Orientation tilt

Pinilla et al. studied the structure and dynamics of [MMIM]Q confined between two parallel solid walls [146], this was the first simulation of an ionic-hquid/solid interface. Simulations were performed at various interwall distances. Mass and charge density along the confinement axis revealed a structure of foyers parallel to the walls and a corresponding oscillatory profile of electrostatic potential. In particular, the potential drop between a point inside the sohd wall and the center of the liquid slab was —0.5 V. Orientational correlation functions indicated that, at the interface, cations orient tilted with respect to the surface but that such orientational order is lost thereafter. A rather singular result vras that the ionic diffusion under confinement was faster than in the bulk, at least for the non[Pg.239]

The alkyl side-chains also appear, from polarized IR measurements, to be well ordered in a trans configuration and are oriented tilted to the backbone. Significant side-chain interdigitation has also been... 

Based on the molecular orientational tilting of LCs, various structures have been suggested to vary voltages onto LCs and thus change optical properties [5,6]. Choi et al. fabricated a two-dimensional LC microlens array using a surface relief structure of ultraviolet (UV)-curable polymer [5]. The microlens array cell was made with indium tin oxide (TTO) glass substrates. One substrate had a specific surface relief structure of the photopolymer used and the other had only the ITO layer. 

The second functionality, available in the lower part of the window, is dedicated to the adjustment of the weather radar orientation (Tilt angle). This can be done in an automatic way or manually (Auto/manual buttons). Additionally, a stabilization function aims to keep the radar beam stable even in case of turbulences. The right-hand part of Fig. 1 presents an image of the controls used to configure radar display, particularly to set up the range scale (right-hand side knob with ranges 20, 40,. .. nautical miles). 

Perpendicular to the interface one observes positional (protrusion) and orientation (tilt angle) fluctuations of the lipids, but also fluctuations in the water density and structure, which is non-homogeneous in any case. These types of fluctuations inhibit several advanced experimental techniques that attempt to obtain ionic profiles close to the interface. Lateral fluctuations may be less of a problem. The scale of the well-known capillary waves is usually much larger than the molecular scale, so that at the molecular level the curvature of the monolayer can be ignored. Lipid monolayer density fluctuations are short-lived. Fluctuations that occur because of lipid clustering in the case of multiple lipid binding to cations may become an issue. 

The refractograp of figure 4 shows highly oriented micro cracks of a polystyrene sample. The orientation of the cracks is perpendicular to the mechanical strain direction. The X-ray refracted intensitiy can be interpreted as crack density, i.e. the inner surfaces within a unit volume. Changing the tilt angle (of polystyrene and polystyrene blend samples) with respect to the primary beam leads to significantly different distributions of crack orientation (Fig. 5). 

The external reflection of infrared radiation can be used to characterize the thickness and orientation of adsorbates on metal surfaces. Buontempo and Rice [153-155] have recently extended this technique to molecules at dielectric surfaces, including Langmuir monolayers at the air-water interface. Analysis of the dichroic ratio, the ratio of reflectivity parallel to the plane of incidence (p-polarization) to that perpendicular to it (.r-polarization) allows evaluation of the molecular orientation in terms of a tilt angle and rotation around the backbone [153]. An example of the p-polarized reflection spectrum for stearyl alcohol is shown in Fig. IV-13. Unfortunately, quantitative analysis of the experimental measurements of the antisymmetric CH2 stretch for heneicosanol [153,155] stearly alcohol [154] and tetracosanoic [156] monolayers is made difflcult by the scatter in the IR peak heights. 

There has been much activity in the study of monolayer phases via the new optical, microscopic, and diffraction techniques described in the previous section. These experimental methods have elucidated the unit cell structure, bond orientational order and tilt in monolayer phases. Many of the condensed phases have been classified as mesophases having long-range correlational order and short-range translational order. A useful analogy between monolayer mesophases and die smectic mesophases in bulk liquid crystals aids in their characterization (see [182]). 

This region has been divided into two subphases, L and S. The L phase differs from the L2 phase in the direction of tilt. Molecules tilt toward their nearest neighbors in L2 and toward next nearest neighbors in L (a smectic F phase). The S phase comprises the higher-ir and lower-T part of L2. This phase is characterized by smectic H or a tilted herringbone structure and there are two molecules (of different orientation) in the unit cell. Another phase having a different tilt direction, L, can appear between the L2 and L 2 phases. A new phase has been identified in the L 2 domain. It is probably a smectic L structure of different azimuthal tilt than L2 [185]. 

LS. In the LS phase the molecules are oriented normal to the surface in a hexagonal unit cell. It is identified with the hexatic smectic BH phase. Chains can rotate and have axial symmetry due to their lack of tilt. Cai and Rice developed a density functional model for the tilting transition between the L2 and LS phases [202]. Calculations with this model show that amphiphile-surface interactions play an important role in determining the tilt their conclusions support the lack of tilt found in fluorinated amphiphiles [203]. 

Grazing incidence excitation of a fluorescent probe in a phospholipid monolayer can also be used to indicate order. The collective tilt of the molecules in a domain inferred from such measurements is indicative of long-range orientational order [222]. 

Figure Bl.22.8. Sum-frequency generation (SFG) spectra in the C N stretching region from the air/aqueous acetonitrile interfaces of two solutions with different concentrations. The solid curve is the IR transmission spectrum of neat bulk CH CN, provided here for reference. The polar acetonitrile molecules adopt a specific orientation in the air/water interface with a tilt angle that changes with changing concentration, from 40° from the surface nonnal in dilute solutions (molar fractions less than 0.07) to 70° at higher concentrations. This change is manifested here by the shift in the C N stretching frequency seen by SFG [ ]. SFG is one of the very few teclnhques capable of probing liquid/gas, liquid/liquid, and even liquid/solid interfaces.

The relative intensities of the bands in the transmission and RAIR spectra were used to determine the orientation of the long axis of the 4-MPP molecules with respect to the normal to the gold surface. It was found that this tilt angle was about 21°, a value that was similar to that obtained from molecular dynamics simulations [11]. 

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