Palmitic acid and hexadecylamine molecules assdsorbed on titania surface in hybrid composites. Effect of surfactants using density functional theory

A complete theoretical characterization of the factors involved in the stabilization of hybrid laminar composites is reported, with the purpose of understanding the main interactions involved in the formation of these types of systems. The models were built using two different surfactants: hexadecylamine (HDA) and palmitic acid (PAc), on a cluster of titania (TiO₂) which represents its anatase crystalline phase. We explored the relevance of the titania-surfactant and surfactant-surfactant interactions, and of the protonation state of the palmitic acid in the stabilization of the hybrid laminar composites. The results were rationalized using energy decomposition analysis (EDA) and the non-covalent interaction index (NIC). The interaction energies between HDA and the TiO₂ surface without the contribution of the dispersion forces were −25 kcal/mol and −29 kcal/mol for the (1 0 0) and (1 01 ) surfaces, respectively. These results suggest the formation of a donor-acceptor bond between HDA-TiO₂ allowing its stabilization. On the other hand, the interaction energy between PAc and the TiO₂ (1 0 1) surface was about −45 kcal/mol without the dispersion term. Thus, the PAc-TiO₂ model has a higher covalent character than the HDA-TiO₂ interaction, as confirmed by the energy decomposition analyses. The results suggest new hybrid laminar composites which are better when carboxylic acid was used instead of amines. Therefore, the use of surfactants with the carboxyl functional group yielded more stable TiO₂ hybrid composites. The interaction energy between surfactant chains was about −13 kcal/mol, confirming the importance of dispersion forces in the stabilization of these types of systems. The study of systems with palmitic acid was complemented by building some anionic models to show the importance of acid protons in those systems. However, when the acidic hydrogens were removed, the interaction energy between the PAc molecules considering the dispersion correction increased to 45 kcal/mol, with a highly repulsive character. These results suggest that the acid hydrogens of the PAc surfactants play a fundamental role in terms of correctly representing the carboxylic acid-TiO₂ interface and the stabilization of the laminar hybrid composites.