Dynamics of Octadecylphosphonate Monolayers Self-Assembled on Zirconium Oxide:  A Deuterium NMR Study

Abstract

Deuterium NMR spectroscopy has been used to probe the dynamics of deuterated octadecylphosphonate (-1,1-d 2 ) (ODPA-d 2 ) monolayers on nonporous ZrO 2 powder (surface area ≈ 40 m 2 /g) over the temperature range of 200-340 K. At 200 K, a broadened Pake doublet with distinct nonrigid characteristics and a horn splitting of approximately 120 kHz was observed. With increasing temperature, the 2 H spectrum gradually transforms into a relatively narrow and featureless peak. Spectral simulations are performed with the help of plausible motional models, and the results show that, over the whole temperature range studied, the C 1 -D bonds have substantial motional freedom with respect to the characteristic 2 H NMR time scale. In addition, at each temperature, a weighted superposition of several simulated line shapes with different rates and site populations is required to account for the observed spectral features, indicating the presence of considerable motional heterogeneity within the ODPA monolayers. The results are further discussed in terms of the known characteristics of the ZrO 2 surface and of the conformational transitions in the octadecyl chain.

Organo-lined alumina surface from covalent attachment of alkylphosphonate chains in aqueous solution

“…26 In fact, these chemical shifts are sensitive not only to the number of Ti-O-P bonds, each supplementary condensation between P-OH and Ti-OH leading to an upfield shift (displacement towards lower chemical shift), but also to the interaction of the PQO groups with surface Lewis or Brønsted acid sites, which leads to downfield shifts (displacement towards higher chemical shift) and to the variations of the O-P-O bond angles. Nevertheless, by using 17 O MAS NMR spectroscopy of enriched 17 O phosphonic acid on titania, the authors have demonstrated that the bonding mode, as previously suggested on titania or zirconia, 47 is predominantly tridentate. In our case, from 31 P spectra only, we can only suggest that octylphosphonate chains are grafted on alumina, and that the grafting modes of the O-P-O bonds are progressively changing by increasing chain density.…”

“…[44][45][46] However, this assumption, as well as the exact mechanism of binding, is still debated, and likely depends on the nature of the metal oxide. 31,47 Some authors suggest from 31 P MAS NMR spectra a preference for a mixed regime of both bi-and tridentate states for alkylphosphonates in SAMs on TiO 2 , due to residual detection of PQO and P-OH groups. 20 However, Mutin et al have pointed out the impossible identification of the different phosphorous sites by spectra simulation if only the 31 P chemical shifts are considered.…”

“…The variation of the surface polarity of surface-modified mineral oxides as a function of both surface loading and anchorage mode has been particularly analyzed in the case of silane grafted on silica. 47 The C BET parameter, used as a polarity indicator for polarisable adsorbed molecules, such as nitrogen, was calculated from the BET equation, applied in the 0.1 o p/p1 o 0.3 relative pressure range of the nitrogen adsorption isotherm. The C BET parameter is usually expressed as in eqn (3)…”

Effects of Fluorination on Self-Assembled Monolayer Formation from Alkanephosphonic Acids on Aluminum: Kinetics and Structure

“…The quintessential SAM systems, alkanethiols on gold and alkylsilanes on silica, have been scrutinized using virtually every tool in the surface science arsenal, and a voluminous literature exists . Another important class of self-assembling materials, phosphonic acids, is somewhat less well characterized, but is becoming of great practical interest because of the ability of these materials to produce robust, well-anchored monolayers on a wide range of metal oxide surfaces. 4e,− Phosphonic acids undergo monolayer formation on many substrates by condensation reactions of the acid function with metal hydroxyl species to form bound phosphonate salts. ,,,,,, Much of the phosphonic acid literature to date is devoted to the characterization of hydrocarbon systems, and little information is available on the behavior of fluorinated phosphonic acid SAMs.…”

“…4e, Phosphonic acids undergo monolayer formation on many substrates by condensation reactions of the acid function with metal hydroxyl species to form bound phosphonate salts. 9,10,17,18,23,28,29 Much of the phosphonic acid literature to date is devoted to the characterization of hydrocarbon systems, and little information is available on the behavior of fluorinated phosphonic acid SAMs.…”