Neutron beam measurements reveal microgel miniaturization in colloids

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This graphical simulation shows the microgel particles (in green) self-aligning in the liquid, with ion clouds (red) overlapping on their surface. Credit: Urs Gasser

Researchers at PSI and the University of Barcelona have explained the microgel’s strange behavior. Their measurements using neutron beams pushed this measurement technique to its limits. The results open up opportunities for new applications in materials and pharmaceutical research.

They flow through our arteries, add color to our walls or make milk taste good: tiny particles or droplets are very finely distributed in the solvent. Together they form a glue. While the physical properties of colloids related to hard particles, such as color pigments in emulsified paints, are well understood, colloids related to soft particles, such as hemoglobin, red element in the blood or fat drops in milk, there are some surprising surprises.

An experiment carried out 15 years ago showed that soft particles made of polymers—called microgels—shrink abruptly when their concentration in the solvent rises above a certain threshold. When this happens, the large particles shrink until they are the same size as their smaller neighbors. Amazingly, this happens even when the particles are not actually in contact with each other. Confused researchers: How does a gel bead know how big its neighbor is without even touching it? Is there some kind of «telepathy» going on between the microgels?

The 2016 hypothesis is confirmed

«Of course not,» said Urs Gasser. This physicist has been studying the miraculous contraction of microgels in colloids for the past ten years. Together with a group of researchers, he published a paper in 2016 explaining the phenomenon.

Briefly explained, in this situation, the polymer particles consist of long carbon chains. They carry a weak negative charge at one end. These chains form a ball, the microgel. This can be thought of like a ball of wool, with the properties of a sponge. Thus, this hologram contains negative point charges that attract positively charged ions in the liquid. The so-called reaction arranges itself around the negative charges in the ball, forming a positively charged cloud on the surface of the microgel. As the microgels come close to each other, their charge clouds overlap (see image above). This in turn increases the pressure inside the liquid, which compresses the microgel particles until a new equilibrium is reached.

However, at the time, the team could not provide experimental evidence of a reactive cloud. Along with his PhD. Students Boyang Zhou and Alberto Fernandez-Nieves of the University of Barcelona, ​​Gasser have now provided that evidence—and it impressively supports the 2016 hypothesis. The results have been published in the journal natural communication.

The SINQ neutron source is important for puzzle solving

This was made possible with neutrons from PSI’s SINQ collision source—along with an experimental trick. Because the reactive cloud in the colloid is so rare, it is not really visible in images of scattered neutrons. The reactions make up no more than 1% of the mass of a microgel. So, Gasser, Zhou and Fernandez-Nieves examined two samples: a colloid in which all the reactions are sodium ions, and another where they are ammonium ions (NH).4).

Both of these ions also occur naturally in microgels—and they disperse neutrons differently. Subtracting one image from another leaves the signal of the response. «This seemingly simple solution requires the utmost care in preparing colloids to make the ion clouds visible. No one has ever measured an ion cloud before,» said Boyang Zhou. That’s rare.»

Applications in cosmetics and pharmaceuticals

Knowing how soft microgels work in colloids means they can be tailored to suit a wide variety of applications. In the oil industry, they are pumped into underground storage tanks to regulate oil viscosity in wells and facilitate extraction. In cosmetics, they give the cream the desired consistency. Smart microgels, which could contain drugs, could also be envisioned. For example, the particles can react with stomach acid and release the drug by contracting.

Or a microgel could shrink into a small, dense polymer ball as the temperature rises, a ball that reflects light differently from its swollen state. This can be used as a temperature sensor in narrow liquid channels. Other sensors may be designed to respond to changes in pressure or contamination. «There are no limits to the imagination,» Gasser said.

More information:
Boyang Zhou et al., Measuring the response cloud of soft microgels by SANS with contrast variation, natural communication (2023). DOI: 10.1038/s41467-023-39378-5

Journal information:
natural communication

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