Adsorption Properties of SAMs Supported on TiO₂ and ZrO₂

This work investigates vapor-phase adsorption on self-assembled monolayers (SAMs) supported on TiO 2 (rutile and anatase) and on ZrO 2 (monoclinic). Synthesis of the adsorbents was made via reactions of organosilicon hydrides (RSiH 3, R = C 18 H 37; C 8 H 17; C 6 F 13 (CH 2 ) 2; CH 2 CH(CH 2 ) 6; H 3 Si(CH 2 ) 8 ) with metal oxide powders. The reactions yielded closely packed surfaces with high grafting densities of organic groups (up to 4.85 groups/nm 2 ). Adsorption properties of the materials obtained were studied by low-temperature nitrogen adsorption. All the adsorption isotherms obtained belonged to the physical adsorption isotherms (type II) and in the region of relative pressures p / p 0 ∼ 0.05−0.3 fit the BET equation. For the modified surfaces the adsorption isotherms were substantially less convex than those for bare metal oxides, indicating a significant decrease in the energy of the adsorption interactions. According to the C constants of the BET equation, the energies of the adsorption interactions for different SAMs range as follows: C 18 H 37 ≤ C 8 H 17 ≤ C 6 F 13 (CH 2 ) 2 < CH 2 CH(CH 2 ) 6 ≈ H 3 Si(CH 2 ) 8 ≪ bare MO 2 . The low values of C constants observed suggested that nitrogen molecules interacted primarily with terminal groups of the closely packed SAMs and did not interact with the metal oxide substrate. C constants increased as grafting density of SAMs decreased. No substantial differences in the adsorption behavior were found for the SAMs supported on different crystalline forms of metal oxide (rutile and anatase) or on different metal oxides. Comparison of the specific surface areas before and after surface modification suggested different space requirements for nitrogen molecules adsorbed on SAMs and on bare metal oxide. The best agreement was found for a N 2 (SAMs) ≈ 1.2 a N 2 (MO 2 ).