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Surfactants at the water-air interface: Exploring the intriguing arrangement of molecules at the liquid skin

by News Staff
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Surfactants play a vital role in our daily lives, particularly as key components in soaps. Their unique structure, consisting of hydrophilic and hydrophobic parts, allows them to gather at the interface between water and air. This accumulation enables them to influence various processes, such as the rate of evaporation of a solution or the efficiency of gas molecule absorption by the solution. The incorporation of carbon dioxide into the oceans, for example, heavily relies on surfactants.

The arrangement of surfactant molecules at the water-air interface has captivated scientists for centuries. Even Benjamin Franklin, who observed the calming effect of cooking oil on water surfaces, and Agnes Pockels, who conducted pioneering experiments on the topic in the late 19th century, were fascinated by this phenomenon.

Understanding the precise organization of surfactant molecules at the water-air interface poses a significant challenge. It requires employing techniques that focus on the outermost layers of water, where surfactant molecules reside in an extremely thin layer measuring only a few billionths of a meter.

Elastically scattered electrons offer the clue

A recent collaborative study conducted by scientists from the Fritz Haber Institute in Berlin, spanning the Departments of Inorganic Chemistry, Molecular Physics, and Theory, has introduced a novel approach to tackle the aforementioned challenge. Their method is based on the elastic scattering of photoelectrons emitted when the water-surfactant-vapor interface is irradiated with X-rays.

The specific surfactant under investigation in their study was perfluorinated pentanoic acid. In the core level photoelectron spectrum of the C 1s (inner shell), it was possible to distinguish four out of the five carbon atoms in the molecule. Particularly, the hydrophilic and hydrophobic ends of the surfactant could be differentiated during the experiment.

Perfluorinated pentanoic acid belongs to the group of “forever chemicals,” which have gained attention as prominent pollutants in natural bodies of water. These compounds are challenging to remove and pose environmental hazards. The measurements were conducted at two synchrotron radiation light sources: BESSY-II in Berlin and SOLEIL near Paris. The X-ray beamlines utilized in the experiment allowed for manipulation of the linear polarization direction of the X-rays.

The angle between the polarization direction and the electron detector determined the intensity of the detected electron signal. By examining the intensity distribution as a function of the angle, valuable insights were obtained regarding the number of elastic “collisions” encountered by the electrons on their path to the detector.

Due to water’s density, electrons originating from the parts of the surfactant molecule immersed deeper in water experienced more elastic scattering than those emerging from sections of the molecule exposed to air, which is significantly less dense than water. The experiments demonstrated the sensitivity of elastic scattering in observing variations in scattering from neighboring carbon atoms in the molecule, despite being separated by a mere one-tenth of a nanometer (0.1 nm).

Molecular dynamics simulations provide the ruler

Although the experiments provided a qualitative understanding of the molecule’s orientation, with the hydrophobic end facing air and the hydrophilic end facing water, they lacked the ability to quantitatively determine the average position of the molecule relative to the water-air interface. This crucial information was obtained through molecular dynamics simulations, which track the trajectories of water and surfactant molecules over time, effectively creating a molecular-scale “movie.”

By analyzing numerous snapshots from the simulation, the average position of the surfactant in relation to the sample surface could be calculated and compared to the elastic scattering data. Remarkably, the theoretical calculations and experimental results exhibited excellent agreement.

These promising findings pave the way for future investigations that will delve into the interplay between surfactant molecules and dissolved ions in water. This phenomenon is prevalent at the water-air interfaces of various natural systems, including oceans, rivers, and aqueous aerosol droplets.

The research outcomes have been published in the prestigious journal Physical Review Letters.

Source: Max Planck Society

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