Polyelectrolyte fields, electrical properties

A simple thin film technique has been developed to measure the electrical properties of polyelectrolyte solutions under sinusoidal electric fields of 100-500 v/cm at frequencies of. 10-10 KHz. Ohmic heating is largely avoided by the rapid transfer of heat to the electrodes and by the high surface to volume ratios. The resulting temperature is not sufficient to damage the medium. Current and voltage wave forms are monitored directly so that dispersion and nonlinear phenomena of the medium can be viewed directly as functions of frequency, voltage, and concentration of the solution. Possible mechanisms for the observed phenomena are discussed. 

Among the early examples of the successful use of electric fields to probe ionic structures and electrical and optical anisotropies are the linear polyelectrolytes. Basic information about macromolecular dimensions, size, and shape have been derived from the relaxation of field-induced changes in optical properties and in electrical parameters of the electrically and optically anisotropic systems. The analysis of electric conductivity measurements has demonstrated that linear polyelectrolytes are electrically anisotropic. It was established that the extremely large dipole moments, which the electric field produces by displacement of the counterion atmosphere parallel to the long axis of the polyions, are responsible for their orientations in the direction of the external field. 

Polyelectrolytes such as the ion exchange plastics form an interesting group of materials because of their ability to interact with water solutions. They have been used in medical applications involving the removal of heavy metal ions from the human body. They can be used to interact with external electric fields and change their physical properties drastically as is illustrated by the fact that some electrically active liquid crystals are polyelectrolytes of low molecular weight. 

Electric field-induced deformation of polyelectrolyte gels has attracted much attention because of the property of smartness. If the size and shape of gels can be controlled as we hope, this may open a new door for gel technology. In this Section, studies on electric field-induced deformation of gels will first be surveyed. 

Another effect of the influence of the electric field on the properties of charged networks was described in a recent publication by Osada. He discovered the phenomenon of contraction of polyelectrolyte networks under the influence of direct current in a good solvent [55, 76, 77]. 

Proteins are polyelectrolytes they carry positive and negative charges, and this property is ascribed to their polar amino acid side chains. Thus, proteins can be titrated, and the number of acidic and basic residues present can thereby be estimated. In addition, proteins migrate in an electrical field, and this movement is termed electrophoresis. Such migration, of course, depends on the net charge of the protein at the time of electrophoresis, which in turn depends on the pH of the medium. 

In recent publications, it has been shown that the conformational state of polyelectrolyte networks can be controlled by external physical factors such as the electric field [69] and visible light [70,71], This fact opens new possibilities for the control of the properties of charged networks and may be of significant practical interest. The first experiments in this direction were made by Tanaka... 

The rotation of large molecules in solution together with their hydration is the basis of many of the properties of solutions containing polyions. One has to ask questions, however, about the decrement of the dielectric constant in the case of linear polyelectrolytes, which include DNA. Substances such as this exhibit dielectric decrements as expected but it is difficult to account for their magnitude in terms of hydration. Thus, there might be a rotation but this cannot be about the long axis because in such a case <5 (Section 2.24) should increase when the molecules are oriented perpendicularly to the electric field and this is not found to be the case. 

Table 8). This permits the interpretation of experimental data by using the electro-optical properties of flexible-chain polymers in terms of a worm-like chain model However, EB in solutions of polyelectrolytes is of a complex nature. The high value of the observed effect is caused by the polarization of the ionic atmosphere surrounding the ionized macromolecule rather than by the dipolar and dielectric structure of the polymer chain. This polarization induced by the electric field depends on the ionic state of the solution and the ionogenic properties of the polymer chain whereas its dependence on the chain structure and conformation is slight. Hence, the information on the optical, dipolar and conformational properties of macromoiecules obtained by using EB data in solutions of flexible-chain polyelectrolytes is usually only qualitative. Studies of the kinetics of the Kerr effect in polyelectrolytes (arried out by pulsed technique) are more useful since in these... 

Bio-macromolecules such as DNA, RNA and proteins exhibit the properties of polyelectrolytes in aqueous solutions. The migration of fragments hydrolyzed from bio-macromolecules in the aqueous gel can be oriented by the weak electric field. Long chains drift slowly, short chains drift fast, and their difference in the speed results in a characteristic spectmm. This is the principle of gel electrophoresis. As a fundamental method in gene engineering, gel electrophoresis has been widely applied in the identification and analysis of DNA and protein characteristic sequences. 

More generally, gels can undergo reversible order-disorder transitions, induced by changes either in temperature, irradiation, electric fields, pH (by chemical or electrochemical activation) or solvent properties. Figure 6.95 lists such different types of stimuli enabling a mechanical response of a polyelectrolyte gel. 

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