Solvation Time Research Articles

Tracking Cavity Formation in Electron Solvation: Insights from X-ray Spectroscopy and Theory.

We present time-resolved X-ray absorption spectra of ionized liquid water and demonstrate that OH radicals, H3O+ ions, and solvated electrons all leave distinct X-ray-spectroscopic signatures. Particularly, this allows us to characterize the electron solvation process through a tool that focuses on the electronic response of oxygen atoms in the immediate vicinity of a solvated electron. Our experimental results, supported by ab initio calculations, confirm the formation of a cavity in which the solvated electron is trapped. We show that the solvation dynamics are governed by the magnitude of the random structural fluctuations present in water. As a consequence, the solvation time is highly sensitive to temperature and to the specific way the electron is injected into water.

Solute dynamics of a hydrophobic molecule in a menthol-thymol based type-V deep eutectic solvent: effect of composition of the components.

Type-V deep eutectic solvents (DESs) are a newly emerging unique class of solvents obtained by physical mixing and heating of non-ionic components. These solvents show deviation from the thermodynamic ideality. Compared to type-I to IV DESs, type-V DESs are less explored and their physical chemistry is in its nascent stage. In this work, we have chosen a type-V DES based on menthol-thymol (MT) for our working media. Solvent and rotational dynamics were studied with varying temperature using a well-known solvatochromic probe, Coumarin 153 (C153). We prepared the MT-based DES using a reported procedure at three molar ratios: 1 : 1 (M1T1), 1 : 1.5 (M1T1.5), and 2 : 1 (M2T1) of menthol (M) and thymol (T). Time-resolved emission spectra (TRES) were constructed with varying temperature. Utilizing TRES, the decay of the solvent correlation function (C(t)) was plotted. We have correlated the solvent relaxation time in these DESs as a function of viscosity. The time-resolved anisotropy decays were also collected to perceive the rotational relaxation dynamics of C153 as a function of temperature. The decay of solvent relaxation was found to be bi-exponential, and the average solvation time (〈τs〉) in M2T1 was found to be longer than those of M1T1.5 and M1T1. The rotational reorientation times (〈τrot〉) also follow the same trend. We have analysed the rotational dynamics of C153 in type-V DESs employing the Stokes-Einstein-Debye (SED) hydrodynamic model. The rotational dynamics in DESs demonstrate a good correlation with the SED model with a little deviation. In MT-based DESs, the solute's rotational relaxation times approach hydrodynamic stick boundary condition at low viscosity (or at high temperatures) for all molar compositions. Using the Arrhenius-type equations, we have correlated the activation energies for the rotational motion of C153, along with the viscous flow and non-radiative pathways for all the DESs.

The shift of excitation spectra at blue edge of emission (BEEmS) as a new methodology to probe heterogeneity

Many current methods for detecting molecular-level heterogeneity are complex and require stringent data analysis, limiting their widespread use. Herein, we propose a novel edge effect, i.e., the shift of excitation spectra at the blue edge of emission (which we termed as Blue Edge Emission Shift, BEEmS), to perceive the structural heterogeneity. This method is simple and can be easily implemented with commonly available fluorimeter. Red edge excitation shift (REES), a related technique, is already in use, but like most other known techniques, its usefulness is constrained by its dependence on environmental rigidity. Our method does not suffer from this drawback significantly. We showed the generality of the proposed method taking various chemically and biologically heterogenous systems including molecular liquid, deep eutectic solvents, organic cavitand, micelle and protein. BEEmS certainly comes out as a more effective sensor of heterogeneity than REES in certain cases (like denatured protein, hydrophobic deep eutectic solvent and SDS micelle) where solvation time is not sufficiently slow to be detected by REES, but can be measured though BEEmS. Furthermore, unlike most existing techniques, domain-specific heterogeneity of a model multi-domain protein is successfully measured.

Diffusion theory of molecular liquids in the energy representation and application to solvation dynamics.

The generalized Langevin equation (GLE) formalism is a useful theoretical fundament for analyzing dynamical phenomena rigorously. Despite the systematic formulation of dynamics theories with practical approximations, however, the applicability of GLE-based methods is still limited to simple polyatomic liquids due to the approximate treatment of molecular orientations involved in the static molecular liquid theory. Here, we propose an exact framework of dynamics based on the GLE formalism incorporating the energy representation theory of solution, an alternative static molecular liquid theory. A fundamental idea is the projection of the relative positions and orientations of solvents around a solute onto the solute-solvent interaction, namely the energy coordinate, enabling us to describe the dynamics on a one-dimensional coordinate. Introducing systematic approximations, such as the overdamped limit, leads to the molecular diffusion equation in the energy representation that is described in terms of the distribution function of solvents on the energy coordinate and the diffusion coefficients. The present theory is applied to the solvation dynamics triggered by the photoexcitation of benzonitrile. The long-time behavior of the solvation time correlation function is in good agreement with that obtained by the molecular dynamics simulation.

Assessing the impact of increase in the number of hydroxyl groups on the microscopic behaviors of ammonium-based room temperature ionic liquids: A combined fluorescence up-conversion, fluorescence correlation and NMR spectroscopic study

The current study aims to understand the impact of variation of number of hydroxyl groups on the cationic head of ammonium-based room temperature ionic liquids (RTILs) towards inter/intra molecular hydrogen bonding interaction, local structural organization and dynamics of the solvent systems. For this purpose, three hydroxyl functionalized ammonium based RTILs (HFILs) bearing different numbers of hydroxyl groups on the cationic head as well as a non-hydroxyl ammonium based RTIL have been examined using both ensembled average and single-molecule spectroscopy techniques. Investigation of solvent relaxation dynamics of all the concerned RTILs by the combination of femto-second fluorescence up-conversion techniques and time corelated single photon counting techniques reveals a bimodal solvent relaxation behaviour having a very fast sub-picosecond and a relatively slower picosecond to nanosecond solvation time component for all the RTILs. Interestingly, analysis of the rotational and translational diffusion dynamics of a few particular solutes shows that the solvent–solvent interaction in the poly-hydroxyl-based HFILs is substantially stronger than the solute–solvent interaction. Additionally, analysis of the rotational diffusion data reveals that all RTILs exhibit significant dynamic heterogeneity, which increases with increase the number of hydroxyl groups on the cationic head of HFILs. Moreover, PFG-NMR study have shown that the HFILs had considerably greater hydrodynamic radii than the non-hydroxyl RTILs, which is likely due to the result of stronger hydrogen bonding interaction between the hydroxyl groups and the constituent of the HFILs. The outcomes of all these investigations have essentially demonstrated that subsequent addition of hydroxyl functionalities to the cationic head of the RTILs significantly alter the intra/inter molecular interaction, local structural organisation and heterogeneity of the medium.

Temperature-dependent ultrafast solvation dynamics of choline chloride-based deep eutectic solvent (DES) and hydroxyl functionalized room temperature ionic liquids (RTILs): Exploring the difference in solvent response between DES and RTILs

The present study aims to provide a comprehensive understanding of the difference in solvent relaxation behavior between DESs and RTILs. For this purpose, dynamics of solvation have been investigated in a choline chloride-based DES and three hydroxyls functionalized ILs, keeping the hydroxyl functionalities same in both classes of media, so that the role of various inter/intra molecular interactions, including hydrogen bonding interaction on the solvent relaxation behavior can be understood. Fluorescence up-conversion spectroscopy (FLUPS) techniques coupled with time-correlated single-photon counting (TCSPC) techniques have been exploited to study the complete Stokes shift dynamics of a dipolar probe, coumarin 153 (C153) dissolved in these media. The solvent response functions generated from the complete dynamic Stokes shift data reveal a bimodal solvent relaxation behaviour having a very fast sub-picosecond and a relatively slower picosecond to sub-nanosecond solvation time component for both DES and hydroxyl ILs. The relatively slower solvation time component, which correlates with bulk viscosity of the concerned medium, is found to arise due to the diffusional motion of the constituents of both DES and hydroxyl ILs. However, temperature-dependent solvent relaxation measurements have revealed that at nearly isoviscous conditions, the solvent relaxation is much faster in the DES as compared to that in hydroxyl IL indicating the differences in solvent relaxation behaviour at the later part of the solvent response. Interestingly, when the early part of the dynamics is monitored, the amplitude associated with the ultrafast component for the DES increases significantly with the increase in temperature, whereas the same remains almost unchanged for the hydroxyl IL. This essentially indicates that even at shorter time scale, the process of solvent relaxation is considerably different for both DES and hydroxyl ILs. All these investigations have essentially demonstrated that despite having similar functionalities, different motions related to the solvent relaxation, operating at microscopic level in both these classes of solvents, are significantly different from each other.

Dynamics of a PEG based polymer gel Electrolyte: A combined frequency dependent dielectric relaxation and Time-resolved fluorescence spectroscopic study

• Industrially relevant polymer gel electrolyte studied via dielectric relaxation and time-resolved fluorescence spectroscopy. • Relaxation dynamics monitored by using non-reactive fluorescent probe coumarin153. • Prominent impact of polymer concentration on solution dynamics. • Decoupling of probe solvation dynamics from medium viscosity and probe rotational dynamics. • Signatures of temporal heterogeneity in these polymer gel electrolyte systems observed. Here we investigate the dynamics of a polyethylene glycol ( P E G ) based polymer gel electrolyte (PGE) media and its dependence on polymer concentration and temperature via frequency dependent dielectric relaxation (DR) and time-resolved fluorescence (TRF) spectroscopic measurements. The polymer gel electrolyte system consists of propylene carbonate (PC; organic solvent), lithium perchlorate ( L i C l O 4 ; electrolyte) and P E G 300 (polymer). The impact of polymer on the dynamics of the medium has been monitored via gradually increasing the polymer concentration in the PGE media keeping the P C / L i C l O 4 molar ratio (11.2) unchanged. Coumarin 153 ( C 153 ), a well-known fluorescent probe molecule, has been employed as a local reporter for the fluorescence based experiments. Although the static dielectric constant of the medium decreases systematically upon addition of P E G 300 in the electrolyte media, no such systematic variation with polymer concentration in the fractional power dependence of the measured average DR time with medium viscosity is observed for these electrolyte systems. Steady state fluorescence measurements in these PGE systems indicate substantial effect of polymer addition on the organic electrolyte system, while no signature of spatial heterogeneity has been detected up to 40 wt% polymer concentration. Interestingly, fractional power dependence, similar to DR findings, of average rotational time for C 153 on the medium viscosity has been observed for both the processes, one by varying temperature of the medium at a fixed polymer concentration and other by changing the polymer concentration at a fixed temperature. Surprisingly, the average solvation time shows negative power dependence to the temperature reduced medium viscosity.

Hydrophobicity-Dependent Heterogeneous Nanoaggregates and Fluorescence Dynamics in Room-Temperature Ionic Liquids.

The hydrophobicity of room-temperature ionic liquids (RTILs) has been shown to have a very significant effect on the optical and structural properties of and in RTILs. The average excited state lifetime of neat RTILs has been shown to be increasing with increasing hydrophobicity of the RTILs. By employing pico-nanosecond-based fluorescence anisotropy decay, the volume of the nanoaggregates in neat RTILs have been calculated. The volume of these nanoaggregates have been shown to be decreasing with increase in hydrophobicity of the RTILs. Thus, hydrophobicity has been shown to have an important role, i.e., hydrophobicity can be used as a handle to tune the properties of RTILs as designer solvents. Moreover, the excited-state lifetime of red-emitting fluorophores, i.e., whose fluorescence emission is not perturbed by the inherent emission of RTILs, has been shown to increase with the increasing hydrophobicity of the RTILs. Highly hydrophobic RTILs have been shown to exhibit positive deviation and highly hydrophilic RTIL has been shown to exhibit negative deviation from the linear correlation between average solvation time (τs) versus viscosity/temperature (η/T).

Dynamics in condensed phase for systems involving phase functions obeying Gaussian statistics

The study of non-equilibrium processes in condensed phase has been fascinating and has attracted considerable attention of experimentalists as well as theoreticians during the last few decades. Since the theoretical treatment using Liouville equation involves multi-dimensional phase space functions, it is very difficult to visualize and also to obtain analytical expressions for the time-dependent distribution functions in Liouville space, which has led to formulation of various reduced space descriptions. The bottleneck in describing the non-equilibrium processes in the reduced space lies in the appearance of coupled velocity and position correlation functions in the equations, which are difficult to evaluate in general. In this work, we have developed a scheme to decouple the complicated multi-point coupled correlation function into two-point velocity and position correlation functions for situations characterized by a time-dependent phase function obeying Gaussian distribution, by making use of the Novikov's theorem. As an illustrative application, we have investigated the problem of optically controlled solvation dynamics and obtained exact expressions for the non-equilibrium solvation time correlation functions showing explicit dependence on the excitation frequency, as observed experimentally. We have shown that the linear response theory result is valid as long as the energy gap obeys the Gaussian distribution, rescuing the linear response theory based result from the domain of validity for small fluctuations in solvation coordinate.

Does urea modify microheterogeneous nature of ionic amide deep eutectics? Clues from non-reactive and reactive solute-centered dynamics

The impact of urea on the microheterogeneous nature of acetamide + LiNO3 deep eutectic solvents (DESs) was investigated via following the viscosity coupling of the average timescales of solute rotation, solvation and cis–trans isomerization reaction occurring in such media. Picosecond resolved fluorescence measurements were carried out to access these average relaxation timescales and the solutes employed were the non-reactive probe, coumarin153 (C153), and a reactive probe, trans- 2-[4′-(Dimethylamino) styryl]benzothiazole (DMASBT). DMASBT undergoes fast cis–trans isomerization reaction upon photo-excitation. The coupling to the solution viscosity (η) of the experimentally determined average timescales was then analyzed via the relation, 〈τx〉∝ηp, x denoting rotation, solvation or reaction, and a fractional p considered as solution microheterogeneity reflector. A comparison of the findings between the urea containing and non-urea ionic DESs revealed that addition of urea did not alter the p value for C153 rotation and DMASBT reaction times, while the C153 average solvation times showed a stronger decoupling. This stronger decoupling has been ascribed to the faster rate of solvation upon addition of urea via the participation of the amide group through the collective H-bond excitations. Addition of electrolyte at a fixed amide concentration, on the other hand, makes the fractional viscosity dependence more pronounced for both solvation and rotational dynamics, although the viscosity coupling (p value) of isomerization reaction timescale remains unchanged. DMASBT reveals, through steady state fluorescence measurements, stronger spatial heterogeneity in these solutions than that by C153, and DMASBT better senses the urea or electrolyte induced modulation of spatial heterogeneity.

Protein dynamics as a sensor for macromolecular crowding: Insights into mixed crowding

Understanding the effect of mixed crowding is of prime importance with regards to addressing the manner in which the crowded cellular interior influences the structure, function and dynamics of proteins. In this study we have used the internal dynamics of the multidomain serum protein, bovine serum albumin (BSA), labeled with the solvation probe 7-diethylamino-3–4-maleimidophenyl-4-methylcoumarin (CPM), as a sensor for binary crowding mixtures. Both synthetic (Ficoll 70, Dextran 70, Dextran 40 & PEG 8000) and protein based (unlabeled BSA) crowders have been used. The ‘BSA + Ficoll 70’ mixture had the maximum retardation effect on the protein dynamics with the average solvation time being more than that of the sum of the individual crowding agents. On the contrary, all other binary mixtures had the reverse effect with the excluded volume induced in their presence being lesser than that of the component crowders taken together. Further support was obtained from the overall protein dynamics based on the acrylamide quenching of emission of the tryptophan residues of BSA. Our results have been discussed in the light of the compatibility and lack in thereof, of the individual components of the mixtures, an aspect that has been further supported by excess viscosity (ηE) estimations based on our time resolved anisotropy data. Till date, crowding induced excluded volume has been sensed mostly through diffusion-based experiments or FRET based sensors. As shown in this study, internal protein dynamics can also add another dimension to the manner in which the degree of macromolecular congestion can be sensed, with the possibility of mapping site-specific response along the polypeptide backbone.

Real-Time Electron Solvation Induced by Bursts of Laser-Accelerated Protons in Liquid Water.

Understanding the mechanisms of proton energy deposition in matter and subsequent damage formation is fundamental to radiation science. Here we exploit the picosecond (10^{-12} s) resolution of laser-driven accelerators to track ultrafast solvation dynamics for electrons due to proton radiolysis in liquid water (H_{2}O). Comparing these results with modeling that assumes initial conditions similar to those found in photolysis reveals that solvation time due to protons is extended by >20 ps. Supported by magnetohydrodynamic theory this indicates a highly dynamic phase in the immediate aftermath of the proton interaction that is not accounted for in current models.

Intramolecular photo-driven charge transfer in a series of pyridyl substituted phenyloxazoles. Structural relaxation in meta-substituted ethylpyridinium derivative of phenyloxazole.

A series of pyridyl (pyridinium) substituted benzoxazoles were studied by steady state absorption, fluorescence spectroscopy, time-resolved fluorescence spectroscopy, fs pulse absorption and polarization spectroscopy, and quantum-chemical calculations. The spectral and kinetic parameters of the fluorophores in MeCN and EtOAc were obtained experimentally and were calculated by means of DFT and TDDFT methods. A scheme including four transient excited states was proposed for the interpretation of differential absorption kinetics of the charged fluorophores. Expressions describing the actual kinetics graphs, the decay associated spectra, and the species-associated spectra were derived. The charge shift step was found to be dependent on average solvation times. A charge shift followed by the formation of the twisted conformer was found for the excited 1-ethyl-3-(5-phenyloxazol-2-yl)pyridinium 4-methyl-1-benzenesulfonate in MeCN and EtOAc. Conformational analysis confirms a large amplitude motion of the meta-substituted ethylpyridinium group as an additional structural relaxation path producing an abnormally large fluorescence Stokes shift.

Interrogating the mechanism of the solvation dynamics in BmimBF4/PC mixtures: A cooperative study employing time-resolved fluorescence and molecular dynamics

In this article, the solvation in ionic liquid/molecular solvent (IL/MS) mixtures is investigated. The steady-state and time-resolved measurements of the solvation of coumarin 153 (C153) in BmimBF4/propylene carbonate are presented. A slight bathochromic shift was observed in the steady-state spectra upon increasing the IL molar fraction xIL. The time correlated photon counting TCSPC and fluorescence upconversion techniques were used to obtain the experimental solvation response functions (SRFs). The SRFs were found to be multi-exponential and the average solvation times increased with xIL. The MD simulations were performed to help the interpretation of the experimental data. An unprecedentedly good agreement between the measured and the calculated SRFs was achieved. The partition of the coulombic energy of interaction between the atoms of C153 and cations, anions and PC molecules provides a deep understanding of the time signatures of the mixture components participation in the solvation response. Indeed, the slow portion of the SRFs was found to be dominated by the IL. These time signatures change in function of xIL and represent the previously established regimes of IL/MS mixtures: electrolyte solution (xIL< 0.2) and lubricated IL (xIL> 0.2). These results will be of crucial interest for the understanding of the molecular rearrangements occurring in IL/MS electrolyte models for dye sensitized solar cells (DSSCs) after photo-excitation.

Assessing rotation and solvation dynamics in ethaline deep eutectic solvent and its solutions with methanol.

Steady-state and time-resolved fluorescence were used to investigate the solvation of coumarin 153 (C153) and coumarin 343 (C343) in methanol + ethaline binary solutions, a deep eutectic solvent composed of a 1:2 molar ratio choline chloride + ethylene glycol. In addition, time-resolved anisotropy decays were used to determine the solute's rotational reorientation time as a function of viscosity. Measurements were made in solutions covering the entire range of mole fraction. Viscosity measurements were used to characterize the bulk solvent properties, and as expected, addition of methanol resulted in an decreased viscosity, showing an exponential decrease with mole fraction, up to ∼50-fold at xMeOH = 1.0. Probe rotational reorientation times were found to be biexponential at xMeOH < 0.3 for C153 and xMeOH < 0.5 for C343 and monoexponential at richer methanol content. In proportion to viscosity, C153 and C343 average rotation times decreased ∼30-fold from xMeOH = 0 to 0.9 and showed a power law dependence of ∼η0.85. Rotation times approached the stick boundary limit on dilution with methanol. Time-resolved Stokes shifts quantified the solvation dynamics and were nearly single exponential for C153 but were clearly biexponential for C343. Solvation times also tracked with viscosity according to a power law dependence, with exponents of 0.3 and 0.4 for C153 and C343, respectively. The dilution effect of methanol was not linear in proportion to the viscosity change and alone cannot account for the change in solvation. Dilution also showed a different correlation to solvation than did temperature variations to govern the viscosity change.

Solvation shell dynamics explains charge transport characteristics of LIB electrolytes

Electrolytes made up of LiPF6 salt in various mixtures of carbonate solvents such as ethylene carbonate (EC) and dimethyl carbonate (DMC) are generally used in lithium ion batteries (LIBs). The electrolyte composed of EC and DMC in the ratio of 3:7 exhibits peak ionic conductivity. Despite its wide usage in LIBs, the reason for observed peak is not well established. We probe 1 M LiPF6 electrolytes of various EC:DMC solvent compositions ranging from 1:9 to 9:1 by moles using the molecular dynamics method to find that electrolyte of EC:DMC = 3:7 offers highest ionic conductivity. Though bulk properties such as viscosity and dielectric constant, and various structural properties vary with composition of the solvents, these properties do not explain the observed peak in ionic conductivity. However, the residence times of solvent molecules in ionic solvation shells show their minima for 3:7 electrolyte. It suggests that faster dynamics of solvation shell of ions (frequent exchange of solvent molecules) results in better ionic conductivity for 3:7 electrolyte. Insights gained in the present work, viz. the relation between ionic conductivity and solvation time of ions can be of help in designing novel high-performance electrolytes for next generation LIBs.

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.

Solvation dynamics in simple fluids: Effect of solute size and potential

While solvation dynamics plays an important role in many processes, most available studies focus on the solvent response to the variations of charge distribution on dissolved solute, and the solvent response to the structural and geometrical variations of solute is rarely explored. Herein, by using dynamical density functional theory, we study the solvation dynamics of simple solvent in response to the variations of solute size and potential strength. We find that increasing solute size causes opposite effect of increasing its potential strength on solvation time. On the solvent structure, varying solute’s potential strength brings a long-range yet weak influence while changing solute size generates a short-range yet strong effect, and therefore the penalty in solvent free energy is generally dominated by the change in solute’s potential strength while the variations of coordination number and solvation shell radius are dominated by the change in solute size. This work unravels the nonpolar solvation dynamics in response to the variation of solute geometry, and casts helpful insights on the design and control of solvation-dynamics related applications in molecular engineering.

Below Band Gap Formation of Solvated Electrons in Neutral Water Clusters?

Below band gap formation of solvated electrons in neutral water clusters using pump–probe photoelectron imaging is compared with recent data for liquid water and with above band gap excitation studies in liquid and clusters. Similar relaxation times on the order of 200 fs and 1–2 ps are retrieved for below and above band gap excitation, in both clusters and liquid. The independence of the relaxation times from the generation process indicates that these times are dominated by the solvent response, which is significantly slower than the various solvated electron formation processes. The analysis of the temporal evolution of the vertical electron binding energy and the electron binding energy at half-maximum suggests a dependence of the solvation time on the binding energy.

Partial Viscosity Decoupling of Solute Solvation, Rotation, and Translation Dynamics in Lauric Acid/Menthol Deep Eutectic Solvent: Modulation of Dynamic Heterogeneity with Length Scale.

Deep eutectic solvents (DESs) are new-generation media that can be fine-tuned to have desired properties circumventing economic and environmental issues. Typically, these are ionic, and only recently, nonionic DESs, having interesting properties, are being explored. In this report, we examined the structure and dynamics of a nonionic lauric acid/menthol (LA/Men) DES through steady-state emission, solvation dynamics, time-resolved fluorescence anisotropy, and translational diffusion dynamics. The zero shift in the emission spectra of coumarin 153 (a solvatochromic dye) as a function of the excitation wavelength suggests that LA/Men DES is spatially homogenous. Decoupling (p = 0.63) of the average solvation time, ⟨τs⟩, from medium viscosity suggests the presence of mild dynamic heterogeneity in the system. Rotational time, ⟨τr⟩, which reflects the nature of the first solvation shell, shows little decoupling (p = 0.81), suggesting it to be fairly dynamically homogeneous at a shorter length scale. An Arrhenius-type analysis also proves that rotation is mainly controlled by medium viscosity. Translational diffusion time, ⟨τD⟩, which provides information at a larger length scale, is strongly decoupled from medium viscosity (p = 0.29). This indicates that at a larger length scale, the DES is quite dynamically heterogeneous. The slow component of solvation time, which is believed to originate at a larger length scale, correlates well with the translational diffusion timescale having similar activation energies. This suggests that their origin is same. Expectedly, for the long component of solvation time, the decoupling is quite strong (p = 0.30). Overall, our result demonstrates the structure and dynamics of the nonionic LA/Men DES, and the existence of length scale-dependent heterogeneity has been proposed.