Maxwell Displacement Current Measurement Research Articles

P18: Influence of vitamin C on alcohol binding to phospholipid monolayers

The simple model of the biological membrane is provided by well-controlled lipid monolayers at the air–water interface. The Maxwell displacement current technique (MDC) provides novel approach to conformation study of the membrane models. The effect of alcohols is interaction with membrane molecules, mainly with the lipid head group and consequent changes in physical–chemical properties of the membrane. The aim of study is to detect changes in structural, electrical and mechanical properties of dipalmitoyl-phosphatidylcholine (DPPC) monolayer on the subphase of methanol–water and ethanol–water mixtures before and after addition of antioxidant agent, vitamin C. Monolayers properties are investigated by a surface pressure analysis (including mechanical properties evaluation) and the Maxwell displacement current measurement, the dipole moment projection calculation. Surface pressure–area isotherms show similar behaviour of the DPPC monolayer on alcohol–water mixtures independently on presence of vitamin C. Binding/adsorption process induces change of electron density distribution across monolayer and thus the molecular dipole moment. We observe small or negligible binding of methanol molecules on oxygen bonds of DPPC. Thus the antioxidant, vitamin C, has no significant effect. For ethanol–water mixtures is observed recovery of electrical properties in presence of antioxidant agent. We suppose that vitamin C regulates DPPC–ethanol molecules interaction.

Influence of vitamin C on alcohol binding to phospholipid monolayers

The simple model of the biological membrane is provided by well-controlled lipid monolayers at the air-water interface. The Maxwell displacement current technique (MDC) provides novel approach to conformation study of the membrane models. The effect of alcohols is interaction with membrane molecules, mainly with the lipid head group and consequent changes in physical-chemical properties of the membrane. The aim of study is to detect changes in structural, electrical and mechanical properties of dipalmitoyl-phosphatidylcholine (DPPC) monolayer on the subphase of methanol-water and ethanol-water mixtures before and after addition of antioxidant agent, vitamin C. Monolayers properties are investigated by a surface pressure analysis (including mechanical properties evaluation) and the Maxwell displacement current measurement, the dipole moment projection calculation. Surface pressure-area isotherms show similar behaviour of the DPPC monolayer on alcohol-water mixtures independently on presence of vitamin C. Binding/adsorption process induces change of electron density distribution across monolayer and thus the molecular dipole moment. We observe small or negligible binding of methanol molecules on oxygen bonds of DPPC. Thus the antioxidant, vitamin C, has no significant effect. For ethanol-water mixtures is observed recovery of electrical properties in presence of antioxidant agent. We suppose that vitamin C regulates DPPC-ethanol molecules interaction.

Domain observation and molecular orientation of chiral dipalmitoylphosphatidylcholine monolayer by Brewster angle microscopy and Maxwell displacement current

To understand the relationship between molecular chirality and electrical properties of monolayers, Maxwell displacement current (MDC) behaviors and domain shapes of chiral DPPC monolayers composed of dextro- ( d-) and levo- ( l-) rotatory molecules at the air–water interface are investigated during monolayer compression using MDC measurement and the Brewster angle microscopy (BAM) system. The experimental results show that for DPPC monolayers of the two pure enantiomers, the π-A isotherms and the MDC behaviors are similar to each other, while the domain patterns of them are mirror shapes of different sizes. This reveals that MDC behaviors due to molecular spontaneous polarization have no relation to chirality but domain shapes closely depend on the molecular chirality. Moreover, the observed domain shapes verify the domain shape theory which was recently developed by Iwamoto et al. [M. Iwamoto, Z.C. Ou-Yang, Phy. Rev. Letts, 93 (2004) 206101].

Generation of Maxwell displacement current across 10, 12- tricosadynoic acid monolayer before and after polymerization

Surface pressure-area (π- A) isotherms and Maxwell displacement current (MDC) of 10,12-tricosadiynoic acid (TDA) monolayer at the air–water interface were measured. UV-polymerization was carried out at various surface pressures and polymerization time. After polymerization, larger molecular area expansion, higher collapse pressures and corresponding changes of MDC signals were observed. The flow behavior of TDA monolayer and its resulting polymerized TDA (PTDA) monolayer were studied in detail during the course of monolayer compression and expansion by means of π- A isotherms and MDC measurements. In-plane molecular flow induced reorientation of TDA monolayer was not reversible, whereas the in-plane molecular flow induced reorientation of PTDA monolayer was reversible. The effects of applied surface pressure and polymerization time on the experimental results of π- A isotherms and MDC measurements were discussed in association with the conformation change and it was interpreted in terms of molecular packing in the monolayer.

Molecular organization of phospholipid monolayers on the water surface by Maxwell displacement current measurement

The monolayer of organic molecules at the air–water interface has been studied using the Maxwell displacement current (MDC) technique. The materials used in this study were the biological materials of phosphatidyl ethanolamine (PE) and phosphatidic acids (PA). The configuration of the experimental set-up consists of the metal/air-gap/monolayer/metal coupled with the Langmuir method. This measurement enables the detection of current without destroying the monolayer. The phase transition and molecular orientation of the phospholipid monolayers were investigated using MDC measurement without mechanical contact between electrodes and the materials. Direct evidence of phase transition from gaseous to the polar ordering phase can be obtained across phospholipid monolayers even though at very low surface pressure. Relaxation process of the phospholipid monolayers was investigated by using the step compression on the MDC signals.

SHG and MDC spectroscopy of chiral organic monolayer at the air–water interface

Dielectric properties of the S-citronelloxy-cyanobiphenyl (S-CCB) and 4 ′ -hexyloxy-4-cyanobiphenyl (6OCB) monolayer were investigated with the optical second harmonic generation (SHG) and Maxwell displacement current (MDC) measurement. With the MDC measurement coupled with the SHG, molecular motion of S-CCB monolayer on the water surface was observed during compression process. SHG circular dichroism (SHG-CD) measurement and the polarized angle dependence measurement were performed to reveal the chirality of the monolayer on the silica substrate and the water surface, respectively. Opposite dichroic ratio observed for S-CCB and R-CCB LB monolayer indicates that the capability of detecting the chirality of LB monolayer. According to the polarized angle dependence measurement, relationship between two tensor components, s 14 and s 15 , can be obtained as s 14 / s 15 ≈ 1 / 30 for the S-CCB monolayer on the water surface where the tensor component s 14 exhibits the molecular chirality.

Monolayer dielectrics and generation of maxwell-displacement current and optical second harmonics

Optical second harmonic generation (SHG) and Maxwell displacement current (MDC) spectroscopy for the exploration of the dielectric polarization phenomena in organic monolayer film are described. It is shown that MDC and SHG are generated from the monolayers due to their noncentrosymmetric structure; the spontaneous polarization and the nonlinear polarization are the main contributions to the MDC and SHG, respectively. The developed MDC measurement coupled with the SHG measurement are shown to be helpful for the study of the orientation order and the phase transition of the monolayers, and thus helpful for the understanding of surface polarization phenomena. In order to show the effect of the SHG-MDC spectroscopy, the dielectric polarization of organic monolayer with C/sub /spl infin//-symmetry is theoretically analyzed with a simple rod-like model. The spontaneous and nonlinear polarization induced in this modeled monolayer system is expressed in orientational order parameters defined by the thermodynamic averages of Legendre polynomials of the orientation angle. The experiments on the SHG and MDCs of mesogenic liquid crystal (LC) monolayers of 4'n-octyl-4-cyanobiphenyl (8CB) on the air-water interface are shown. Finally, piezoelectric effect and flexoelectric effect in monolayers are briefly discussed.

Interfacial phenomena in polymer films and generation of maxwell displacement current

AbstractInterfacial electrostatic phenomena in ultrathin polyimide films have been examined, and the space charge distribution and electronic density of states have been determined. The presence of excess negative charges at the film‐metal interface of nanometer thickness has been revealed and the alignment of the surface Fermi level of polymer films and Fermi level of metals have been elucidated. Taking into account the interfacial space charge, a step structure observed in the I‐V characteristic of metal‐polyimide‐rhodamine‐polyimide‐metal junction, very similar to Coulomb staircase, is well explained. Furthermore, the electrical breakdown mechanism of a nanometer‐thick polyimide film is found quite different from that of micrometer‐thick films, owing to the presence of this interfacial nanometric space charge. Finally, for a profound understanding of the behaviour of surface monolayer, the Maxwell displacement current measurement coupled with optical second harmonic generation measurement has been employed.

PHOTOINDUCED MOLECULAR MOTION OF THE AZOBENZENE MONOLAYER INVESTIGATED BY MAXWELL DISPLACEMENT CURRENT TECHNIQUE

Photoinduced molecular motion of the azobenzene command surface polymer was investigated by means of the Maxwell displacement current (MDC) technique. Two kinds of command surface polymer, 6AZn-PVA and 6AZn-CBm-PVA, were investigated in order to study the microscopic interaction between photochromic molecules and liquid crystal (LC) molecules. The MDC measurements were carried out during monolayer compression and ultra-violet photoirradiation. For photoirradiation measurements, the direction of current flowing closed circuit was reversed for these two kinds of polymers and this indicates that the orientational motion of LC molecules was successfully detected by the MDC measurement.

Study of Orientational Order Parameters of 4-Octyl-4'-N-cyanobiphenyl langmuir films by Maxwell displacement current measurement coupled with Optical Second Harmonic Generation Measurement

Using Maxwell displacement current (MDC) and optical second harmonic generation (SHG) measurements, the orientational order parameters S1 and S3 of 4-octyl-4′-n-cyanobiphenyl (8CB) monolayers on the water surface were determined by monolayer compression.

A Study on the Dielectric Relaxation Time of Arachidic Acid Monolayers by MDC Measurement

Maxwell displacement current (MDC) measurement has been employed to study the dielectric property of Langmuir-films. A method for determining the dielectric relaxation time of floating monolayers on the water surface is presented. MDC flowing across the monolayers is analyzed using a rod-like molecular model. It is revealed that the dielectric relaxation time of monolayer in the isotropic polar orientational phase is determined using a linear relationship between the monolayer compression speed and the molecular area Am . Compression speed α- was about 30, 40, 50mm/min.

Generation of Maxwell Displacement Current across Liquid Crystal Monolayers and Control of Liquid Crystal Alignment

The orientational transition process of p-pentyl-p'-cyano-biphenyl (5CB) liquid crystal monolayers at the air-water surface during monolayer compression and the cis-trans photoisomerization process of surface azobenzene monolayer were examined using the Maxwell displacement current measurement. Using this measurement, the orientational transition process of 5CB molecules in monolayers mixed with azobenzene molecules was examined by alternating irradiation with ultra-violet and visible light. Subsequently, we examined the orientational transition process of 5CB molecules in Liquid crystal (LC) cells triggered by the conformational change of the surface azobezene monolayer on polyimide (PI) Langmuir-Blodgett (LB) films. It was shown that the optical transmittance and the capacitance of the LC cells with 1-layer PI LB films changed reversibly by the alternating irradiation of ultra-violet (UV) and visible light, whereas those of LC cells with more than 3-layer PI LB layers did not change reversibly. These results were discussed taking into account the physico-chemical property of PI LB films.

Analysis of the Dielectric Relaxation Property of Phospholipid Monolayers by Maxwell Displacement Current Measurement

Using the Maxwell displacement current (MDC) measurement with alternating monolayer compression and expansion process, the dielectric property of monolayers has been investigated. The monolayer expansion and compression processes were analyzed on the basis of the Debye theory, and the relaxation time of dl-α-dilaurylphosphatidylcholine (DLPC) was determined using a whole curve of MDCs. It was suggested that the interaction among molecules makes a contribution to the dielectric relaxation phenomena, although the change in the orientational order of polar molecules is a main contribution of MDC.

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 [α-phosphatidylcholine dipalmitoyl (DPPC)] monolayers at the air–water interface were investigated. It was found that at a temperature of 20 °C, the chiral and racemic phospholipid monolayers exhibited similar pressure-area isotherms with a pressure plateau between 65 and 85 Å2, 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.

Presence of In-Plane Orientation in Single Azobenzene Monolayers by Maxwell-Displacement-Current Measurement

The presence of in-plane orientation of azobenzene molecules in single monolayers of polymer films was detected by means of Maxwell-displacement-current (MDC) measurement with linearly polarized photoirradiation. The magnitude of MDC generated by photoirradiation polarized in a direction parallel to the dipping direction of the azobenzene monolayer was greater than that generated by photoirradiation normal to the dipping direction. The in-plane orientational change in azobenzene molecules in single polymeric monolayers was produced as a result of alternating polarized photoirradiation of UV and visible light for a sufficiently long time, and it was also detected by means of MDC measurement.