Hydrophobization of Inorganic Oxide Surfaces via Ring-Opening Polymerization of Cyclic Siloxane Vapor.

The ability to control the surface chemistry of inorganic oxides has a profound impact on numerous applications, including lubrication, antifouling, and anticorrosion. While often overlooked as potential modifying agents given their lack of traditional functional groups, siloxanes have recently been shown to react readily with and covalently attach to inorganic oxide surfaces. Herein, we examine the reactions of cyclic siloxane vapor with solid interfaces via a ring-opening polymerization (ROP) initiated by the inherent acid/base characteristics of several smooth inorganic oxide surfaces. Surfaces are characterized by ellipsometry, dynamic contact angle analysis, and X-ray photoelectron spectroscopy (XPS). This technique requires no additional solvents and very little reactant to produce nanometer-thick hydrophobic surfaces that exhibit low contact angle hysteresis. Additional studies with particulate surfaces suggest that this method prepares conformal coatings regardless of surface architecture.

Poly(isobutylene) nanoparticles via cationic polymerization in nonaqueous emulsions.

The preparation of poly(isobutylene) (PIB) nanoparticles via cationic emulsion polymerization is presented. As a requirement, an oil-in-perfluoroalkane nonaqueous emulsion is developed, which is inert under the carbocationic polymerization conditions. To stabilize the dichloromethane/hexane droplets in the fluorinated, continuous phase, an amphiphilic block copolymer emulsifier is prepared containing PIB and 1H,1H-perfluoroalkylated poly(pentafluorostyrene) blocks. This system allows for the polymerization of isobutylene with number-average molecular weights (Mn) up to 27,000 g mol(-1). The particle morphologies are characterized via dynamic light scattering and electron microscopy. For Mn > 20,000 g mol(-1), the particles exhibit shape-persistence at room temperature and are ≈100 nm in diameter.

Effects of chemical structure on the dynamic and static surface tensions of short-chain, multi-arm nonionic fluorosurfactants.

Fluorinated surfactants with short perfluoroalkyl chains (R(F)) as potential substitutes for the environmentally questionable, long R(F) systems are presented. Three types of nonionic hydrophilic-fluorophilic amphiphiles are synthesized and evaluated based on surface activity in equilibrated (static) and non-equilibrated (dynamic) states. Furthermore, several mono- and disaccharide-based fluorosurfactants are also examined as potential non-bioaccumulative alternatives. A correlation between the chemical structure and resulting surface properties is made by comparing R(F) length, number and size, alkyl-spacer, and hydrophilic moieties. Based on dynamic and static surface tension experiments, the effects of surfactant structure are summarized to provide a basis for the future design of fluorosurfactants. We have found that surfactants with more perfluorinated chains tend to have a higher surface tension reduction, but typically result in slower dynamic behaviors. Using the presented structural characteristics, surfactants with R(F)<4 can be prepared with static surface tensions as low as 18.1 mN/m or reduce surface tension within milliseconds.

Hydrophobization of inorganic oxide surfaces using dimethylsilanediol.

Dimethylsilanediol is a stable crystalline solid that was described in 1953. As the monomer of an important class of commercial products (poly(dimethylsiloxanes)-silicones, PDMS) and as a simple molecule in its own right (the silicon analog of acetone hydrate), it has been neglected by several fields of fundamental and applied research including the hydrophobization of inorganic oxide surfaces. We report that dimethylsilanediol is a useful reagent for the surface modification (hydrophobization) of oxidized silicon and other oxidized metal surfaces and compare the wetting properties of modified solids with those of conventionally modified surfaces. That water is the only byproduct of this modification reaction suggests that this and likely other silanediols are useful surface-modification agents, particularly when substrate corrosion or the competitive adsorption of byproducts is an issue. We note that dimethylsilanediol is volatile with a significant vapor pressure at room temperature. Vapor-phase surface modifications are also reported.

Rediscovering silicones: "unreactive" silicones react with inorganic surfaces.

Chemical reactions of linear trimethylsilyl-terminated poly(dimethylsiloxane)s with the surfaces of oxidized silicon, titanium, aluminum, and nickel are reported. These reactions lead to covalently attached poly(dimethylsiloxane) polymer chains and to hydrophobized inorganic surfaces. Linear silicones of this type (silicone oils) are generally not considered to be reactive with inorganic oxide surfaces, and an enormous research effort over the last 50 years to develop other silicon-containing reagents with reactive functional groups did not consider the simple alternative that we report. In retrospect, with the acknowledgment of the facile equilibration of siloxane chains with either acid or base catalysis (that was well-known in the 1940s and 1950s), the synthetic approach to functionalized inorganic surfaces by use of linear silicones is obvious. We also report the reactions of poly[3,3,3-trifluoropropyl)methylsiloxane], poly[(3-aminopropyl)methylsiloxane-co-dimethylsiloxane], poly(phenylmethylsiloxane-co-dimethylsiloxane), and poly(dimethylsiloxane-block-ethylene oxide) with oxidized silicon surfaces, which suggest that this reaction is general for silicones.

Dip-coating crystallization on a superhydrophobic surface: a million mounted crystals in a 1 cm2 array.

Silicon wafers (silicon dioxide surfaces) were patterned by photolithograpy to contain 3 µm (width) × 6 µm (length) × 40 µm (height) staggered rhombus posts in a square array (20 µm center-to-center spacing). These surfaces were hydrophobized using a vapor phase reaction with tridecafluorooctyldimethylchlorosilane and exhibit "superhydrophobicity" (water contact angles of θ(A)/θ(R) = 169°/156°). When a section of a wafer is submerged in and withdrawn from water, the superhydrophobic surface emerges, apparently completely dry. If the same procedure is performed using aqueous sodium chloride as the liquid bath, individual crystals of the salt can be observed on the top of each of the posts. "Dip-coating crystallization" using an aqueous sodium chloride solution of 4.3 M produces crystals with ∼1 µm dimensions. A less concentrated solution, 1 M NaCl, renders crystals with ∼500 nm dimensions. These experiments suggest that superhydrophobic surfaces that emerge from water and are "apparently completely dry" are, in fact, decorated with micrometer-size (several femtoliters) sessile water drops that rapidly evaporate. This simple technique is useful for preparation of very small liquid drops or puddles (of controlled composition) and for preparation of arrays of controlled size, crystalline substances (dip-coating crystallization).

Contact angle hysteresis on superhydrophobic surfaces: an ionic liquid probe fluid offers mechanistic insight.

Silicon/silicon dioxide surfaces containing 3 µm (width) × 6 µm (length) × 40 µm (height) staggered rhombus posts were prepared using photolithography and hydrophobized using a perfluoroalkyl-containing monofunctional silane. These surfaces exhibit water contact angles of θ(A)/θ(R) = 169°/156°. Water drops come to rest on a carefully aligned horizontal sample but roll when the surface is tilted slightly. No visible trail or evidence of water "left behind" at the receding edge of the drop is apparent on surfaces that water drops have rolled on or on samples removed from water through the air-water interface. When dimethylbis(ß-hydroxyethyl)ammonium methanesulfonate (N(+)S(-), a nonvolatile ionic liquid) is used as the liquid probe fluid (instead of water), contact angles of θ(A)/θ(R) = 164°/152° are observed and ∼3-µm-diameter sessile drops are visible (by scanning electron microscopy - SEM) on the top of every post of a sample drawn out of this liquid. We interpret the formation of these sessile microdrops as arising from microcapillary bridge failure that occurs during receding events and emphasize that the capillary bridges rupture in primarily a tensile failure mode. Smaller sessile drops could be prepared using mixtures of water and N(+)S(-). Microdroplets of N(+)S(-) were also observed to form selectively at particular features on surfaces containing square holes separated by ridges. This suggests that pinning sites can be identified using microscopy and this ionic liquid probe fluid.

Contact angle hysteresis: a different view and a trivial recipe for low hysteresis hydrophobic surfaces.

Contact angle hysteresis is addressed from two perspectives. The first is an analysis of the events that occur during motion of droplets on superhydrophobic surfaces. Hysteresis is discussed in terms of receding contact line pinning and the tensile failure of capillary bridges. The sign of the curvature of the solid surface is implicated as playing a key role. The second is the report of a new method to prepare smooth low hysteresis surfaces. The thermal treatment of oxygen plasma-cleaned silicon wafers with trimethylsilyl-terminated linear poly(dimethylsiloxane) (PDMS - commercial silicone oils) in disposable glass vessels is described. This treatment renders silicon/silica surfaces that contain covalently attached PDMS chains. The grafted layers of nanometre scale thickness are liquid-like (rotationally dynamic at room temperature), decrease activation barriers for contact line motion and minimize water contact angle hysteresis. This simple method requires neither sophisticated techniques nor substantial laboratory skills to perform.

Displacement reactions of covalently attached organosilicon monolayers on Si.

The covalently attached monolayers of alkylsilanes (R(CH(3))(2)SiX) on Si undergo complete displacement by the solutions of different organosilanes (R'(CH(3))(2)SiX). By varying the reaction time, the degree of displacement can be controlled offering a convenient method for the preparation of surfaces with mixed functionalities (R and R').