Periodic Reporting for period 2 - WatFun (Water at Oxide Surfaces: A Fundamental Approach)
Okres sprawozdawczy: 2022-07-01 do 2023-12-31
The water/oxide interface and the molecular processes that happen there regulate everything from environmental chemistry and the sequestration of CO2 to the cohesion of man-made structures. The properties of individual surface sites govern reactivity, so probing chemistry at this level is necessary to understand natural processes better and ultimately improve technologies where this interface plays a central role.
In this project, we take a radically new approach to investigate the water/oxide interface at the most fundamental, the atomic, scale: we have found a way to integrate bulk liquid water into ultrahigh vacuum (UHV) setups, where an arsenal of highly-developed techniques is available to investigate surfaces. Most notably, the latest developments in non-contact Atomic Force Microscopy (ncAFM) provide atomically-resolved images of insulating materials. This offers the opportunity to accurately determine fundamental quantities that were hitherto inaccessible and to obtain clear-cut experimental results for interpreting and predicting molecular-scale processes.
Our objective is to develop novel measurement concepts and apply them to minerals. Following a broad work plan, we are:
- measuring the surface tension of neat water and the surface free energies of solids with unprecedented purity;
- devising a method to determine, site-by-site, the intrinsic proton affinity, the fundamental property that determines the point of zero charge of oxides in solutions, and their Brønsted acidity in gas-phase reactions;
- investigating, at the atomic scale, how liquid water affects surface structure and how oxides become hydroxylated, dissolve, and ‘age’;
- discover how ice nucleates on the mineral aerosol surfaces that are crucial in cloud formation;
- study how dissolved CO2 reacts with natural minerals, which affects the global carbon cycle;
- address the hydrated oxides that form the basis of cements in concrete.
While this project focuses on providing a fresh view of environmentally relevant surface chemistry, our approach can impact a much wider range of areas.
In this project, we take a radically new approach to investigate the water/oxide interface at the most fundamental, the atomic, scale: we have found a way to integrate bulk liquid water into ultrahigh vacuum (UHV) setups, where an arsenal of highly-developed techniques is available to investigate surfaces. Most notably, the latest developments in non-contact Atomic Force Microscopy (ncAFM) provide atomically-resolved images of insulating materials. This offers the opportunity to accurately determine fundamental quantities that were hitherto inaccessible and to obtain clear-cut experimental results for interpreting and predicting molecular-scale processes.
Our objective is to develop novel measurement concepts and apply them to minerals. Following a broad work plan, we are:
- measuring the surface tension of neat water and the surface free energies of solids with unprecedented purity;
- devising a method to determine, site-by-site, the intrinsic proton affinity, the fundamental property that determines the point of zero charge of oxides in solutions, and their Brønsted acidity in gas-phase reactions;
- investigating, at the atomic scale, how liquid water affects surface structure and how oxides become hydroxylated, dissolve, and ‘age’;
- discover how ice nucleates on the mineral aerosol surfaces that are crucial in cloud formation;
- study how dissolved CO2 reacts with natural minerals, which affects the global carbon cycle;
- address the hydrated oxides that form the basis of cements in concrete.
While this project focuses on providing a fresh view of environmentally relevant surface chemistry, our approach can impact a much wider range of areas.
In the first project period, we successfully adapted ncAFM to image the surfaces of several key minerals with atomic resolution (see figure). Using the qPlus sensor (depicted in the center of the figure) allows unmatched image quality and resolution. A critical technical requirement is to ‘functionalize’ the very end of the ‘tip’ (the needle mounted at the end of the sensor) with one specific atom or molecule. Another challenge has been to prepare the samples’ surfaces via cleaving. We also computational techniques (including machine learning) to interpret our experimental data.
As the representative images in the figure show, we have successfully mastered this for alpha-alumina, mica, and feldspars.
Specifically, we have (a) found that the surface of alumina is reconstructed because of strain effects, (b) evaluated the short-range ordering of K+ cations on muscovite mica in its most pristine, as-cleaved condition, and (c) provided the very first images of alkali feldspars (orthoclase and microcline, a superior ice nucleator), where we found that their surfaces readily hydroxylate even when cleaved under UHV conditions.
We have also started to build a dedicated apparatus to measure the surface tension of liquid water with the highest accuracy.
As the representative images in the figure show, we have successfully mastered this for alpha-alumina, mica, and feldspars.
Specifically, we have (a) found that the surface of alumina is reconstructed because of strain effects, (b) evaluated the short-range ordering of K+ cations on muscovite mica in its most pristine, as-cleaved condition, and (c) provided the very first images of alkali feldspars (orthoclase and microcline, a superior ice nucleator), where we found that their surfaces readily hydroxylate even when cleaved under UHV conditions.
We have also started to build a dedicated apparatus to measure the surface tension of liquid water with the highest accuracy.
The results obtained so far concern materials that have not been traditionally investigated with surface science techniques. In new collaborations, we are reaching out to geologists and geochemists, who we provide with insights that cannot be obtained any other way. Our experimental data also provide the basis for computational modeling of the mineral-water interface, and we expect that our surface tension measurements will provide a most fundamental values with highest purity and accuracy.