Orientational Relaxation Time Research Articles

Effects of urea on the aqueous solvation structure and dynamics of cis/trans-N-methylformamide: An insight from molecular dynamics simulation study

We have performed classical molecular dynamics simulation of aqueous cis/trans-N-methylformamide (NMF) systems in the presence of urea at 298 K by considering the Smith and Duffy models of urea with varying concentrations, ranging from 0 to 13.26 m. It is found that irrespective of urea models, the donating ability of the hydrogen atom of NMF to the oxygen of urea is higher than the oxygen of water, while it is comparatively higher in Duffy model. The overall urea–water interaction is significant for the Smith model, whereas the Duffy model favors the urea–urea interactions. The center of mass radial distribution functions [gCM(r)] of cis/trans-NMF suggest water is preferred in the near vicinity over urea in the solvation shell of these two conformers. A shoulder around 3.50 Å in gCM(r) of trans-NMF and water is mainly due to the hydrogen bonding contribution from the water molecules. The translational and rotational dynamics of cis/trans-NMF are slower with the addition of urea, but the slowing down is significant in trans-NMF. A similar behavior is also observed for water and urea. The higher dipole orientational relaxation time of urea for the Duffy model indicates its aggregation behavior. The hydrogen bond lifetime of Otrans…Hwat is higher compared to Ocis…Hwat for both the urea models. It is also observed that a significant increase in water–water hydrogen bond lifetime with varying concentrations of the Smith model of urea. It is also found that the stability of all these hydrogen bonds comparatively higher in Smith model.

Self-assembling of glutathione in aqueous environment: A combined experimental and theoretical study

Detailed concentration-dependent measurements of transient and stationary ultrasonically induced birefringence were combined with molecular docking and DFT techniques to study the formation of glutathione (GSH) self-aggregates in aqueous solutions. Molecular docking revealed the formation of dimers through hydrogen bonding with a binding free energy equal to −3.44 kcal/mol. The behavior of stationary birefringence measurements with concentration allowed us to assign the observed relaxation process to self-association mechanism between glutathione molecules. Systematic analysis of the birefringence traces in the transient region permitted the evaluation of the orientational relaxation times and the corresponding hydrodynamic volumes. Combining the concentration dependent spectroscopic data with supplementary measurements of several physical properties, such as mass density, shear viscosity, sound speed, isentropic compressibility, specific conductivity, and pH, the self-association scheme has been established. The chemical reactivity and the biological activity of the GSH monomer and dimer species were also evaluated and discussed in the context of the fair competence of GSH molecules to interact forming aggregates in aqueous environment.

Hydrogen bonding behavior of ethanol-trifluoroethanol binary mixtures and its effects on the water structure and dynamics in ternary aqueous-ethanol-trifluoroethanol solutions

The hydrogen bonding structure and dynamics of binary ethanol (EtOH) – trifluoroethanol (TFE) mixtures and in ternary aqueous-EtOH-TFE solutions are investigated by classical molecular dynamics simulations. Presence of water reduces hydrogen bonds between EtOH and TFE due to formation of new alcohol-water hydrogen bonds. In aqueous-EtOH-TFE mixtures, the tetrahedral order of water increases as we move from water-EtOH to water-TFE solution. Our calculated translational and rotational data suggests a slower dynamic of the species in their binary as well as in the ternary mixtures. The steep rise of the orientational relaxation times for TFE indicates a stronger aggregation among TFE molecules in both binary and ternary mixtures. A faster anisotropic decay of water is also observed with increasing TFE concentrations in the ternary solutions. The hydrogen bond lifetime associated with the hydrogen of TFE (HT⋯OT/OE/OW) is multiple times higher than that of H-bonds formed by the hydrogen of EtOH (HE)/water (HW).

Concentration Dependence of Dynamics and Structure among Hydrated Magnesium Ions: An Ultrafast Infrared Study.

The dynamics of aqueous magnesium chloride solutions, from relatively dilute (0.5 m) to near saturated (4.2 m) concentrations, were investigated using ultrafast two dimensional infrared and polarization selective pump-probe spectroscopies. The experiments were performed on two spectrally distinct nitrile stretch frequencies of the selenocyanate vibrational probe, corresponding to the CN nitrogen lone pair being associated with water and with Mg2+. No chemical exchange of the two species was observed over the experimental time scale (∼100 ps), enabling straightforward analysis of their dynamics. The dynamics reported by the Mg2+-associated peak are slower than those of the water-associated peak, suggesting that the immediate environment of the hydrated Mg2+ is different from the rest of the solution. Notably, the Mg2+-associated peak displays three spectral diffusion time scales, the slowest being ∼30 ps, while the water-associated peak decays as a faster biexponential. From the complete orientational relaxation time and hydrodynamic theory, a magnesium hydration number of six was obtained, which is in good agreement with NMR and X-ray diffraction studies. This hydration number holds for all concentrations until near saturation, when the linewidths and the dynamics deviate from linear trends, indicative of Mg2+ solvation structure changes resulting from a shortage of water molecules needed for full solvation.

Kinetics of orientation processes in nematic liquid crystals in a periodically changing magnetic field

The method of obtaining a magnetic field, the intensity vector of which moves along a conical surface at different values of the angle between the intensity vector and the axis of rotation of the magnetic field, the values of the intensity and angular velocity of rotation of the magnetic field, is described. An increase in the phase shift between the intensity vector and the director is found with an increase in the angle at the apex of the magnetic field cone. In the studied range of angular velocities of rotation of the magnetic field at angles between the intensity vector and the axis of rotation of the magnetic field less than 60 °, the director moves synchronously with the field. An exponential character of the dependence of the orientation relaxation time, which characterizes the rapid action of liquid crystal technical devices, on pressure and temperature is established.

Dielectric properties of the non-glycated and in vitro methylglyoxal-glycated cornea of the rabbit eye

The dielectric properties of the non-glycated and in vitro methylglyoxal-glycated cornea of the rabbit eye were tested in the frequency range of 200 Hz to 100 kHz of the electric field and at temperatures of 25 to 140 °C. The denaturation temperature (Td) for the non-glycated cornea and the non-enzymatically glycated cornea are approximately 45 and 55 °C, respectively. The mechanism of proton conduction up to Td in a glycated cornea requires more energy, i.e. more than twice the activation energy (ΔH) than in non-glycated tissue. The dielectric spectra for both examined tissues showed the same characteristic frequency of about 7 kHz assigned to the orientation relaxation time of the polar side groups inside the corneal stroma. These results may be useful in the surgical treatment of the cornea using conductive keratoplasty and in tissue engineering for clinical applications to regenerate this tissue. The medical use of these physico-biological techniques is important because the human cornea protects all eye tissues from various environmental factors.

A nonlinear constitutive model for entangled symmetric dendrimers

An orientation-stretch-coupled constitutive equation is suggested for symmetric Cayley tree-like dendrimers, which can self-consistently describe the linear relaxation spectrum and nonlinear viscoelastic behavior. The molecular stress is determined by orientation-stretch-coupled conformation tensors of all segments. The linear relaxation spectrum is determined according to hierarchical arm retraction with branch point hopping and dynamic tube dilation. Under strong flows, the orientation relaxation time and the stretch relaxation time are affected by the convective constraint release effect and branch point withdrawal. The coupling between segmental orientation and stretch in each generation is represented in the evolution of the coupled conformation tensor, while the possible stretch coupling among different generations is ignored. The predictions on both linear and nonlinear rheological behaviors are consistent with experiments, proving the rationality in treating coupling problems.

Acoustically induced birefringence in polymer aqueous solutions: The case of polyvinyl alcohol

Transient and stationary ultrasonically induced birefringence traces of various relatively dilute poly-vinyl alcohol aqueous solutions were measured over a range 500–1400 kHz of ultrasonic frequencies and 0.02–0.13 W/cm2 of acoustic field amplitudes. The intrinsic value of the stationary birefringence for all polymer solutions investigated in this study was found to be proportional to the solution mass concentration, the square root of the ultrasonic intensity and the frequency of the ultrasound in agreement with theoretical predictions. Systematic analysis of the transient birefringence signals caused by the applied ultrasonic field has been performed in order to evaluate the orientational relaxation times from the viewpoint of dynamics of polymer chain orientation and anisotropy changes in polarizability. The results are discussed in the context of recent experimental studies and theoretical predictions of the ultrasonically induced birefringence in terms of transport phenomena.

Modifying of the dielectric properties of adsorbed water in the force field of a solid surface

The results of thermodynamic and acoustoelectric studies of changes in the dielectric properties of adsorbed water under the influence of a solid surface force field are presented. It is noted that the dielectric properties of adsorbed water depend not only on the characteristic size of the system, but also on the interaction energy with the solid surface. Near the hydrophilic solid surface, the density of adsorbed water is increased and the mobility of molecules is reduced. The opposite effect occurs for hydrophobic surfaces. The frequency dependences of the dielectric properties of adsorbed water, as in the case of water in the solid and liquid phase, can be characterized using Debye relaxation theory. It is shown that a significant increase in the orientational dielectric relaxation time in adsorbed water can be explained by taking into account the energy of interfacial boundaries.

Study of adsorption effect on orientational and relaxation properties of finite polymer chains near the solid surface

To study dynamic properties of a polymer coating, we consider a polymer chain with finite length. We take into account a chain bending rigidity and assume that the chain is located near an adsorbing flat surface of a solid. One part of the chain is fixed on the surface, and the second one remains free. It is supposed that the cause of chain stretching is the internal effective mean (molecular) field formed as a result of intermolecular interactions with free ends of other chains. Kinetic equations based on the Kubo method are obtained to calculate the relaxation time of the segments. The dependences of the long-range orientational order parameter and relaxation time on the adsorption parameter, the mean field coefficient and chain bending rigidity parameters are calculated. It is shown that a first-order phase transition occurs at the critical point, which is associated with a drastic change in the degree of the chain elongation. We discovered the “critical deceleration” effect, which consists in a sharp increase in the relaxation time near the critical point during the formation of surface polymer layers on the surface of a solid.

Dynamic of orientation processes in liquid crystal systems of technical devices

The relaxation properties of nematic liquid crystals by the pulsed method of a fixed distance in a rotating magnetic field are investigated. The adequacy of the solution of the equation of motion of the nematic director in the conditions of asynchronous motion of the director and the magnetic field induction vector was experimentally confirmed. The values of the rotational viscosity coefficient and the orientation relaxation time that characterize the parameters of devices with a liquid-crystal working medium in a wide range of pressures and temperatures are calculated. In the framework of the free volume theory, the dependence of the rotational viscosity coefficient on temperature and pressure is analyzed.

Capturing the Effects of Explicit Waters in Implicit Electrostatics Modeling: Qualitative Justification of Gaussian-Based Dielectric Models in DelPhi.

Our group has implemented a smooth Gaussian-based dielectric function in DelPhi (J. Chem. Theory Comput. 2013, 9 (4), 2126-2136) which models the solute as an object with inhomogeneous dielectric permittivity and provides a smooth transition of dielectric permittivity from surface-bound water to bulk solvent. Although it is well-understood that the protein hydrophobic core is less polarizable than the hydrophilic protein surface, less attention is paid to the polarizability of water molecules inside the solute and on its surface. Here, we apply explicit water simulations to study the behavior of water molecules buried inside a protein and on the surface of that protein and contrast it with the behavior of the bulk water. We selected a protein that is experimentally shown to have five cavities, most of which are occupied by water molecules. We demonstrate through molecular dynamics (MD) simulations that the behavior of water in the cavity is drastically different from that in the bulk. These observations were made by comparing the mean residence times, dipole orientation relaxation times, and average dipole moment fluctuations. We also show that the bulk region has a nonuniform distribution of these tempo-spatial properties. From the perspective of continuum electrostatics, we argue that the dielectric "constant" in water-filled cavities of proteins and the space close to the molecular surface should differ from that assigned to the bulk water. This provides support for the Gaussian-based smooth dielectric model for solving electrostatics in the Poisson-Boltzmann equation framework. Furthermore, we demonstrate that using a well-parametrized Gaussian-based model with a single energy-minimized configuration of a protein can also reproduce its ensemble-averaged polar solvation energy. Thus, we argue that the Gaussian-based smooth dielectric model not only captures accurate physics but also provides an efficient way of computing ensemble-averaged quantities.

Effect of Polymer Concentration on the Structure and Dynamics of Short Poly(N,N-dimethylaminoethyl methacrylate) in Aqueous Solution: A Combined Experimental and Molecular Dynamics Study

A combined experimental and molecular dynamics (MD) study is performed to investigate the effect of polymer concentration on the zero shear rate viscosity η0 of a salt-free aqueous solution of poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA), a flexible thermoresponsive weak polyelectrolyte with a bulky 3-methyl-1,1-diphenylpentyl unit as the terminal group. The study is carried out at room temperature (T = 298 K) with relatively short PDMAEMA chains (each containing N = 20 monomers or repeat units) at a fixed degree of ionization (α+ = 100%). For the MD simulations, a thorough validation of several molecular mechanics force fields is first undertaken for assessing their capability to accurately reproduce the experimental observations and established theoretical laws. The generalized Amber force field in combination with the restrained electrostatic potential charge fitting method is eventually adopted. Three characteristic concentration regimes are considered: the dilute (from 5 to 10 wt %), the semidilute (from 10 to 20 wt %), and the concentrated (from 20 to 29 wt %); the latter two are characterized by polymer concentrations cp higher than the characteristic overlap concentration cp*. The structural behavior of the PDMAEMA chains in the solution is assessed by calculating the square root of their mean-square radius of gyration ⟨Rg2⟩0.5, the square root of the average square chain end-to-end distance ⟨Ree2⟩0.5, the ratio ⟨Ree2⟩/⟨Rg2⟩, and the persistence length Lp. It is observed that at low polymer concentrations, PDMAEMA chains adopt a stiffer and slightly extended conformation because of excluded-volume effects (a good solvent is considered in this study) and electrostatic repulsions within the polymer chains. As the polymer concentration increases above 20 wt %, the PDMAEMA chains adopt more flexible conformations, as the excluded-volume effects seize and the charge repulsion within the polymer chains subsides. The effect of total polymer concentration on PDMAEMA chain dynamics in the solution is assessed by calculating the orientational relaxation time τc of the chain, the center-of-mass diffusion coefficient D, and the zero shear rate viscosity η0; the latter is also measured experimentally here and found to be in excellent agreement with the MD predictions.

The mesoscopic collective motion of self-propelling active particle suspension confined in two-dimensional micro-channel

The mesoscopic collective motion of self-propelling active particle suspension confined in high aspect ratio two-dimensional micro-channel is numerically studied through coupled equation by considering background thermal fluctuation, inter-particle interaction, self-propulsion and micro-channel confinement. Both the self-propulsion and micro-channel confinement are the factors driving the system away from equilibrium and sustaining heterogeneous motion. In such system, the propulsion induced particle accumulation around the channel walls is a universal phenomenon with spatial heterogeneity, where large fraction of particles are caged inside the accumulated cluster with local oscillation coexisting with few fast propelling particles in the center region. Although the formation mechanism of the induced accumulation is well studied, post the cluster formation, how the cluster evolves and its dynamical properties is rarely discussed. Based on the merits of equation, the dynamical evolution of induced accumulation is revealed by particle trajectories. It is found that the induced accumulation can be dissociated through the slow re-orientation process of few jammed particles. By using the idea of force chain network, how the transverse confinement couples the transverse displacement with the longitudinal displacement is evidenced. It is further verified by the statistical measurement of correlation probability between transverse and longitudinal displacements. The suppressed displacements in both directions is the origin leading to the slow dynamics of cluster evolution. Temporally, within the orientational relaxation time, this system exhibits non-trivial anomalous diffusion under the competition between the counter effects of self-propulsion (enhanced diffusion) and micro-channel confinement (suppressed diffusion). Additionally, by considering the orientational coupling, the deep hysteresis of accumulation has been found even for very weak orientational coupling strength.

Dipole-relaxation dynamics in a modified polythiourea with high dielectric constant for energy storage applications

Enhancing dipole polarization has been demonstrated as an effective approach to increase the dielectric constant of polymer dielectrics and thus to improve their discharged energy density for energy storage applications in electrical power and electronic systems. However, further optimization to get a higher dielectric constant and lower polarization loss hinges upon a more insightful understanding of the dynamics of dipole relaxation. Here, we demonstrate an approach, based on the Dissado-Hill dielectric response model, to probe the dynamics of dipole polarization in an all organic polythiourea (PDTC-HK511) composed of p-phenylene diisothiocyanate (PDTC) as rigid segments and Jeffamine® HK511 as flexible segments. Our results reveal that PDTC-HK511 possesses strong subglass transition β and γ relaxation processes in conjunction with the quasi-DC diffusion process at relatively high temperature and a low frequency range. The γ relaxation enhances the dielectric constant by 20%–25% but causes no apparent loss at the operating temperature and frequency conditions due to the high flexibility and short relaxation time of the dipole orientation. In comparison to β relaxation, the weak interactions and much shorter relaxation time of dipoles in γ relaxation evidenced the large flexibility of dipole movement. This work provides deeper insight into the dipole movement and aids future rational designs of polymers for dielectric energy storage.

Structure and Dynamics of Complex Ions in YbCl3–MCl (M = K, Cs) Melts Based on Raman Spectroscopy Data

Structural information on high-temperature ionic melts YbCl3–MCl (M = K, Cs) was generated in situ using Raman spectroscopy. Force constants of Yb –Cl bonds and orientational relaxation times of $$ YbC{l}_6^{3-} $$ complex ions were calculated. Empirical equations describing the correlation between $$ YbC{l}_6^{3-} $$ dynamic behavior and the conductivity of the studied melts were obtained.

A conductive mechanism of PVA (Mowiol 10-98) filled with ZnO and MWCNT nanoparticles

Polyvinyl alcohol (PVA) hybrid nanocomposites are prepared via an ex situ approach with ZnO and MWCNT nanoparticle fillers and their conductive mechanisms have been investigated. The tailored hybrid nanocomposite conformation and their microstructural disparities for different filler concentrations were studied using an X-ray diffractometer. The direct current (DC) conductivity studies show an increase in the conductivity from \(1.0528\times 10^{-11}\) to \(2.1514\times 10^{-8}\hbox { S cm}^{-1}\) up to a percolation threshold filler concentration of \(x=7.5\) wt%. The dielectric constant substantially indicates a decreasing trend with increasing frequency. The exaggerated dielectric constant values of 11.8 at 5 kHz, 6.3 at 100 kHz, 5.86 at 500 kHz and 2 at 1 MHz are observed for 7.5 wt% filler hybrid nanocomposites, which indicates their potential application as a gate material in metal-oxide-semiconductor field-effect transistors (MOSFETs). The alternating current (AC) electrical conductivity demonstrates an increasing behaviour up to \(x=7.5\) wt% filler concentration. The smaller values observed in the real part of the electric modulus (\({M}^{\prime })\) indicates a riddance in electrode polarization. The observed higher frequency shift in the imaginary part of the electric modulus for increasing the filler concentration up to \(x=7.5\) wt%, decreases the relaxation time of the dipole orientation thereby increasing the conductivity mechanism of the hybrid nanocomposites. Apart from these, its small relaxation time with high electrical conductivity favours this material PVA/(x)MWCNT(\(15-x\))ZnO to have prospective application in microwave absorption appliances. The increase in the surface roughness of the film seen from the AFM images up to \(x=7.5\) wt% concentration supports an enhancement in the crystalline nature of the filler. Differential scanning calorimeter studies show an enhancement in glass transition temperature (\(T_{\mathrm{g}})\), melting temperature (\(T_{\mathrm{m}})\) and decomposition temperature (\(T_{\mathrm{d}})\) for PVA filled with MWCNTs and ZnO composites for optimum filler concentration \(x=7.5\) wt%.

Counterion Effect on Vibrational Relaxation and the Rotational Dynamics of Interfacial Water and an Anionic Vibrational Probe in the Confined Reverse Micelles Environment

Vibrational relaxation and the rotational dynamics of water molecules encapsulated in reverse micelles (RMs) have been investigated by ultrafast infrared (IR) spectroscopy and two-dimensional IR (2D IR) spectroscopy. By changing the counterion of the hydrophilic headgroup in the RMs formed by Aerosol-OT (AOT) from Na+ to K+, Cs+ and Ca2+, we could determine the specific counterion effects on the rotational dynamics of water molecules. The orientational relaxation time constant of water decreases in the order Ca2+ > Na+ > K+ > Cs+. The SCN- anionic probe and counterion can form ion pairs at the interfacial region of the RMs. The rotational dynamics of SCN- anion significantly decreases because of the synergistic effects of confinement and the surface interactions in the interfacial region of the RMs. The results can provide a new understanding of the cationic Hofmeister effect at the molecular level observed in biological studies.

Behaviour of cis- and trans-N-methylformamide in liquid mixture: Dynamical properties at varying pressure and temperature, and ion solvation scenario

Classical molecular dynamics simulations have been carried out to investigate dynamical properties of cis-trans mixture of liquid N-methylformamide at eight different temperatures (278–423 K) and four pressures ranging from 0.1 MPa to 300 MPa. At 298 K and 0.1 MPa, it is observed that cis-NMF is impeded by approximately 4.12% in its diffusion rate than trans-NMF. Higher pressures evoke slower dynamics concomitantly with enhanced caging effect, which is appreciable in case of cis-NMF. Calculated activation energy values for diffusion (13.465 ± 0.21–15.641 ± 0.51 kJ/mol) are in agreement with experimental observations, particularly at 0.1 MPa. Unlike self-diffusion behaviour, the orientational relaxation time of NH bond vector follows Arrhenius behaviour even at lower temperature and higher pressure. Hydrogen bonding patterns are observed to be different for H-bonds involving amide-hydrogen of either trans-NMF or cis-NMF molecules and overall interaction probability is boosted by the participation of cis-NMF conformers. Solvation scenario of ions of different charge and size in cis/trans-NMF mixture has also been explored. It is found that cations favour oxygen of cis-NMF and anions are attracted towards hydrogen sites of trans-NMF while intermediately-sized potassium and bromide ions may deviate in their preferences from the usual trend.

Parameters of LC molecules’ movement measured by dielectric spectroscopy in wide temperature range

Dielectric properties of a nematic liquid crystal (NLC) mixture ZhK-1282 were investigated in the frequency range of 102–106Hz and a temperature range of −20 to 80 °С. On the basis of the Debye’s relaxation polarization model dielectric spectra of temperature dependence of the orientational relaxation time τ and the dielectric strength δe were numerically approximated at the parallel orientation of a molecular director relative to alternating electric field. Influence of ester components in the mixture plays crucial role in relaxation processes at low temperature and external field frequency. The activation energy of the relaxation process of a rotation of molecules around their short axis was measured in a temperature interval of −20 to +15 °С in which the dispersion of a longitudinal component of the dielectric constant takes place. The energy of potential barrier for polar molecules rotation in the mesophase was calculated. The value of the transition entropy from the nematic to isotropic phase was obtained from this calculation. The values of the coefficient of molecular friction and rotational diffusion were obtained by different methods. The experimental data obtained are in a satisfactory agreement with the existing theoretical models.