A number of investigators have studied the electrokinetic potential (ζ) of kaolinite as a function of pH. These investigators all agree that the ζ is negative except at quite acid pH values. The actual shapes of the curves reported by the several authors vary considerably.
In spite of the general negative character of kaolinite the mineral has a rather high anion exchange capacity. The cation exchange capacity is the lowest for the clay minerals. It has been shown that anionic surfactants adsorb readily onto the mineral even up to neutrality, often in quantities exceeding the reported anion exchange capacity.
The kaolinite studied was obtained commercially and was from the American Petroleum Institute standard locality at Lewiston, Montana (project 49, sample 17). Infrared, X-ray diffraction and microscopic examinations of the material have indicated that it is a high grade kaolinite. The material is the same as was used by Choi, Smith and Wen in their study of surfactant adsorption on kaolinite.
During the past decade a large number of studies have been made of ζ of oxide minerals in aqueous solutions by the streaming potential technique. Limitations of the method, such as the measured ζ being too low because of surface conductance if ionic strength is low, have been adequately discussed by many authors.
Streaming potentials are determined by measuring the difference of potential between electrodes positioned at the ends of a porous plug of mineral particles when an aqueous solution is forced to stream through the plug. Bright platinum electrodes in the form of discs containing many small holes were us’ed in the present study. An estimate of the electrokinetic potential (ζ) can be determined from streaming potential data by use of the following equations :
ζ = 4πµλE/DP
where ζ = electrokinetic potential
µ = viscosity of the aqueous solution
λ = specific conductance of the solution within the plug
E = streaming potential
D = dielectric constant of the solution
P = driving pressure (forcing the aqueous solution through the plug)
In aqueous solutions at 25° C using standard constants this equation becomes
ζ = 9.69 x 10 4 Eλ/P
where ζ is in mV, E is in mV, λ is in ohms-¹ cm-¹ and P is in cm of mercury. For the equation to hold true the flow of fluid through the plug must be laminar and the size of the pores must be considerably larger than double layer thickness.
The major competing method to the streaming potential method for measuring ζ of mineral particles is the microelectrophoresis method.
In the m.e.p. procedure very finely divided mineral particles are placed in a clear glass or plastic cell. Electrodes are positioned at both ends of the cell and a potential difference is applied across them. By use of a microscope focused on a proper zero velocity layer within the cell it is possible to note the velocity at which particles migrate toward either the anode or the cathode. The migration velocity is generally expressed as microelectrophoretic mobility in µ/sec./v/cm. Electronegative particles migrate to the anode.
Several formulas are available for obtaining ζ from electrophoretic mobility such as the simple Helmholtz-Smoluchowski equation an equation proposed by Henry and one proposed by Overbeek. These last two equations correct for such items as relaxation and surface conductance. In the present work the Helmholtz-Smoluchowski equation
ζ = V/E 4π µ/D
is used where
ζ = electrokinetic potential
V = particle velocity
µ = viscosity of the solution
D = dielectric constant
For water at 25°C using standard values for constants, the equation reduces to
ζ = 12.85 V/E
where ζ is in mV, V is in µ/sec and E is in V/cm
ζ versus pH for kaolinite as determined by both electrokinetic methods in the absence of surfactants. Also shown on the figure are curves for quartz determined by both methods (s.p. from present work and m.e.p. from data of Iwasaki, et al) and curves for αAl2O3 determined by both methods (s.p. from data of Fuerstenau and Modi, and m.e.p. determined from data of Johansen and Buchanan). It is probable that all streaming potential curves are in error between about pH 4 and pH 10 because of low total ionic strength and the problem of surface conductance. It should be noted that the ZPC of kaolinite appears to be at about pH 2.2 by the m.e.p. method and could not be determined by the s.p. method.
Although the overall surface charge of kaolinite remains negative in the pH range pH3-4.2, it is interesting to note that addition of the negatively charged dodecylsulfate anion (DS) causes this surface charge to become more negative. The results are consistent with adsorption studies which show strong SDS adsorption in this pH range. The earlier works noted that adsorption onto positive edge sites was probably not sufficient to account for all SDS adsorption. Yet Thiesen presents evidence that kaolinite faces posses only negative (and presumably neutral) sites even at acid pH values.