Solute-solvent Coupling Research Articles

The role of memory-dependent friction and solvent viscosity in isomerization kinetics in viscogenic media

Molecular isomerization kinetics in liquid solvent depends on a complex interplay between the solvent friction acting on the molecule, internal dissipation effects (also known as internal friction), the viscosity of the solvent, and the dihedral free energy profile. Due to the absence of accurate techniques to directly evaluate isomerization friction, it has not been possible to explore these relationships in full. By combining extensive molecular dynamics simulations with friction memory-kernel extraction techniques we consider a variety of small, isomerising molecules under a range of different viscogenic conditions and directly evaluate the viscosity dependence of the friction acting on a rotating dihedral. We reveal that the influence of different viscogenic media on isomerization kinetics can be dramatically different, even when measured at the same viscosity. This is due to the dynamic solute-solvent coupling, mediated by time-dependent friction memory kernels. We also show that deviations from the linear dependence of isomerization rates on solvent viscosity, which are often simply attributed to internal friction effects, are due to the simultaneous violation of two fundamental relationships: the Stokes-Einstein relation and the overdamped Kramers prediction for the barrier-crossing rate, both of which require explicit knowledge of friction.

Artificial Intelligence Resolves Kinetic Pathways of Magnesium Binding to RNA.

Magnesium is an indispensable cofactor in countless vital processes. In order to understand its functional role, the characterization of the binding pathways to biomolecules such as RNA is crucial. Despite the importance, a molecular description is still lacking since the transition from the water-mediated outer-sphere to the direct inner-sphere coordination is on the millisecond time scale and therefore out of reach for conventional simulation techniques. To fill this gap, we use transition path sampling to resolve the binding pathways and to elucidate the role of the solvent in the binding process. The results reveal that the molecular void provoked by the leaving phosphate oxygen of the RNA is immediately filled by an entering water molecule. In addition, water molecules from the first and second hydration shell couple to the concerted exchange. To capture the intimate solute–solvent coupling, we perform a committor analysis as the basis for a machine learning algorithm that derives the optimal deep learning model from thousands of scanned architectures using hyperparameter tuning. The results reveal that the properly optimized deep network architecture recognizes the important solvent structures, extracts the relevant information, and predicts the commitment probability with high accuracy. Our results provide detailed insights into the solute–solvent coupling which is ubiquitous for kosmotropic ions and governs a large variety of biochemical reactions in aqueous solutions.

On the presence of solute-solvent transport coupling in reverse osmosis

The present work demonstrates that experimental salt passage data for reverse osmosis (RO) shows better agreement with models capturing solute-solvent transport coupling (convective coupling), especially as the applied pressure is increased. This conclusion is drawn based on five RO data sets using cellulose acetate and polyamide membranes that were modeled using the classical solution diffusion (SD) model, which does not include convection, as well as the convection-inclusive SD model introduced by Paul in 2004 and the pore-based model, which also includes convective coupling. The improved model-to-experimental data agreement with solute-solvent coupling is more easily noticed from salt passage variation with pressure than from the most commonly studied RO metric, salt rejection ratio. The importance of solute-solvent coupling in RO indicates that free volume in the membrane influences the description of membrane transport, and that these voids ‘open up’ as applied pressure is increased. The derivation of the pore-based and SD models from the Maxwell-Stefan equations is shown, and major differences in assumptions used in their derivation are discussed. A study of these differences should aid membrane researchers in selecting the most appropriate modeling approach for a given solute-solvent-membrane system.

Numerical Analysis of Solute–Solvent Coupling Magnitude in the Thermally Backward Ring Closing Reaction of Spirooxazines

A fundamental involvement of solvent fluctuations often arises in solution reactions, where the coupling between the solute structural change and solvent fluctuations plays a decisive role in reaction kinetics. In this study, the rate constants associated with the ring-closing reaction of 1,3,3-trimethylspiro(indoline-2,3′-naphtho[2,1-b][1,4]oxazine) (SNO) are numerically analyzed by means of the one-dimensional Grote–Hynes model and two-dimensional Fokker–Planck equation with sink term model, respectively. The solute–solvent coupling parameter Ccoupled was evaluated in terms of the non-equilibrium solvation energy, which was directly computed by molecular dynamics (MD) simulations. The correlation between the solute–solvation coupling magnitude and the ring closure mechanism in solution are discussed.

Defect-mediated colloidal interactions in a nematic-phase discotic solvent.

Interactions between colloidal inclusions dispersed in a nematic discotic liquid-crystalline solvent were investigated for different solute–solvent coupling conditions. The solvent was treated at the level of Gay–Berne discogens. Colloidal inclusions were coupled to the solvent with a generalized sphere-ellipsoid interaction potential. Energy strengths were varied to promote either homeotropic or planar mesogenic anchoring. Colloid–colloid interactions were modeled using a soft, excluded-volume contribution. Single-colloid and colloid-pair samples were evolved with Molecular Dynamics simulations. Equilibrium trajectories were used to characterize structural and dynamical properties of topological defects arising in the mesomorphic phase due to colloidal inclusions. Boojums were observed with planar anchoring, whereas Saturn rings were obtained with homeotropic anchoring. The manner in which these topological defects drive colloidal interactions was assessed through a free energy analysis, taking into account the relative orientation between a colloidal dyad and the nematic-field director. The dynamical behavior of defects was qualitatively surveyed from equilibrium trajectories borne from computer simulations.

Solvent scaling scheme for studying solvent restructuring thermodynamics in solvation processes

We consider the solvent-solvent interaction energy change associated with the solvent restructuring in response to a progressive change in the interaction between the solute and the solvent molecules. We derive expressions for this solvent restructuring energy using a new solvent scaling scheme in which the solvent-solvent potential energy is scaled relative to its reference value. We find that we can naturally extend the familiar single-coupling parameter methodology with a second coupling parameter that controls the solvent-solvent interactions. In doing so we can not only access information about the solvent restructuring energy, but also quantify the statistical correlations between the fluctuations of the solute-solvent and the solvent-solvent interaction energies. Based on these findings we implement a molecular dynamics simulation strategy to evaluate the solvent restructuring energetics associated with turning on the electrostatic interactions between one initially uncharged water molecule dissolved in liquid water, using both the TIP4P-2005 and OPC water models. We also use the theory to explore the dependence of the solvent restructuring energy on solute-solvent coupling, both in general and in the special case in which the solute-solvent coupling is accurately described by the linear response approximation.

Femtosecond coherent nuclear dynamics of excited tetraphenylethylene: Ultrafast transient absorption and ultrafast Raman loss spectroscopic studies.

Ultrafast torsional dynamics plays an important role in the photoinduced excited state dynamics. Tetraphenylethylene (TPE), a model system for the molecular motor, executes interesting torsional dynamics upon photoexcitation. The photoreaction of TPE involves ultrafast internal conversion via a nearly planar intermediate state (relaxed state) that further leads to a twisted zwitterionic state. Here, we report the photoinduced structural dynamics of excited TPE during the course of photoisomerization in the condensed phase by ultrafast Raman loss (URLS) and femtosecond transient absorption (TA) spectroscopy. TA measurements on the S1 state reveal step-wise population relaxation from the Franck-Condon (FC) state → relaxed state → twisted state, while the URLS study provides insights on the vibrational dynamics during the course of the reaction. The TA spectral dynamics and vibrational Raman amplitudes within 1 ps reveal vibrational wave packet propagating from the FC state to the relaxed state. Fourier transformation of this oscillation leads to a ∼130 cm-1 low-frequency phenyl torsional mode. Two vibrational marker bands, Cet=Cet stretching (∼1512 cm-1) and Cph=Cph stretching (∼1584 cm-1) modes, appear immediately after photoexcitation in the URLS spectra. The initial red-shift of the Cph=Cph stretching mode with a time constant of ∼400 fs (in butyronitrile) is assigned to the rate of planarization of excited TPE. In addition, the Cet=Cet stretching mode shows initial blue-shift within 1 ps followed by frequency red-shift, suggesting that on the sub-picosecond time scale, structural relaxation is dominated by phenyl torsion rather than the central Cet=Cet twist. Furthermore, the effect of the solvent on the structural dynamics is discussed in the context of ultrafast nuclear dynamics and solute-solvent coupling.

Computational Analysis of Solute–Solvent Coupling Magnitude in the Z/E Isomerization Reaction of Nitroazobenzene and Benzylideneanilines

The dynamic solvent effect often arises in solution reactions, where coupling between chemical reaction and solvent fluctuation plays a decisive role in the reaction kinetics. In this study, the Z/E isomerization reaction of nitoroazobenzene and benzylideneanilines in the ground state was computationally studied by molecular dynamics simulations. The non-equilibrium solvation effect was analyzed using two approaches: (1) metadynamics Gibbs energy surface exploration and (2) solvation Gibbs energy evaluation using a frozen solvation droplet model. The solute–solvent coupling parameter (Ccoupled) was estimated by the ratio of the solvent fluctuation Gibbs energy over the corresponding isomerization activation Gibbs energy. The results were discussed in comparison with the ones estimated by means of the analytical models based on a reaction–diffusion equation with a sink term. The second approach using a frozen solvation droplet reached qualitative agreement with the analytical models, while the first metadynamics approach failed. This is because the second approach explicitly considers the non-equilibrium solvation in the droplet, which consists of a solute at the reactant geometry immersed in the pre-organized solvents fitted with the solute at the transition state geometry.

Introducing QMC/MMpol: Quantum Monte Carlo in Polarizable Force Fields for Excited States.

We present for the first time a quantum mechanics/molecular mechanics scheme which combines quantum Monte Carlo with the reaction field of classical polarizable dipoles (QMC/MMpol). In our approach, the optimal dipoles are self-consistently generated at the variational Monte Carlo level and then used to include environmental effects in diffusion Monte Carlo. We investigate the performance of this hybrid model in describing the vertical excitation energies of prototypical small molecules solvated in water, namely, methylenecyclopropene and s-trans acrolein. Two polarization regimes are explored where either the dipoles are optimized with respect to the ground-state solute density (polGS) or different sets of dipoles are separately brought to equilibrium with the states involved in the electronic transition (polSS). By comparing with reference supermolecular calculations where both solute and solvent are treated quantum mechanically, we find that the inclusion of the response of the environment to the excitation of the solute leads to superior results than the use of a frozen environment (point charges or polGS), in particular, when the solute-solvent coupling is dominated by electrostatic effects which are well recovered in the polSS condition. QMC/MMpol represents therefore a robust scheme to treat important environmental effects beyond static point charges, combining the accuracy of QMC with the simplicity of a classical approach.

Structural Order of Water Molecules around Hydrophobic Solutes: Length-Scale Dependence and Solute-Solvent Coupling.

It has been suggested that the structure and thermodynamics of the water molecules in the hydration layer of simple hydrophobic solutes undergo an order-disorder transition around a nanometer length-scale of the solute size. Using extensive atomistic molecular dynamics (MD) and replica exchange molecular dynamics (REMD) simulation studies, we have probed this order-disorder transition around model hydrophobic solutes of varying size and shape (spherical, planar, and linear), as well as flexible hydrophobic homopolymer chains (n-alkanes), where the conformational fluctuations are likely to create both spatial and temporal heterogeneity on the solvent accessible surface. We have explored the structural response of the water molecules in the hydration shell due to the local variations of the length-scale (or curvature) upon hydrophobic collapse and/or local conformational changes of these polymers. We have shown that the tetrahedral order of the water molecules in the hydration shell is practically independent of the polymer size in the extended state of the polymer due to the availability of a subnanometer cross-sectional length-scale, allowing the water molecules to form hydrogen bonds around the polymer chain. Beyond a certain length of the polymer chains, the collapsed states (associated with larger solute length-scale) start to induce disorder in the surface water molecules. We demonstrate that the local structure (both local number density and tetrahedral order) of the hydration layer is dynamically coupled to the local topology of the polymer. Thus, we envisage that in a flexible (bio)polymer, the hydration shell properties will be sensitive to the local conformational state of the molecule (both spatially and temporally), and the overall observed water structure and dynamics will be dependent on the topological/chemical heterogeneity, and the time-scale of fluctuations in the local curvature (length-scale) of the solvent accessible surface. Moreover, we have demonstrated the direct coupling between the local density fluctuations of water and the local hydrophobic collapse of the polymer. For the extended state of the polymer, the local solvent density fluctuation is practically independent of the solute coordinate (length-scale), and the hydrophobic collapse of the polymer is prompted by a "local dewetting" process induced by these fluctuations.

Lithium solvation in dimethyl sulfoxide-acetonitrile mixtures.

We present molecular dynamics simulation results pertaining to the solvation of Li(+) in dimethyl sulfoxide-acetonitrile binary mixtures. The results are potentially relevant in the design of Li-air batteries that rely on aprotic mixtures as solvent media. To analyze effects derived from differences in ionic size and charge sign, the solvation of Li(+) is compared to the ones observed for infinitely diluted K(+) and Cl(-) species, in similar solutions. At all compositions, the cations are preferentially solvated by dimethyl sulfoxide. Contrasting, the first solvation shell of Cl(-) shows a gradual modification in its composition, which varies linearly with the global concentrations of the two solvents in the mixtures. Moreover, the energetics of the solvation, described in terms of the corresponding solute-solvent coupling, presents a clear non-ideal concentration dependence. Similar nonlinear trends were found for the stabilization of different ionic species in solution, compared to the ones exhibited by their electrically neutral counterparts. These tendencies account for the characteristics of the free energy associated to the stabilization of Li(+)Cl(-), contact-ion-pairs in these solutions. Ionic transport is also analyzed. Dynamical results show concentration trends similar to those recently obtained from direct experimental measurements.

Understanding THz Spectra of Aqueous Solutions: Glycine in Light and Heavy Water

THz spectroscopy of aqueous solutions has been established as of recently to be a valuable and complementary experimental tool to provide direct insights into the solute-solvent coupling due to hydrogen-bond dynamics involving interfacial water. Despite much experimental progress, understanding THz spectra in terms of molecular motions, akin to mid-infrared spectra, still remains elusive. Here, using the osmoprotectant glycine as a showcase, we demonstrate how this can be achieved by combining THz absorption spectroscopy and ab initio molecular dynamics. The experimental THz spectrum is characterized by broad yet clearly discernible peaks. Based on substantial extensions of available mode-specific decomposition schemes, the experimental spectrum can be reproduced by theory and assigned on an essentially quantitative level. This joint effort reveals an unexpectedly clear picture of the individual contributions of molecular motion to the THz absorption spectrum in terms of distinct modes stemming from intramolecular vibrations, rigid-body-like hindered rotational and translational motion, and specific couplings to interfacial water molecules. The assignment is confirmed by the peak shifts observed in the THz spectrum of deuterated glycine in heavy water, which allow us to separate the distinct modes experimentally.

Probing the Microscopic Aspects of 1-Butyl-3-Methylimidazolium Trifluoroacetate Ionic Liquid and Its Mixture with Water and Methanol: A Photophysical and Theoretical (DFT) Study

Considering the potential of mixed ionic liquid-cosolvent systems in wide range of applications, photophysical and theoretical studies on an industrially important ionic liquid, 1-butyl-3-methylimidazolium trifluoroacetate (BMIMTFA), and also its mixture with water and methanol have been investigated. Two organic dipolar solutes coumarin 153 (C153) and 2-aminonitrofluorene (ANF) have been used as the probe molecule for the present study. Steady-state absorption and emission spectral behavior of C153 has not been significantly influenced by both the cosolvents. However, excitation wavelength dependent measurements with ANF in the BMIMTFA-water and BMIMTFA-methanol show entirely different photophysical response. For BMIMTFA-methanol system the average solvation and rotational time is found to be less than that in BMIMTFA-water system. Quite interestingly, time-resolved fluorescence anisotropy measurements reveal two different solute-solvent coupling constant (C(obs)) even if same mole fraction of water and methanol is used for the mixed solvent systems. Theoretical calculations also reveal stronger intermolecular interaction between IL and methanol than that between IL and water. The present combined photophysical and theoretical calculations seem to suggest different microscopic structural organization in the two binary systems.

Fluorescence Anisotropy of a Nonpolar Solute in 1-Alkyl-3-Methylimidazolium-Based Ionic Liquids: Does the Organized Structure of the Ionic Liquid Influence Solute Rotation?

Temperature-dependent fluorescence anisotropies of a nonpolar solute 9-phenylanthracene (9-PA) have been measured in 1-alkyl-3-methylimidazolium-based ionic liquids with anions such as bis(trifluoromethylsulfonyl)imide ([Tf2N–]), tris(pentafluoroethyl)trifluorophosphate ([FAP–]), tetrafluoroborate ([BF4(–)]), and hexafluorophosphate ([PF6(–)]) to find out if the organized structure of the ionic liquid has a bearing on solute rotation. Analysis of the experimental data using the Stokes–Einstein–Debye hydrodynamic theory indicates that there is no significant variation in the solute–solvent coupling constants (Cobs) with an increase in the length of the alkyl chain on the imidazolium cation for the ionic liquids with [Tf2N–] and [FAP–] anions. However, in the case of ionic liquids with [BF4(–)] and [PF6(–)] anions, the rotation of 9-PA for a given viscosity at constant temperature becomes progressively faster and C(obs) decreases by a factor of 2.4 from ethyl to octyl derivatives. Quasihydrodynamic theories of Gierer–Wirtz and Dote–Kivelson–Schwartz could not account for the significant decrease in the C(obs) values. The observed behavior has been rationalized in terms of the organized structure of the ionic liquids having [BF4(–)] and [PF6(–)] anions, which results as a consequence of the high charge-to-size ratio of these anions compared to [Tf2N–] and [FAP–].

Rotational Diffusion of Nonpolar and Ionic Solutes in 1-Alkyl-3-Methylimidazolium Bis(trifluoromethylsulfonyl)imides: Is Solute Rotation Always Influenced by the Length of the Alkyl Chain on the Imidazolium Cation?

In an attempt to find out whether the length of the alkyl chain on the imidazolium cation has a bearing on solute rotation, temperature-dependent fluorescence anisotropies of three structurally similar solutes have been measured in a series of 1-alkyl-3-methylimidazolium (alkyl = methyl, ethyl, propyl, butyl, and hexyl) bis(trifluoromethylsulfonyl)imides. Solute-solvent coupling constants obtained from the experimentally measured reorientation times with the aid of Stokes-Einstein-Debye hydrodynamic theory indicate that there is no influence of the length of the alkyl chain on the rotation of nonpolar, anionic, and cationic solutes 9-phenylanthracene (9-PA), fluorescein (FL), and rhodamine 110 (R110), respectively. It has also been noticed that the rotational diffusion of 9-PA is closer to the predictions of slip hydrodynamics, whereas the rotation of negatively charged FL and positively charged R110 is almost identical and follows stick hydrodynamics in these ionic liquids. Despite having similar shape and size, ionic solutes rotate slower by a factor of 3-4 compared to the nonpolar solute. Interplay of specific and electrostatic interactions between FL and the imidazolium cation of the ionic liquids, and between R110 and the bis(trifluoromethylsulfonyl)imide anion, appear to be responsible for the observed behavior. These results are an indication that the length of the alkyl chain on the imidazolium cation does not alter their physical properties in a manner that has an effect on solute rotation.

Solute rotation in polar liquids: Microscopic basis for the Stokes-Einstein-Debye model

Here, we develop a framework for a molecular level understanding of the celebrated Stokes-Einstein-Debye (SED) formula. In particular, we explore reasons behind the surprising success of the SED model in describing dipolar solute rotation in complex polar media. Relative importance of solvent viscosity and solute-solvent dipolar interaction is quantified via a self-consistent treatment for the total friction on a rotating solute where the hydrodynamic contribution is modified by the friction arising from the longer ranged solute-solvent dipolar interaction. Although the solute-solvent dipolar coupling is obtained via the Mori-Zwanzig formalism, the inclusion of solvent structure via the wave vector dependent viscosity in the hydrodynamic contribution incorporates solvent molecularity in the present theory. This approach satisfactorily describes the experimental rotation times measured using a dipolar solute, coumarin 153 (C153), in protic and aprotic polar liquids, and more importantly, provides microscopic explanation for insignificant contribution of electrical interactions on solute rotation, in contrast to the substantial role played by the translational dielectric friction in the context of ionic mobility. It is also discussed on how the present theory can be suitably extended to study the rotation of a realistic solute in media other than dipolar solvents.

Coupled reverse draw solute permeation and water flux in forward osmosis with neutral draw solutes

Understanding the simultaneous forward water flux and reverse draw solute flux in osmotically driven membrane processes is essential to the development of these emerging technologies. In this work, we investigate the reverse fluxes of three neutral draw solutes-urea, ethylene glycol, and glucose-across an asymmetric forward osmosis membrane. Our experiments reveal that for the rapidly permeating draw solutes (urea and ethylene glycol), an additional resistance to mass transfer develops due to external concentration polarization on the feed side of the membrane. A model to predict the reverse flux for these highly permeable solutes is derived and then validated using independently determined transport parameters. The experimentally measured water fluxes generated by these solutes are consistently lower than those predicted by theories for calculating the water flux in osmotically driven membrane processes-even when the effects of external concentration polarization are taken into account. These results are indicative of a coupling between the forward water flux and reverse solute flux, and a reflection coefficient is introduced to account for the solute–solvent coupling. Interestingly, our experiments demonstrate that solute–solvent coupling does not significantly impact the reverse flux of draw solute. The effect of this solute–solvent coupling on the reverse flux selectivity (i.e., the ratio of the forward water flux to the reverse solute flux) is demonstrated.

Glycine in aqueous solution: solvation shells, interfacial water, and vibrational spectroscopy from ab initio molecular dynamics

An aqueous glycine solution is studied with ab initio molecular dynamics to investigate the structural aspects of the different solvation shells within the zwitterion and their impact on the infrared spectrum. The individual contributions to the total IR spectrum from glycine and solvation water are decomposed systematically using the standard schemes in terms of maximally localized Wannier orbitals to define approximate molecular dipole moments in solution. The IR spectra of the aqueous solution and of the solvated zwitterionic glycine molecule itself are compared to those stemming from neutral glycine in the gas phase and a virtual "isolated" zwitterionic glycine molecule vertically transferred from solution into vacuum. Furthermore, electronic polarization effects due to solute-solvent coupling are discussed in detail for the solute and for the interfacial solvent molecules based on dipole moment distribution functions.

Effect of ultrafast electron transfer on photon echo signal: Decoherence process in electron-donating solvents

The effect of ultrafast electron transfer (ET) on the coherent state was investigated by means of three pulse photon echo for an azaporphyrin (AzP) derivative, 2,7,12,17-Tetra- tert-butyl-5,10,15,20-tetraaza-21 H,23 H-porphine, in electron-donating solvents. The peak shift and fwhm of the echo signal substantially decreased in the reactive solvents, indicating that the ultrafast ET affected the coherent state of the chromophore significantly. By integrating the present results in reactive and inert solvents with those previously performed for other systems, the role of the rapid reaction in the decoherence process was discussed from the viewpoints of the strength of the solute–solvent coupling.

Standard electrode potential, Tafel equation, and the solvation thermodynamics

Equilibrium in the electronic subsystem across the solution-metal interface is considered to connect the standard electrode potential to the statistics of localized electronic states in solution. We argue that a correct derivation of the Nernst equation for the electrode potential requires a careful separation of the relevant time scales. An equation for the standard metal potential is derived linking it to the thermodynamics of solvation. The Anderson-Newns model for electronic delocalization between the solution and the electrode is combined with a bilinear model of solute-solvent coupling introducing nonlinear solvation into the theory of heterogeneous electron transfer. We therefore are capable of addressing the question of how nonlinear solvation affects electrochemical observables. The transfer coefficient of electrode kinetics is shown to be equal to the derivative of the free energy, or generalized force, required to shift the unoccupied electronic level in the bulk. The transfer coefficient thus directly quantifies the extent of nonlinear solvation of the redox couple. The current model allows the transfer coefficient to deviate from the value of 0.5 of the linear solvation models at zero electrode overpotential. The electrode current curves become asymmetric in respect to the change in the sign of the electrode overpotential.