Zeta potential
Zeta potential is the electrical potential at the slipping plane. This plane is the interface which separates mobile fluid from fluid that remains attached to the surface.
Zeta potential is a scientific term for
The zeta potential is caused by the net
because these are defined at different locations. Such assumptions of equality should be applied with caution. Nevertheless, zeta potential is often the only available path for characterization of double-layer properties.The zeta potential is an important and readily measurable indicator of the
Zeta potential can also be used for the pKa estimation of complex polymers that is otherwise difficult to measure accurately using conventional methods. This can help studying the ionisation behaviour of various synthetic and natural polymers under various conditions and can help in establishing standardised dissolution-pH thresholds for pH responsive polymers.[8]
Magnitude of Zeta potential (mV) | Stability behavior |
---|---|
0 to 5 | Rapid coagulation or flocculation |
10 to 30 | Incipient instability |
30 to 40 | Moderate stability |
40 to 60 | Good stability |
>61 | Excellent stability |
Measurement
Some new instrumentations techniques exist that allow zeta potential to be measured. The Zeta Potential Analyzer can measure solid, fibers, or powdered material. The motor found in the instrument creates an oscillating flow of electrolyte solution through the sample. Several sensors in the instrument monitor other factors, so the software attached is able to do calculations to find the zeta potential. Temperature, pH, conductivity, pressure, and streaming potential are all measured in the instrument for this reason.
Zeta potential can also be calculated using theoretical models, and an experimentally-determined
Electrokinetic phenomena and electroacoustic phenomena are the usual sources of data for calculation of zeta potential. (See Zeta potential titration.)
Electrokinetic phenomena
This velocity is measured using the technique of the laser
Electrophoresis
Electrophoretic mobility is proportional to electrophoretic velocity, which is the measurable parameter. There are several theories that link electrophoretic mobility with zeta potential. They are briefly described in the article on electrophoresis and in details in many books on colloid and interface science. [3][4][5][11] There is an IUPAC Technical Report[12] prepared by a group of world experts on the electrokinetic phenomena. From the instrumental viewpoint, there are three different experimental techniques: microelectrophoresis, electrophoretic light scattering, and tunable resistive pulse sensing. Microelectrophoresis has the advantage of yielding an image of the moving particles. On the other hand, it is complicated by electro-osmosis at the walls of the sample cell. Electrophoretic light scattering is based on dynamic light scattering. It allows measurement in an open cell which eliminates the problem of electro-osmotic flow except for the case of a capillary cell. And, it can be used to characterize very small particles, but at the price of the lost ability to display images of moving particles. Tunable resistive pulse sensing (TRPS) is an impedance-based measurement technique that measures the zeta potential of individual particles based on the duration of the resistive pulse signal.[13] The translocation duration of nanoparticles is measured as a function of voltage and applied pressure. From the inverse translocation time versus voltage-dependent electrophoretic mobility, and thus zeta potentials are calculated. The main advantage of the TRPS method is that it allows for simultaneous size and surface charge measurements on a particle-by-particle basis, enabling the analysis of a wide spectrum of synthetic and biological nano/microparticles and their mixtures.[14]
All these measuring techniques may require dilution of the sample. Sometimes this dilution might affect properties of the sample and change zeta potential. There is only one justified way to perform this dilution – by using equilibrium
Streaming potential, streaming current
The streaming potential is an electric potential that develops during the flow of liquid through a capillary. In nature, a streaming potential may occur at a significant magnitude in areas with volcanic activities.[15] The streaming potential is also the primary electrokinetic phenomenon for the assessment of the zeta potential at the solid material-water interface. A corresponding solid sample is arranged in such a way to form a capillary flow channel. Materials with a flat surface are mounted as duplicate samples that are aligned as parallel plates. The sample surfaces are separated by a small distance to form a capillary flow channel. Materials with an irregular shape, such as fibers or granular media, are mounted as a porous plug to provide a pore network, which serves as capillaries for the streaming potential measurement. Upon the application of pressure on a test solution, liquid starts to flow and to generate an electric potential. This streaming potential is related to the pressure gradient between the ends of either a single flow channel (for samples with a flat surface) or the porous plug (for fibers and granular media) to calculate the surface zeta potential.
Alternatively to the streaming potential, the measurement of streaming current offers another approach to the surface zeta potential. Most commonly, the classical equations derived by Maryan Smoluchowski are used to convert streaming potential or streaming current results into the surface zeta potential.[16]
Applications of the streaming potential and streaming current method for the surface zeta potential determination consist of the characterization of surface charge of polymer membranes,[17] biomaterials and medical devices,[18][19] and minerals.[20]
Electroacoustic phenomena
There are two electroacoustic effects that are widely used for characterizing zeta potential: colloid vibration current and electric sonic amplitude.[5] There are commercially available instruments that exploit these effects for measuring dynamic electrophoretic mobility, which depends on zeta potential.
Electroacoustic techniques have the advantage of being able to perform measurements in intact samples, without dilution. Published and well-verified theories allow such measurements at volume fractions up to 50%. Calculation of zeta potential from the dynamic electrophoretic mobility requires information on the densities for particles and liquid. In addition, for larger particles exceeding roughly 300 nm in size information on the particle size required as well.[citation needed]
Calculation
The most known and widely used theory for calculating zeta potential from experimental data is that developed by
- Detailed theoretical analysis proved that Smoluchowski's theory is valid only for a sufficiently thin double layer, when the Debye length, , is much smaller than the particle radius, :
- The model of the "thin double layer" offers tremendous simplifications not only for electrophoresis theory but for many other electrokinetic and electroacoustic theories. This model is valid for most nanometers in water. The model breaks only for nano-colloids in a solution with ionic strengthapproaching that of pure water.
- Smoluchowski's theory neglects the contribution of surface conductivity. This is expressed in modern theories as the condition of a small Dukhin number:
The development of electrophoretic and electroacoustic theories with a wider range of validity was a purpose of many studies during the 20th century. There are several analytical theories that incorporate surface conductivity and eliminate the restriction of the small Dukhin number for both the electrokinetic and electroacoustic applications.
Early pioneering work in that direction dates back to Overbeek[22] and Booth.[23]
Modern, rigorous electrokinetic theories that are valid for any zeta potential, and often any , stem mostly from Soviet Ukrainian (Dukhin, Shilov, and others) and Australian (O'Brien, White, Hunter, and others) schools. Historically, the first one was Dukhin–Semenikhin theory.[24] A similar theory was created ten years later by O'Brien and Hunter.[25] Assuming a thin double layer, these theories would yield results that are very close to the numerical solution provided by O'Brien and White.[26] There are also general electroacoustic theories that are valid for any values of Debye length and Dukhin number.[5][11]
Henry's equation
When κa is between large values where simple analytical models are available, and low values where numerical calculations are valid, Henry's equation can be used when the zeta potential is low. For a nonconducting sphere, Henry's equation is , where f1 is the Henry function, one of a collection of functions which vary smoothly from 1.0 to 1.5 as κa approaches infinity.[12]
References
- ^ "Colloidal systems – Methods for Zeta potential determination". ISO International Standard 13099, Parts 1,2 and 3. International Organization for Standardization (ISO). 2012.
- ^ ISBN 978-0-12-460529-9.
- ^ ISBN 978-0-521-42600-8. [page needed]
- ^ ISBN 978-0-444-63908-0. [page needed]
- ISBN 978-0-521-11903-0.[page needed]
- S2CID 98812224.
- ^ PMID 31517289.
- ISBN 978-0-08-100557-6.
- ^ "Zeta Potential Using Laser Doppler Electrophoresis". Malvern.com. Archived from the original on 7 April 2012.
- ^ ISBN 978-0-19-855189-8. [page needed]
- ^ S2CID 16513957.
- ^ "Zeta Potential Measurement With TRPS". Izon Science.
- PMID 29234015.
- ISSN 1687-885X.
- ISBN 978-3-200-03553-9.
- ISSN 0011-9164.
- ISSN 0927-7757.
- PMID 29868575.
- ISSN 0022-3654.
- ^ Smoluchowski M (1903). "Przyczynek do teoryi endosm ozy elektrycznej i kilku zjawisk pokrewnych" [Contribution to the theory of electro-osmosis and related phenomena] (PDF) (in Polish). Archived from the original (PDF) on August 10, 2017.
- ^ Overbeek JT (1943). "Theory of electrophoresis — The relaxation effect". Koll. Bith.: 287.
- PMID 18898334.
- ^ Semenikhin NM, Dukhin SS (January 1975). "Polarization of a Moderately Thin Double-Layer Around Spherical-Particles and Its Influence on Electrophoresis". Colloid Journal of the USSR. 37 (6): 1013–1016.
- doi:10.1139/v81-280.
- .