Zeta Potential of Stearic Acid Monolayer at the Air–Aqueous Solution Interface

Abstract

The zeta (ζ) potential of an insoluble monolayer of stearic acid at the air–water interface was measured as a function of pH in the presence of 0.0001, 0.001, and 0.01 M NH4NO3. The ζ potential was measured by means of the plane interface technique which involved the determination of the electrophoretic velocity profile of reference (polystyrene latex) particles along the solution depth in a rectangular open quartz cell. The ζ vs pH relationship at 0.001 and 0.01 M NH4NO3 was analyzed in terms of the Gouy–Chapman–Stern–Grahame model for electrical double-layer incorporating a simple site-binding model used previously for many oxide and latex colloid studies. The dissociation constant (pKa) of stearic acid monolayer and double-layer parameters such as integral capacitances of inner and outer Helmholtz layers and the complexation constant of counterion complexes were also estimated.