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

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 characterized3–8, 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, Tc, 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 Tc is analogous to the triple point in the phase diagram of Langmuir monolayers.