Binary monolayer mixtures: Modification of nanopores in silicon-supported tris(trimethylsiloxy)silyl monolayers

Chemically grafted monolayers were prepared by reaction of tris(trimethylsiloxy)chlorosilane (tris-TMSCl) with silicon wafers both in the vapor phase and in toluene solution. Denser (more closely packed) monolayers, as assessed by carbon content (determined using X-ray photoelectron spectroscopy) and contact angle analysis, were obtained using the vapor phase reaction. Contact angles (theta(A)/theta(R)) for water, methylene iodide, and hexadecane on the vapor phase modified surfaces (96 degrees/87 degrees; 66 degrees/56 degrees; 33 degrees/31 degrees) indicate the hydrophobic and oleophobic nature of this surface. Dynamic contact angles of 30 different probe fluids were measured on the tris-TMS surfaces. A plot of contact angle hysteresis (the difference between advancing and receding contact angles) versus molar volume of the probe fluid shows a sharp decrease in hysteresis in the region of 180-190 cm(3)/mol for tris-TRIS monolayers prepared in vapor phase. Probe fluids with lower molecular volume exhibit hysteresis of 8-12 degrees, while liquids of larger molecular volume show hysteresis of 2-3 degrees. This size-exclusion contact angle hysteresis behavior argues for the presence of holes (nanopores) with a cross section of similar to 0.5 nm(2) that are accessible to the probes of smaller dimension. No size-exclusion effect was found for the tris-TMS monolayer (of lower degree of surface coverage) prepared by liquid-phase silanization. Contact angle hysteresis on this surface (8-13 degrees) decreases gradually with increasing molecular volume of the probe fluid. The results suggest that penetration of molecules of fluid into the monolayers is responsible for contact angle hysteresis. The holes in the tris-TMS monolayers expose silanols that can be reacted with smaller silanizing reagents to yield uniformly mixed (one molecule of silane per hole) binary monolayers. Several binary monolayers were prepared by subsequent modification of tris-TMS surfaces with alkyl-, bromoalkyl-, fluoroalkyl-, and aminoalkyl-functionalized silanes. Functional silane/tris-TMS binary monolayers are formed with a similar to 1:10 molar ratio from the vapor phase synthesized tris-TMS surface. Mixed monolayers with similar to 1.4 molar ratio of silane/tris-TMS were obtained for binary monolayers prepared from the liquid phase synthesized tris-TMS surface. The extent of incorporation of the subsequently reacted silane is controlled primarily by the density of the tris-TMS monolayer and is almost independent of the chemical nature of the reagent used subsequently.