Electroacoustic measurement of surface potentials

The electrokinetic behaviour of compact ceramic surfaces is mainly characterised by streaming potential measurements using flow parallel to the surface. For these measurements the electrokinetic analyser (EKA) most frequently is used. Often this method shows experimental difficulties. With respect to characterising filtration membranes the process determining effect comes not only from the surface. The open porosity of the membrane as a whole also influences the interaction with charged species to be filtrated. Suspensions are nowadays characterised using an electroacoustic method. To compare the electrokinetic results measured on suspensions with measurements on compact surfaces the use of an electroacoustic method was tested. The potentials measured that way we call “surface acoustic potential” (SAP). What we claim is that an acoustic wave meeting perpendicularly a planar electrically charged surface causes the development of an oscillating electrical dipole between the charge on the surface and the countercharge in the double layer. Responding to the ultrasonic wave the dipole oscillates because of the mobile countercharge ions primarily. But we assume that also under certain conditions ions of the internal volume of the solid may contribute to the dipole strength. Measurements on filtration membranes with complex structure and ceramic pellets with different sintering treatments and, therefore, different porosities and surface conditions are compared with the electrokinetic behaviour of the suspensions of the same material they are made from. The properties of the electrically chargeable MOH groups on the surface changes during thermal treatment resulting in a shift of the isoelectric point. These changes are clearly detectable using the SAP method.