Molecular reorientations, local

A simple composite model of slow motions and molecular reorientations is the so-called slowly relaxing local structure model by Freed.156,157 This... 

Relaxation dispersion data for water on Cab-O-Sil, which is a monodis-perse silica fine particulate, are shown in Fig. 2 (45). The data are analyzed in terms of the model summarized schematically in Fig. 3. The y process characterizes the high frequency local motions of the liquid in the surface phase and defines the high field relaxation dispersion. There is little field dependence because the local motions are rapid. The p process defines the power-law region of the relaxation dispersion in this model and characterizes the molecular reorientations mediated by translational displacements on the length scale of the order of the monomer size, or the particle size. The a process represents averaging of molecular orientations by translational displacements on the order of the particle cluster size, which is limited to the long time or low frequency end by exchange with bulk or free water. This model has been discussed in a number of contexts and extended studies have been conducted (34,41,43). 

The correlation function involves the elements aaf of the molecular polarizability tensor in the laboratory fixed coordinate system. The aif change with time because of molecular reorientation. Note that the only q dependence on the right-hand side of Eq. (7.1.3) is in the translational factor Fs(q, t). The (0)try (/)> is purely local in character and hence does not depend on q. In the remaining sections of this chapter we evaluate this correlation function for various combinations of molecular symmetries and models of reorientation in fluids. 

Regarding local molecular dynamics, it is well established that all surfactant self-assemblies are characterized by very rapid short-range dynamics. The surfactant chains, as well as oil and water molecules, are in a liquidlike state. Local molecular reorientation occurs on time scales of 10 s, similar to pure water or hydrocarbon. On the other hand, due to the presence of the surfactant films, the reorientation is more or less anisotropic. Local translational motion is also quite fast, but long-range translation is strongly dependent on the presence of the surfactant films. 

In our experiment, we used a 250-jiim-thick sample of 5CB sandwiched between glass slides treated with dimethyl-w-octadecyl-3-aminopro-pyltrimethoxysilyl-chloride to give homeotropic molecular alignment. The sample was situated in an oven with a temperature stability of 0.05 K. A cw Ar laser was used as the pump beam to induce the molecular reorientation. At high Ar" laser intensity 350 W/cm ) local heating... 

Distinct characters of the static structure enter the specification of this more complex motion. Same as for librational oscillations, the symmetry of tumbling reorientations proceeds from both the symmetry of the intermolecular torques and the symmetry of the molecules themselves through the values of their distinct molecular moments of inertia. The most prominent role of inertial factors, and more recently that of torques too, have been made especially clear by W.A. Steele (1). Apart from these molecular and local factors, collective properties of the liquid enter the dynamics through the rate of reorientation. This rate itself is a combination of two factors, generally undistin-guishable in the observed mean value these are the velocity of the angular rotation and the rate of occurence of elementary rotations. Hereafter in section V we will come back to experimental approaches specifying the rate of occurence dependence upon the liquid equation of state (i.e. its temperature and density dependence). ... 

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