Hydrogen-bonded surface macroclusters of carboxylic acid on silica in cyclohexane

The macrocluster formation of propionic acid adsorbed onto silica (glass and silicon oxide) surfaces from propionic acid-cyclohexane mixtures has been studied using surface forces measurement, adsorption excess isotherm measurement and Fourier transform infrared spectroscopy (FTIR) in the attenuated total reflection (ATR) and the transmission (TS) mode. In pure cyclohexane, the interaction extended to only 2-3 nm and described as the van der Waals force. On the other hand, unusually long-range attraction was observed in the presence of propionic acid in the concentration range of 0.1-20 mol%. At 0.5 mol% propionic acid, the attraction appeared at 67+8 nm and turned into repulsion below 2 nm upon compression. This attraction was accounted for in terms of the bridging of opposed adsorption layers of propionic acid on the surfaces. The thickness of the adsorption layer was estimated as half of the attraction range to be 34±4 nm for 0.2-1.0 mol% propionic acid, which agreed well with the thickness calculated from the adsorption excess amount of propionic acid, 38 ± 9 nm, assuming that the adsorption layer consisted of pure propionic acid. Chemical interactions involved in the formation of the adsorption layers of propionic acid were investigated by FTIR-ATR spectroscopy using silicon oxide as a substrate. The hydrogen-bonded OH absorption of propionic acid around 3120 cm^-1 was observed at 0.1 mol% propionic acid where the contribution from the bulk solution was small. The linear structure of the surface cluster initiated from the surface silanol groups was demonstrated from the peak position and the dichroic analysis of the hydrogen-bonded OH absorption. An application of this phenomenon with respect to the nano-scale coating technique was demonstrated by the photo-polymerization of the acrylic acid monomer adsorbed on the silica surfaces. Photo-irradiation of the silica substrates for 20 min in 0.1 mol% acrylic acid produced extremely uniform and flat polymer films with a thickness of ≈ 25 nm, which exhibited an average difference in the height of less than 0.2 nm for a 3 μm × 3 μm area on the surface.