What Is the Driving Force behind the Adsorption of Hydrophobic Molecules on Hydrophilic Surfaces?

The adsorption of limonene, a common organic compound found in indoor air, on hydrophilic surfaces such as glass (SiO₂), a prevalent surface in the indoor environment, is poorly understood. In this study, we have investigated the interaction of limonene and three other cyclic hydrocarbons (cyclohexane, cyclohexene, and benzene) on hydroxylated SiO₂ using infrared spectroscopy and ab initio molecular dynamics (AIMD) simulations. Experimental results show that there is an interaction between these cyclic hydrocarbons and surface hydroxyl groups. AIMD simulations demonstrate that all of the cyclic molecules, except for cyclohexane, π-hydrogen bond with surface hydroxyl groups while cyclohexane interacts with the surface OH groups through dispersion forces. According to experiments and simulations, the intermolecular interaction between limonene and SiO₂ is significantly stronger than those of other compounds explored. This study provides an understanding of some of the driving forces behind the formation of organic coatings on glass surfaces important in indoor environments.