Interface molecular orientation at the

Various methods of statistical mechanics are applied to the calculation of surface orientation of asymmetric molecules, by introducing an angular dependence to the intermolecular potential function. The Boltzmann distribution can also be used to estimate the orientational distribution of molecules. The pair potential V(r) may be written as V(r, 6) if it depends on the mutual orientation of two anisotropic molecules, and then we can write for the angular distribution of two molecules at a fixed distance, r, apart 

This type of interaction aligns the molecules mutually, such as placing the solvent molecules around a dissolved solute molecule in a solution. 

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The strength of weld lines in injection-molded articles is generally weaker than that of the bulk. This is due to the incomplete entanglement of polymer chains at the two impinging fountain flow fronts in the mold cavity. When the two flow fronts meet in the cold cavity, the molecular orientation at the interface remains parallel to the weld line as evidenced by the frozen-in molecular orientation found in the molded article under proper injection-molding conditions... 

Hi) Detection of surface chemical composition and reactivity due to the unique molecular orientation at the interface. 

As in the case of molecular orientation at the interface of neat liquids, solute molecular orientation can provide insight into the local intermolecular interactions at the interface, which, in turn, is useful for interpreting dynamics, spectroscopy, and reactivity. The simple picture that the hydrophilic part of an asymmetric solute molecule tends to point toward the bulk aqueous phase, while the hydrophobic part points toward the opposite direction, has been confirmed in both simulations and experiments. Polarization-dependent SHG and SFG nonlinear spectroscopy can be used to determine relative as well as absolute orientations of solute molecules with significant nonlinear hyperpolarizability. The technique is based on the fact that the SFG and the SHG signals coming from an interface depend on the polarization of the two input and one output lasers. Because an interface with a cylindrical symmetry has only four elements of the 27-element second-order susceptibility tensor being nonzero, these elements (which depend on the molecular orientation) can be measured. This enables the determination of different moments of the orientational distribution ... 

In chemicals like salol the molecules are elongated (non-spherical) and a lot of energy is needed to rotate the randomly arranged liquid molecules into the specific orientations that they take up in the crystalline solid. Then q is large, is small, and the interface is very sluggish. There is plenty of time for latent heat to flow away from the interface, and its temperature is hardly affected. The solidification of salol is therefore interface controlled the process is governed almost entirely by the kinetics of molecular diffusion at the interface. 

It has been pointed out321-324 that the two groups of solvents differ by some definite structural features. In particular, ED, 1,2-BD, and 1,3-BD possess vicinal OH groups that can form intramolecular hydrogen bonds. For these solvents, the ability of the organic molecule to interact with neighboring molecules is reduced. This results in the possibility of a different orientation at the interface because of different interactions of the OH groups with the Hg surface.323 The different molecular structure leads to different dipolar cooperative effects. As a result, the dependence of C on the bulk permittivity follows two different linear dependencies. 

Figure 11.42 (Top) Structures of spider web dendritic amphiphiles. (Bottom) Schematic representation of the molecular orientation at the water-air interface. Reprinted from Ariga et al. (2004). Copyright 2004 American Chemical Society.

The molecular-level stmcture of the electrode/electrolyte interface was studied using two- and three- phase systems, including membrane/vapor, membrane/vapor/catalyst and membraneAfapor/ graphite systems. The simulations of a membraneAfapor interface show a region of dehydration near the interface. The interfacial thickness measured from the water density profile was found to decrease in width with increasing humidity. Hydronium ions displayed a preferential orientation at the interface, with the oxygen exposed to the vapor phase. 

The structure of the liquid- liquid interfadal layer depends on the difference in polarity between the two liquids (Kaeble, 1971). Asymmetric molecules of some liquids display a molecular orientation on the interface which is indicative of their structure. Thus, interfacial tension at the octane-water interlace is SO.S nm/m whereas at the octanol-water interne it is only 8.8 nm/m. Reduction of inter dal tension in the latter case points to the orientation of octanol hydroxyl groups toward water, in other words to the structure and polarity of the interfadal layer. Because of such an orientation, the stimulus for adsorption of other asymmetric molecules on the interface is decreased. A similar pattern is typical of the homologous series of lower attcy] acrylates at the interface with water the carbonyl groups of their asymmetrical molecules are oriented toward water this orientation is more eSective the higher the polarization of the carbonyl, i.e the smaller the alkyl. Interfadal tension decreases in the same order from 27.2 nm/m for hexyl acrylate (Yeliseyeva et at, 1978) to 8 nm/m for methyl acrylate (datum from our laboratory by A, Vasilenko). 

Methylated cyclodextrins promote the hydroformylation of higher olefins, too. Molecular dynamics simulations show that the reaction takes place right at the interface and that cyclodextrins act as both surfactants and receptors that favour the meeting of the catalyst and the olefin. The methylated cyclodextrin adopts specific amphiphilic orientations at the interface, with the wide rim pointing towards the water phase. This orientation makes easier the formation of inclusion complexes with the reactant (1-decene), the key reaction intermediate [Rh(H)CO(TPPTS)2-decene)] and the reaction product (undecanal). ... 

As we stated at the beginning of this section, most of the work in this area has focused upon the study of molecular orientation at the liquid-vapor interface and how this is affected by details of the intermolecular forces. Thompson and Gubbins have carried out molecular dynamics simulations of the vapor-liquid interface for homonuclear 12-6 diatomic molecules and for such molecules with point-charge quadrupoles. They find that in the case of the nonpolar molecules, there is a tendency for molecules in the liquid to align perpendicular to the surface and those in the vapor to align parallel to the surface. The addition of a quadrupole to the 12-6 diatomic " reverses this effect. A study of the vapor-liquid interface for an interaction site model of -octance leads to similar conclusions as for the nonpolar diatomic. These effects are reproduced qualitatively by all the theoretical approximations, with the exception of the influence of the quadrupole, which can only be predicted at first order within the context of the perturbation theory based upon division of the Mayer function Eq. (3.5.2). 

The spectrum of CO on Pt(poly) is shown in Fig. 5.9 [2]. This spectrum is significant as it demonstrated the possibility of obtaining SFG from electrochemical interfaces with very good S/N ratio. Further, the results are comparable to those obtained using infrared spectroscopy. In the same report, Tadjeddine and coworkers also presented potential-dependent SFG spectra of CN" on a Pt electrode. Since the SFG process is coherent, the phase of each resonance is present in the spectra. As noted earlier, phase information can be used to determine molecular orientation at the surface. For example, the phase information can determine whether CN is surface coordinated through the nitrogen or the carbon atom [24]. 

Through the thickness dependence of elastic, inelastic, and quasielastic scattering in the millielectronvolt (or picosecond) region, it was found that the fast picosecond dynamics of polymer thin films was not described by homogenous dynamics but by heterogeneous dynamics, which were attributed to the orientation of molecular chains at the interface between polymer thin film and substrate. 

The area occupied on the water surface by a single alkanoic acid molecule within the monolayer is about 0.2 nm. This value corresponds closely to the cross-sectional area of the carboxyl end of the molecule as shown in Figure 12.10c. This implies that the floating film is indeed monomolecular in dimension. Snch measurements have been made for a huge number of different materials, providing insight into the relationships between the detailed molecular structure and the molecular orientation at the air-water interface. 

Fig. 2.5 Top panel, a Model of a complex between P450 and NADPH-cytochrome P450 oxidoreductase (FOR). A complex of P450 (red) and Mol A of the hinge-deletion mutant of POR(ATGEE), denoted as PORT e [53]). the flavin mononucleotide (FMN) domain (blue) and flavin adenine dinucleotide (FAD) domain (yellow)] and an enlarged view showing the relative orientation of the EMN and heme, b and c Open-book representation of molecular surface at the interface of P450 (b) and the EMN domain of POR (c). Five salt-bridge pairs are shown with same let-...

There is still some discussion as to the exact nature of the adsorption barrier, and whether or not molecular reorientation at the interface is an important process. In 1983, van den Tempel and Lucassen-Reynders [22] published a review of advances up to that point. They noted that for small molecules, any orientation at the interface was fast, and so diffusion would still be the rate-determining step. A more significant aspect was the distinction made between a kinetic process resulting in surfectant molecules being (a) adsorbed from the subsurface layer to the interface (usually faster than diffusion from the bulk to the subsurface) and (b) reoriented when at the interface, usually a slower process involving cooperative motions. Serrien and loos [23] explained the slow... 

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