Electro-sterically
dispersing carbon particles in non-polar media
For
the past two decades, carbon black in hydrocarbon liquids with adsorbed
polybutene polyamine succinimide (PIBSA-PAM for short notation; see Figure 1)
dispersant has been investigated as a model non-aqueous colloidal dispersion.
A group of researchers have earlier proposed that such colloidal systems
may be thermodynamically stabilized by a combination of the steric effect
of the adsorbed layer and the electrostatic repulsive forces created via
charge exchange between the carbon surface and dispersant micelles in solution.
More recently, another group, however, utilizing a surface force
apparatus showed that the repulsive forces between the carbon surfaces coated
with the polymeric dispersant is significantly short-ranged (i.e., ~ a few
nanometers), and deduced that the stabilization of the non-aqueous colloids is
thus mainly due to the steric contribution which exceeds the attractive van der
Waals forces between particles.
Figure
1. Molecular
structure of PIBSA-PAM dispersant in ball presentation (grey = carbon; beige =
hydrogen; blue = nitrogen; red = oxygen).
In an effort to better understand the prevailing mechanism for the colloidal stability control, our ongoing research project explores the behavior of the carbon black dispersions under various physicochemical conditions through a wide range of experimental techniques including rheology, infrared spectroscopy, dynamic light scattering, optical microscopy, electron microscopy, atomic force microscopy, and dynamic electrophoresis. What follows briefly describes two specific topics of our current interest comprising a broad approach to the investigation of the physics of colloidal stabilization from a non-conventional perspective.
Molecular Organization of Adsorbed Dispersants at the Carbon Black Surfaces Dynamic light scattering measurements indicate that PIBSA-PAM dispersants form micelles in free solution. This inverse micellization behavior exhibits strong dependence with molecular characteristics of dispersant. A primary aim of this study is to validate and understand how this bulk solution self-assembly can be correlated to the structure of adsorbed dispersant layer. Recently, there have been many reports of various unanticipated morphologies of adsorbed surfactants and block copolymers at solid-liquid interfaces, contradicting the conventional picture of laterally uniform monolayers (Figure 2A). Our atomic force microscopy measurements indicate that the dispersant does not adsorb onto graphite as a simple monolayer of individual molecules, but rather as a cluster or hemi-micelle (Figure 2B). The shape or form of the cluster could impact the efficiency of the dispersants in inhibiting carbon black agglomeration. Our working hypothesis is that the surface self-assembly observed with an atomically flat substrate can be translated to structuring of adsorbed dispersants on the carbon black surfaces. By means of dynamic light scattering, we are studying PIBSA-PAM adsorbed on carbon black particles as a function of molecular weight, composition, and surface binding moiety of the dispersant. Preliminary results are consistent with the new picture of micellar adsorption, which may serve as a basis for extending technological strategies for developing new dispersants or dispersant boosters.
Figure
2A. Schematic
illustration of possible morphologies of dispersant layers at solid-liquid
interfaces.
Figure
2B. AFM
phase
(left) and height (right) images of adsorbed PIBSA-PAM layer structure on
graphite in tapping mode. 2μm scan.
Surface Charging of the Carbon Particles in Nonpolar Media Prior experiments have given evidence of surface charging of carbon black in oil with added PIBSA-PAM dispersant. This aspect is well illustrated in Figure 3 which displays electrically driven motions in a carbon black sample between two electrodes recorded with optical microscopy. A further investigation into charge in our systems is being made using light scattering-based instruments to statistically measure the electrophoretic mobility and zeta potential. Figure 4 presents such data obtained at various temperatures as an example. In order to evaluate the role that the electrostatic repulsion plays in the carbon black stability, dynamic electrophoretic measurements are being performed on systems containing dispersants with varying molecular structures.
Figure
3.
Click
here
to see a video clip demonstrating electrophoretic mobility of carbon black
particles. Sinusoidal
field was applied at 27V and 2Hz.
Data obtained in collaboration with Eric Dufresne.
Figure
4. Plot of zeta potential (ζ)
versus temperature (T) for carbon
black dispersed in hexadecane with dispersant: upward triangles, heated from 25°C;
downward triangles, cooled from 75°C. The
lines serve as guides to the eye. Data
obtained with generous permission and assistance from Dr. Craig Herb (E-Ink).
All of the above described is a work in progress, and any comments are very welcome. Also, a manuscript discussing our previous results on a relevant topic will soon be available as a downloadable file: "Effect of Temperature on Carbon Black Agglomeration in Motor Oil with Adsorbed Dispersant" (with Steve Meeker, Veronique Trappe, Dave Weitz, Nancy Diggs, and Jack Emert). For further information, please contact us.
Postdoctoral
Fellow
Division
of Engineering & Applied Sciences
Harvard
University
Engineering
Sciences Laboratory 223
40
Oxford Street
Cambridge,
MA 02138
Tel:
(617)495-7598
Fax:
(617)496-3088