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CO2 as a line active agent in a nanopore

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Scientific news from INP CNRS and UGA
Just like the effect of surfactants acting on the surfaces between two media such as water and air, there can be a lineactant effect on the lines separating three media. Using molecular dynamics simulations, and thanks to a new approach based on a force measurement, physicists from the Interdisciplinary Laboratory of Physics in Grenoble (LIPhy, CNRS/UGA) show for the first time such an effect for carbon dioxide (CO2) in contact with a drop of water confined in a nanopore.

Surface tension characterizes the energy at the interface between two bodies, such as the surface of a drop of water in air or of an air bubble in water. A surfactant, such as soap or detergents, is a product that attaches itself to the interface. It modifies its surface tension and more generally the mechanical properties of the interface. It thus facilitates the formation of bubbles and stabilizes the foam. When there is a third body, as in the case of a drop of water in the air resting on a solid substrate, a line appears at the contact of these three bodies. To characterize the energy of this contact line, we speak of line tension. In practice, the effects of line tension mainly concern nanometer-sized objects and, although the concept of line tension was proposed by J. W. Gibbs 150 years ago, its properties are still poorly understood and difficult to apprehend: the measurement of line tension remains a challenge both experimentally and numerically.

Using molecular dynamics simulations, and thanks to a new approach based on a force measurement, physicists from the Interdisciplinary Laboratory of Physics in Grenoble (LIPhy, CNRS/UGA) have achieved for the first time a precise and sensitive estimation of the effect of a gas on the line tension. They demonstrate that CO2 behaves as a line active agent. In the case of a water drop confined in a nanopore with a CO2-rich vapor, they calculated the change in line tension due to the accumulation of CO2 molecules at the contact line surrounding the water drop confined within a nanopore (figure). Just as surfactants lower the surface tension, the presence of CO2 lowers the line tension, and significantly more in the case of a hydrophilic than a hydrophobic wall. This study is published in the journal PNAS.

These results show that the mechanical effects induced by a lineactant on the line tension can play a key role on nucleation phenomena. The concerned domains are diverse. A common point is the contribution of nanometric structures such as nanopores or nanoparticles in contact with different phases: CO2 sequestration in porous rocks, water purification and desalination by nanoporous membranes, porous electrodes and electrolysis phenomena, opening/closing phenomena of biological nanopores or even the formation of clouds by condensation on atmospheric nanoparticles, to name only a few examples. The question is now open to know if other gases than CO2 act in a similar way!

Figure : CO2 concentration field calculated for a water drop confined in a nanometric pore. We visualize with the warm colors the CO2 accumulation in the vicinity of the two contact lines between the solid, liquid and gas phases (perpendicular to the image). Calculations show that the line tension can be significantly modified, up to a factor 10 for a CO2 pressure of 5 MPa (50 bar) and for a hydrophilic wall.

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View online : Carbon dioxide as a line active agent: its impact on line tension and nucleation rate. R. Bey, B. Coasne et C. Picard, PNAS, paru le 17 août 2021.