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Modification of Silane: Hydrophobicity or Oleophobicity


Silane is highly sensitive to water and can form stable chemical bonds with inorganic substrates, while changing the structure of organic functional groups to give the substrate surface different properties. If the surface does not absorb water or is not wetted by water, it is hydrophobic. If the surface adsorbs water or is wetted by water, it is hydrophilic. Specifically, these terms describe the interaction between the solid phase boundary layer and liquid water or steam. Silane can be used to control the interaction between the solid boundary layer and water to affect different degrees of hydrophobicity or hydrophilicity.


Hydrophobic surface treatment of silanes


The factors that affect the ability of organic silane to create hydrophobic surfaces include its organic substitution, surface coverage range, residual unreacted groups (from silanes and surfaces), and distribution of teos silane on the surface.


Alkyl or fluoroalkyl substituents are hydrophobic functional groups that induce teos silane to form hydrophobic surfaces. In order to generate a hydrophobic surface, the organic substitution of teos silane must be non-polar.


The surfaces modified to be hydrophobic are typically polar, with hydrogen bonding sites distributed. A successful hydrophobic coating must eliminate or reduce hydrogen bonding and shield the polar surface from interacting with water by creating a nonpolar interface. Hydroxyl groups are the most common position for hydrogen bonding, and the hydrogen on hydroxyl can be eliminated by the oxylanes bond formed with organic silanes. The effectiveness of silanes reacting with hydroxyl groups not only affects hydrophobic behavior by eliminating hydroxyl as a water adsorption site, but also by eliminating hydroxyl as a water adsorption site to achieve hydrophobicity. Typically, alkyl silicones and fluorinated alkyl silicones provide better hydrophobic properties than linear alkyl silicones.


Superhydrophobic and oleophobic surface treatment of silanes


Hydrophobicity is usually related to lipophilicity (the affinity of a substance for oil) because nonpolar organic substitutions are usually hydrocarbons and have structures similar to many oils, which can control hydrophobic and lipophilic interactions. The surface with a critical surface tension of 20-30mN/m will be wetted by hydrocarbon oil and has water resistance. When the critical surface tension is less than 20mN/m, the hydrocarbon oil no longer spreads, and the surface is both hydrophobic and oleophobic. The contact angle presented by superhydrophobic surfaces is greater than 120°, and the surface is completely hydrophobic (contact angle is 180°). The most oleophobic silane surface treatment includes fluorinated long-chain alkyl silanes and methylated mid-chain alkyl silanes.


Hydrophilic surface treatment of silanes


Most surfaces are hydrophilic. Water is ubiquitous in the environment, but the exact nature of water's interaction with a specific surface is largely unknown. Water adsorption may be uniform or isolated platelets, and may be driven by many different physical and chemical processes. Other adsorbates present in the environment may impede the surface's adsorption of water. Hydrophilic surface treatments are used to control the nature and extent of water's interaction with a surface.


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