1H fast MAS NMR studies of hydrogen-bonding interactions in self-assembled monolayers

The structures formed by the adsorption of carboxyalkylphosphonic acids on metal oxides were investigated by H-1 fast magic angle spinning (MAS), heteronuclear correlation (HETCOR), and H-1 doublequantum (DQ) MAS solid-state NMR experiments. The diacids HO2C(CH2)(n)PO3H2 (n = 2, 3, 11, and 15) were adsorbed on TiO2 and two types of ZrO2 powders having average particle sizes of 20, 30, and 5 nm, respectively. Carboxyalkylphosphonic acids bind selectively via the phosphonate group, forming monolayers; with pendant carboxylic acid groups. Whereas dipolar coupled P-OH protons are detected on TiO2, there are only isolated residual P-OH groups on ZrO2, reflecting the relative binding strengths of phosphonic acids on these two substrates. From a comparative 1H MAS NMR study with an analogous monolayer system, HO2C(CH2)(7)SH coated gold nanoparticles, the hydrogen-bonding network at the monolayer/air interface is found to be quite disordered, at least for SAMS deposited on nonplanar substrates. Whereas only hydrogen-bonded homodimers occur in the bulk diacids, hydrogen bonding between the carboxylic and phosphonic acid groups is present in multilayers of the diacids on the ZrO2 nanopowder.