Zeta potential experimental techniques

The work of Larson et al. (62) represented the first detailed study to show agreement between AFM-derived diffuse layer potentials and ((-potentials obtained from traditional electrokinetic techniques. The AFM experimental data was satisfactorily fitted to the theory of McCormack et al. (46). The fitting parameters used, silica and alumina zeta-potentials, were independently determined for the same surfaces used in the AFM study using electrophoretic and streaming-potential measurements, respectively. This same system was later used by another research group (63). Hartley and coworkers 63 also compared dissimilar surface interactions with electrokinetic measurements, namely between a silica probe interacting with a polylysine coated mica flat (see Section III.B.). It is also possible to conduct measurements between a colloid probe and a metal or semiconductor surface whose electrochemical properties are controlled by the experimenter 164-66). In Ref. 64 Raiteri et al. studied the interactions between... 

Determination of the Electrophoretic Mobility, To evaluate the equation for the double-layer interaction (eq 5), the zeta potential, must be known it is calculated from the experimentally measured electrophoretic mobility. For emulsions, the most common technique used is particle electrophoresis, which is shown schematically in Figure 4. In this technique the emulsion droplet is subjected to an electric field. If the droplet possesses interfacial charge, it will migrate with a velocity that is proportional to the magnitude of that charge. The velocity divided by the strength of the electric field is known as the electrophoretic mobility. Mobilities are generally determined as a function of electrolyte concentration or as a function of solution pH. 

Adsorption isotherms are habitually obtained using the solution depletion method, which consists of comparing the solute concentrations before and after the attainment of adsorption equilibrium. Electrokinetic or zeta potentials are determined by two techniques microelectrophoresis [12,14,17] and streaming potential [13,58,59]. The former is employed to measure the mobility of small particles of chemically pure adsorbents, whereas the latter is adopted to investigate the electrophoretic behaviour of less pure coarser mineral particles. A correlation between the adsorption and electrophoretic results is usually examined with the aim of sheding light on the mechanism by means of which the surfactants are adsorbed at the solution-solid interface. This implies the necessity of maintaining the same experimental conditions in both experiments. For this purpose, the same initial operational procedure is applied. 

Kirby, B.J. and Hasselbrink, E.F., Zeta potential of microfluidic substrates 1. Theory, experimental techniques, and effects on separations. Electrophoresis, 2004, 25 187-202. 

Electro osmosis This technique involves the movement of a liquid relative to a stationary charged surface (e.g. a capillary or porous plug) by the application of an electric field. Experimentally, zeta potentials may be measured by this method by means of an apparatus such as that shown in Figure 10.10. The potential is supplied by electrodes, as shown in the schematic, and the transport of liquid across the tube is observed through the motion of an air bubble in the capillary providing the return flow. For water at 25°C, a field of about 1500 V/cm is needed to produce a velocity of 1 cm/s if the surface potential (xjro) is 100 mV. 

Microfluidic and nanofluidic chips have a wide range of applications in the chemical, biomedical, environmental, and biology areas, where a variety of chemical solutions are used. With the development of microfabrication technology, many new materials such as PDMS and poly (methyl methacrylate) (PMMA) are also employed for chip fabrication. Since each pair of sohd-liquid interface has its unique zeta potential and electroosmotic mobility, which have significant influences on flow control in such small-scale devices, it is very important to experimentally determine these two parameters using the current monitoring technique in order to develop microfluidic and nanofluidic devices for various applications. 

From the discussions outlined so far, it is apparent that the zeta potential happens to be one of the key parameters that dictates the overall potential distribution within the EDL. Several experimental techniques have been reported in the literature for an accurate measurement of this important parameter. In the subsequent discussions, the general principle behind these techniques is briefly outlined. The existing methodologies for the measurement of C primarily rely on the measurement of electroosmotic flow velocities through microfluidic conduits. When an electrical field is applied tangentially to a solid—liquid interface, an electrical body force is exerted on the excess counterions in the diffuse EDL. These ions move under the action of the applied electrical field, pulling the liquid with them. For thin EDLs (relative to the half channel width), the average electroosmotic flow velocity can be expressed as... 

Concentration/separation of sample solutes is one of most important functions in micro- and nanofluidic systems. TGF has proved to be a promising technique that can achieve concentration and separation in microfiuidic devices. However, so far very limited experimental and theoretical investigations have been reported. Experimentally, it is highly desirable to develop various microfiuidic structures that can be utilized by the TGF technique to cmicentrate different samples. Furthermore, more experiments should be carried out to characterize the thermoelectrical properties of buffers and samples so as to obtain the temperature-dependent electroosmotic mobility and electrophoretic mobility, as well as buffer conductivity, viscosity, and dielectric permittivity for each individual sample and buffer solution. In addition, the development of reliable, accurate, high-resolution, experimental techniques for measuring fiow, temperature, and sample solute concentration fields in microfiuidic channels is needed. Theoretically, the model development of TGF is still in its infancy. The models presented in this study assume the dilute solute sample and linear mass flux-driving forces correlations. However, when the concentrations of the sample solute and the buffer solution are comparable, the aforementioned assumptions break down. Moreover, the channel wall zeta potential in this situation may become nonconstant. More comprehensive models should be developed to incorporate the solute-buffer and solute-channel wall... 

Zeta Potential Measurement, Fig. 2 Schematic of the experimental setup for the current-monitoring technique (a)... 

Some of the experimental techniques employed in these studies have included determining the change in surfactant concentration in the bulk solution npon adsorption, zeta potential measurements, and probe tecbniqnes (electron spin resonance and fluorescence). Attempts to describe the adsorption behavior exhibited in tbe adsorption isotherms has led to the development of several mathematical models [26, 30-33]. To date, none of the models are capable of fully accounting for all of the phenomena which affect surfactant adsorption without introducing ad hoc assnmptions and adjustable parameters, but they bave offered some interesttug insights. 

Additional to the sedimentation behaviour, the zeta-potential was measured for each suspension. This was conducted by means of an electroacoustic measurement technique (Sect. 2.3.7.2). The technique yields an effective zeta-potential of the binary suspension, which is calculated from the electroacoustic raw signals by assuming effective particle properties (e.g. for the permittivity and density of the solid material). When the two particle components contribute independently to the electroacoustic signal and do not affect each other with regard to the interfacial properties, it is possible to calculate the effective zeta-potential from the zeta-potentials of the single components. The comparison between such calculated zeta-potential values with experimental ones allows a first evaluation of the interfacial phenomena in the binary suspension. 

Zeta potential is a fundamental parameter for modelling and characterizing electrokinetic flows in a variety of microfluidics and Lab-on-a-Chip devices. Because the zeta potential depends on so many factors (pH, concentration, liquid, surface etc.) more measurements are required for a variety of surface-liquid combinations particularly, biological and biochemical fluids. Since measurements can vary greatly between methods and experiments, multiple tests should be employed to accurately determine the zeta potential. In addition, measurements performed using multiple techniques can be corroborated which will reduce errors between results. The ultimate goal is to develop an accurate database of zeta potential measurements for various solid-liquid interfaces, however this will most likely require the development of new theoretical models and experimental methods to increase the accuracy and throughput of current devices. 

As mentioned above, one of the main criteria for electrostatic stabflity is the high surface or zeta potential, which can be experimentally measured (vide infra). Before describing the experimental techniques for measuring the zeta potential it is essential to consider the electrokinetic effects in some detail, describing the theories that can be used to calculate the zeta potential from the particle electrophoretic mobility [21]. 

Learn the fundamentals of some common experimental methods in colloidal science light-scattering techniques, zeta potential measurements, and rheology. 

The determination of electrophoretic velocities may be carried out experimentally by the use of methods suitable for transport number measurements. Moving boundary techniques have proved useful despite the problem of a difficulty in selecting suitable indicator ions. Reliable estimates of electrophoretic velocities make possible the determination of zeta-potentials. Since colloids migrate at characteristic rates under the influence of an electric field, electrophoresis provides an important means of separation. Coatings, such as rubber or graphite, may be deposited on metal electrodes by this means and additives to these may be co-deposited. 

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