Electric quadrupole model on the formation of molecular chirality dependent domain shapes of lipid monolayers at the air-water interface

Shapes and orientational deformation of a lipid monolayer domain have been analyzed taking into account the surface pressure, line tension, and electrostatic energy due to the spontaneous polarization and electric quadrupole density generated from the domain. The electrostatic energy due to the generation of spontaneous polarization and electric quadrupole density contributes to the formation of orientational deformation as the Frank elastic energy and spontaneous splay, respectively. Since the orientational configuration of the electric quadrupole density and in-plane spontaneous polarization is dependent on the molecular chirality, and the positive splay deformation of electric quadrupole density is induced by the spontaneous splay, the bending direction of in-plane spontaneous polarization depends on the chirality of constituent lipids. The electrostatic energy due to the in-plane spontaneous polarization is dependent on the orientational deformation of in-plane spontaneous polarization, and bends the domain shape towards the bending direction of the in-plane spontaneous polarization. It has been demonstrated that the chiral dependence of the domain shapes of lipid monolayers originated from the chiral dependence of orientational structure due to the electric quadrupole density.     

Orientational and interaction induced dynamics in the isotropic phase of a liquid crystal: polarization resolved ultrafast optical Kerr effect spectroscopy

The ultrafast dynamics of the isotropic phase of a liquid crystal 4'-pentyl-4-p-biphenylcarbonitrile (5CB) have been investigated using polarization resolved optical Kerr effect spectroscopy. Measurements were made as a function of both temperature and dilution in nonpolar solvents. To separate single molecule and interaction induced components to the relaxation of the induced birefringence, measurements of both the anisotropic and isotropic response were made. The isotropic response was found to be dominated by a damped low-frequency mode of intramolecular origin. There is a minor additional component assigned to an interaction induced contribution. There is at most an extremely weak isotropic signal beyond 1 ps, showing that the picosecond time scale dynamics of 5CB are dominated by orientational relaxation. The isotropic response is independent of temperature in the range studied (0.2-50 K above the nematic to isotropic phase-transition temperature). The anisotropic response exhibits relaxation dynamics on time scales spanning subpicosecond to several hundred picoseconds and beyond. The fastest components are dominated by a librational response, but there are smaller contributions from three low-frequency intramolecular modes, and a contribution from interaction induced effects. The low-frequency spectral density extracted from these data are independent of temperature in the range studied, 0.2-30 K above the phase-transition temperature, but shift to lower frequency on dilution in alkane solvents. In neat 5CB the picosecond time scale orientational dynamics are dominated by temperature-independent reorientation within the pseudonematic domains, while in solution these are disrupted, and the orientational response becomes faster and temperature dependent.     

Temperature- and solvation-dependent dynamics of liquid sulfur dioxide studied through the ultrafast optical Kerr effect

The ultrafast dynamics of liquid sulphur dioxide have been studied over a wide temperature range and in solution. The optically heterodyne-detected and spatially masked optical Kerr effect (OKE) has been used to record the anisotropic and isotropic third-order responses, respectively. Analysis of the anisotropic response reveals two components, an ultrafast nonexponential relaxation and a slower exponential relaxation. The slower component is well described by the Stokes-Einstein-Debye equation for diffusive orientational relaxation. The simple form of the temperature dependence and the agreement between collective (OKE) and single molecule (e.g., NMR) measurements of the orientational relaxation time suggests that orientational pair correlation is not significant in this liquid. The relative contributions of intermolecular interaction-induced and single-molecule orientational dynamics to the ultrafast part of the spectral density are discussed. Single-molecule librational-orientational dynamics appear to dominate the ultrafast OKE response of liquid SO2. The temperature-dependent OKE data are transformed to the frequency domain to yield the Raman spectral density for the low-frequency intermolecular modes. These are bimodal with the lowest-frequency component arising from diffusive orientational relaxation and a higher-frequency component connected with the ultrafast time-domain response. This component is characterized by a shift to higher frequency at lower temperature. This result is analyzed in terms of a harmonic librational oscillator model, which describes the data accurately. The observed spectral shifts with temperature are ascribed to increasing intermolecular interactions with increasing liquid density. Overall, the dynamics of liquid SO2 are found to be well described in terms of molecular orientational relaxation which is controlled over every relevant time range by intermolecular interactions.     

Dynamics of ultrasonically induced birefringence of in rod-like colloidal solutions

This review summarized our experimental studies of ultrasonically induced birefringence on aqueous solution of rigid rod-like colloids and rod-like micelles from the view point of dynamics of particle orientation. For rigid rod-like colloids in dilute concentration, the orientational relaxation time was described in terms of the Debye–Einstein equation. This indicated that the ultrasonically induced birefringence could be one of useful tools for determining the size of particles of anisotropic shape. For rod-like micelles, the birefringence showed anomalous damping oscillation when the rod-like micelle was in an entangling state. The mechanism of damping oscillation was discussed using the results of the rheological measurements.     

Local orientational mobility in dendrimers. Theory and computer-aided simulation

The orientational mobility of segments in dendrimers are studied by the method of Brownian dynamics, and the results are analyzed in terms of an earlier proposed analytical theory. The orientational autocorrelation function for the cosine of an angle of segmental rotation in dendrimers of a given generation P ₁(t) is controlled by three relaxation processes with the corresponding relaxation times. Characteristic times and the contribution from the above processes to P ₁(t) are calculated. The first process refers to the local mobility of a selected segment; the second process, to the rotations of a dendrimer branch, which originates from the selected segment of a given generation; and the third process, to the rotation of a dendrimer macro-molecule as a whole. The proposed approach makes it possible to estimate the relaxation spectrum of a dendrimer by studying the orientational mobility of segments in different generations. The relaxation times can be used to describe various relaxation processes observed by different experimental methods, such as dielectric relaxation, NMR, dynamic birefringence, and polarized luminescence.     

In search of temporal power laws in the orientational relaxation near isotropic-nematic phase transition in model nematogens

Recent Kerr relaxation experiments by Gottke et al. have revealed the existence of a pronounced temporal power law decay in the orientational relaxation near the isotropic-nematic phase transition (INPT) of nematogens of rather small aspect ratio, kappa (kappa approximately 3-4). We have carried out very long (50 ns) molecular dynamics simulations of model (Gay-Berne) prolate ellipsoids with aspect ratio 3 in order to investigate the origin of this power law. The model chosen is known to undergo an isotropic to nematic phase transition for a range of density and temperature. The distance dependence of the calculated angular pair correlation function correctly shows the emergence of a long range correlation as the INPT is approached along the density axis. In the vicinity of INPT, the single particle second rank orientational time correlation function exhibits power law decay, (t(-alpha)) with exponent alpha approximately 2/3. More importantly, we find the sudden appearance of a pronounced power-law decay in the collective part of the second rank orientational time correlation function at short times when the density is very close to the transition density. The power law has an exponent close to unity, that is, the correlation function decays almost linearly with time. At long times, the decay is exponential-like, as predicted by Landau-de Gennes mean field theory. Since Kerr relaxation experiments measure the time derivative of the collective second rank orientational pair correlation function, the simulations recover the near independence of the signal on time observed in experiments. In order to capture the microscopic essence of the dynamics of pseudonematic domains inside the isotropic phase, we introduce and calculate a dynamic orientational pair correlation function (DOPCF) obtained from the coefficients in the expansion of the distinct part of orientational van Hove time correlation function in terms of spherical harmonics. The DOPCF exhibits power law relaxation when the pair separation length is below certain critical length. The orientational relaxation of a local director, defined in terms of the sum of unit vectors of all the ellipsoidal molecules, is also found to show slow power law relaxation over a long time scale. These results have been interpreted in terms of a newly developed mode coupling theory of orientational dynamics near the INPT. In the present case, the difference between the single particle and the collective orientational relaxation is huge which can be explained by the frequency dependence of the memory kernel, calculated from the mode coupling theory. The relationship of this power law with the one observed in a supercooled liquid near its glass transition temperature is explored.     

A first principles simulation study of fluctuations of hydrogen bonds and vibrational frequencies of water at liquid-vapor interface

We present a theoretical study of the structure and dynamics of water-vapor interface by means of ab initio molecular dynamics simulations. The inhomogeneous density, hydrogen bond and orientational profiles, voids and vibrational frequency distributions are investigated. We have also studied various dynamical properties of the interface such as diffusion, orientational relaxation, hydrogen bond dynamics and vibrational frequency fluctuations. The diffusion and orientational relaxation of water molecules are found to be faster at the interface which can be correlated with the voids present in the system. The hydrogen bond dynamics, however, is found to be slightly slower at the interface than that in bulk water. The correlations of hydrogen bond relaxation with the dynamics of vibrational frequency fluctuations are also discussed.     

Brownian dynamics investigation of magnetization and birefringence relaxations in ferrofluids

Brownian dynamics simulations are used to investigate the dynamics of orientational properties of real charge-stabilized ferrofluids, i.e. stable colloidal dispersions of magnetic nanoparticles. The relaxation times of the magnetization and of the birefringence, data accessible by experimental techniques, have been computed at several volume fractions. Besides, the effect of hydrodynamic interactions has been investigated. Equilibrium simulations without field are found to be inadequate to determine the aforementioned relaxation times for the systems under study, the dipolar interaction being too weak. Thus a nonequilibrium simulation procedure that mimics the experimental operating mode has been developed. After equilibrium simulations under a magnetic field, both birefringence and magnetization decays are recorded once the field is suppressed. Birefringence and magnetization decays are markedly impeded as the volume fraction increases, whereas they are barely enhanced when the intensity of the initial magnetic field is increased at a fixed volume fraction. Eventually, hydrodynamic interactions exhibit a slight but systematic lengthening of the relaxation times.     

Molecular dynamics investigation of a density-driven glass transition in a liquid crystal system

Molecular dynamics simulations are carried out to address the density-driven glass transition in a system of rodlike particles that interact with the Gay-Berne potential. Since crystallization occurs in this system on the time scale of the simulations, direct simulation of the glass transition is not possible. Instead, glasses with isotropic orientational order are heated to a temperature T, and the relaxation times by which nematic orientational order develops are determined. These relaxation times appear to diverge at a critical density rho(c); i.e., the system can equilibrate at rhorho(c) (at the temperature T). The relaxation times follow a power-law scaling as the critical density is approached, suggesting that this density-driven glass transition concurs with mode coupling theory.     

Characteristic behavior of short-term dynamics in reorientation for Gay-Berne particles near the nematic-isotropic phase transition temperature

A specific transition behavior was found in the tumbling motion near the nematic-isotropic phase boundary using molecular dynamics simulations of the Gay-Berne mesogenic model under isobaric conditions at a reduced pressure P* of 2.0. The relaxation time for the motion obtained from the second-rank orientational time correlation function and the rotational diffusion coefficient showed a clear jump at the nematic-isotropic phase transition temperature. Regardless of the temperature dependence of the relaxation time, the change in the rotational diffusion coefficient evaluated from the orientational order parameters and the relaxation time agreed qualitatively with that of real mesogens. The rotational viscosity coefficients gamma(1) and gamma(2) were obtained from the simulation data for the relaxation time for the short-term dynamics and for the rotational diffusion coefficients. gamma(1) was proportional to (2), where is the second-rank orientational parameter. Furthermore, the rotational behavior of the model was compared with that of the Debye approximation in the isotropic phase.     

Nuclear magnetic resonance and dielectric noise study of spectral densities and correlation functions in the glass forming monoalcohol 2-ethyl-1-hexanol

The spectral densities related to various relaxation processes of the glass former 2-ethyl-1-hexanol (2E1H), a monohydroxy alcohol, are probed using several nuclear magnetic resonance (NMR) experiments as well as via dielectric noise spectroscopy (DNS). On the basis of the spectral density relating to voltage fluctuations, i.e., without the application of external electrical fields, DNS enables the detection of the structural relaxation and of the prominent, about two decades slower Debye process. The NMR-detected spectral density, sensitive to the orientational fluctuations of the hydroxyl deuteron, also reveals dynamics slower than the structural relaxation, but not as slow as the Debye process. Rotational and translational correlation functions of 2E1H are probed using stimulated-echo NMR techniques which could only resolve the structural dynamics or faster processes. The experimental results are discussed with reference to models that were suggested to describe the dynamics in supercooled alcohols.     

Solvent and rotational relaxation of coumarin 153 in a protic ionic liquid dimethylethanolammonium formate

The solvent relaxation and orientational dynamics of coumarin 153 (C-153) was investigated in N,N-dimethylethanolammonium formate (DAF) with a variation of temperature. DAF is a protic room-temperature ionic liquid, comprised of nonaromatic cations. Both solvent relaxation and orientational dynamics of C-153 in DAF are linearly well-correlated with the bulk viscosity at different temperatures. We optimized the geometry of DAF using quantum chemical calculations using density functional theory methods. The optimized structure of DAF shows a nonbonded interaction between cation and anion, which suggests that a hydrogen bond is formed between hydrogen atoms attached to the nitrogen atom of the cation with the oxygen atom of the anion in DAF.     

Multiple short time power laws in the orientational relaxation of nematic liquid crystals

Relaxation in the nematic liquid crystalline phase is known to be sensitive to its proximity to both isotropic and smectic phases. Recent transient optical Kerr effect (OKE) studies have revealed, rather surprisingly, two temporal power laws at short to intermediate times and also an apparent absence of the expected exponential decay at longer times. In order to understand this unusual dynamics, we have carried out extensive molecular dynamics simulations of transient OKE and related orientational time correlation functions in a system of prolate ellipsoids (with aspect ratio equal to 3). The simulations find two distinct power laws, with a crossover region, in the decay of the orientational time correlation function at short to intermediate times (in the range of a few picoseconds to a few nanoseconds). In addition, the simulation results fail to recover any long time exponential decay component. The system size dependence of the exponents suggests that the first power law may originate from the local orientational density fluctuations (like in a glassy liquid). The origin of the second power law is less clear and may be related to the long range fluctuations (such as smecticlike density fluctuations)--these fluctuations are expected to involve small free energy barriers. In support of the latter, the evidence of pronounced coupling between orientational and spatial densities at intermediate wave numbers is presented. This coupling is usually small in normal isotropic liquids, but it is large in the present case. In addition to slow collective orientational relaxation, the single particle orientational relaxation is also found to exhibit slow dynamics in the nematic phase in the long time.     

Orientational dynamics and energy landscape features of thermotropic liquid crystals: An analogy with supercooled liquids

Recent optical kerr effect (OKE) studies have revealed that orientational relaxation of rodlike nematogens near the isotropic-nematic (I-N) phase boundary and also in the nematic phase exhibit temporal power law decay at intermediate times. Such behaviour has drawn an intriguing analogy with supercooled liquids. Here, we have investigated the single-particle and collective orientational dynamics of a family of model system of thermotropic liquid crystals using extensive computer simulations. Several remarkable features of glassy dynamics are on display including non-exponential relaxation, dynamical heterogeneity, and non-Arrhenius temperature dependence of the orientational relaxation time. Over a temperature range near the I-N phase boundary, the system behaves like a fragile glass-forming liquid. Using proper scaling, we construct the usual relaxation time versus inverse temperature plot and explicitly demonstrate that one can successfully define a density dependent fragility of liquid crystals. The fragility of liquid crystals shows a temperature and density dependence which is remarkably similar to the fragility of glass forming supercooled liquids. Energy landscape analysis of inherent structures shows that the breakdown of the Arrhenius temperature dependence of relaxation rate occurs at a temperature that marks the onset of the growth of the depth of the potential energy minima explored by the system.     

Relaxation and jump dynamics of water at the mica interface

The orientational relaxation dynamics of water confined between mica surfaces is investigated using molecular dynamics simulations. The study illustrates the wide heterogeneity that exists in the dynamics of water adjacent to a strongly hydrophilic surface such as mica. Analysis of the survival probabilities in different layers is carried out by normalizing the corresponding relaxation times with bulk water layers of similar thickness. A 10-fold increase in the survival times is observed for water directly in contact with the mica surface and a non-monotonic variation in the survival times is observed moving away from the mica surface to the bulk-like interior. The orientational relaxation time is highest for water in the contact layer, decreasing monotonically away from the surface. In all cases the ratio of the relaxation times of the 1st and 2nd rank Legendre polynomials of the HH bond vector is found to lie between 1.5 and 1.9 indicating that the reorientational relaxation in the different water layers is governed by jump dynamics. The orientational dynamics of water in the contact layer is particularly novel and is found to undergo distinct two-dimensional hydrogen bond jump reorientational dynamics with an average waiting time of 4.97 ps. The waiting time distribution is found to possess a long tail extending beyond 15 ps. Unlike previously observed jump dynamics in bulk water and other surfaces, jump events in the mica contact layer occur between hydrogen bonds formed by the water molecule and acceptor oxygens on the mica surface. Despite slowing down of the water orientational relaxation near the surface, life-times of water in the hydration shell of the K(+) ion are comparable to that observed in bulk salt solutions.     

Testing the core/shell model of nanoconfined water in reverse micelles using linear and nonlinear IR spectroscopy

A core/shell model has often been used to describe water confined to the interior of reverse micelles. The validity of this model for water encapsulated in AOT/isooctane reverse micelles ranging in diameter from 1.7 to 28 nm (w0 = 2-60) and bulk water is investigated using four experimental observables: the hydroxyl stretch absorption spectra, vibrational population relaxation times, orientational relaxation rates, and spectral diffusion dynamics. The time dependent observables are measured with ultrafast infrared spectrally resolved pump-probe and vibrational echo spectroscopies. Major progressive changes appear in all observables as the system moves from bulk water to the smallest water nanopool, w0 = 2. The dynamics are readily distinguishable for reverse micelle sizes smaller than 7 nm in diameter (w0 = 20) compared to the response of bulk water. The results also demonstrate that the size dependent absorption spectra and population relaxation times can be quantitatively predicted using a core-shell model in which the properties of the core (interior of the nanopool) are taken to be those of bulk water and the properties of the shell (water associated with the headgroups) are taken to be those of w0 = 2. A weighted sum of the core and shell components reproduces the size dependent spectra and the nonexponential population relaxation dynamics. However, the same model does not reproduce the spectral diffusion and the orientational relaxation experiments. It is proposed that, when hydrogen bond structural rearrangement is involved (orientational relaxation and spectral diffusion), dynamical coupling between the shell and the core cause the water nanopool to display more homogeneous dynamics. Therefore, the absorption spectra and vibrational lifetime decays can discern different hydrogen bonding environments whereas orientational and spectral diffusion correlation functions predict that the dynamics are size dependent but not as strongly spatially dependent within a reverse micelle.     

The combined effect of cations and anions on the dynamics of water

We studied the orientational relaxation of the OD-stretch vibration of HDO molecules in concentrated solutions of alkali-halide salts (NaCl, NaI, CsCl and KI) in isotopically diluted water (4% D(2)O in H(2)O), using polarization-resolved femtosecond infrared pump-probe spectroscopy (fs-IR). We were able to distinguish the orientational dynamics of the water molecules solvating the halide ions from the dynamics of the bulk water and the water solvating the cations. We found that the reorientation of the halide-bound molecules shows two strongly different components (2.0 ± 0.3 ps and 9 ± 1 ps), related to a wiggling motion of the OD group hydrogen-bonded to the anion, and rotational diffusion of the molecule over the charged anion surface, respectively. The relative amplitudes of the two components are dependent on the nature of both the anion and cation, and on the concentration. These results show that cations can have a profound effect on the solvation shell dynamics of their counter-ions.     

Orientational correlations in two-dimensional liquid crystals studied by molecular dynamics simulation

Orientational correlations in Langmuir monolayers of nematic and smectic-C liquid crystal (LC) phases are investigated by molecular dynamics simulation. In both phases, the orientational correlation functions decay algebraically yet with the different exponents of 1.9 and 0.2 for the nematic and the smectic-C monolayers, respectively. The power law decay, i.e., the absence of long-range orientational order, means the both monolayers should be the ideal 2D system with a continuous symmetry, whereas the large difference in the exponents of power law gives rise to the crucial difference in their optical properties; the nematic monolayer is optically isotropic while the smectic-C monolayer exhibits an anisotropy on the length scale of visible light. Since the exponent is inversely proportional to the molecular exchange energy, the averaged molecular interaction in the nematic monolayer should be an order of magnitude smaller than that in the smectic-C monolayer, which is ascribed to the low molecular density and the weak molecular dipole due to the water molecule. The relation between the molecular interaction and the orientational correlation calculated for the 2D LC system offers much information not only about the 2D LCs but also on the bulk system.     

Orientational–deformational nature of flow birefringence in solutions of rigid-chain polymers

The orientational–deformational nature of flow birefringence (FB) in solutions of rigid-chain polymers is confirmed experimentally. The possibility of a direct determination of the internal viscosity parameter by the dependence of the FB relaxation time on the solvent viscosity is demonstrated. Indirect manifestation of internal viscosity in the FB relaxation time is confirmed by the dependence of the form factor on the degree of coiling. © 1993 John Wiley & Sons, Inc.     

Simulations of the dynamics of thermal undulations in lipid bilayers in the tensionless state and under stress

The relaxation processes of height undulations and density fluctuations in a membrane have been studied by molecular dynamics simulations of a coarse grained amphiphilic bilayer model. We observe a double exponential decay in their time correlations, with relaxation rates in good quantitative agreement with the theory by Seifert and Langer [Europhys. Lett. 23, 71 (1993)]. Intermonolayer friction due to slippage between the two monolayers is shown to be the dominant dissipative mechanism at the high wave numbers, q>10 mum(-1), typically encountered in computer simulations. We briefly discuss the ramifications of the slow undulatory relaxation process for the calculation of bending rigidities from the static undulation structure factors. The relaxation rates are sensitive to the surface tension, and at high elongations an oscillatory contribution is observed in the time correlation of the undulations.