Investigation of Surfactant Mediated Acid–Base Charging of Mineral Oxide Particles Dispersed in Apolar Systems

Abstract

The current work examines the role of acid-base properties on particle charging in apolar media. Manipulating the polarity and magnitude of charge in such systems is of growing interest to a number of applications. A major hurdle to the implementation of this technology is that the mechanism(s) of particle charging remain a subject of debate. The authors previously conducted a study of the charging of a series of mineral oxide particles dispersed in apolar systems that contained the surfactant AOT. It was observed that there was a correlation between the particle electrophoretic mobility and the acid-base nature of the particle, as characterized by aqueous point of zero charge (PZC) or the isoelectric point (IEP). The current study investigates whether or not a similar correlation is observed with other surfactants, namely, the acidic Span 80 and the basic OLOA 11000. This is accomplished by measuring the electrophoretic mobility of a series of mineral oxides that are dispersed in Isopar-L containing various concentrations of either Span 80 or OLOA 11000. The mineral oxides used have PZC values that cover a wide range of pH, providing a systematic study of how particle and surfactant acid-base properties impact particle charge. It was found that the magnitude and polarity of particle surface charge varied linearly with the particle PZC for both surfactants used. In addition, the point at which the polarity of charge reversed for the basic surfactant OLOA 11000 was shifted to a pH of approximately 8.5, compared to the previous result of about 5 for AOT. This proves that both surfactant and particle acid-base properties are important, and provides support for the theory of acid-base charging in apolar media.

Effect of synergists on organic pigment particle charging in apolar media

“…In dispersions containing OLOA 11000, they charged negatively with a peak zeta potential of −37 ± 3 mV. This charging behavior is consistent with the proposed acid-base charging mechanism observed with mineral oxides [16,17], even though the chemistry of the cyan particle is quite different. The conjugated aromatic pi-bonds in cyan are known to act as a weak base, and it appears that the AOT and Span 80 are able to act as either proton donors or electron receivers, yielding a positive particle surface charge.…”

“…To do this, we examine the charging behavior of two commonly used organic pigments, magenta and cyan, and their corresponding synergists. The charge stabilizing agents used in this study are the common commercial surfactants Span 80, Aerosol‐OT (AOT), and OLOA 11000, all three of which have been used extensively in the past to study apolar charging .…”

Nonlinear Electrophoresis of Highly Charged Nonpolarizable Particles

“…In addition, the relaxation effect is field dependent, complicating the interpretation of experimental results even further (see Supplemental Material [13]). This nonlinear effect, which is different from the nonlinear elec-trokinetics found for polarizable particles [20], is not well understood for intermediate to large κa in contrast to the case of small κa in nonpolar electrolytes [21][22][23][24][25][26]. Theoretical analyses have predicted that the electrophoretic velocity becomes nonlinear to the applied field at a moderate to high field β = aE/φ th 1 [14,[27][28][29].…”

Magnetically-responsive electrophoretic silica organosols

“…Furthermore, mineral oxides can be charged by surfactants in nonpolar solvents [3][4][5][6][7][8]. Of the many possible inorganic nanoparticles, silica has by far been the most studied [7,[9][10][11][12][13][14][15]. Despite the extensive studies of charged nanoparticles in organic solvents, the detailed origin of the production of charge on the particle surfaces is still not fully understood [16,17].…”