Understanding of the Role of Specific Intermolecular Interactions on the Formation and Binding Strength of Interfacial Molecules

Herein, we explored the impact of specific interactions on the formation of the interfacial layer in two types of materials, monohydroxyl alcohols (2-phenyl-1-ethanol, 2Ph1E, and 2-cyclohexyl-1-ethanol, 2C1E) and their acetate counterparts (2-phenylethyl acetate, 2PhA, and 2-cyclohexylethyl acetate, 2CA, van der Waals liquids), infiltrated within silica mesoporous templates of d ∼ 5 nm using Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry, molecular dynamics (MD) simulations, and DFT computations. Collected thermograms revealed the presence of two glass transitions, T_g, interpreted as a proof that each material confined in mesoporous templates forms an interfacial layer. This was further confirmed by FTIR measurements that showed the splitting of the band related to the stretching vibrations of the carbonyl group into two components due to the creation of C═O···H-O intermolecular interactions between pore walls and acetates derivatives. To measure the strength of the adsorption of guest molecules to the host matrix, we compared the deviation of the high T_g from the bulk values and performed further evaporation experiments. The first approach indicated that 2C1E followed by 2Ph1E binds to the interface in the most efficient way, while in acetates derivatives, the interfacial layer is not so strongly bonded to the pore walls. This was confirmed by the evaporation experiments that demonstrated that only alcohol molecules attached to the pore walls survived prolonged annealing at T = 363 K. These results, although quite intuitive, seemed surprising considering that both FTIR experimental investigations and additional MD and DFT calculations indicated the formation of the strong H-bonds between each material and the silica. Furthermore, the binding energy between host and guest molecules was comparable. Nevertheless, unexpectedly, it was found that the cooperativity of the hydrogen bonds between the interface and alcohol molecules plays a significant role in the formation of the strongly adsorbed interfacial layer and its stability during prolonged annealing.