Mystery of microgels solved

Researchers at PSI and the University of Barcelona have managed to explain the strange behaviour of microgels. Their measurements using neutron beams have pushed this measuring technique to its limits. The results open up opportunities for new applications in materials and pharmaceutical research.

They flow through our arteries, add colour to our walls or make milk tasty: tiny particles or droplets that are very finely distributed in a solvent. Together they form a colloid. Whereas the physics of colloids involving hard particles – such as colour pigments in emulsion paint – is understood well, colloids involving soft particles – such as haemoglobin, the red pigment in blood, or droplets of fat in milk – hold some startling surprises. An experiment carried out 15 years ago showed that soft particles made of polymers – so-called microgels –shrink abruptly when their concentration in a solvent is increased above a certain threshold. When this happens, large particles contract until they are the size of their smaller neighbours. Amazingly, this happens even when the particles are not actually in contact with each other. The researchers were puzzled: How does a gel particle know how big its neighbour is without touching it? Is there some sort of “telepathy” going on between microgels?

Hypothesis of 2016 confirmed

“Of course not,” smiles Urs Gasser. The physicist has been studying the miraculous shrinking of microgels in colloids for the past ten years. Together with a team of researchers, he published a paper in 2016 explaining the phenomenon. Briefly, in this situation, the polymer particles consist of long carbon chains. These carry a weak negative charge at one end. These chains form a ball, the microgel. This can be thought of as resembling a ball of wool, with the properties of a sponge. This three-dimensional tangle therefore contains negative point charges that attract positively charged ions in the liquid. These so-called counterions arrange themselves around the negative charges in the ball, forming a positively charged cloud on the surface of the microgel. When the microgels come close together, their charge clouds overlap (see image).  This in turn increases the pressure inside the liquid, which compresses the microgel particles until a new equilibrium is reached.

At the time, however, the research team was unable to provide experimental proof of the cloud of counterions. Together with his PhD student Boyang Zhou and Alberto Fernandez-Nieves of the University of Barcelona, Gasser has now furnished that evidence – and it impressively supports the 2016 hypothesis. The results have been published in the journal Nature Communications.

SINQ neutron source crucial to solving the puzzle

This was possible thanks to the neutrons from PSI’s spallation source SINQ – along with an experimental trick. Because the cloud of counterions in the colloid is so rarefied that it is not actually visible in the image of the scattered neutrons. The counterions account for no more than one percent of the mass of a microgel. So Gasser, Zhou and Fernandez-Nieves examined two samples: one colloid in which all the counterions were sodium ions, and another in which they were ammonium ions (NH₄). Both these ions also occur naturally in microgels – and they scatter neutrons differently. Subtracting one image from the other leaves the signals of the counterions. Boyang Zhou: “This seemingly simple solution requires the utmost care in preparing the colloids so as to make the ion clouds visible. No one has ever measured such a rarefied ion cloud before.”

Applications in cosmetics and pharmaceuticals

Knowing how soft microgels behave in colloids means that they can be tailored to fit many different applications. In the oil industry, they are pumped into underground reservoirs to adjust the viscosity of the oil in the well and facilitate its extraction. In cosmetics, they give creams the desired consistency. Smart microgels are also conceivable, which could be loaded with medicines. The particles could react to gastric acid, for example, and release the drug by shrinking. Or else a microgel could shrink into a small, densely packed polymer ball when the temperature increases, one that reflects light differently than in its swollen state. This could be used as a temperature sensor in narrow fluid channels. Other sensors could be designed to respond to changes in pressure or contamination. “There are no limits to the imagination,” says Urs Gasser.

Text: Bernd Müller

About PSI

The Paul Scherrer Institute PSI develops, builds and operates large, complex research facilities and makes them available to the national and international research community. The institute's own key research priorities are in the fields of matter and materials, energy and environment and human health. PSI is committed to the training of future generations. Therefore about one quarter of our staff are post-docs, post-graduates or apprentices. Altogether PSI employs 2200 people, thus being the largest research institute in Switzerland. The annual budget amounts to approximately CHF 420 million. PSI is part of the ETH Domain, with the other members being the two Swiss Federal Institutes of Technology, ETH Zurich and EPFL Lausanne, as well as Eawag (Swiss Federal Institute of Aquatic Science and Technology), Empa (Swiss Federal Laboratories for Materials Science and Technology) and WSL (Swiss Federal Institute for Forest, Snow and Landscape Research). Insight into the exciting research of the PSI with changing focal points is provided 3 times a year in the publication 5232 - The Magazine of the Paul Scherrer Institute.

Contact

Dr Urs Gasser

Laboratory for Neutron Scattering and Imaging
Paul Scherrer Institute, Forschungsstrasse 111, 5232 Villigen PSI, Switzerland
Telephone: +41 56 310 32 29, e-mail: urs.gasser@psi.ch
 

Original publication

Measuring the counterion cloud of soft microgels using SANS with contrast variation

Boyang Zhou, Urs Gasser, Alberto Fernandez-Nieves

Nature Communications, 07.07.2023

DOI: 10.1038/s41467-023-39378-5