Chen-Xu Wu

Rate-equation theory of azobenzene monolayer photoisomerization

Abstract We perform a rate-equation description of the photoisomerization of an azobenzene monolayer, and find that the corresponding displacement current consists of two exponential-decay components. This result depicts the existing experimental results well.

Polar orientational phase transition and differential dielectric constant of smectic monolayers on a water surface

We build up a self-consistent equation and derive a general equation of orientational phase transition between a normal-director phase and a tilted-director phase for smectic monolayers on a water surface under monolayer compression with a consideration of molecular configuration. A differential dielectric constant for monolayer films is discovered and discussed. The change of the first orientational order parameter and the dielectric constant at the polar orientational phase transition point is also examined. It is found that the influence of the polar orientational phase transition on the first orientational order parameter in monolayer films is clear compared with that on the dielectric constant. This reveals that Maxwell-displacement-current measuring technique would be a useful way to measure such a phase transition.

Nonlinear dependence of Maxwell displacement current across chiral phospholipid mixed monolayers on molar ratio

Mixed Langmuir films composed of the left- and right-handed chiral enantiomers are analyzed using a model based on the Bragg–Williams theory for binary mixtures, and the contribution of the mixing effect to the generation of Maxwell displacement current (MDC) and surface pressure-area isotherm is discussed. It is revealed that the formation of the homogeneous and intimate mixture of the left- and right-handed chiral enantiomers gives rise to the additional MDC generation due to the dipole–dipole interaction between the left- and right-handed chiral enantiomer molecules by the molecular twist transition during monolayer compression.

Modeling analysis of molecular chiral effect detected by Maxwell-displacement-current measurements

Based on Maxwell displacement current (MDC) and surface pressure measurements by monolayer compression, the physicochemical properties of chiral and racemic phospholipid [alpha-phosphatidylcholine dipalmitoyl (DPPC)] monolayers at the air-water interface were investigated. It was found that at a temperature of 20 degrees C, the chiral and racemic phospholipid monolayers exhibited similar pressure-area isotherms with a pressure plateau between 65 and 85 Angstrom(2), usually attributed to the two-dimensional (2D) phase transition from expanded monolayer to condensed one. Nevertheless, with MDC measurement some critical peaks in the 2D pressure plateau region with different amplitude and sign were detected related to the chirality of the phospholipid samples. This result clearly demonstrates that microscopic chirality affects the electrical properties of monolayers. To investigate the chiral effect theoretically, a twist-conformation molecular model was raised by a modification of the simple rodlike model and biaxial rodlike model, which were developed in our previous work. The analysis based on this modified model reveals that the monolayer properties, especially the MDC behavior by compression, are profoundly influenced by the chirality of molecules

Linear and nonlinear dielectric constant tensor for monolayer films in terms of orientational order parameters

We give a general expression of the dielectric constant tensor in orientational orders in the applied static or low-frequency electric field for the semi-two-dimensional monolayer films with C∞v symmetry. The dielectric anisotropic orientational tensor and orientational polarization tensor are derived in orientational order parameters. The nonlinear electro-optical Pockels effect including the local-field influence is also discussed using the orientational orders. A numerical calculation result shows that the first orientational order parameter S1 is a destructive order while the second orientational orientational order parameter S2 is a constructive one to the dielectric constant in the normal direction of monolayer films.

Tilting phase transition of Langmuir monolayers: a competition between steric repulsion and interaction of dipolar molecules with liquid surface

Abstract Monolayers of amphiphilic molecules at the liquid–air interface are investigated, taking into account the steric repulsion between molecules and the Coulomb attractive interaction of dipoles with the liquid surface. It is shown that the former favors a perpendicular alignment of molecules at the surface (SmAp phase) and the latter gives a tilted orientation (SmCp phase). As the temperature decreases, the competition between these two interactions leads to a second-order phase transition from SmAp to SmCp. The large pretransitional fluctuation of the tilt angle at a critical molecular area is predicted.

Determination of orientatinal order parameters of 4-alkyl-4 '-cyanobiphenyl monolayers at the air-water interface by Maxwell-displacement-current and second harmonic generation measurements

Abstract Using Maxwell displacement current (MDC) and second harmonic generation (SHG) measurements, the linear and nonlinear dielectric properties of organic monolayer films on a material surface can be examined. In this study, we have developed a novel experimental system which can detect MDC and SHG signals, and examined the molecular motion of 4-cyano-4′-n-alkyl-biphenyls (5CB) on the water surface by monolayer compression. The orientational order parameters expressed by using Legendre polynomials were successfully determined from both MDC and SHG signals, and the difference in the determined order parameters was discussed in association with the molecular motion of 5CB monolayers.

Separation of chiral phases by compression: kinetic localization of the enantiomers in a monolayer of racemic amphiphiles viewed as mixing cholesteric liquid crystals

A monolayer of amphiphiles is studied as a film of cholesteric liquid crystal (ChLC) mixed by the left-and right-handed chiral enantiomers. The equilibrium equations for the chiral phase separation (CPS) order parameter and molecular orientation pattern are derived by minimizing the free energy of the mixing ChLC. We found a one-dimensional (1D) periodic localization of the enantiomers, a stripe-type of CPS. By incorporating the free energy of the stripe-CPS into the thermodynamic calculation, we show that its pressure-area isotherms behave like a spatial 2D van der Waals equation of state, and must have a pressure-plateau region for the phase transition from racemic phase to CPS. This gives a clear explanation of the CPS in a racemic monolayer

Analysis of thermally stimulated Maxwell-displacement current across organic monolayers

Dielectric relaxation processes in organic monolayer films during heating are analyzed on the basis of the rotational Debye–Brownian motion equation. Theoretical formula for the thermally stimulated current (TSC) is obtained. The polar orientational disordering of polarized and nonpolarized monolayers is examined by taking into account the dielectric relaxation time τ of monolayers. TSCs across phospholipid monolayers of L-α-dimyristoyl, phosphatidylcholine are discussed on the basis of the analysis developed here.

Space charge distribution and capacitance-voltage characteristics of metal/polyimide Langmuir-Blodgett film/metal structure

The capacitance-voltage (C-V) characteristics of the metal/polyimide (PI) Langmuir-Blodgett (LB) film/metal structure are analyzed, taking into account the interfacial electrostatic phenomena and the presence of the interfacial electronic states. The capacitance (/spl Cscr/) and the additional relative capacitance [(/spl Cscr/-/spl Cscr//sub 0/)//spl Cscr//sub 0/] at various applied external voltages (V/sub ex/) are calculated, under the assumption that the electron acceptor density of states (DOS) at the interfaces has a Gaussian profile centered at the energy level of the lowest unoccupied molecular orbital (LUMO). The results reveal that when a positive bias (V/sub ex/>0) is applied to a metal/insulator (PI LB film)/metal (MIM) element, the capacitance and the additional relative capacitance decrease as the thickness of PI LB film increases, while a negative bias (V/sub ex/<0) gives rise to slight changes of the capacitance and the additional relative capacitance. In addition, it is found that the density of space charge also experiences slight changes as the applied external bias increases. The calculated results of the C-V characteristics of the MIM element show good agreement with the experimental results.