Self-assembled monolayer (SAM) structures and properties are dominated by two interactions: those between the substrate and adsorbate, and those between the adsorbates themselves.
Researchers at Penn State University and the Sigma-Aldrich company have found a way to control geometry and stability by making SAMs out of different carboranethiol isomers, which are cage-like molecules [Hohman, J. N., et al., ACS Nano. (2009) 3 (3), 527.]
Each of the carborane cages consists of ten boron atoms and two carbon atoms. The carbon atoms have an unusual hexa-coordinated structure that gives them a positive charge. As a result, the carborane cages have a dipole moment – a measure of the separated charge of a molecule that helps determine its interaction with water and other molecules – that is either nearly parallel to the surface of the SAM (for M1, in which sulfur is attached to a carbon atom in the carborane cage) or nearly perpendicular to the surface of the SAM (for M9, in which sulfur is attached to a boron atom in the carborane cage).
“Both the M1 and M9 molecules form the same structure on the surface of the SAM, but the M1 molecules make the SAM more stable, while the M9 molecules make the SAM more hydrophilic, or wettable,” said Paul Weiss, the corresponding author. “Each of these properties is beneficial for different applications.”
The group determined that the M1 molecules interacted more strongly with one another than the M9 molecules interacted with each other due to dipole interactions between the M1 cages. As a result, more energy was required to remove a M1 molecule from the SAM, thus creating a more stable SAM. On the other hand, the M9 SAM, although less stable, was able to become wet more easily than the M1 SAM because the dipole was oriented toward the outside of the SAM.
These properties were investigated by dynamic contact angle goniometry, Kelvin probe force microscopy, and grazing incidence Fourier transform infrared spectroscopy.
“Carborane chemistry is so well-understood that there is a lot we can do to exploit these SAMs,” said Weiss. “For example, we can add more chemical groups onto selected spots on SAMs to determine the supra-molecular, or overall, properties of a SAM that go beyond the properties of a single molecule.”