Cycloalkanes and Aromatics on a Model Astrophysical Surface: Wetting, Dewetting and Thermal Desorption

The interaction of small aromatic molecules (benzene and naphthalene), and of small cycloalkanes (cyclohexane and decalin), with an amorphous silica (aSiO₂) surface as a model of interstellar silicate grain materials has been explored. Experimental studies of the interaction of cycloalkanes with the aSiO₂ surface, employing temperature-programmed desorption to explore directly the binding of the adsorbates to the surface, are combined with data derived from previously published studies of the small aromatics. This reveals that, in the monolayer, the aromatic species wet the aSiO₂ surface while the cycloalkanes tend to dewet. The results of a simple computational comparison of the two reported herein suggest that this behavior results from the donation of charge from the extended π electron structure on the aromatic molecule to Lewis acid Si sites on the aSiO₂ surface forming a dative interaction and making these Si atoms hypervalent. Such behavior is not possible with the cycloalkanes and their interaction with the aSiO₂ surface is dominated by weak van der Waals interactions. Reduction of the binding energy of the cycloalkanes compared to the aromatic sees the surface residence time of the former significantly reduced compared to the latter. We speculate as to the possible impact of the role of Lewis acid-base behavior in weakening the bonding in the aromatics consequently reducing the activation energy for their surface hydrogenation in a catalytic fashion.