Evidence of a transition temperature for the optimum deposition of grafted monolayer coatings

Abstract

TECHNIQUES for surface modification are of considerable technological interest for the fabrication of water-repellent and anti-fouling coatings. Silanization¹ (the chemical grafting of organic molecules onto a substrate via a trichlorosilane group) stands out among these techniques by virtue of its ability to provide highly compact coatings of optical quality, extreme chemical inertness and adjustable wettability². Although the silanization reaction has been extensively characterized^3–8, the properties of the grafted layers are still too variable for most commercial applications; for example, the quality of the grafted layers depends critically on the presence of trace amounts of water, and on the temperature at which the silanization reaction takes place⁹. Here we provide evidence for the existence of a near-ambient temperature threshold, T_c, which represents an upper bound for obtaining the highest-quality films. This threshold temperature is found to be an intrinsic property of the silane molecules: it depends linearly on their chain length, but is independent of the solvent used for the reaction. We suggest that T_c is analogous to the triple point in the phase diagram of Langmuir monolayers.