SILANIZATION OF SOLID SUBSTRATES - A STEP TOWARD REPRODUCIBILITY

Evidence is given for a transition temperature T-c which controls the,formation of grafted alkylsilane monolayers on hydrophilic substrates. When the silanization reaction is performed below T-c, layers of high surface quality are obtained. They are characterized by an extremely low surface tension gamma(c) = 20.5 +/- 0.5 mN/m and low contact angle hysteresis (less than 1 degrees) between the advancing and receding contact angles of sessile drops of test liquids. This temperature T-c is an intrinsic property of the silane molecules: it depends on their chain length but is independent of the nature of the solvents used for the grafting reaction. We have related T-c to the triple point observed in the phase diagram of amphiphilic molecules spread at the air-water interface. On the basis of this correspondence, we suggest the existence, in the early stages of the adsorption process, of large in-plane inhomogeneities of the molecular surface density, which should be observable by fluorescence microscopy. Upon further adsorption, close-packed monolayers are reached if the reaction is performed at low enough temperatures. The presence of a film of water on the bare substrate, which has been suggested earlier and is confirmed by our experiments, supports the analogy with Langmuir monolayers. Preliminary experiments on fluorinated trichlorosilanes seem to show the same trend for optimum grafting. A gamma(c) value of 6 +/- 1.2 mN/m, together with low hysteresis, has been reached by proceeding at low temperatures.