Microgel mystery solved

July 10, 2023

(Nanowerk News) Researchers at PSI and the University of Barcelona have succeeded in explaining the strange behavior of microgels. Their measurements using a beam of neutrons have pushed this measurement technique to its limits. The results open up opportunities for new applications in pharmaceutical materials and research.

They flow through our arteries, add color to our walls or make milk palatable: tiny particles or droplets distributed very finely in a solvent. Together they form colloids. While colloid physics involving hard particles – such as the color pigments in emulsion paint – is well understood, colloids involving soft particles – such as hemoglobin, the red pigment in blood, or fat droplets in milk – hold some surprising surprises.

Experiments carried out 15 years ago showed that soft particles made of polymers – called microgels – shrink suddenly when their concentration in the solvent increases above a certain threshold. When this happens, the large particle contracts until it is about the size of its smaller neighbor. Amazingly, this occurs even when the particles are not actually in contact with one another. Researchers are perplexed: How do gel particles know how big their neighbors are without touching them? Is there some kind of “telepathy” going on between the microgels?

The 2016 hypothesis was confirmed

“Of course not,” smiled Urs Gasser. The physicist has been studying the miraculous shrinkage of microgels in colloids for the past ten years. Together with a team of researchers, he published a paper in 2016 explaining the phenomenon. In short, in this situation, the polymer particles consist of long carbon chains. It carries a weak negative charge at one end. These chains form a ball, the microgel. It can be thought of as resembling a ball of wool, with the properties of a sponge. Therefore, these three-dimensional tangles contain negative point charges which attract positively charged ions in the liquid. These so-called counterions arrange themselves around the negative charges in the sphere, forming a cloud of positively charged on the surface of the microgel. When the microgels are close together, the charge clouds overlap (see figure). This in turn increases the pressure inside the liquid, which compresses the microgel particles until a new equilibrium is reached. This graphical simulation shows microgel particles (green) arranging themselves in a liquid, with overlapping ion clouds (red) on their surface. (Image: Urs Gasser)

However, at that time, the research team was unable to provide experimental evidence of counter clouds. Together with his PhD students Boyang Zhou and Alberto Fernandez-Nieves of the University of Barcelona, ​​​​Gasser has now completed the evidence – and impressively supports the 2016 hypothesis. The results have been published in the journal Nature Communications (“Measuring counterion cloud from soft microgels using SANS with varying contrast”).

The SINQ neutron source is important for solving the puzzle

This is possible thanks to the neutrons from the PSI SINQ spalation source – along with an experimental trick. Because counterion clouds in colloids are so thin that they are actually invisible in the scattered neutron image. The number of counterions is not more than one percent of the mass of the microgel. So Gasser, Zhou and Fernandez-Nieves examined two samples: one colloid in which all the counter ions were sodium ions, and another in which they were ammonium (NH4) ions. These two ions also occur naturally in microgels – and they scatter neutrons differently. Subtracting one image from another leaves a counterion signal.

Boyang Zhou: “This seemingly simple solution requires careful preparation of the colloid to make the ion cloud visible. No one had ever measured a rarefied ion cloud before.”

Application in cosmetics and medicine

Knowing how soft microgels behave in colloids means that they can be adapted to suit 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, creams provide the desired consistency. Smart microgels are also conceivable, which can be filled with drugs. The particles can react to stomach acid, for example, and release medication by shrinking. Or the microgels can shrink into tiny, dense polymer balls as the temperature increases, balls that reflect light differently from their swollen state. It can be used as a temperature sensor in narrow liquid passages. Other sensors can be designed to respond to changes in pressure or contamination.

“Imagination is limitless,” said Urs Gasser.

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