Specific birefringence constants

The main difficulty in the experimental investigations of EB in flexible-chain polymer solutions is due to the low value of the observed effect. Specific Kerr constants for a flexible-chain polymer bearing no charge are K 10 cm g (V/300) even for polar macromolecules and, hence, the difference between birefringence in a dilute solution and the Kerr effect in the solvent alone is very slight. 

Here M is the molecular weight and V the partial specific volume of the solute, N the Avogadro number, k the Boltzmann constant, and T the absolute temperature s and D are the sedimentation and translational diffusion coefficients (after extrapolation to infinite dilution). The translational frictional coefficients from both measurements are regarded as identical, i.e., f, = fd. The rotary frictional coefficient, designated as f, can be determined from either flow birefringence or non-Newtonian viscosity measurements. 

Since the simplest oil-in-water (O/W) and water-in-oil (W/O) microemulsions are ternary systems in which the particles are swollen direct and reverse micelles, respectively, the examples given for the application of electrical birefringence will include both microemulsions and micelles. As the studies reveal, the experiments are usually carried out to find answers to specific questions instead of the complete physical characterization of the particles. Often, however, interesting additional information is derived such as the mechanism of phase separation or the elasticity constant of the monolayer in W/O microemulsions. 

The time course of orientational changes induced by electric fields contains information on the orientation mechanism, and on the electrical and geometrical properties (main dipole axis, length) of the aligning and deorienting molecules. For instance, permanent dipole orientation of a given particle type in the presence of a constant electric field builds up with zero slope and has two modes, whereas the build-up of induced dipole orientation starts with maximum slope and is characterized by only one time constant. The deorientation relaxation of a system of identical particles, after termination of the step pulse, is monophasic, independently of the presence of permanent or induced dipoles. Table 3 summarizes the characteristic features of the rotational kinetics indicated by electric dichroism and birefringence for small perturbations. We see that there are a number of specific relationships to differentiate between permanent and induced dipole mechanism. In particular, the technique of field-reversal is a sensitive indicator for the relative contributions of permanent or induced dipoles. 

Determination of molecular symmetry. Molecular symmetry of P. can be determined from measurements of viscosity, streaming birefringence, rates of sedimentation and diffusion, or directly by electron microscopy, For a known M, the frictional coefficient can be calcnlated from ultracentrifugal measurements, e.g. from the sedimentation /= [Mfl - vp)]/S, where v is the partial specific volume, p is the density and S is the sedimentation constant. The axial ratio a/b of a P. can be derived from the frictional ratiowhereis the/of a spherical molecule. The value of a/b for most globular P. is between 2 and 20, and greater than 20 for fibrous R, e, g. the axial ratio of fibrinogen is 30. 

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