Orientation at interface/tilt angle

Ion-association adsorption of water-soluble porphyrin was studied using time-resolved TIR fluorometry. Interfacial adsorption of anionic or cationic surfactants succeeded in attracting the oppositely charged porphyrin from the mixture of cationic and anionic porphyrins in the aqueous phase, tetrakis(sulphonatophenyl)porphyiin (TPPS) adsorbed with the hexadecyltrimethylammonium ion and tetrakis(iV-methylpyridyl)porphyrin (TMPyP) with the hexadecanesulfonate [44], In addition, it was found that the application of an external electric field across the interface enabled to control the interfacial ion-association adsorption of ionic porphyrin [45]. Furthermore, TIR Raman microspectroscopy revealed the interfacial concentration of meso-tetrakis(A -methylpyridyl) porphyrinato-manganese(III) (Mn(tmpyp) +) adsorbed with dihexadecyl hydrogenphosphate (DHP) at the toluene/water interface and also revealed that the orientation of the tilt angle was 65° from the interface normal [46]. 

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. 

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.

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. 

FIGURE 11.9. (a) Definition of the angles 6,0 and

Tilt angles of a a function of the mole fraction of complex a at the interface, simulated by using the orientation parameter D. For TMA system, L species ate dominant and the tilt angle is estimated as 31°. 

An analysis of the surfactant tilt angles revealed that no significant difference appears between the molecular orientations of the hydrophobic C12 chain at the air/water and the oil/water interface. This result is in discrepancy with experimental investigations of the interaction of C12E5 and dodecane. Neutron reflection measurements suggest an more upright arrangement of the C12 chains if dodecane penetrates into the C12 layer. 

When the fibers are oriented perpendicular to or placed under a small angle to the applied load direction, the following deformation processes are possible see Fig. 7.3. If the interfacial adhesion is poor, microvoid formation can occur at the fiber interface see case (a) in Fig. 7.3. Fibers tilted at a certain angle can separate from the matrix see case (b) in Fig. 7.3. If the interfacial is good, the crack propagates along the interface with the fibers or inside the matrix material with the result of a smooth fracture surface see case (c) in Fig. 7.3 [1]. 

In the macroscopic approach, which ignores the detailed structure of this interface, the expression of the interfacial energy has to be found following thermodynamic and symmehy considerations. is aperiodic function of the azimuthal angle

tilt angle d (with respect to the surface normal) defining the orientation of the director at the surface, can thus be developed in a Fourier series [2, 10] ... 

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. 

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