Creating a hydrophobic surface is dependent on an organosilane’s organic substitution, surface coverage, residual unreacted groups (both from the silane and the surface), and distribution on the surface.
Aliphatic hydrocarbon substituents or fluorinated hydrocarbon substituents enable silanes to induce hydrophobicity. Surface hydrophobicity is achieved with a nonpolar organic substitution, but more subtle distinctions can be made as well. The hydrophobic effect of the organic substitution is related to the free energy of transfer of hydrocarbon molecules from an aqueous phase to a homogeneous hydrocarbon phase. For nonpolar entities, van der Waals interactions are predominant factors in interactions with water and compete with hydrogen bonding in ordering of water molecules. Van der Waals interactions for solid surfaces are primarily related to the instantaneous polarizability of the solid which is proportional to the dielectric constant or permittivity at the primary UV absorption frequency and the refractive index of the solid. Entities with sterically closed structures that minimize van der Waals contact are more hydrophobic than open structures that allow van der Waals contact. Both polypropylene and polytetrafluoroethylene are more hydrophobic than polyethylene. Similarly, methyl-substituted alkylsilanes and fluorinated alkylsilanes provide better hydrophobic surface treatments than linear alkyl silanes.
Surfaces to be rendered hydrophobic are usually polar with a distribution of hydrogen bonding sites. A successful hydrophobic coating must eliminate or mitigate hydrogen bonding and shield polar surfaces from interaction with water by creating a nonpolar interphase. Hydroxyl groups are the most common sites for hydrogen bonding. The hydrogens of hydroxyl groups can be eliminated by oxane bond formation with an organosilane. Hydrophobic behavior is affected by the silane’s ability to react with hydroxyls to eliminate hydroxyls as water adsorbing sites and to provide anchor points for the nonpolar organic substitution of the silane which shields the polar substrates from further interaction with water.
Hypothetical Trimethylsilylated Surfaces: pyrogenic silica has 4.4-4.6 OH/nm2. Typically < 50% are reacted. Other substrates have fewer opportunities for reaction.
Strategies for silane surface treatment depend on the population of hydroxyl groups and their accessibility for bonding. A simple conceptual case is the reaction of organosilanes to form a monolayer. If all hydroxyl groups are capped by the silanes and the surface is effectively shielded, a hydrophobic surface is achieved. Realistically, not all of the hydroxyl groups will react leaving residual sites for hydrogen bonding. Further, there may not be enough anchor points on the surface to allow the organic substituents to effectively shield the substrate. Therefore, the substrate reactive groups of the silane, the conditions of deposition, the ability of the silane to form monomeric or polymeric layers, and the nature of the organic substitution all play a role in rendering a surface hydrophobic. The minimum requirements for hydrophobicity with the economic restrictions for various applications further complicate selection.
Hydrophobicity is frequently associated with oleophilicity (the affinity of a substance for oils) since nonpolar organic substitution is often hydrocarbon in nature and shares structural similarities with many oils. The hydrophobic and oleophilic effect can be differentiated and controlled. At critical surface tensions of 20-30 mN/m, surfaces are wetted by hydrocarbon oils and are water repellent. At critical surface tensions < 20 mN/m, hydrocarbon oils no longer spread and the surfaces are both hydrophobic and oleophobic. The most oleophobic silane surface treatments have fluorinated long-chain alkyl silanes and methylated medium-chain alkyl silanes.
Automotive side windows are treated with fluoroalkylsilanes to provide self-cleaning properties. Water beads remove soil as they are blown over the glass substrate during acceleration.
Superhydrophobic surfaces present apparent contact angles that exceed the theoretical limit for smooth surfaces, i.e. > 120°. The most common examples of superhydrophobicity are associated with surfaces that are rough on a sub-micron scale and contact angle measurements are composites of solid surface asperities and air—denoted as the Cassie state. Perfectly hydrophobic surfaces (contact angles of 180°) have been prepared by hydrolytic deposition of methylchlorosilanes as microfibrillar structures.
Superhydrophobic Surface – Cassie state
Perfect Hydrophobicity – 180°: Toluene-swollen crosslinks covalently attached methylsilicone: The methylsilicone phase separates in ethanol to form a covalently attached fibrillar network. Fiber diameter is ~20 nm. Ellipsometry indicates a film thickness of ~20 nm.
SEM image. T. McCarthy, J. Am. Chem. Soc., 2006, 128, 9052.
Although silane- and silicone-derived coatings are very hydrophobic, they maintain a high degree of permeability to water vapor. This allows coatings to breathe and reduces deterioration at the coating interface associated with entrapped water. Since ions are not transported through nonpolar silane and silicone coatings, they offer protection to composite structures ranging from pigmented coatings to rebar-reinforced concrete.