Contribution of the transverse ionic dipole moment to the electric field orientation of rod-shaped particles as demonstrated by reversing-pulse electric birefringence

A new reversing-pulse electric birefringence (RPEB) theory is proposed. This theory considers the contribution of the ionic electric dipole moment (q1) in the transverse direction (short axis) in addition to the permanent (p3) and ionic electric dipole moment (q3) in the longitudinal direction (long axis) for the prolate ellipsoid of revolution in solution. This RPEB theory is a combination of the previous Tinoco-Yamaoka-Matsuda (TYM) theory with the recent Yamaoka-Sasai-Kohno (YSK) ion-fluctuation theory. The new theory (termed the extended YSK theory) takes into account the relaxation times for ion-fluctuations of an ionized particle (molecule), not only along the long axis (τI3), but also along the short axis (τI1), together with the relaxation time (τθ) for the overall molecular rotation. The TYM theory agrees with the extended YSK theory, if the ionic relaxations along the long and short axes are much faster than the rotational relaxation. With those electric and hydrodynamic parameters, RPEB curves are calculated by using the new YSK theory. A large number of new signal patterns are simulated in the buildup and reverse processes with the theoretical expressions. Three experimental RPEB signals, which have hitherto remained abnormal, are elucidated well. The extraordinary RPEB signal profiles observed for a β-FeOOH particle of prolate-ellipsoidal shape and for a medium-size and slightly flexible double-stranded DNA are fitted satisfactorily by considering both the longitudinal q3 and transverse q1 dipole moments. An unusual signal with a shallow hump in the buildup and a distinct hump in the reverse process, observed for a charged α-helical poly(l-lysine)·HBr in a mixed solvent system, is fitted by considering the transverse moment q1, together with the longitudinal p3 and q3 moments. The relative magnitude between τI3, τI1 and τθ greatly affects the signal profiles intricately, producing dips and humps both in the buildup and reverse processes.