Adsorption Properties of SAMs Supported on TiO₂ and ZrO₂

Abstract

This work investigates vapor-phase adsorption on self-assembled monolayers (SAMs) supported on TiO2 (rutile and anatase) and on ZrO2 (monoclinic). Synthesis of the adsorbents was made via reactions of organosilicon hydrides (RSiH3, R ) C18H37; C8H17; C6F13(CH2)2; CH2dCH(CH2)6; H3Si(CH2)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 18H37 e C8H17 e C6F13(CH2)2 < CH2dCH(CH2)6 ≈ H3Si(CH2)8 , bare MO2. 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) ≈ 1.2aN 2 (MO2).

Adsorption and wetting characterization of hydrophobic SBA-15 silicas

“…For hydroxylated bare SBA-15 silica, C constants were $100-150, indicating high-energy, polar surfaces of Si-OH groups [48,49]. The C constants of SBA-15 grafted with CAMs of alkyldimethylsilanes were in the range of 15-25, demonstrating non-polar, low-energy organic surfaces of closely packed alkyl groups [49][50][51][52]. SBA-15 grafted with SAMs of alkyltriethoxy-silanes showed C constants of $40-60, demonstrating hydrophobic surfaces with accessible polar groups (residual Si-OH from the silica and newly formed Si-OH from the incomplete condensation of tri-functional silanes).…”

Hydrophobic/superhydrophobic oxidized metal surfaces showing negligible contact angle hysteresis

“…Metal surfaces containing native oxide layers have been modified with monolayers/self-assembled monolayers (SAMs) of functional organosilanes (general formula R n SiX 4Àn , n = 1-3, X = usually Cl or OR) [13,[14][15][16][17][18][19][20][21][22][23][24], phosphonic acids (RPO(OH) 2 ) [15][16][17]22,23,, alkanehydroxamic and alkanecarboxylic acids [25,[46][47][48][49][50][51][52] and isocyanates [24,53]. Among the various surface modification techniques reported so far, the use of hydridosilanes (R n SiH 3Àn ) and its analogues [13,15,17,[54][55][56][57][58][59][60][61] are of interest because they are covalently attached to the surfaces by metal (M)-O-Si bonds and produce well-ordered monolayers with densities as high as SAMs [57]. Another advantage of this reaction is that it is clean because the byproducts are only H 2 and/or H 2 O [59].…”

Preparation of silane-coated TiO2 nanoparticles in supercritical CO2

“…Next, the observed weight loss in the range 525-650 K, occurring for both as-prepared and washed samples, was assigned to the desorption of hydrolyzed silane chains interacting by hydrogen bonding with the OHs on the TiO 2 surface. In addition, within this temperature region some of the weight loss was attributed to the elimination of water coming from the condensation of silanol groups interacting either with OHs on the surface or laterally with other silanols [8,12,35,39]. Moreover, some desorption of residual TiO 2 physisorbed water could also take place, although most of the adsorbed water was expected to be consumed during the silanization reaction [40].…”