Zeta Potential Analyzers, classification and areas of applicability.

Zeta potential analyzers is an analytical device for characterizing zeta potential as a property of interfacial layer in liquid particulates (dispersions, emulsions) and porous bodies. There is ISO Standard -13099 being prepared that would describe the main principles underlying functioning of such devices.  Zeta potential is not directly measurable parameter.  It should be calculated from other experimentally measurable properties of the system using appropriate theory. There is no Zeta Potential analyzer that would be capable of working with all possible liquid based heterogeneous systems. All of them have limitation. It might be quite challenging to find an optimum device for particular application. Here we present some brief classification of these devices pointing out some obvious restrictions.  

All these devices must have means for moving liquid relative to the solid surface for disturbing double layer (1) and also means for monitoring generated signals (2).

(1). Relative motion of the liquid can be induced by external electric field or mechanical pressure field.

Instruments relying on electric field for this purpose are based on phenomenon of Electrophoresis (dilute systems) or ElectricSonic Amplitude (concentrated systems). Electrophoresis based instruments apply DC or low frequency (<10 KHz) AC field. In contrary, ESA instruments apply high frequency AC field on scale of MHz, usually below 10 MHz.  Practically all such devices are suitable for liquid particulate systems, not for porous bodies. 

Instrument relying on mechanical pressure field are suitable for both, particulate systems and porous bodies. In the case of particulates a high frequency ultrasound (MHz range) is used for moving particles relative to the liquid. This is so-called Colloid Vibration Current effect. In the case of porous body constant gradient of pressure could move liquid relative to the porous matrix, which generates Streaming current in isochoric mode. Propagation of ultrasound through a porous body generates electric signal as well, known as Seismoelectric effect. It can be defined as non-isochoric streaming current.  These instruments can be used only with concentrated systems. There are several USA Patents (see below [1-3]) describing such instruments.

There is an approximate threshold between diluted and concentrated system : 1 % vl.

(2). Monitoring can be achieved using optical means, electrical or mechanical. 

Optical devices use either dynamic light scattering  (ELS) or image analysis for extracting information on the speed of the particles motion in the electric field. These instruments are suitable only for dilute particulate systems. Instrument design should prevent liquid from thermal motion, which usually restricts high limit of the ionic strength. Possible electrosmotic flow on the measuring chamber walls is another complicated factor, especially for image analysis devices.

Electric output is used in streaming current/potential devices for porous bodies and in the CVI Zeta probe for both, particulates and porous bodies. Streaming current is monitored usually in DC mode or at very low frequency. CVI, on the other side is high frequency AC current, which requires measurement of the magnitude and phase.

Mechanical output is used in form of measured ultrasound signal generated by ESA devices.       

References:

1.  Dukhin, A.S. and Goetz, P.J. “Method and device for characterizing particle size distribution and zeta potential in concentrated system by means of Acoustic and Electroacoustic Spectroscopy”, patent USA, 6,109,098 (2000)

2.  Dukhin, A.S. and Goetz, P.J. “Method and device for Determining particle size Distribution and zeta potential in Concentrated Dispersions”, patent USA, 6,449,563 (2002)

3.  Dukhin, A.S. and Goetz, P.J. “Method for Determining Electric Properties of Particles in Liquids by means of Combined Electroacoustic and Complex Conductivity Measurement.” patent USA, 6,915,214 B2 (2005)