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 degrees C and 633 nm in aqueous suspensio ns 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 th e sign-inversion point, E-si. Above E-si, the sign became positive, whereas the signal profile usually showed a hump in the buildup process and a larg er hump in the reverse process. A complex signal pattern appeared at the hi ghest MK-10 concentration (0.145 g/L), For the multicomponent equilibrium s ystem, theoretical RPEB expressions were derived in terms of three electric dipole moments: the permanent moment, the fluctuating counterion polarizab ility (sigma(3)) with ionic relaxation time (tau(I)) along the symmetry axi s, and the covalent polarizability anisotropy (Delta alpha'). Analysis of p rofiles of the observed RPEB signals showed that the MK-10 particle possess es no permanent dipole moment. The ratio sigma(3)/Delta alpha' was less tha n -1, and tau(1) was comparable with the overall rotational relaxation time tau of the disklike MK-10 particle. RPEB signals at 0.145 gn 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 fie ld-strength dependence of the steady-state birefringence was fitted with SU SID orientation functions. The field orientation mechanism of MK-10 particl es in aqueous media was discussed, (C) 1999 Academic Press.