REVERSING-PULSE ELECTRIC BIREFRINGENCE OF MONTMORILLONITE PARTICLES SUSPENDED IN AQUEOUS-MEDIA - INSTRUMENTATION AND THE EFFECT OF PARTICLECONCENTRATION, IONIC-STRENGTH, AND VALENCE OF ELECTROLYTE ON FIELD ORIENTATION

The electrooptical property of clay particles dispersed in aqueous med ia was studied with a newly constructed reversing-pulse generator, in which a field-effect transistor (abbreviated as MOS-FET) rather than a vacuum tube was used as the switching element. This pulse generator h as the following characteristics: (1) a stable output voltage in the 5 -300 V range, (2) a widely variable output pulse duration in the 10 mu s-1.2 s range, and (3) a very short time constant of electric pulse ( generally 150 ns for buildup and reverse and 50 ns for decay). Moreove r, this generator could deliver a single rectangular pulse and/or a re versing pulse to high ionic strength samples, e.g,, 0.2 mol dm(-3) NaC l of MgCl2, because of a high electric current resistance of MOS-FET. By using this new reversing-pulse generator, a study was carried out o n the effect of the ionic strength, valence of electrolyte, and partic le concentration on the electrooptical and hydrodynamic properties of montmorillonite particles dispersed in aqueous media. The analysis of measured reversing-pulse electric birefringence signals of Na-montmori lonite showed that the particle possesses no permanent dipole moment b ut that an ion polarization makes a large contribution to the field or ientation, The sodium ion, trapped on the cation-exchangeable site, ma y be responsible for this ion polarization; the Na-montmorillonite par ticles also form aggregates of card house structure at increased conce ntrations of added sodium ions. In contrast to the Na-montmorillonite particle, reversing-pulse electric birefringence signals of Mg-montmor illonite suspensions showed longer average rotational relaxation times . The signals could not be analyzed theoretically because of larger ag gregates of book house structure, The optical anisotropy of the clay p article was increased with an increase in Mg2+ concentration.