Controlling surface interactions between self-assembled monolayers and mica via polarization mediated ordering of ions and hydration layers.
Why do some surfaces stick together while others repel each other? At scales far too small to see with the bare eye, this question is controlled by a complex interplay of intermolecular forces that arise when charged particles, called ions, and water organize themselves at the boundary between a solid and a liquid. Understanding and controlling this behavior is essential for technologies ranging from lubricants and coatings to sensors and electronics.
With a new study in Journal of the American Chemical Society, Valentina Wieser, first author of the study and colleagues at National Taiwan University used this structuring phenomenon to electrically switch molecular adhesion between a specifically designed aromatic surface adsorbent layer and a mica surface immersed in sodium containing electrolyte.
By adjusting the voltage applied on the gold surface beneath the molecules, the researchers were able to control where the ions moved and hence modulate the interaction between the molecules and the mica surface. When negative potential was applied, positively charged sodium ions were directed into the molecule layer where the particular molecular structure kept them anchored. This led to the formation of a very stable, cushion-like barrier of ordered co-ions and water. As a result, the mica surface was strongly pushed away.
In contrast, when positive potential was applied, the cations were expelled out of the layer and the broadly structured ion and hydration layer at the interface collapsed, allowing the two surfaces to attract each other.
In effect, the researchers created an electromechanically controlled adhesion switch, a surface that becomes either repulsive or “sticky” solely depending on the applied voltage and the specific ion structuring.
This discovery shows how precise control of ions and water at interfaces can be used to design smart, responsive surfaces with switchable mechanical properties.
“This system shows a very peculiar mechanism for specific ion structuring and capturing that opens up fascinating new possibilities for understanding and utilizing interface phenomena,” says Prof. Hsiu-Wei Cheng, co-corresponding author of the study.
Prof. Hsiu-Wei Cheng's email address: [email protected]
