Orientational order study of monolayers at the air–water interface by Maxwell-displacement current and optical second harmonic generation

Electrostatic origin of the Frank elastic energy and anisotropic line tension of the domains in monolayers at the air-water interface

The free energy of condensed phase domains in monolayers at the air-water interface has been analyzed by taking into account the surface pressure, line tension, and electrostatic energy due to the normal and in-plane spontaneous polarization. We found that this free energy was reduced to the sum of the Frank elastic energy and anisotropic line tension, which were used to reproduce the shapes of domains in fatty acid monolayers, in the limit of small orientational deformation. Domains in monolayers are interesting systems as a meeting point between the physics of liquid crystals and electrostatics.

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.

Shape and stability of two-dimensional lipid domains with dipole-dipole interactions

We study the general energy and shape of the two-dimensional solid monolayer domains with the dipole-dipole interactions. Compared with the domain energy without tilted dipole moments [M. Iwamoto and Z. C. Ou-Yang, Phys. Rev. Lett. 93, 206101 (2004)], the general dipolar energy is not only shape and size but also boundary orientation dependent. The general shape equation derived by this energy using variational approach predicts a circular solution and an equilibrium shape grown from this circle. In particular, the latter is composed of two branches: a translation-induced growth of all odd harmonic modes and a pressure-induced cooperative deformation by all even harmonic modes. The good qualitative agreement between our prediction and the experimental observations shows the validity of the present theory.

Compression induced chiral symmetry breaking of monolayers comprised of banana-shaped achiral molecules at an air-water interface: Williams-Bragg approach

Chirality of monolayers comprised of banana-shaped achiral molecules at an air-water interface was investigated theoretically, and a forming mechanism of chiral structure as an assembly of achiral molecules was argued. A model of such monolayers was constructed taking into account the short-range repulsive interaction between constituent banana-shaped achiral molecules, and the free energy density functional of the model was derived as a generalization of Williams-Bragg approach. It was predicted that chiral symmetry breaking occurs by monolayer compression, where two-dimensional characteristics of monolayers at an interface plays an important role in the formation of chiral structure by banana-shaped achiral molecules.

Monolayer alignment on azobenzene surfaces during UV light irradiation: Analysis of optical polarized absorption measurement results and theoretical treatment

The influence of the charge separation during the trans-cis conformational change on the surface of azobenzene 6Az10PVA monolayer on the polar liquid-crystal monolayer film, such as 4-n-pentyl-4'-cyanobiphenyl(5CB), is investigated. The effective anchoring energy (in the Rapini-Papolar form) is phenomenologically described in the framework of the molecular model, which takes into account the interaction between the surface polarization and surface electric field, for number of conformational states of the boundary surface. It is shown, using the experimental data for the voltage across the 6Az10PVA+5CB film, provided by the surface-potential technique, that the charge separation during the conformational changing, caused by the UV irradiation, may lead to changing of the surface alignment of liquid-crystalline molecules. The influence of the photoisomerization process on the orientational order parameter S2(t) using the optical polarized absorption measurement is also investigated.

Local orientations of fluctuating fluid interfaces

Thermal fluctuations cause the local normal vectors of fluid interfaces to deviate from the vertical direction defined by the flat mean interface position. This leads to a nonzero mean value of the corresponding polar tilt angle which renders a characterization of the thermal state of an interface. Based on the concept of an effective interface Hamiltonian we determine the variances of the local interface position and of its lateral derivatives. This leads to the probability distribution functions for the metric of the interface and for the tilt angle which allows us to calculate its mean value and its mean-square deviation. We compare the temperature dependences of these quantities as predicted by the simple capillary-wave model, by an improved phenomenological model, and by the microscopic effective interface Hamiltonian derived from density-functional theory. The mean tilt angle discriminates clearly between these theoretical approaches and emphasizes the importance of the variation of the surface tension at small wavelengths. Also the tilt angle two-point correlation function is determined which renders an additional structural characterization of interfacial fluctuations. Various experimental accesses to measure the local orientational fluctuations are discussed.

Detection of flexoelectric effect from 4-heptyloxy-4′-cyanobiphenyl monolayers at an air-water interface by Maxwell displacement current and optical second harmonic generation

The flexoelectric effect of 4-heptyloxy-4'-cyanobiphenyl (7OCB) monolayers at the air-water interface is studied by Maxwell displacement current (MDC) and optical second harmonic generation measurements. Though MDC was expected to increase during the compression of 7OCB monolayers in L2L2' phase from the MDC theory developed previously, decrease of MDC was detected in these phases. This abnormalous phenomenon is found to be due to the quench of flexoelectric effect by the flow orientation of monolayers.

Orientational relaxation phenomena in Langmuir-Blodgett films at the air-water interface

The nature of the orientational relaxation process of the director n to its equilibrium orientation n(eq), in the mono-(multi)layer(s) Langmuir-Blodgett film, during the lateral compression in absence of flow, is investigated. The relaxation time, during compression of 4-n-pentyl-4(')-cyanobiphenyl mono-(multi)layer(s) film on the water surface, using the Ericksen-Leslie theory, has been calculated for the number of dynamic regimes.

Compression-shear-induced tilt azimuthal orientation of amphiphilic monolayers at the air-water interface: a C(infinity)-->C2v transition in the flow of a two-dimensional hexatic structure

Compression-shear-induced tilt azimuthal orientation of amphiphilic monolayer in tilting phases (L2 and L'2) at the air-water interface is analyzed as dynamical equilibrium of the elastic distortion of orthogonally hexagonal structure of the molecules under compression-induced shear flow. It is shown theoretically that the compression can induce molecular tilts lying along and/or against the flow direction. All these tilts makeup the initial random tilt azimuth of the molecular tails along a uniform direction. At a threshold compression speed, it causes a C(infinity)-->C(2v)-symmetry transition at the air-water interface. With Maxwell displacement current and optical second-harmonic generation measurements, the above theoretical results are verified experimentally in a monolayer of 4-heptyloxy-4(')-cyanobiphenyl.

Vibrational spectroscopic investigation of the phase diagram of a biomimetic lipid monolayer

The phase behavior of a biomimetic monolayer consisting of diphospholipid molecules on water is investigated using vibrational sum-frequency generation and fluorescence microscopy. In addition to the transition from the-molecularly disordered-liquid phase to the highly ordered and oriented con-densed phase, a novel, extremely sharp transition is observed at low compression, which is attributed to the uncurling of the hydrophobic alkane chains upon compression.

Monolayers at the air-water interface: Maxwell displacement current and optical second-harmonic generation studies and theoretical treatment

The structural properties and the surface pressure pi-A isothermal diagram have been investigated during compression of the 4-n-pentyl-4'-cyanobiphenyl (5CB) monolayer film on the water surface, both experimentally, using both the Maxwell displacement current (MDC) and optical second-harmonic generation (SHG) techniques, and theoretically, in the framework of the molecular model, assuming that the randomly tilted polar 5CB molecules on the water surface are replaced by the collective tilted array of unit vectors. The average angle is evaluated based on the both MDC and SHG techniques. The reasonable agreement between the calculated and experimental values of pi(A) was obtained.