The battery of different pieces of high-tech equipment found in modern laboratories measure a dizzying array of parameters. Every factor is probed in an attempt to help researchers understand what, exactly, they have got in their sample tube. Zeta potential is one of the more obscure measurements, but it is assuming increasing importance. Stuart Nathan reports.
Zeta potential arises in colloidal systems; suspensions of particles in a liquid, where the particles are larger than those in a solution and smaller than those in a suspension. Colloids are important in several branches of the life sciences.
Most physiological fluids are colloids; several important manufacturing produce their products in a colloidal form; drugs are frequently formulated as colloids. The behaviour of these systems is governed by the complex interplay of electrical charges between the molecules of the solvent, any ions that might be dispersed in it, and the charges that exist on the colloidal molecules.
Zeta potential is an electrical property characteristic of a colloid, resulting from these interactions.
Simply stated, it is the difference in electrical potential between the layer of liquid bound to the surface of a colloidal particle and the bulk liquid. It can tell researchers how stable a colloid is whether the particles are likely to coagulate and sink to the bottom, for example.
Repulsion forces
A zeta potential of about 50mV indicates that a colloid is very stable, because the repulsion forces between the particles are strong. The closer the zeta potential gets to zero, the more likely the colloid is to coagulate.
In can also provide other information: in biological systems, for example, the zeta potential gives clues to how cells are interacting and whether drug active agents, for example, are binding to their targets properly.
Moreover, measurement of zeta potential can be used to derive the size distribution of particles in a colloid. This is often a vital property. Particle size is imperative to understanding how drugs are absorbed into the body, for example. It also gives process engineers important insights into how a process is working, and whether any adjustments are needed to enhance the efficiency or improve the performance.
The factors governing zeta potential are extremely complex and employ almost as much high-level maths as predicting the behaviour of black holes.
Potential difference
Fortunately, it can be measured fairly easily by looking at how a colloid behaves when it is subjected to an electrical field. As colloidal particles invariably carry a charge, applying a potential difference across a sample of the colloid will force the particles to drift, with positively-charged particles heading for the negative electrode inducing the field, and vice-versa.
The zeta potential is, at least approximately, directly proportional to the electrophoretic mobility of the particles that is, the ratio of the particles' velocity under the influence of the electrical field to the field strength.
These properties are proving useful to researchers like Mike Garvey of the University of Liverpool's surface science research centre. A former senior scientist with Unilever, Garvey is now working on water treatment techniques.
He is using zeta potential to study the interaction of charged particles in untreated water and the compounds used to clarify the water, generally metal salts such as precipitated aluminium hydroxide. "These particles are generally positively charged, and most of the natural organic matter in untreated water the material that gives the brown coloration is negatively charged.
Positive particles
The negatively charged material attaches itself to the positively charged particles, so that these particles becomes less positive or more negative charged. This tells us that something is happening at that surface and we use colloid science expertise to interpret what is involved.“
Zeta potential is proving an ideal tool to study this system. The quantities involved in experimentation are extremely small, so it's difficult to detect whether the colloidal particles are absorbing organic matter using conventional techniques, says Garvey. “Zeta potential measurement is extremely sensitive, so any material that adsorbs on particle's surface will give a change in its potential,“ he adds.
The goal of Garvey's research is to use the electrical data from his colloid studies to devise new ways of treating water.
"The negatively charged material the natural organic material in water is the material which is most readily removed, because the coagulant has a positive charge. You add the coagulant powder and it settles down. It sinks to the bottom and takes these impurities all the brown coloration with it,“ he says.
"Our hypothesis is that the uncharged organic matter slips through, and while that hasn't posed a problem up until now, tightening legislation means that new technology might be required to remove this too.
"The route that we are pursuing is to take that uncharged material and then oxidise it making it negatively charged. Then, essentially, we will perform the same operation again with more coagulant to remove the additional floc of organic matter.“
Garvey anticipates that this technique could help the water companies surpass even the most stringent regulations. Water quality trends are tightening everywhere, with new water quality directives from the European Union demanding lower levels than ever of substances like nitrogen compounds and lead residues in drinking water.
In the UK, meanwhile, the Environment Agency has launched regulations to greatly increase the level of monitoring for microorganisms such cryptosporidium, and to reduce the amount of these species which survive water treatment.
Moreover, regulations in the US are tightening even further, and Garvey notes that Europe tends to take its lead from the US. "What happens there, we know will come here eventually.“
Upgrades
The versatility of the equipment is likely to make it useful in other research, too. Because the behaviour of particles under electrophoresis is also related to the particle size, zeta potential analysers can also be upgraded to sophisticated measurement devices.
Garvey's research will soon link up with another part of Liverpool University with is developing new methods in nanotechnology using nanometre-scale particles to build microscopic structures that could transform the fields of high-tech ceramics, microelectronics and medicine.
"Future collaboration in this field would make a great deal of use of this equipment,“ he says.
