Nanoparticles (NPs) are microscopic materials dispersed in a liquid, and their behavior is governed largely by the electrical charge they carry on their surface. The zeta potential ($\zeta$) is a measurement that quantifies this electrical charge, providing a single value that represents the net electrical potential at the boundary of a nanoparticle moving through a liquid. This value is a fundamental property for any nanoparticle system, directly determining how the particles interact with each other and with the surrounding medium. The zeta potential is a powerful predictor of how stable a nanoparticle formulation will be in a liquid environment.
Understanding the Physics of Zeta Potential
When a nanoparticle is suspended in a liquid, it acquires a surface charge, attracting oppositely charged ions from the solution. This creates a structured region of charge known as the Electric Double Layer (EDL), which is the physical basis for the zeta potential measurement. The EDL is divided into the inner, tightly bound Stern layer and the outer, more mobile diffuse layer.
The Stern layer consists of ions strongly adsorbed to the particle surface that move with the particle as a single unit. The diffuse layer contains a looser atmosphere of ions whose concentration decreases further away from the particle. The “slipping plane” is the imaginary boundary separating the liquid that moves with the particle from the bulk liquid that remains stationary.
The zeta potential is defined as the electrical potential measured precisely at this slipping plane. Because the EDL neutralizes the particle’s initial surface charge over a distance, the zeta potential is generally lower in magnitude than the particle’s true surface potential.
Zeta Potential and Nanoparticle Stability
The zeta potential’s most practical function is predicting the colloidal stability of a nanoparticle dispersion. A high magnitude of zeta potential (strongly positive or negative) indicates strong electrostatic repulsion between particles. This repulsive force overcomes natural attractive forces, such as van der Waals forces, keeping the particles well-separated and ensuring a stable, dispersed system.
Generally, a zeta potential magnitude greater than $\pm 30$ millivolts (mV) is the threshold for a highly stable colloidal suspension. Conversely, a value close to zero indicates minimal repulsive force, allowing attractive forces to dominate when particles collide. When the zeta potential approaches zero, particles quickly overcome the repulsive energy barrier, leading to aggregation, flocculation, and eventual settling out of the solution.
The balance between these attractive and repulsive forces is described by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. This theory posits that the total energy between two approaching particles is the sum of the attractive van der Waals energy and the repulsive electrostatic energy. A highly negative or positive zeta potential ensures the repulsive energy barrier is sufficiently high to prevent particles from aggregating, thereby maintaining the formulation’s stability.
How Zeta Potential is Measured
The zeta potential is not measured directly but is calculated from electrophoretic mobility. This measurement relies on electrophoresis, where a charged particle suspended in a liquid moves when an electric field is applied. The particle’s velocity is directly proportional to the strength of the electric field and its electrophoretic mobility.
The standard laboratory technique for determining this mobility is Electrophoretic Light Scattering (ELS). In this method, a voltage is applied across a cell, causing charged particles to move toward the electrode of the opposite charge. A laser beam is directed through the sample, and the light scattered by the moving particles undergoes a Doppler frequency shift.
This frequency shift is directly proportional to the speed of the particles, allowing the instrument to calculate their electrophoretic mobility. Mobility is then converted into the zeta potential value using established mathematical models, such as the Smoluchowski equation. The accuracy of the final value depends on accurately measuring particle velocity and knowing the viscosity and dielectric constant of the liquid medium.
Manipulating and Applying Zeta Potential
Scientists intentionally manipulate the zeta potential to control nanoparticle behavior for specific applications. The two most common external factors used to modify the zeta potential are the pH and the ionic strength of the surrounding liquid. Adjusting the pH changes the concentration of hydrogen and hydroxyl ions, which alters the net charge by protonating or deprotonating chemical groups on the particle surface.
Ionic strength, the concentration of dissolved salts, influences the thickness of the Electric Double Layer. Increasing the salt concentration compresses the diffuse layer, shrinking the particle’s electric shield and reducing the magnitude of the zeta potential. This reduction can intentionally destabilize a formulation, causing particles to aggregate at a specific time.
Controlling the zeta potential is important in targeted drug delivery systems. For example, a formulation may be engineered to have a slightly negative zeta potential to ensure stability while circulating in the bloodstream, preventing premature clearance. The particle’s surface can be designed to become less stable in the low-pH environment of a tumor or cellular compartment, which causes the particle to destabilize and release its drug payload at the target site. This control is also applied in material science to formulate stable products like paints, coatings, and ceramics, where particles must remain uniformly dispersed.