Definition of zeta potential analyzer
Zeta potential analyzers is an analytical device for characterizing zeta potential, which is a property of interfacial layer in liquid particulates (dispersions, emulsions) and porous bodies. There is International Standard ISO 13099, Parts 1,2 and 3: 2012 “Colloidal systems – Methods for Zeta potential determination”: describing the main principles underlying functioning of such devices. Zeta potential is not directly measurable parameter. Therefore, it should be calculated from other experimentally measurable properties of the system using an appropriate theory. Obviously, there is no Zeta Potential analyzer that would be capable of working with all possible liquid based heterogeneous systems. Unfortunately, all of them have limitations. 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.
Principles of functioning
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). Liquid motion relative to the interface occurs due to application of either electric field or mechanical stress.
Instruments relying on electric field for this purpose are based on the phenomenon of Electrophoresis (dilute systems) or ElectricSonic Amplitude (concentrated systems). Electrophoresis based instruments apply DC or low frequency (<10 KHz) AC electric field. Meantime, ESA instruments apply high frequency AC electric field on a 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 stress for disturbing DL are suitable for both, particulate systems and porous bodies. In the case of particulates, a high frequency ultrasound (MHz range) causes particles motion relative to the liquid. This is so-called Colloid Vibration Current effect (CVI). Instead, in the case of porous material, the pressure gradient of ultrasound wave moves liquid relative to the porous matrix. Consequently, this motion generates Streaming Current, which is also known as a known as Seismoelectric effect in non-isochoric mode. These instruments work only with concentrated systems. There is an approximate threshold between diluted and concentrated system when volume fraction of particles is approximately 1 % vl. There are several USA Patents describing such instruments.
Principles of monitoring particle motion
(2). Monitoring of the particle motion and related DL disturbance requires using either optical means, or electrical, or mechanical.
Optical devices use either dynamic light scattering (ELS) or image analysis for extracting information on the speed of the particles when they move in the electric field. Therefore, these instruments are suitable only for dilute particulate systems. Instrument design should prevent liquid from thermal motion, which usually imposes limit of the high ionic strength. Possible electroosmotic flow along the sample cell walls is another factor affecting measurements, especially for the image analysis devices.
Electric output is used in the Streaming Current/Potential devices for porous bodies. Electroacoustic devices in the CVI mode also employ electric output. They function for both, particulates and porous bodies.
Streaming Current is monitored usually in DC mode or at very low frequency.
Meanwhile, CVI is the high frequency AC current, which requires measurement of the magnitude and phase. Photo below presents one of such devices, DT-300 Zeta Potential Probe.
Subsequently, mechanical output in a form of ultrasound pulse is the measured signal of ESA devices.