Polar liquids, molecular orientation

Molecular Orientation and Surface Tension. One disconcerting fact is the molecular orientation of nonsymmetrical and polar liquids. With orientation, the surface of these liquids may represent only a part of the molecules. For this type of liquid, a correlation between the surface and the bulk properties could present a problem. The original assumption made by Hildebrand and Scott (9) was that nonpolar liquid molecules assume a spherical symmetry. The general application of their equations to liquids without a spherical symmetry is really remarkable. However, we noted (17) that deviations were found for nearly all oxygenated liquids. In the case of polymers, the molecular orientation is expected to play a more important role. This could be one of the reasons that the relationship between surface and bulk properties of polymers is rather sporadic. In the Experimental section, we shall explain the importance of the induced orientation (3,26) with respect to obtaining a surface tension reading for a polar polymer. 

Before leaving the subject of distribution of electrons within molecules, and its attribution to the origin of molecular polarity, with consequent effect on intermolec-ular forces (with further consequent effects on solubilities and melting points), it is pertinent to remind ourselves of two significant challenges faced by chemistiy instractors (i) to graphically represent forces of attraction between molecules and (ii) to develop the imagery that in the liquid state, orientation of molecules toward each other because of polarities is transitory, even if more probable, as they move past each other. 

Other noncontact AFM methods have also been used to study the structure of water films and droplets [27,28]. Each has its own merits and will not be discussed in detail here. Often, however, many noncontact methods involve an oscillation of the lever in or out of mechanical resonance, which brings the tip too close to the liquid surface to ensure a truly nonperturbative imaging, at least for low-viscosity liquids. A simple technique developed in 1994 in the authors laboratory not only solves most of these problems but in addition provides new information on surface properties. It has been named scanning polarization force microscopy (SPFM) [29-31]. SPFM not only provides the topographic stracture, but allows also the study of local dielectric properties and even molecular orientation of the liquid. The remainder of this paper is devoted to reviewing the use of SPFM for wetting studies. 

A report on the change in molecular orientation after grafting is presented by Hayakawa et al. (28), through the angular distribution of polarization of fluorescence on Nylon films grafted with methyl methacrylate, vinyl acetate, and vinyl pyrrolidone, respectively, in liquid phase. 

For this discussion, several points should be stressed here. Most importantly, there is no polar order along the director in any known liquid crystal phase, including the C phase. Thus, functional arrays with large P along the director are not oriented along a polar axis in the FLC phase. This is our interpretation of the small of DOB AMBC and other FLC materials. There are other possible problems as well, however. For example, though DOBAMBC possesses substantial dipoles oriented normal to the director, it s observed macroscopic polarization (-0.009 D/molecule) is very small. This could be due to poor molecular orientation in the FLC phase, which in turn could represent a fundamental problem in design of FLCs for x<2). 

When a substance is placed in an electric field, such as exists between the plates of a charged capacitor, it becomes to some extent electrically polarized. The polarization results at least in part from a displacement of electron clouds relative to atomic nuclei polarization resulting from this cause is termed electronic polarization. For molecular substances, atomic polarization may also be present, owing to a distortion of the molecular skeleton. Taken together, these two kinds of polarization are called distortion polarization. Finally, when molecules possessing permanent dipoles are present in a liquid or gas, application of an electric field produces a small preferential orientation of the dipoles in the field direction, leading to orientation polarization. 

Polarized analysis There is useful spectral information arising from the analysis of polarization of Raman scattered light. This, typically called as polarized analysis, provides an insight into molecular orientation, molecular shape, and vibrational symmetry. One can also calculate the depolarization ratio. Overall, this technique enables correlation between group theory, symmetry, Raman activity, and peaks in the corresponding Raman spectra. It has been applied successful for solving problems in synthetic chemistry understanding macromolecular orientation in crystal lattices, liquid crystals or polymer samples and in polymorph analysis. 

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). 

When liquid crystalline specimens are viewed between crossed polars, it is the positions of extinction bands, and how the positions change as the crossed polars are rotated, that is used to find the point-to-point variation in molecular orientation. It is appropriate to examine the principles on which this analysis is based ... 

In the presence of a strong external electric field, E, the molecular dipoles of a polar fluid acquire a small average orientation parallel to the applied field. When the electric and magnetic fields are parallel the l4N NMR signal of a pure polar liquid is split by AV... 

The above examples illustrate that continuum models such as the Kirkwood model are reasonably successful in describing the static permittivity, provided one has an independent means of estimating the correlation parameter Unfortunately, these estimates are available for only a few polar solvents, so that gK must be considered an independent parameter. The version of Kirkwood s theory presented here only considers orientational polarization. When distortional polarization, that is, the effect of molecular polarizability, is included, interpretation of experimental results is less clear. Since the approach taken here involves continuum concepts, it is necessarily limited. In the following section, a simple model based on a molecular description of a polar liquid is presented. 

When a polar solvent is placed in a changing electrical field, the molecules must realign so that their dipole vectors maintain the orientation corresponding to minimum energy. Because of intermolecular forces, this process does not occur infinitely fast but on a time scale which depends on the properties of the medium and which is usually on the order of 1-100 ps. Dielectric relaxation experiments provide very useful information about molecular motion in polar liquids and the ability of the solvent molecules to respond to changing electrical conditions. 

To relate Dff to the anisotropic motion of a molecule in a liquid crystalline solvent, we employ the function P(d, < ), defined as the probability per unit solid angle of a molecular orientation specified by the angles 6 and <3>, the polar coordinates of the applied magnetic field direction relative to a molecule-fixed Cartesian coordinate system. We expand P(0, ) in real spherical harmonics ... 

Orthoscopic examination with crossed polars is carried out first of all to determine the isotropism or the anisotropism of a sample. The polarization colors, the defects and variation in molecular orientation, and the orientation pattern or texture of liquid crystals are observed in this examination. With a heating stage the temperature of phase transition is also determined. In addition, with use of a compensator, the determination of vibration directions of the ordinary and extraordinary rays, the determination of relative retardation and birefringence are possible. In this section, the optical basics for orthoscopic observations are briefly outlined. The description of textures frequently observed for polymeric liquid crystals is given in Section 4.1.4. 

Solids and dipole relaxation of defects in crystals lattices Molecules which become locked in a solid or rigid lattice cannot contribute to orientational polarization. For polar liquids such as water, an abrupt fall in dielectric permittivity and dielectric loss occur on freezing. Ice is quite transparent at 2.45 GHz. At 273 °K, although the permittivity is very similar (water, 87.9 ice, 91.5) the relaxation times differ by a factor of 10 (water, 18.7 x 10 s ice, 18.7 x 10 s). Molecular behavior in ordinary ice and a feature which may be relevant to a wide variety of solids has been further illuminated by the systematic study of the dielectric properties of the nu-... 

The birefringence of textile fibers provides a measure of the molecular orientation of the polymer chain of the fiber. It might also be something of an indicator of the degree of crystallinity of the polymer chain. Birefringence is the difference between the refractive indices of the fiber in a direction parallel to the fiber axis and in a direction at right angles to the fiber axis. Details of how this refractive index is obtained are discussed elsewhere [3], but briefly the fiber is mounted in a liquid whose refractive index is known (or has been previously determined) and the fiber is then examined in plane-polarized monochromatic... 

This form would be particularly useful for the case of polarized light passing through an anisotropic medium. In many experiments, however, unpolarized light is passing through an isotropic medium as a liquid solution, or a gas. One then averages over all molecular orientations or equivalently over all photon directions, and also over polarizations ... 

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