Probing the Structure of Colloidal Doublets by Electrophoretic Rotation

Abstract

A doublet formed by the coagulation of two colloidal spheres rotates toward alignment with an applied electric field. The rotation rate is proportional to the difference in the zeta potentials of the two spheres and a geometric coefficient N. This coefficient can be calculated from the electrohydrodynamic equations and depends on the kinematic boundary condition imposed on the spheres. If the doublet rotates as a single rigid body, so that the surfaces of the two spheres do not move relative to one another, then the value of N is a factor of 2 or 3 smaller than if the two spheres are free to rotate relative to one another. Thus, one can evaluate the rigidity of the doublet by determining the value of N experimentally. We have tracked the motion of individual doublets formed from micrometer-size polystyrene latex spheres differing in zeta potential and determined the value of N for different solution conditions. The influence of Brownian motion was taken into account when interpreting the rotational trajectories of the doublets. Over a range of electrolyte concentration, pH, sphere size, and electric field, all the values of N agree with the theoretical values for rigid-body rotation. This result is surprising because in many of the experiments the conditions were such that the two spheres should have been in a secondary minimum with a gap between 5 and 20 nm according to classical DLVO theory. In these cases the freely-rotating condition on each sphere is expected. Possible explanations for the rigid-body behavior of the doublets are suggested.

Interactions between Two Spherical Particles with Nonuniform Surface Potentials: The Linearized Poisson–Boltzmann Theory

“…This conclusion differs from the one reached by Holt and Chan (14), who considered the same doublet experiments (4,5). They argue that surface heterogeneities could result in torques of order thousands of kT "at separations comparable to the secondary DLVO minimum."…”

“…We now consider whether our analysis of single-mode surfaces can explain the unusually strong interactions between particles in doublets observed by Velegol et al (4). They measured their particles' zeta-potentials using electrophoresis and applied a standard DLVO calculation to show that the particles in each doublet ought to sit in a secondary minimum.…”

“…For example, measurements using the surface force apparatus and the atomic force microscope have shown the existence of long-range attractive forces which are not explained by the classical theory (1)(2)(3). Other experiments have shown the existence of unusually strong interactions between particles in doublets (4,5). The standard DLVO theory has also been inadequate for some simulations of coagulation (6).…”

Electrophoresis of Spherical Particles with a Random Distribution of Zeta Potential or Surface Charge

“…Thus, the ζ potential is often obtained from mobility measurements and used to predict colloidal stability. However, experiments have shown that even when the mobility measurements indicate a high ζ potential, the suspension is often unstable (2,3). This and other discrepancies (e.g., in Ref.…”

Measurement and Interpretation of Electrokinetic Phenomena (IUPAC Technical Report)

“…The electrokinetic theory for this type of surface has been considered by several authors [118,119,125,126]. Anderson et al [127][128][129] have also performed experimental checks on the validity of some theoretical predictions. In the case of spherical particles, it has been demonstrated [130] that their motion depends on the first, second, and third moments of the ζ-potential distribution along the surface of the particle.…”