Molecular orientation/tilt angle

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 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. 

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]. 

For monolayers and LB films, the distribution of the molecular orientation is expected to be sharply peaked at a tilt angle < > between the molecular axis and the film normal. Using the tilt angle < >, then, the ratio a is also presented by... 

The values of the envelopes of SH intensity at 45 ° incidence in the mixed monolayers are plotted against the fraction of C180AZ0N02 in Fig.ll. The molecular tilt angle <(> evaluated by the above-mentioned procedure is also shown in this figure. The SH intensity of the single-component monolayer of C180AZ0N02 is very small. This result means that the orientation of amphiphile... 

A series of SAMs formed on Au from mono- and dithiol conjugated aromatic molecules was characterized by cyclic voltammetry, grazing incidence Fourier transform infrared spectroscopy, contact angle measurement, and ellipsometry.43 The analyses indicated that the molecular orientation of conjugated phenylene- and thophene-based dithiols became less tilted with respect to the surface normal as the chain length of the organic molecules increased. 

Modern methods of vibrational analysis have shown themselves to be unexpectedly powerful tools to study two-dimensional monomolecular films at gas/liquid interfaces. In particular, current work with external reflection-absorbance infrared spectroscopy has been able to derive detailed conformational and orientational information concerning the nature of the monolayer film. The LE-LC first order phase transition as seen by IR involves a conformational gauche-trans isomerization of the hydrocarbon chains a second transition in the acyl chains is seen at low molecular areas that may be related to a solid-solid type hydrocarbon phase change. Orientations and tilt angles of the hydrocarbon chains are able to be calculated from the polarized external reflectance spectra. These calculations find that the lipid acyl chains are relatively unoriented (or possibly randomly oriented) at low-to-intermediate surface pressures, while the orientation at high surface pressures is similar to that of the solid (gel phase) bulk lipid. 

The structure and tilt angle of the molecules relative to the surface normal were determined by their FTIR spectra. In helical peptides, the transition moment of amide-I band lies nearly parallel to the helix axis and that of amide-II perpendicular. Since transition moments, which lie parallel to the gold surface, cannot be detected in grazing angle FTIR, the ratio between the intensities of the amide-I band (1,665 cm-1) and amide-II band (1,550 cm-1) indicates to what extend the molecules in the monolayer are oriented perpendicular to the gold surface. Based on the FTIR spectra it was possible to calculate the tilt angle, namely the angle between the molecular axis and the surface normal. The frequencies of amide-I and amide-II vibrations indicate that the monolayer is indeed in an a helix form (Fig. 2a). 

The input polarization dependence of p- and s-polarized SH light was measured to elucidate the molecular orientation of L and ML species. The results can be fitted well to the theory and the relative values of three non-zero elements of the surface susceptibility [63-65], which determine the molecular orientation, are obtained from the fitting. Since hyperpolarizability of azobenzene dyes is known to be dominated by a single element which is an element of the hyperpolarizability along the tc-jt moment direction [52], the tilt angle 0 can be estimated from the relative values of the surface susceptibility. 

Cholesteric liquid crystals are similar to smectic liquid crystals in that mesogenic molecules form layers. However, in the latter case molecules lie in two-dimensional layers with the long axes parallel to one another and perpendicular or at a uniform tilt angle to the plane of the layer. In the former molecules lie in a layer with one-dimensional nematic order and the direction of orientation of the molecules rotates by a small constant angle from one layer to the next. The displacement occurs about an axis of torsion, Z, which is normal to the planes. The distance between the two layers with molecular orientation differing by 360° is called the cholesteric pitch or simply the pitch. This model for the supermolecular structure in cholesteric liquid crystals was proposed by de Vries in 1951 long after cholesteric liquid crystals had been discovered. All of the optical features of the cholesteric liquid crystals can be explained with the structure proposed by de Vries and are described below. 

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