Orientation Of Permanent Dipoles Research Articles

Molecular dynamics simulations of the dielectric constants of salt-free and salt-doped polar solvents.

We develop a Stockmayer fluid model that accounts for the dielectric responses of polar solvents (water, MeOH, EtOH, acetone, 1-propanol, DMSO, and DMF) and NaCl solutions. These solvent molecules are represented by Lennard-Jones (LJ) spheres with permanent dipole moments and the ions by charged LJ spheres. The simulated dielectric constants of these liquids are comparable to experimental values, including the substantial decrease in the dielectric constant of water upon the addition of NaCl. Moreover, the simulations predict an increase in the dielectric constant when considering the influence of ion translations in addition to the orientation of permanent dipoles.

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We report experimental studies on optical (birefringence, Δn), dielectric (Δε) and bend-splay elastic anisotropies (ΔK = K33 − K11) of a few mixtures of two nematic liquid crystals, namely CCH-7 and CCN-47, made of highly polar molecules with identical cores and antagonistic orientation of permanent dipoles. In particular, the polar group (-CN) attached to the bicyclohexane core of CCH-7 is oriented along the longitudinal direction whereas, in CCN-47, it is oriented along the transverse direction. We show that apart from the significant contribution to the optical and dielectric anisotropies, the antagonistic orientation of strongly polar groups plays a crucial role in determining the bend-splay elastic anisotropy. The elastic properties are explained based on a model proposed by Priest, considering the effect of intermolecular association and the resulting length-to-width ratio of the molecules.

Studies on binary mixtures of nematic liquid crystals made of strongly polar molecules with identical cores and antagonistic orientation of permanent dipoles

We report experimental studies on optical (birefringence, Δn), dielectric (Δε) and bend-splay elastic anisotropies (ΔK = K33 − K11) of a few mixtures of two nematic liquid crystals, namely CCH-7 and CCN-47, made of highly polar molecules with identical cores and antagonistic orientation of permanent dipoles. In particular, the polar group (-CN) attached to the bicyclohexane core of CCH-7 is oriented along the longitudinal direction whereas, in CCN-47, it is oriented along the transverse direction. We show that apart from the significant contribution to the optical and dielectric anisotropies, the antagonistic orientation of strongly polar groups plays a crucial role in determining the bend-splay elastic anisotropy. The elastic properties are explained based on a model proposed by Priest, considering the effect of intermolecular association and the resulting length-to-width ratio of the molecules.

Electron transfer in nonpolar media.

The Marcus theory of electron transfer views fluctuating orientations of permanent dipoles as the nuclear mode bringing the donor and acceptor into the tunneling resonance. Electronic polarization of the solvent is excluded as the fast mode adiabatically following the electronic density. This view, valid for solids, does not apply to molecular liquids where molecular translations (density fluctuations) modulate the induction interaction of the donor-acceptor complex with the solvent. This mechanism of promoting radiationless electronic transitions is considered here in the framework of the perturbation liquid-state theory. The reorganization energy of electron transfer in nonpolar solvents is nonzero and reaches the values of 0.1-0.3 eV for typical molecular sizes and solvents used in applications. The reorganization energy scales quadratically with the molecular polarizability of the solvent and decays as the inverse fifth power with the size of the donor and acceptor. The combination of the entropic character of the density fluctuations, driven by re-packing of molecular cores, with the short range of induction solute-solvent interactions leads to the violation of the fluctuation-dissipation theorem for the variance of the donor-acceptor energy gap. An explicit, approximately hyperbolic, dependence of the reorganization energy on temperature is predicted. It leads to a non-Arrhenius kinetic law for the rate constant of electron transfer in nonpolar liquid solvents.

Pyro-Electrification of Freestanding Polymer Sheets: A New Tool for Cation-Free Manipulation of Cell Adhesion in vitro.

Localized electric fields have become, in recent years, a source of inspiration to researchers and laboratories thanks to a huge amount of applications derived from it, including positioning of microparticles as building blocks for electrical, optical, and magnetic devices. The possibility of producing polymeric materials with surface charge thus opens new perspectives for applications where process simplicity and cost-effectiveness of flexible electronics are of fundamental importance. In particular, the influence of surface charges is widely studied and is a critical issue especially when new materials and functional technologies are introduced. Here, we report a voltage-free pyro-electrification (PE) process able to induce a permanent dipole orientation into polymer sheets under both mono- and bipolar distribution. The technique makes use of the pyroelectric effect for generating electric potentials on the order of kilovolts by an easy-to-accomplish thermal treatment of ferroelectric lithium niobate (LN) crystals. The PE allows us to avoid the expensive and time-consuming fabrication of high-power electrical circuits, as occurs in traditional generator-based techniques. Since the technique is fully compatible with spin-coating-based procedures, the pyro-electrified polymer sheets are easily peeled off the surface of the LN crystal after PE completion, thus providing highly stable and freestanding charged sheets. We show the reliability of the technique for different polymers and for different applications ranging from live cell patterning to biofilm formation tests for bacteria linked to food-processing environments.

Rearrangement of the molecular orientation of Alq3 in organic light-emitting diodes under constant current aging investigated using sum frequency generation spectroscopy

Sum frequency generation (SFG) spectroscopy has been used to study the operational degradation of 4, 4′-bis[N-(1-naphthyl)-N-phenylamino]-biphenyl (α-NPD)/tris(8-hydroxylquinoline) aluminum (Alq3) organic light-emitting diodes (OLEDs). A reduction in the SFG peak strengths of Alq3 was found in aged OLEDs, indicating that the permanent dipole orientation in the Alq3 layer was randomized as a result of long-term continuous operation. In contrast, the SFG signals derived from the α-NPD in aged samples were enhanced as compared to those of pristine OLEDs, and it can be attributed to an increase of the internal electric field near the interface by the long-term operation.

A percolation context evidenced by thermostimulated currents in oil-resin art media

Degradation over time of home-made thixotropic oil/resin mixtures, similar to those used as natural media by the art painter J.M. William Turner (1775–1851) is investigated by using a combination of dielectric methods, i.e., low-frequency dielectric spectroscopy (LFDS) and the technique of thermostimulated currents (TSC). These address the direct conductivity (d.c.), i.e., the circulation of free charges, and the anelastic orientation of permanent dipoles, respectively. The oil/resin mixtures can be regarded as conducting/insulating-like composite systems with respect to the two conductivities. This provides the opportunity to follow selectively the behavior of the oil, at any composition. In fresh mixtures, LFDS measurements give evidence for three d.c. regimes, delimited by the critical resin-contents (R) R1∼30% and R2∼60%. This parallels observation with the naked eye of phase separation at R R2 in four-year-old naturally-aged samples. Such a stable composition-dependent context with two marked thresholds encourages a discussion of the time-behavior in terms of percolation, i.e., relative spatial organization of the parent substances, rather than by using a microscopic approach at the molecular scale. At R>R1 the surveillance of the TSC signal indicates that the oil clusters tend to coalesce so as to form bigger clusters with aging. The coalescence process appears to be more efficient when the resin forms a continuum (R>R2) than a dispersion (R<R2).

Vibrational spectra of magnesia cement, phase 3

AbstractInfrared and Raman spectra of magnesia cement, Mg2(OH)3Cl · 4H2O (phase 3), were observed at room temperature. The formation process of phase 3 was investigated from the change in OH− stretching vibrations by comparison with those of phase 5. Broad infrared bands were observed at ∼1300 cm−1 (H system) and 1050 cm−1 (D system) with some structures in phase 3, which is considered to result from the coupling of the OH stretching vibration with lattice modes. The vibrational analysis was carried out for the lattice vibrations (translational modes) of phase 3, assuming OH− and H2O as the point masses on the basis of the valence force field. The Raman intensity was analyzed by the modified many‐body model. The infrared intensity was also analyzed using known parameters, by adding the dipole moments due to electric charges of ions, the orientation of permanent dipoles (OH− and H2O) and induced dipoles by electric charges. The force constants were determined in consistency with the calculated wavenumbers and the calculated infrared and Raman intensities. Copyright © 2001 John Wiley &amp; Sons, Ltd.

Transient electric birefringence of agarose gels. I. Unidirectional electric fields.

The orientation of agarose gels in pulsed electric fields has been studied by the technique of transient electric birefringence. The unidirectional electric fields ranged from 2 to 20 V/cm in amplitude and 1 to 100 s in duration, values within the range typically used for pulsed field gel electrophoresis (PFGE). Agarose gels varying in concentration from 0.3 to 2.0% agarose were studied. The sign of the birefringence varied randomly from one gel to another, as described previously [J. Stellwagen & N.C. Stellwagen (1989), Nucleic Acids Research, Vol. 17, 1537-1548]. The sign and amplitude of the birefringence also varied randomly at different locations within each gel, indicating that agarose gels contain multiple subdomains that orient independently in the electric field. Three or four relaxation times of alternating sign were observed during the decay of the birefringence. The various relaxation times, which range from 1 to approximately 120 s, can be attributed to hierarchies of aggregates that orient in different directions in the applied electric field. The orienting domains range up to approximately 22 microns in size, depending on the pulsing conditions. The absolute amplitude of the birefringence of the agarose gels increased approximately as the square of the electric field strength. The measured Kerr constants are approximately 5 orders of magnitude larger than those observed when short, high-voltage pulses are applied to agarose gels. The increase in the Kerr constants in the low-voltage regime parallels the increase in the relaxation times in low-voltage electric fields. Birefringence saturation curves in both the low- and high-voltage regimes can be fitted by theoretical curves for permanent dipole orientation. The apparent permanent dipole moment increases approximately as the 1.6 power of fiber length, consistent with the presence of overlapping agarose helices in the large fiber bundles orienting in low-voltage electric fields. The optical factor is approximately independent of fiber length. Therefore, the marked increase in the Kerr constants observed in the low-voltage regime is due to the large increase in the electrical orientation factor, which is due in turn to the increased length of the fiber bundles and domains orienting in low-voltage electric fields. Since the size of the fiber bundles and domains approximates the size of the DNA molecules being separated by PFGE, the orientation of the agarose matrix in the applied electric field may facilitate the migration of large DNA molecules during PFGE.

The initial slope of the transient birefringence of rigid macromolecules under the action of a reversing electric pulse of arbitrary field strength

A solution is presented for the rotational diffusion equation of rigid macromolecules in solution under the action of a reversing electric pulse. The distribution function is expanded into a functional series composed of products tiPj of the power of time (ti) and Legendre polynomial (Pj). The coefficients of the series are then easily obtained up to a higher order of time from the simple recursion formulas. The initial slope of normalized reversing birefringence at an arbitrary field strength is simply expressed by two averages of the first order Legendre polynomial and the second order Legendre polynomial, &amp;lt;P1≳ and &amp;lt;P2≳, initial slope=2Θβ2/γ (1−&amp;lt;P2≳)/&amp;lt;P2≳−6Θβ/γ &amp;lt;P1≳/&amp;lt;P2≳. Furthermore, the initial slope for pure permanent dipole orientation (γ=0) is found to be independent of the electric field. That is, initial slope=−12Θ. On the other hand, the initial slope obtained by neglecting the Brownian motion at high fields is larger by 6Θ. For example, it is equal to −6Θ for pure permanent dipole orientation. This discrepancy is also discussed.

Field strength dependence of the electric birefringence of rigid and nonrigid molecules

The field strength dependence of the electric birefringence for a system of noninteracting rigid or nonrigid molecules is calculated on the basis of the most general model, following the formalism of Nagai and Ishikawa. The results are expressed as power series of the field strength. Simple equations are obtained in the cases of permanent dipole orientation and induced dipole orientation. Applications to 1,2-dichloroethane and the nonrigid trumbell model are presented as examples.

Rotation of cells and ion exchange beads in the MHz-frequency range.

Whereas human red blood cells and protoplasts of leaves of Avena sativa show rotation only at discrete frequencies in the kHz-range of a linear alternating electric field, rotation of these cells is observed at practically every frequency in the MHz range (15 to 200 MHz). The cell rotation in the MHz-range can be explained in terms of field induced polarisation and orientation of permanent dipoles within the membrane and the cell. This interpretation is supported by the finding that Chelex beads (polystyrene cross-linked with divinylbenzene and coupled to iminodiacetic acid) do exhibit rotation in the MHz-range, but not in the kHz-range. It is most interesting that the Cu2+- and Ca2+-forms of the Chelex beads show a shift in the rotation frequency spectrum in addition to an increase in the magnitude of the rotation speed with respect to the Na+-form. At frequencies in the MHz-range at which rotation of the Chelex beads is not observed, formation of chains of beads from the electrodes occurs instead. This is due to positive polarisation of the beads leading to positive dielectrophoresis. The results support the view that rotation needs negative polarisability which is determined by the functional groups.

A small DNA restriction fragment with an apparent permanent dipole moment

The electric birefringence of two DNA restriction fragments, each containing 147 base-pairs, has been investigated. The decay of the birefringence was the same for the two fragments, with a relaxation time corresponding to the reorientation of fully extended, rod-like molecules. However, the birefringence saturation behavior of the two fragments was markedly different: one fragment oriented by the expected induced-dipole mechanism, while orientation of the other fragment followed the theoretical curve for permanent dipole orientation. This difference in behavior must be due to differences in the base-pair sequences of the two fragments.

Electrical birefringence study of monodisperse collagen solutions.

AbstractAcid‐soluble collagen solution in 0.1 M acetic acid are studied by electrical birefringence. Specific birefringence is independant of concentration (for c &lt; 100 mg/l.) and follows Kerr's law at low fields. Birefringence decays present a single relaxation time, and reversing pulse experiments show a very low contribution of induced moment compared to permanent dipole orientation. This result is also confirmed by birefringence rise measurements.

Electric properties of macromolecules. XIII. Transient electric birefringence of sodium polyethylenesulfonate solutions

AbstractThe electric birefringence of sodium polyethylenesulfonate in water has been studied. For a low molecular weight fraction, the electric field dependence followed the Kerr law quite closely. For a higher molecular weight fraction, the specific Kerr constant was much larger, and pronounced saturation of the electric birefringence was observed in fields of 15 kv./cm. The specific Kerr constant increased with decreasing polyelectrolyte concentration in pure water. On the other hand, it decreased on isoionic dilution. The saturation effect was the more marked, the lower the polyelectrolyte concentration. The saturation behavior resembled that of permanent dipole orientation, but this mechanism was not supported by the build‐up of the birefringence. The magnitudes of specific Kerr constants of various types of macromolecules in solution are tabulated and discussed.

Phospholipid model membranes. II. Permeability properties of hydrated liquid crystals

The rates of diffusional exchange of trapped ions through hydrated liquid crystals and related vesicular structures composed of various naturally occurring, purified phospholipids were determined. Correlations were observed between the permeability characteristics and structure of the particles as reported in the preceding article. The results were consistent with the view that the particles are śclosed’, i.e. the internal aqueous compartments are completely separated from the bulk aqueous phase by bimolecular lipid lamellae. Apparently only phosphatidylethanolamine formed incompletely sealed particles. Admixture of other phospholipids with phosphatidylethanolamine resulted in the usual closed structures. Chloride diffusion through phosphatidylcholine, phosphatidylserine, and mixtures of phosphatidylcholine, phosphatidylethanolamine and cholesterol was considerably faster than that of K + or Na +, while the cations diffused faster through phosphatidic acid and phosphatidylinositol. Explanation of the Cl −/K + differential was sought in the possible orientation of permanent dipoles and of charged groups. The presence of Ca 2+ was shown to have a marked effect on the permeability properties of the acidic phospholipids — phosphatidylserine, phosphatidylinositol, and phosphatidic acid. It was concluded, on the basis of the above observations, that this system constitutes a valid model for many biological membrane phenomena.

Experiments on the Carnauba Wax Electret

The strength of the applied electric field, the thickness of the slab being electrified, the time of electrification and the low temperature holding time before removing the field were varied for carnauba wax electrets. After approximately one hour the discharge current varied inversely as a power of the time since turning off the forming field, the power being close to one. The magnitude of this current depended on both the forming field strength and on the thickness of the electret. A very large discharge was obtained when the wax was electrified and discharged while not completely solid. Dielectric constant measurements were made at temperatures between 27°C and 86°C, frequencies between 50 cps and 13 kc, and with superposed dc field strengths in the wax of zero, 3700 and 8100 volts per cm. The application of dc field was found to raise the dielectric constant at the higher temperatures. The case for permanent dipole orientation and that for ionic space charges as the cause of the stored charge of the electret is examined, and it is shown that there are objections to each mechanism. An alternate explanation is suggested.