Search

The hydrolytic stability of C18 monolayers supported on TiO2 and ZrO2 was studied. Three types of monolayers were prepared from the following octadecyl modifiers:  (1) octadecyldimethylchlorosilane (C18H37Si(CH3)2Cl); (2) octadecylsilane (C18H37SiH3); and (3) octadecylphosphonic acid (C18H37P(O)(OH)2). The hydrolysis of the surfaces prepared was studied under static conditions at 25 and 65 °C at pH 1−10. On the basis of the loss of grafted material, the stability of the monolayers fall in the following range:  C18H37P(O)(OH)2 ≥ C18H37SiH3 ≫ C18H37Si(CH3)2Cl. At 25 °C, monolayers from C18H37P(O)(OH)2 showed only ~2−5% loss in grafting density after one week at pH 1−10. The high stability of these monolayers was explained because of the strong interactions of the phosphonic acids with the substrates. Monolayers from C18H37Si(CH3)2Cl showed poor hydrolytic stability at any pH, which was explained because of the low stability of Ti−O−Si and Zr−O−Si bonds. Unlike monofunctional silanes, trifunctional silane (C18H37SiH3) yielded surfaces that showed good hydrolytic stability. This suggests that the stability of the monolayers from trifunctional silanes is primarily due to “horizontal” bonding (Si−O−Si or Si−OH...HO−Si) rather than due to bonding with the matrix (M−O−Si). At 65 °C, all C18 surfaces become more susceptible to hydrolysis; however, the trend observed for 25 °C remained unchanged. Low-temperature nitrogen adsorption was used to study the adsorption properties of the monolayers as a function of their grafting density. The energy of adsorption interactions showed a significant increase as the grafting density of the monolayers decreased. The order of the alkyl groups in the monolayers, as assessed from CH2 stretching, decreased as the grafting density of the monolayers decreased.

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; CH2&dbd;CH(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/nm²). 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/p0 ~ 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:  C18H37 ≤ C8H17 ≤ C6F13(CH2)2 < CH2&dbd;CH(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 aN2(SAMs) ≈ 1.2aN2(MO2).

This work investigates the preparation of the self-assembled monolayers (SAMs) of organosilicon hydrides (RSiH3) supported on transition metal oxides. The reactions of alkyl-, fluoroalkyl-, and ω-alkenyl-silanes and α,ω-bis-hydridosilanes with nonporous high surface area TiO2 (rutile and anatase), ZrO2 (monoclinic), and HfO2 (monoclinic) powders were studied, and supported SAMs were characterized by Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), and chemical analysis. SAMs of organosilicon hydrides were closely packed and well-ordered (assessed from CH2 stretchings). Grafting densities of the monolayers were ~4.6−4.8 group/nm² for C18H37 SAMs and ~3.6−3.7 group/nm² for C6F13 SAMs, which are close to the ultimate values reported in the literature for the best organosilicon SAMs. The reactions with organosilicon hydrides proceed in a noncorrosive environment (no HCl), which distinguishes RSiH3 from RSiCl3 coupling agents. Synthesis of the monolayers was highly reproducible, and SAMs of high quality were prepared from the metal oxides of different surface area and particle size, of different vendors, and of different crystalline forms. Titania, zirconia, and hafnia reacted with RSiH3 unexceptionally, and no influence of the metal oxide on the SAMs' quality was found. According to FTIR, the reactions of RSiH3 with metals oxides yielded cross-linked monolayers (with Si−O−Si bonds) grafted to the metal oxides (via Si−O−MS bonds). The mechanism of the self-assembly of organosilicon hydrides on the surfaces is discussed. Kinetics of the monolayer formation in the solution phase was investigated. The reactions were found to follow first-order kinetics with two rate constants. According to the rate constants, the following range of reactivity was established:  H3Si(CH2)8SiH3 > C6F13(CH2)2SiH3 > CH2&dbd;CH(CH2)6SiH3 ≈ C8H17SiH3 > C18H37SiH3. According to TGA, SAMs supported on titania, zirconia, and hafnia showed good thermal and oxidative stability and no mass loss was observed below 180−200 °C in air. Temperatures of the maximum mass loss rate were independent of the metal oxide and vary from ~250−300 °C for alkyl- to ~390 °C for fluoroalkyl-modified surfaces. It is suggested that the degradation of the surfaces proceeds by oxidative destruction of the organic groups and yields silica-like surfaces supported on the metal oxide.