Zeta potential

The main focus of zeta potential analysis of macroscopic solid surfaces is to gain information on the surface charge. This charge is established on the surface of a solid material when it gets in contact with water. Furthermore, the zeta potential provides information on surface functionality, the specific interaction of dissolved compounds with the solid surface, and liquid-on-solid surface adsorption processes.

The zeta potential is thus important for understanding the behavior of solid materials in many technical processes where aqueous systems play a role, e.g. membranes for water treatment, biomaterials in contact with blood or wet processing of semiconductor wafers. Knowledge of the zeta potential of a material helps you optimize specific surface modification processes for a material to perform at its best when applied.

Charge formation

Charges are formed on a solid surface in contact with a dilute electrolyte solution. This charge formation is either due to dissociation or protonation of functional groups or due to adsorption of ions from the solution. The charges are compensated by oppositely charged ions in the electrochemical double layer.

Charge formation by reactions of functional groups

We distinguish acidic and basic surface groups. Acidic groups such as carboxylic or sulfonic acid dissociate when in contact with water, i.e. the H+ ion is released into the surrounding water and the surface assumes a negative charge. Basic groups such as amine groups get protonated when in contact with water, i.e. the surface assumes a positive charge.

The equilibrium of dissociation and protonation strongly depends on the pH value.

Charge formation
Schematic presentation of charge formation at the solid-liquid interface for a hydrophilic material surface with acidic or basic functional groups

Charge formation by adsorption

The presence of functional groups is not a prerequisite for charge formation. On inert surfaces, negative surface charge is formed due to preferential adsorption of hydroxide ions from water. Inert surfaces are thus negatively charged at neutral and alkaline pH. Only at low pH, where the concentration of hydronium ions becomes dominant, inert surfaces exhibit positive surface charge. Again, charge formation by adsorption strongly depends on the pH value.

Charge formation
Schematic presentation of charge formation at the solid-liquid interface for a hydrophobic material surface without functional groups

The electrochemical double layer

The charging behavior at the solid-liquid interface and the definition of the zeta potential are explained using the model of the electrochemical double layer (EDL): When a surface is put in contact with an aqueous solution, the surface assumes a surface charge. The surface charge gives rise to a surface potential, which decays with increasing distance from the solid surface.

Furthermore, the presence of this surface charge leads to a charge distribution at the interface which is different from that in the bulk liquid phase. In the model of the EDL, the charge distribution is divided into a stationary immobile and a diffuse mobile layer of counterions which compensate the surface charge. A plane of shear separates these layers from each other.

The zeta potential is defined as the potential decay between the solid surface and the bulk liquid phase at the plane of shear.

Electrochemical double layer
Model of the electrochemical double layer at the solid-liquid interface (ΨS…surface potential, ζ…zeta potential)

Streaming potential

Upon relative movement of the liquid with respect to the solid sample, the ions of the electrochemical double-layer are sheared off their equilibrium position and shifted along the solid surface. The resulting charge separation gives rise to electrokinetic effects, one of these is called streaming potential. Streaming potential, or alternatively streaming current data are used to calculate the zeta potential.

Electrolyte flow
Electrolyte flow

 

The fundamental equations that relate the streaming potential and the streaming current to the zeta potential have been derived by Hermann von Helmholtz and Marjan von Smoluchowski.

The equation used for calculation of zeta potential using streaming current data requires exact knowledge about the length and cross-section of the streaming channel, i.e. solid sample size.

\zeta=\frac { dI }{ dp } \times\frac { \eta }{ \epsilon\times\epsilon_{ 0 } } \times\frac { L }{ A }

with
dI/dp __ slope of streaming current vs. differential pressure
η _____ electrolyte viscosity
ε _____ dielectric coefficient of electrolyte
ε0 _____  permittivity
L ______ length of the streaming channel
A ______ cross-section of the streaming channel

The equation is thus well suited for zeta potential investigations of planar solids, but is not suitable for the zeta potential evaluation of irregularly shaped samples.

For sample types where the geometry of the streaming channel is unknown (e.g. planar solids of irregular size, fibers, textiles and granular samples) a derivative of the Helmholtz-Smoluchowski equation is applicable. This equation uses streaming potential data in combination with the electrolyte conductivity.

\zeta=\frac { dU }{ dp } \times \frac { \eta }{ \epsilon\times\epsilon_{ 0 } } \times { \kappa  }_{ B }

with
dU/dp __ slope of streaming potential vs. differential pressure
κB ______ electrolyte conductivity

Zeta potential and its dependences

The zeta potential depends on both, the solid sample itself but also the properties of the liquid phase used for the measurement. The pH dependence of zeta potential is among the most extensively studied dependences of zeta potential. It gives valuable information on the composition of the outermost surface, i.e. the presence of acidic or basic functional groups. The pH value at which the zeta potential is 0 mV is known as the isoelectric point and is used as an indicator for the chemistry of a surface.

Zeta potential and its dependencies
Zeta potential and its dependencies

Zeta potential for adsorption studies

The zeta potential is sensitive to the outermost surface layer of the material. It is thus perfectly suited for monitoring changes in the surface charge upon adsorption of dissolved substances in solution on the solid surface. Both the time- and concentration-dependence of adsorption processes are directly accessible by monitoring the respective change in either streaming potential, streaming current or zeta potential data.

Knowledge about the surface charge and its changes due to liquid-on-solid surface adsorption is important for tuning material properties and optimizing processes. Changes of surface properties due to modification, storage, aging or wear during operation can be investigated.

Applications

The range of applications for zeta potential analysis on solids is vast and diverse: It reaches from membrane characterization, to biocompatibility and protein adsorption studies on biomaterials, optimization of wafer cleaning processes, technological applications in detergency to applications in cosmetics, polymers, minerals and petroleum industries.

Membranes and Filters

Surface charge is a key parameter in membrane surface analysis. Knowledge about the surface charge helps to increase the selectivity of membranes, to optimize the retention in the separation process and to reduce membrane fouling.

>> Enhancement of membrane separation performance
>> Monitoring of membrane fouling with zeta potential
>> The zeta potential of track-etched PVDF membranes
>> Understanding filter efficiency – virus removal in depth filtration

Biomaterials

The interface between a material’s surface and the surrounding fluid determines its compatibility with a biological environment. Interfacial properties usually require separate analyses of the solid surface and the liquid phase.

The benefit of zeta potential studies is a direct solid/water interface analysis. It furthermore visualizes the interaction between proteins in solution and biomaterials and thus helps to tailor biocompatible devices.

>> Inner surface treatment of haemodialysis membranes
>> Tuning of solid surface properties by self-assembled monolayers

Cosmetics and Surfactants

Zeta potential results can be used to correlate the effect of hair care products such as shampoo and conditioner on hair fibers. Understanding the different behavior of virgin or bleached hair is the basis for optimizing formulations of cosmetic products.

>> Effect of bleaching on the streaming potential of human hair

Polymers and Composites

To receive enhanced polymers and reinforced polymers with defined surface properties for e.g. wettability, printability or adhesion, external treatments such as flame treatment, corona discharge or plasma activation are performed. These treatments induce functional groups which have a certain charge. This surface charge can be best determined with zeta potential measurements, which show the effect and efficiency of such treatments.

>> Charge formation on polymer surfaces
>> From inert to “active” monitoring of surface treatment
>> Self-disinfectant surfaces – correlation between antimicrobial activity and zeta potential
>> Surface modification of Kapton® films

Semiconductor materials

Cleanliness in the production of semiconductor materials, the detection of trace impurities and of their removal during cleaning procedures, is one of the most important issues during processing to guarantee proper function of the semiconductor layers. The benefit of using zeta potential measurements is the optimization of wet chemical processes like CMP to ensure the removal of the CMP slurries from the semiconductor surface. Therefore, knowledge of the surface charge of the respective semiconductor is essential. Furthermore, zeta potential measurements are beneficial for the quality determination of deposited SAMs (self-assembled monolayers).

>> Detection of native oxide on silicon nitride wafers

Fibers and Textiles

The surface charge of natural and technical fibres is essential for the success of different process steps like scouring and bleaching of cotton fibres. The surface charge also plays a major role in the subsequent application of dyestuff or of medical fibres/textiles like gauzes. An increased surface compatibility of technical fibres, such as glass or carbon, for their use as reinforcement materials in e.g. polymer matrices is analysed with zeta potential measurements.

>> Monitoring the washing efficiency of stained cotton fabrics
>> Optimization of sizing systems for glass fiber finishing

External links

Electrokinetic Analyzer for Solid Surface Analysis from Anton Paar
Selected literature on surface zeta potential

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