The preparation of ultrahydrophobic and ultralyophobic surfaces using several techniques is described. Plasma polymerization of 2,2,3,3,4,4,4-heptafluorobutyl acrylate on poly(ethylene terephthalate) yields surfaces with water contact angles of θA/θR = 174°/173°. Argon plasma etching of polypropylene in the presence of poly(tetrafluoroethylene) renders surfaces with water contact angles as high as θA/θR = 172°/169°. Surfaces of compressed pellets of submicrometer, variable-diameter spherical particles of PTFE oligomers exhibit water contact angles of θA/θR = 177°/177°, methylene iodide contact angles of θA/θR = 140°/138°, and hexadecane contact angles of θA/θR = 140°/125°. We emphasize that contact angle hysteresis is more important in characterizing lyophobicity than is the maximum achievable contact angle. These surfaces are rough at the micrometer and submicrometer scales, and water drops roll easily on all of them. We make an intuitive argument that the topology of the roughness is important and controls the continuity of the three-phase contact line and thus the hysteresis. We also report smooth ultralyophobic surfaces that are prepared by silanization of silicon wafers with Cl(SiMe2O) n SiMe2Cl (n = 0, 1, 2, and 3), (Me3SiO)3SiCH2CH2Si(CH3)2Cl, and (Me3SiO)2Si(CH3)CH2CH2Si(CH3)2Cl. These surfaces exhibit much lower contact angles but little or no hysteresis, and droplets of water, hexadecane and methylene iodide slide easily off them. We propose that these covalently attached monolayers are flexible and liquidlike and that droplets in contact with them experience very low energy barriers between metastable states.

Chemically grafted monolayers of trialkylsilanes were prepared by reaction of (primarily) alkyldimethylchlorosilanes with silicon wafers under three conditions:  in the vapor phase at elevated temperature (60−70 °C), in toluene in the presence of ethyldiisopropylamine (EDIPA) at room temperature, in toluene/EDIPA at 60−70 °C. It was determined that reactions at the solution−solid interface are very slow in the later stages of the reaction and that long reaction times are necessary to achieve maximum bonding density. The bonding density is determined and can be controlled by the reaction conditions. The highest carbon content on the surface (assessed by X-ray photoelectron spectroscopy) as well as the highest contact angles were obtained using vapor phase reactions. A series of nine H(CH2) n Me2Si− surfaces was prepared with n = 1, 2, 3, 4, 8, 10, 12, 18, and 22. Water contact angles (θA/θR = ∼105°/∼94°) are independent of chain length, indicating that these surfaces project disordered methyl groups toward the probe fluid and that water does not penetrate the monolayers. Hydrophobization is achieved topologically:  the monolayers prevent water from penetrating and interacting with residual silanols. n-Hexadecane and methylene iodide contact angles decrease with increasing chain length for this series, indicating that these probe fluids penetrate the monolayers and interact with methylene groups. These chemically grafted monolayers differ in structure from those prepared by self-assembly in that the distance between molecules is significantly greater and that all molecules are covalently attached to the substrate. The contact angle hysteresis for these surfaces is a function of alkyl group structure and bonding density:  mobile surfaces with flexible chains or rotational mobility and rigid surfaces that pack well exhibit low hysteresis, whereas rigid surfaces that cannot pack well exhibit high hysteresis. We argue that molecular level topography (roughness and rigidity) is responsible for the observed hysteresis.

The hydrolytic stability of C18 monolayers supported on TiO2 and ZrO2 was studied. Three types of monolayers were prepared from the following octadecyl modifiers: (1) octadecyldimethylchlorosilane (C18H37Si(CH3)2Cl); (2) octadecylsilane (C18H37SiH3); and (3) octadecylphosphonic acid (C18H37P(O)(OH)2). The hydrolysis of the surfaces prepared was studied under static conditions at 25 and 65 degrees C at pH 1-10. On the basis of the loss of grafted material, the stability of the monolayers fall in the following range: C18H37P(O)(OH)2 > or = C18H37SiH3 >> C18H37Si(CH3)2Cl. At 25 degrees C, monolayers from C18H37P(O)(OH)2 showed only approximately 2-5% loss in grafting density after one week at pH 1-10. The high stability of these monolayers was explained because of the strong interactions of the phosphonic acids with the substrates. Monolayers from C18H37Si(CH3)2Cl showed poor hydrolytic stability at any pH, which was explained because of the low stability of Ti-O-Si and Zr-O-Si bonds. Unlike monofunctional silanes, trifunctional silane (C18H37SiH3) yielded surfaces that showed good hydrolytic stability. This suggests that the stability of the monolayers from trifunctional silanes is primarily due to "horizontal" bonding (Si-O-Si or Si-OH...HO-Si) rather than due to bondingwith the matrix (M-O-Si). At 65 degrees C, all C18 surfaces become more susceptible to hydrolysis; however, the trend observed for 25 degrees C remained unchanged. Low-temperature nitrogen adsorption was used to study the adsorption properties of the monolayers as a function of their grafting density. The energy of adsorption interactions showed a significant increase as the grafting density of the monolayers decreased. The order of the alkyl groups in the monolayers, as assessed from CH2 stretching, decreased as the grafting density of the monolayers decreased.

The solution-phase reactions of octadecylsilanes with different headgroups (C18H37SiH3, C18H37Si(OCH3)3, C18H37SiCl3, and C18H37Si(CH3)2Cl) and of octadecylphosphonic acid (C18H37PO3H2) with titanium dioxide (anatase) were investigated. Chemical analysis and FTIR suggested that all the reactions, with the exception of that of C18H37Si(CH3)2Cl, yielded closely packed self-assembled monolayers (SAMs). SAMs were characterized with high grafting density (∼4.3-4.8 octadecyl groups/nm 2 ) and with high degree of ordering of alkyl chains. Reaction of C18H37Si(CH3)2Cl yielded less ordered surfaces with grafting density ∼1.5 group/nm 2 . The kinetics plots were similar for all the reactions and illustrated two distinct regions, a rapid attachment followed by a slow growth of the grafting density. The uptake curves were adequately described by the first-order kinetics with two rate constants that differed from each other by 1-2 orders of magnitude. According to the rate constants, the following range of reactivity was established: C18H37SiCl3 . C18H37-PO(OH)2 > C18H37Si(CH3)2Cl > C18H37Si(OCH3)3 > C18H37SiH3. Two distinct types of the SAM growth, uniform and islandlike ones, were proposed on the basis of the FTIR study of the SAMs at submonolayer coverage. Silanes capable of cross-linking (C18H37SiX3, X ) H, Cl, OCH3) gave SAMs with a high degree of ordering at relatively low surface coverage, suggesting nonuniform (islandlike) film growth. For SAMs of C18H37Si(CH3)2Cl and C18H37PO(OH)2, the order gradually improved with coverage and highly ordered SAMs were obtained only for high surface coverage, arguing for the uniform mechanism of the film growth. The thermal stability of the supported monolayers was characterized by TGA. All the SAMs showed good thermal and oxidative stability, and no mass loss was observed below ∼200 °C in air. The temperatures of the maximum mass loss rate were close for all SAMs (∼300 °C).

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