Molecular dynamics simulation of the nematic liquid crystal phase in the presence of an intense magnetic field

The influence of an intense external field on the dynamics of the nematic liquid crystal phase is investigated using a molecular dynamics simulation for the Gay-Berne nematogen under isobaric-isothermal conditions. The molecular dynamics as a function of the second-rank orientational order parameter P<2> for a system consisting of a nematic liquid crystal in the presence of an intense magnetic field is compared with that of a similar system without the field. The translational motion of molecules is determined as a function of the translational diffusion coefficient tensor and the anisotropy and compared with the values predicted theoretically. The rotational dynamics of molecules is analyzed using the first- and the second-rank orientational time correlation functions. The translational diffusion coefficient parallel with respect to the director is constrained by the intense field, although the perpendicular one is decreased as the P<2> is increased, just as it is in the system without the field. However, no essential effect of the strong magnetic field is observed in the rotational molecular dynamics. Further, the rotational diffusion coefficient parallel with respect to the director obtained from the first-rank orientational time correlation function in the simulation is qualitatively in agreement with that in the real nematic liquid crystalline molecules. The P<2> dependence of the rotational diffusion coefficient for the system with the intense magnetic field shows a tendency similar to that for the system without the field.     

Water rotational relaxation and diffusion in hydrated lysozyme

This paper is concerned with the dynamics of water around a small globular protein. Dipolar second-rank relaxation time and diffusion properties of surface water were computed by extensive molecular dynamics simulations of lysozyme in water which lasted a total of 28 ns. Our results indicate that the rotational relaxation of water in the vicinity of lysozyme is 3-7 times slower than that in the bulk depending on how the hydration shell is defined in the calculation. We have also verified that the dynamics of water translational diffusion in the vicinity of lysozyme have retardations similar to rotational relaxation. This is a common assumption in nuclear magnetic relaxation dispersion (NMRD) studies to derive residence times. In contrast to bulk water dynamics, surface water is in a dispersive diffusion regime or subdiffusion. Very good agreement of dipolar second-rank relaxation time with NMRD estimates is obtained by using appropriate dimensions of the hydration shell. Although our computed second-rank dipolar retardations are independent of the water model, SPC/E describes more realistically the time scale of the water dynamics around lysozyme than does TIP3P.     

Subdiffusive dynamics of a liquid crystal in the isotropic phase

The isotropic phase dynamics of a system of 4-n-hexyl-4'-cyano-biphenyl (6CB) molecules has been studied by molecular dynamics computer simulations. We have explored the range of 275-330 K keeping the system isotropic, although supercooled under its nematic transition temperature. The weak rototranslational coupling allowed us to separately evaluate translational (TDOF) and orientational degrees of freedom (ODOF). Evidences of subdiffusive dynamics, more apparent at the lowest temperatures, are found in translational and orientational dynamics. Mean square displacement as well as self-intermediate center of mass and rotational scattering functions show a plateau, also visible in the orientational correlation function. According to the mode coupling theory (MCT), this plateau is the signature of the beta-relaxation regime. Three-time intermediate scattering functions reveal that the plateau is related to a homogeneous dynamics, more extended in time for the orientational degrees of freedom (up to 1 ns). The time-temperature superposition principle and the factorization property predicted by the idealized version of MCT hold, again for both kinds of dynamics. The temperature dependence of diffusion coefficient and orientational relaxation time is well described by a power law. Critical temperatures Tc are 244+/-6 and 258+/-6 K, respectively, the latter is some 10 K below the corresponding experimental values. The different values of Tc we obtained indicate that ODOF freezes earlier than TDOF. This appears due to the strongly anisotropic environment that surrounds a 6CB molecule, even in the isotropic phase. The lifetime of these "cages," estimated by time dependent conditional probability functions, is strongly temperature dependent, ranging from some hundreds of picoseconds at 320 K to a few nanoseconds at 275 K.     

Comparative study of temperature dependent orientational relaxation in a model thermotropic liquid crystal and in a model supercooled liquid

Recent optical Kerr effect experiments have revealed a power law decay of the measured signal with a temperature independent exponent at short-to-intermediate times for a number of liquid crystals in the isotropic phase near the isotropic-nematic transition and supercooled molecular liquids above the mode coupling theory critical temperature. In this work, the authors investigate the temperature dependence of short-to-intermediate time orientational relaxation in a model thermotropic liquid crystal across the isotropic-nematic transition and in a binary mixture across the supercooled liquid regime in molecular dynamics simulations. The measure of the experimentally observable optical Kerr effect signal is found to follow a power law decay at short-to-intermediate times for both systems in agreement with recent experiments. In addition, the temperature dependence of the power law exponent is found to be rather weak. As the model liquid crystalline system settles into the nematic phase upon cooling, the decay of the single-particle second-rank orientational time correlation function exhibits a pattern that is similar to what has been observed for supercooled liquids.     

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.     

Extended rotational diffusion and orientational relaxation of symmetric top molecules in a strong dc electric field: second-rank orientational correlation functions

Second-rank orientational correlation functions (pertaining to Kerr effect relaxation and Raman scattering) are obtained using the extended rotational diffusion (J-diffusion) model of symmetric top polar molecules in a strong constant external field. It is shown that the shape of the molecule noticeably affects all second-rank correlation functions and relaxation times in the rare collision limit. In the opposite limit of frequent collisions, the quantities of interest are shown to be shape independent as a consequence of vanishingly small inertial effects. An interpolation formula for the orientation relaxation times in the intermediate regime between the rare and frequent collision limits is also given.     

Effect of temperature and pressure on the structure, dynamics, and hydrogen bond properties of liquid N-methylacetamide: a molecular dynamics study

The structure and dynamical properties of liquid N-methylacetamides (NMA) are calculated at five different temperatures and at four different pressures using classical molecular dynamics simulations. Our results are analyzed in terms of pressure-induced changes in structural properties by investigating the radial distribution functions of different atoms in NMA molecule. It is found that the first peak and also the second peak of C-O and N-H are well defined even at higher temperature and pressure. It is also observed that the number of hydrogen bonds increase with application of pressure at a given temperature. On the other hand, the calculated hydrogen bond energy (E(HB)) shows that the stability of hydrogen bond decreases with increasing of pressure and temperature. Various dynamical properties associated with translational and rotational motion of neat NMA are calculated and the self-diffusion coefficient of NMA is found to be in excellent agreement with the experiment and the behavior is non-Arrhenius at low temperatures with application of pressures. The single particle orientational relaxation time for dipole vector and N-C vector are also calculated and it is found that the orientational relaxation time follows Arrhenius behavior with a variation of temperature and pressure.     

Exponential probe rotation in glass-forming liquids

Using time resolved optical depolarization, we have studied the rotational behavior of molecular probes in supercooled liquids near the glass transition temperature T(g). Simultaneously, the dynamics of the liquid immediately surrounding these rigid probes is measured by triplet state solvation experiments. This direct comparison of solute and solvent dynamics is particularly suited for assessing the origin of exponential orientational correlation functions of probe molecules embedded in liquids which exhibit highly nonexponential structural relaxation. Polarization angle dependent Stokes shift correlation functions demonstrate that probe rotation time and solvent response time are locally correlated quantities in the case of smaller probe molecules. Varying the size of both guest and host molecules shows that the size ratio determines the rotational behavior of the probes. The results are indicative of time averaging being at the origin of exponential rotation of probes whose rotational time constant is slower than solvent relaxation by a factor of 20 or more.     

Solvation dynamics and electric field relaxation in an imidazolium-PF6 ionic liquid: from room temperature to the glass transition

Time-resolved phosphorescence spectra and anisotropy of quinoxaline were measured in an ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-HFP), in its supercooled state near the glass-transition temperature. The solvation dynamics results are compared with the rotational motion of the probe and with the dielectric behavior of the neat ionic liquid. The dynamics in the viscous state are highly dispersive and show a super-Arrhenius temperature dependence, as typical for glass-forming materials. Combined with room-temperature results, solvation dynamics is observed to follow the structural relaxation times in terms of eta/T for more than 10 decades, from subnanoseconds at room temperature to seconds near the glass-transition temperature T(g). The dielectric modulus relaxation follows this trend only for temperatures T > 1.2T(g) and departs significantly from eta/T in the 1.1T(g) > T > T(g) range. This deviation is reminiscent of the enhanced translational diffusion or fractional Stokes-Einstein behavior observed in many fragile supercooled liquids. Because the electric field relaxation in BMIM-HFP includes dc conductivity, this correlation function involves translational motion and thus displays the effect of enhanced diffusivity. A microscopic model is required for rationalizing the decoupling of solvation dynamics from the longitudinal time scales and the limitation of this effect to the viscous regime with T < 1.2T(g).     

Effects of temperature on the alignment and electrooptical responses of a nematic nanoscale liquid crystalline film

The surface-induced alignment and electrooptical (EO) dynamics of a 50-nm-thick liquid crystalline (4-n-pentyl-4'-cyanobiphenyl; 5CB) film were studied at three temperatures: 25 and 33 degrees C (near the crystalline-nematic and nematic-isotropic transition temperatures, respectively) and 29 degrees C (a median temperature in the stability region of the nematic phase). The ZnSe surfaces that entrap the liquid crystal (LC) film have been polished unidirectionally to produce a grooved surface presenting nanometer-scale corrugations, a structure that induces a planar and homogeneous orientation in the nematic phase. The present work attempts to understand the influences of temperature on the surface-induced alignment and corresponding EO dynamics of the material. Step-scan time-resolved spectroscopy measurements were made to determine the rate constants for the electric-field-induced orientation and thermal relaxation of the 5CB film. The field-driven orientation rates vary sensitively with temperature across a range that spans the stability limits of the nematic phase; the relaxation rates, however, vary very little across this same temperature range. We propose that these differences in LC behavior arise as consequence of the interplay of the temperature dependence of the elastic constants, viscosity, and degree of orientational order of the LC medium. A simple theoretical model provides some understanding of these behaviors.     

The plastic and liquid phases of CCl3Br studied by molecular dynamics simulations

We present a molecular dynamics study of the liquid and plastic crystalline phases of CCl(3)Br. We investigated the short-range orientational order using a recently developed classification method and we found that both phases behave in a very similar way. The only differences occur at very short molecular separations, which are shown to be very rare. The rotational dynamics was explored using time correlation functions of the molecular bonds. We found that the relaxation dynamics corresponds to an isotropic diffusive mode for the liquid phase but departs from this behavior as the temperature is decreased and the system transitions into the plastic phase.     

Relaxation processes in mixtures of liquid crystals and polymers near phase boundaries and during phase separation

We present experimental studies of the relaxation of concentration fluctuations in a semidilute solution of polystyrene (PS) (30% by weight) in 4-cyano-4'-n-octyl-biphenyl (8CB) (70% by weight) using the photon correlation spectroscopy (PCS). In the homogeneous phase there are two modes of relaxation. The slow one (typical time scale is taus = 0.001 s) is due to the diffusion of polymer chains (of molecular mass 65,000) in the LC matrix (of molecular mass 290), while the fast one has the time scale of the order of tauf approximately 0.00001 s. The amplitude of the fast mode is much weaker than the one for the slow mode. Moreover it does not depend on the scattering wave vector, q. The value of the diffusion coefficient, Dc = 1/(tausq2) for the slow mode decreases with temperature according to the Arhenius law until we reach the coexistence curve. Its value close to the coexistence is Dc = 4 x 10(5) nm2/s and the activation energy in the homogeneous mixture is Ec=127 kJ/mol. If we gradually undercool the mixture below the coexistence into the metastable two-phase region without inducing the phase separation we find unexpectedly that Dc does not change with temperature even 4 degrees below the coexistence curve. The characteristic time of the fast mode does not depend on the scattering wave vector indicating that it is related to the transient gel structure. We have shown that it is possible to measure the short time relaxation of concentration fluctuations during the phase separation in the mixture. At low temperature close to the isotropic-nematic phase transition we have observed that the relaxation is well separated in time from the typical time of the domain growth. This relaxation mode is characterized by the large diffusion coefficient D = 2 x 10(8) nm2/s. The mode probably comes from the coupling between the orientational dynamics of liquid crystals and the transient gel structure of polymers.     

Dynamic light scattering study of salt effect on phase behavior of pig vitreous body and its microscopic implication

The salt effects on phase equilibrium and dynamical properties of pig vitreous body was studied by the macroscopic observation of the swelling behavior and dynamic light scattering under various conditions. It was found that the vitreous body collapses with maintaining the shape when the concentrations of salts (NaCl, CaCl2, and MgCl2) were changed. From the observations of the dynamics of light scattered by the pig vitreous body, intensity autocorrelation functions that revealed two diffusion coefficients, Dfast and Dslow were obtained. We developed the theory for describing the density fluctuation of the entities in the vitreous gel system with sodium hyaluronate filled in the meshes of the collagen fiber network. The dynamics of collagen and sodium hyaluronate explains two relaxation modes of the fluctuation. The diffusion coefficient of collagen obtained from Dfast and Dslow is very close to that in aqueous solution, which suggests the vitreous body is in the swollen state. Divergent behavior in the measured total scattered light intensities and diffusion coefficients upon varying the concentration of salt was observed. Namely, a slowing down of the dynamic modes accompanied by increased "static" scattered intensities was observed. This is indicative of the occurrence of a phase transition upon salt concentration.     

Computer simulation studies of anisotropic systems. XI. Second- and fourth-rank molecular interactions

The Maier-Saupe theory for nematic liquid crystals provides a reasonable account of their orientational order and its temperature dependence. The theory is based on second-rank anisotropic interactions and its predictions can be improved by the introduction of higher-rank terms as in the Humphries-James-Luckhurst theory. However comparison with the properties of real nematogens does not allow an unambiguous test of the theory because the form of the pair potential is unknown. This is not the case for computer simulations where the intermolecular potential is defined. We have therefore undertaken a Monte Carlo study of the influence fourth-rank interactions on nematic behaviour and report the results of our simulations here. The model nematogen used as a reference is that developed by Lebwohl and Lasher in which the particles are confined to the sites of a simple cubic lattice and interact via a second-rank anisotropic potential. The simulation gives the internal energy, the heat capacity at constant volume and the second-rank order parameter as a function of temperature, as well as the nematic-isotropic transition temperature. These results are used to provide the first unambiguous test of the Humphries-James-Luckhurst theory. We also discuss the changes in the transition temperature which are caused by the introduction of the fourth-rank term in the pair potential using thermodynamic perturbation theory for the Helmoltz free energy.     

Dynamic arrest in a liquid of symmetric dumbbells: reorientational hopping for small molecular elongations

We present extensive equilibrium and out-of-equilibrium molecular-dynamics simulations of a liquid of symmetric dumbbell molecules, for constant packing fraction, as a function of temperature and molecular elongation. We compute diffusion constants as well as odd and even orientational correlators. The notations odd and even refer to the parity of the order l of the corresponding Legendre l polynomial, evaluated for the orientation of the molecular axis relative to its initial position. Rotational degrees of freedom of order l are arrested if, in the long-time limit, the corresponding orientational l correlator does not decay to zero. It is found that for large elongations translational and rotational degrees of freedom freeze at the same temperature. For small elongations only the even rotational degrees of freedom remain coupled to translational motions and arrest at a finite common temperature. On the contrary, the odd rotational degrees of freedom remain ergodic at all investigated temperatures. Hence, in the translationally arrested state, each molecule remains trapped in the cage formed by its neighboring molecules, but is able to perform 180 degrees rotations, which lead to relaxation only for the odd orientational correlators. The temperature dependence of the characteristic time of these residual rotations is well described by an Arrhenius law. Finally, we discuss the evidence in favor of the presence of the type-A transition for the odd rotational degrees of freedom, as predicted by mode-coupling theory for small molecular elongations. This transition is distinct from the type-B transition, associated with the arrest of the translational and even rotational degrees of freedom for small elongations, and with all degrees of freedom for large elongations. Odd orientational correlators are computed for small elongations at very low temperatures in the translationally arrested state. The obtained results suggest that hopping events restore the ergodicity of the odd rotational degrees of freedom at temperatures far below the A transition.     

Experimental and theoretical analysis of the reorientational dynamics of fullerene C70 in various aromatic solvents

A previous study of C70 in deuterated chlorobenzene generated evidence suggesting C70 was experiencing unique reorientational behavior at given temperatures. The present study explores the possibility that this behavior is present across other solvents. The 13C spin-lattice relaxation rates for four carbon resonances in C70 were analyzed in benzene-d6, chlorobenzene-d5, and o-dichlorobenzene-d4, and as a function of temperature, to probe the reorientational dynamics of this fullerene. Anisotropic behavior was observed at the lowest (283 K) and highest temperatures (323 K), isotropic diffusion was seen between 293 and 303 K, and quasi-isotropic at 313 K. When anisotropic motion was present, diffusion about the figure axis was seen to be higher than diffusion of the figure axis. Experimentally obtained diffusion coefficients generated reorientational correlation times that were in excellent agreement with experimental values. Theoretical predictions generated by a modified Gierer-Wirtz model provided acceptable predictions of the diffusion constants; with DX usually being more closely reproduced and DZ values generally being underestimated. Overall, the results indicate that the factors affecting rotational behavior are complex and that multiple solvent factors are necessary to characterize the overall motion of C70 in these solvents. Although a solvent's viscosity is normally sufficient to characterize the tumbling motion, the spinning motion is less sensitive to solvent viscosity but more responsive to solvent structure. The balance and collective influence of these factors ultimately determines the overall rotational behavior.     

Contribution to understanding of the molecular dynamics in liquids

The dielectric relaxation spectroscopy is used for studying the orientational molecular dynamics in the isotropic (I) and nematic (N) phases of two mesogenic liquids composed of the molecules of similar structure and length, but of an essentially different polarity: n-heptylcyanobiphenyl, C(7)H(15)PhPhCN, 7CB (molecular dipole moment mu approximately 5D) and 4-(trans-4'-n-hexylcyclohexyl)isothiocyanatobenzene, C(6)H(13)CyHxPhNCS, 6CHBT (mu approximately 2.5D); advantageously, the temperatures of the I-N phase transition for the two compounds are very close to each other (T(NI) = 316.6 +/- 0.2 K). It is shown that regardless of the differences in polarity of 7CB and 6CHBT molecules and their abilities in dipolar aggregation, the values and temperature dependences of the relaxation time (corresponding to the rotational diffusion of the molecules around their short axis) are very close to each other, in both the isotropic and nematic phases of the liquids studied. Therefore, the data show that the dielectric relaxation processes occurring in dipolar liquids in the isotropic and nematic states lead through the rotational diffusion of individual molecules and the diffusion seems to be not influenced by the intermolecular interactions.     

Correlation of transverse and rotational diffusion coefficient: a probe of chemical composition in hydrocarbon oils

Measurements of relaxation time and diffusion coefficient by nuclear magnetic resonance are well-established techniques to study molecular motions in fluids. Diffusion measurements sense the translational diffusion coefficients of the molecules, whereas relaxation times measured at low magnetic fields probe predominantly the rotational diffusion of the molecules. Many complex fluids are composed of a mixture of molecules with a wide distribution of sizes and chemical properties. This results in correspondingly wide distributions of measured diffusion coefficients and relaxation times. To first order, these distributions are determined by the distribution of molecular sizes. Here we show that additional information can be obtained on the chemical composition by measuring two-dimensional diffusion-relaxation distribution functions, a quantity that depends also on the shape and chemical interactions of molecules. We illustrate this with experimental results of diffusion-relaxation distribution functions on a series of hydrocarbon mixtures. For oils without significant amounts of asphaltenes, the diffusion-relaxation distribution functions follow a power-law behavior with an exponent that depends on the relative abundance of saturates and aromatics. Oils with asphaltene deviate from this trend, as asphaltene molecules act as relaxation contrast agent for other molecules without affecting their diffusion coefficient significantly. In waxy oils below the wax appearance temperature a gel forms. This is reflected in the measured diffusion-relaxation distribution functions, where the restrictions due to the gel network reduce the diffusion coefficients without affecting the relaxation rates significantly.     

Study on the intermolecular interaction of C60 and simulations on the orientational properties of C60 in crystals

We developed the new intermolecular interaction model of C(60) with the quantitative accuracy for the molecular orientational properties in crystals. The energy difference (DeltaE) and the activation barrier (E(barrier)) between the two stable orientations (P and H orientations) in crystals are in the values of +14.7 and +260 meV in our model, respectively; these values are in fairly good agreement with the experimental values (DeltaE approximately +11 meV, E(barrier)=+235-+290 meV in experiments). The relaxation calculation for C(60) crystals using our model revealed that there is the reversal of the stable orientations between the P and H orientations under the high H-orientation occupancy (p(H)) in crystals, when p(H)>0.83, DeltaE<0. From the molecular dynamics calculations for C(60) crystals using our model, it is found that the phase transition is induced at T(C)=200-260 K, which is consistent with the experimental value of 260 K. Immediately below T(C), we found a great variety of molecular rotational jumps involving that between the P and H orientations every about 10(-9) s due to the thermal activation. In the high temperature phase (>T(C)), all molecules rotate irregularly like in Brownian motion involving the rotational "slumber" for approximately 10(-12)-10(-11) s.