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

Chemically grafted monolayers were prepared by reaction of tris(trimethylsi loxy)chlorosilane (tris-TMSCl) with silicon wafers both in the vapor phase and in toluene solution. Denser (more closely packed) monolayers, as assess ed by carbon content (determined using X-ray photoelectron spectroscopy) an d contact angle analysis, were obtained using the vapor phase reaction. Con tact 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 na ture of this surface. Dynamic contact angles of 30 different probe fluids w ere 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 regio n of 180-190 cm(3)/mol for tris-TRIS monolayers prepared in vapor phase. Pr obe fluids with lower molecular volume exhibit hysteresis of 8-12 degrees, while liquids of larger molecular volume show hysteresis of 2-3 degrees. Th is 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 wa s found for the tris-TMS monolayer (of lower degree of surface coverage) pr epared by liquid-phase silanization. Contact angle hysteresis on this surfa ce (8-13 degrees) decreases gradually with increasing molecular volume of t he 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 h ole) binary monolayers. Several binary monolayers were prepared by subseque nt 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 pha se 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.