Reversing-Pulse Electric Birefringence of Montmorillonite Particles Suspended in Aqueous Media. Instrumentation and the Effect of Particle Concentration, Ionic Strength, and Valence of Electrolyte on Field Orientation

Abstract

To clarify the electrooptical property of clay particle suspended in aqueous media with a reversing-pulse electric birefringence (RPEB) method, the new RPEB transient theories and orientation function were derived, and the reversing pulse generator, which is also applicable to large particle such as clay , was newly constructed. The transient RPEB theory was derived on the basis of the interaction of applied electric pulse with two electric dipole moments of the macroion : one is due to the ion-atmosphere polarizability a with a single relaxation timle f r for the distortion of ion-atmosphere along the longitudinal axis and the other is due to the intrinsic molecular (instantaneously field-induced) polarizability anisotropy Ll a ' (= a 3-a ll) between the longitudinal (3) and transverse (l) axes of the macroion. The new ori entation function was derived for disklike model. It was assumed that the disklike particle possessed the following three electric dipole moments : (1) permanent dipole moment, (2) saturable induced dipole moment, which is induced in a finite time and saturated at an electric fielld strength and (3) unsaturable induced dipole moment, which is instantaneously induced dipole moment by an external electric field. In the newl y constructed reversing pulse generator a field-effect transistor (MOS-FET) was used instead of a vacuum tube as the switching element. This pulse generator has the following characteristics : (I) a stable output voltage in the 5-300Y range, (2) a widely variable output pulse duration in the 10 I1S-1 .2 s range and (3) a very short time constant of electric pulse (in most cases, 150 ns for buildup and reverse and 50 ns for decay). Moreover this generator could deliver a single rectangular pulse and/or a reversing pulse to high-ionic strength samples, e.g ., 0.2 moldm-NaCI or MgCl~, because of a high electric current resistance of MOS-FET. The theories and reversing pulse generator newly developed were applied to study the effects of particle concentration the ionic strength, and valence of electrolyte on the electrooptical and hydrodynamic properties IOf montmorillonite particles suspended in aqueous media. It was clear that both these theories and generator were very useful for the study of clay particle suspended in aqueous media. RPEB data of Na-montmorillonite particle indicated that the particle possesses no pemlanent dipole moment, but an ionic polarization makes a large contribution to the field orientation. Since the contribution of the ionic polarization increased with an increase of ionic strength, the distortion of sodium ions trapped on the cation-exchangeable sites may be responsible for this ionic polarization. The Na-montmorillonite particles form aggregates of card-house structure, which is an irregular structure, at increased concentrations of added sodium ions, since the optical anisotropy decreases and the diameter of particle increases with increasing ionic strength of added sodium ions. In contrast to the Na-montmorillonite particle, the RPEB

Pulsed Electric Linear Dichroism of Triphenylmethane Dyes Adsorbed on Montmorillonite K10 in Aqueous Media

“…The purification of MK-10 as the sodium form, its sonication and fractionation procedure, and the preparation of the aqueous suspension were all described in detail in a previous paper (19). The concentration of the dialyzed stock suspension of MK-10 was 0.29 g/L without added sodium ions.…”

“…Therefore, no electrochromic effect was involved during the pulse application for the present dye-clay system, in contrast with a previous case (2). In addition to the one described previously (19), another home-made pulse generator was also used for delivering a single square-wave electric pulse of higher output intensity (maximal 1.5 kV) with a variable duration for as long as 1.2 s to a highly conductive solution (20 mM NaCl). For this new pulser, whose basic design and circuitry are the same as those described in detail in Ref.…”

“…For this new pulser, whose basic design and circuitry are the same as those described in detail in Ref. (19), the final-stage discharge unit consists of two Hitachi 2SK1835 MOS-FET transistors in tandem in place of the previous 2SK534 transistor (N. Ikuta, Master's thesis submitted to Faculty of Science, Hiroshima University, 1993).…”

“…Three important prerequisites exist for the evaluation of A/A: (1) the fraction of the dye actually bound to clay in a suspension must be determined by dialysis, ultrafiltration, or spectrophotometric titration; (2) the number of the species of dye bound on clay particles and the bound spectrum of each species must be determined; (3) the numbers and directions of the optical transition dipole moments of, preferably, each bound species, or at least the free dye species, must be available from either the stretched film dichroism or quantum-mechanical calculation. Above all, the orientational behavior of the clay particle must be clarified in the absence of the dye beforehand, by working on the electric birefringence of montmorillonite K10 (hereafter abbreviated as MK-10) in aqueous media, which is colorless and, hence, no electric dichroism is observed (19,20). The disklike MK-10 particle was found to be easily oriented by applied electric fields, with its orientation direction transforming from the symmetry axis to the plane, showing a saturation at field strengths near and above 3 kV/cm.…”

“…This information is difficult to obtain from other physico-chemical methods, such as second harmonic generation (SHG) (21), near-edge X-ray absorption spectroscopy (NEXAFS), and X-ray photoelectron spectroscopy (XPS) (22,23). For the present purpose, the ELD spectra of two monocationic triphenylmethane dyes, crystal violet (hereafter abbreviated as CV) and malachite green (hereafter abbreviated as MG), were measured in the presence of MK-10 under conditions closely related to the electric birefringence measurements of MK-10 itself (19,20). These two highly symmetric dyes have been studied extensively for their interaction with a number of biopolymers and related polyelectrolytes (24)(25)(26)(27)(28).…”