(534c) Determination of the Zeta Potential of Planar Solids in Nonpolar Liquids

We have recently been using impedance spectroscopy to determine the electrical properties of alkanes doped with surfactants.[1][2] An important advantage of this technique is that it yields both the fluid conductivity and the double-layer capacitance. The latter property can be converted into the Debye length or ionic strength of the solution. Measurements of the concentration of charge versus the concentration of surfactant give important clues as to the mechanism of charge formation in these micellar solutions.[3]

The Gouy-Chapman model for the counterion cloud is used to convert the differential capacitance into ionic strength. This requires knowledge of the equilibrium (no applied voltage) surface potential of the electrodes. In the absence of such knowledge, we have until now assumed that surface potential was negligibly small compared to the thermal voltage. Under these conditions, the capacitance is insensitive to the surface potential. However, electrophoretic measurements of zeta potentials of micron-sized particles suggest these potentials are not always small in such solutions.[4][5]

Measurements of zeta potentials of planar interfaces (like our electrodes) are rarely reported in the literature compared to those of colloidal particles. The zeta potential of the latter is usually obtained by electrophoresis. Of course, we could grind our electrodes to form colloidal particles but the surfaces exposed by grinding are not necessarily the same as the original flat.

Recently, Sides & Prieve developed a new technique – called ZetaSpin® – for measuring zeta potentials on flat surfaces.[6][7][8] A circular disk having a diameter equal to a couple of centimeters is rotated around its axis and the resulting streaming potential generated on the axis near the disk is measured relative to the solution far away. All previous publications involving this technique used aqueous solutions.

In this paper, we describe modifications (to the aqueous version) which allow ZetaSpin® to measure streaming potentials in nonpolar solutions. Basically, three changes are needed: 1) use of an electrometer with much higher input impedance, 2) a different choice of electrodes and 3) a much longer time (following changes in fluid flowrate) for the readings to reach steady state. Finally, we present experimental results which demonstrate that we are indeed measuring streaming potential in the nonpolar solutions. The ZetaSpin® system emerges as powerful technique to quickly quantify surface charging as required for the study of electrostatic stabilization of particles dispersed in nonpolar liquids.

[1] B.A. Yezer, A.S. Khair, P.J. Sides, D.C. Prieve, Use of electrochemical impedance spectroscopy to determine double-layer capacitance in doped nonpolar liquids, J. Colloid Interface Sci. 449 (2015) 2–12. doi:10.1016/j.jcis.2014.08.052.

[2] B.A. Yezer, A.S. Khair, P.J. Sides, D.C. Prieve, Determination of charge carrier concentration in doped nonpolar liquids by impedance spectroscopy in the presence of charge adsorption, J. Colloid Interface Sci. 469 (2016). doi:10.1016/j.jcis.2016.02.014.

[3] D.C. Prieve, B.A. Yezer, A.S. Khair, P.J. Sides, J.W. Schneider, Formation of Charge Carriers in Liquids, Adv. Colloid Interface Sci. 244 (2017). doi:10.1016/j.cis.2016.11.004.

[4] S. Poovarodom, J.C. Berg, Effect of particle and surfactant acid-base properties on charging of colloids in apolar media, J Colloid Interface Sci. 346 (2010) 370–377. doi:10.1016/j.jcis.2010.03.012.

[5] Q. Guo, J. Lee, V. Singh, S.H. Behrens, Surfactant mediated charging of polymer particles in a nonpolar liquid, J Colloid Interface Sci. 392 (2013) 83–39. doi:10.1016/j.jcis.2012.09.070.

[6] P.J. Sides, J. Newman, J.D. Hoggard, D.C. Prieve, Calculation of the streaming potential near a rotating disk, Langmuir. 22 (2006). doi:10.1021/la061041x.

[7] P.J. Sides, D.C. Prieve, Surface conductivity and the streaming potential near a rotating disk-shaped sample, Langmuir. 29 (2013). doi:10.1021/la402702z.

[8] D.C. Prieve, P.J. Sides, Streaming potential near a rotating porous disk, Langmuir. 30 (2014). doi:10.1021/la5022092.