Reversing-Pulse Electric Birefringence of Montmorillonite K10 in Aqueous Media: Electric Moments, Field Orientation, and Changes in Sign and Profile of Signals Observed in the Low-Field Region.

The reversing-pulse electric birefringence (RPEB) of a montmorillonite K10 (MK-10) sample was measured at 25 degreesC and 633 nm in aqueous suspensions in the concentration range 0.005-0.145 g/L in the 0-200 V/cm region. The sign of the RPEB signals was negative and the profile was associated with a deep dip in the reverse process in all cases at weak fields lower than the sign-inversion point, Esi. Above Esi, the sign became positive, whereas the signal profile usually showed a hump in the buildup process and a larger hump in the reverse process. A complex signal pattern appeared at the highest MK-10 concentration (0.145 g/L). For the multicomponent equilibrium system, theoretical RPEB expressions were derived in terms of three electric dipole moments: the permanent moment, the fluctuating counterion polarizability (varsigma3) with ionic relaxation time (tauI) along the symmetry axis, and the covalent polarizability anisotropy (Deltaalpha'). Analysis of profiles of the observed RPEB signals showed that the MK-10 particle possesses no permanent dipole moment. The ratio varsigma3/Deltaalpha' was less than -1, and tau1 was comparable with the overall rotational relaxation time tau of the disklike MK-10 particle. RPEB signals at 0.145 g/L were analyzed with the two-state equilibrium theory and the MK-10 particle was found to form thin platelike aggregates with partially overlapping surfaces. The field-strength dependence of the steady-state birefringence was fitted with SUSID orientation functions. The field orientation mechanism of MK-10 particles in aqueous media was discussed. Copyright 1999 Academic Press.