤ T/K ≤ 370, for studying the anion and temperature dependencies of heterogeneous dynamics of three different ionic acetamide deep eutectics: acetamide + LiX, X being bromide (Br-), nitrate (NO3 -), and perchlorate (ClO4 -). These systems are chosen because the fractional viscosity dependence of average relaxation rates reported by various measurements has been attributed to the heterogeneous dynamics of these systems. Simulations performed here attempt to characterize the heterogeneous relaxation dynamics in terms of correlated time and length scales and understand the solution inhomogeneity in microscopic terms. Additionally, simulation studies for pure molten acetamide have been performed to understand the impact of ions on motional features of acetamide in these ionic deep eutectic systems. The computed radial distribution functions suggest microheterogeneous solution structure and dependence upon anion identity and temperature. A significant plateau in the simulated time dependent mean squared displacements indicates pronounced cage-rattling and inhomogeneity in relaxation dynamics. Simulated diffusion coefficients for acetamide and ions show decoupling from the simulated viscosities of these deep eutectics. Calculated two- and four-point correlation functions reveal the presence of dynamic heterogeneity even at ∼180 K above the measured thermodynamic glass transition temperature (Tg). Further analyses reveal the existence of multiple timescales that respond strongly to the rise in solution temperature. The simulated dynamic structure factor and overlap function relaxations show strong stretched exponential relaxations. The simulation results support the experimental observation that the bromide system is the most dynamically heterogeneous among these three systems. Correlated length scales show much weaker anion and temperature dependencies with an estimated length of ∼1 nm, suggesting formation of clusters at the local level as the origin for the micro-heterogeneous nature of these ionic deep eutectics.

Does Confinement Modify Preferential Solvation and H-Bond Fluctuation Dynamics? A Molecular Level Investigation through Simulations of a Bulk and Confined Three-Component Mixture.

Exploring the local environment around a dissolved solute in a bulk aqueous solution of alcohol and assessing the impact of confinement on the solvation structure is an important topic yet is much less studied. Such a study is important because it can provide critical information regarding the miscibility of an amphiphilic drug after delivery at a designated nanoscopic site and the subsequent release. The present molecular dynamics simulation study reports an in-depth investigation of the composition-dependent solvation structure around a dissolved hydrophobic solute, coumarin 153 (C153), in ambient binary mixtures of methanol and water in both bulk and under confinement. The confinement is a spherical sodium bis(2-ethylhexyl) sulfosuccinate (AOT) reverse micelle with a diameter of 55 Å. Inter- and intraspecies H-bond fluctuation dynamics have been monitored and compared with those from the corresponding bulk binary mixtures. A systematic comparison of both solvation structure and H-bond dynamics between confined and bulk binary mixtures reveals modulation of both preferential solvation and H-bond relaxation times inside a nanoscopic environment. More specifically, confinement accentuates the preferential solvation phenomenon and facilitates di-mixing of mixture components. In addition, the present study reveals that the tetrahedral H-bond network of neat liquid water becomes severely affected upon addition of methanol, which becomes further distorted under confinement. Confinement severely affects the interspecies hydrogen bonds and makes the corresponding continuous hydrogen bonds much shorter-lived. Interestingly, structural hydrogen bond relaxation timescales become longer in confined binary mixtures than those in bulk binary mixtures.

Dynamic Susceptibility and Structural Heterogeneity of Large Reverse Micellar Water: An Examination of the Core-Shell Model via Probing the Layer-wise Features.

In this work, we explored, via molecular dynamics simulations, layer-wise structural and spatio-temporal heterogeneity features of confined water inside rigid spherical reverse micelles of 55 Å inner diameter. These confined aqueous pools were divided into four fictitious concentric layers of 5 Å thickness and a central core layer. Reverse micellar confinements were constructed using model potentials mimicking AOT (charged) and IGEPAL (neutral) surfactant molecules for encapsulating SPC/E water. Density profiles for confined water were obtained and compared to validate the present simulations. The simulated layer-wise structural features were: dipole orientation distributions, tetrahedral angle distributions, tetrahedral order parameter, and the average number of H-bonds per water molecule and the relevant population distributions. Simulated dynamical features included mean-square displacements, velocity autocorrelation functions, non-Gaussian parameters, single-particle displacement distributions, dynamic susceptibilities, and the collective single-particle reorientational relaxations of first and second ranks. Analyses of simulation results revealed a strong impact of the confinement on bulk water structure and dynamics. The chemical nature of the confinement was found to influence both structure and dynamics. Interfacial water molecules were found to be the most severely affected ones, and the successive progression toward the center revealed a tendency for restoration of the bulk limit, although the bulk values were never fully recovered. A close inspection of the simulated results revealed an overlap among the layer-wise structural and dynamical features. These observations suggest a breakdown of the two-state core-shell model even for large reverse micelles (RMs) where an ample amount of "free" water is available. The simulated collective reorientational relaxations of reverse micellar water agree well with the existing time-resolved two-dimensional infrared (2D-IR) measurements.

Subpicosecond Solvation Response and Partial Viscosity Decoupling of Solute Diffusion in Ionic Acetamide Deep Eutectic Solvents: Fluorescence Up-Conversion and Fluorescence Correlation Spectroscopic Measurements.

Fluorescence up-conversion (∼250 fs instrumental response) coupled with time correlated single photon counting measurements was performed to explore the complete Stokes shift dynamics of a dipolar solute probe, coumarin 153 (C153), in several ionic acetamide deep eutectic solvents (DESs) that contained lithium nitrate/bromide/perchlorate as electrolyte. Combined measurements near room temperature reflected a total dynamic Stokes shift of approximately 800-1100 cm-1 and triexponential solvation response functions. Interestingly, the average rate of solvation became faster upon successive replacement of bromide by nitrate in these deep eutectics, and a subpicosecond time scale emerged in the measured solvation response when bromide was fully replaced by nitrate. Temperature dependent solute diffusion in these deep eutectics at the single molecule level, monitored by tracking the translational motion of rhodamine 6G (R6G) via fluorescence correlation spectroscopic (FCS) technique, revealed pronounced fractional viscosity dependence of the solute's translational motion. Subsequently, this partial decoupling of solute translation was attributed to the microheterogeneous nature of these ionic DESs after examining the diffusion-viscosity relationship via the FCS measurements of R6G in several normal solvents at room temperature and in a liquid amide solvent at different temperatures.

Cloud Point Driven Dynamics in Aqueous Solutions of Thermoresponsive Copolymers: Are They Akin to Criticality Driven Solution Dynamics?

Cloud point driven interaction and relaxation dynamics of aqueous solutions of amphiphilic thermoresponsive copolymers were explored through picosecond resolved and steady state fluorescence measurements employing hydrophilic (coumarin 343, C343) and hydrophobic (coumarin 153, C153) solute probes of comparable sizes. These thermoresponsive random copolymers, with tunable cloud point temperatures (Tcp's) between 298 and 323 K, were rationally designed first and then synthesized via reversible addition-fragmentation chain transfer (RAFT) copolymerization of methyl methacrylate (MMA) and poly(ethylene glycol) monomethyl ether methacrylate (PEGMA). Subsequently, copolymers were characterized by NMR spectroscopy and size exclusion chromatography (SEC). A balance between the hydrophilic (PEGMA) and the hydrophobic (MMA) content dictates the critical aggregation concentration (CAC), with CAC ∼ 2-14 mg/L for these copolymers in aqueous media. No abrupt changes in the steady state spectral features of both C153 and C343 in the aqueous solutions of these polymers near but below the cloud point temperatures were observed. Interestingly, spectral properties of C153 in these solutions show the impact of hydrophobic/hydrophilic interaction balance but not by those of C343. More specifically, C153 reported a blue shift (relative to that in neat water) and heterogeneity in its local environment. This suggested different locations for the hydrophilic (C343) and the hydrophobic (C153) probes. In addition, the excited state fluorescence lifetime (⟨τlife⟩) of C153 increased with the increase of hydrophobic (MMA) content in these copolymers. However, C343 reported no such variations, although fluorescence anisotropy decays for both solutes were significantly slowed down in these aqueous solutions compared to neat water. Anisotropy decays indicated bimodal time-dependent friction for these solutes in aqueous solutions of these copolymers but monomodal in neat water. A linear dependence of the average rotational relaxation rates (⟨krot⟩ = ⟨τrot⟩-1) of the type ⟨krot⟩ ∝ (|T - Tcp|/Tcp)γ with negative values for the exponent γ was observed for both solutes. No slowing down of the solute rotation with temperature approaching the Tcp was detected; rather, rotation became faster upon increasing the solution temperature, suggesting domination of the local friction.