Selection of Silane Coupling Agents on Metal Surfaces
(1) Metals that form stable oxides on the surface such as: Al, Sn, Ti
There are enough hydroxyl groups on the surface to condense with silicon hydroxyl groups, choose methoxy or ethoxy silane coupling agent
(2) Metals that cannot form stable oxides on the surface: Fe, Zn, Cu
Choice of two silanes: one silane (amine group, polycarboxylate, polyamine, etc.) to complex the metal and the other silane to react with the first silane and the organic moiety
(3) Metals that are not easy to form oxides on the surface: Au, Ni and other precious metals
Choose a functional group (P, S, N) silane that facilitates coordination of these metals with another silane that can react with organic moieties
(4) Metals capable of forming stable metal hydrides on the surface: Ti, Zr, Ni
Select silanes containing silicon-hydrogen bonds, which can form a stable chemical structure with metals and release hydrogen gas.

Silane Modification - Hydrophobic and Oleophobic
Silanes are very sensitive to water and can form stable chemical bonds with inorganic substrates. At the same time, changing the structure of organic functional groups can endow substrates with different properties. A surface is hydrophobic if it does not absorb or be wetted by water. A surface is hydrophilic if it absorbs water or is wetted by water. More specifically, these terms describe the interaction of a solid-phase boundary layer with liquid or vapor water. Silanes can be used to highly controllably alter the interaction of solid boundary layers with water, thereby affecting varying degrees of hydrophobicity or hydrophilicity.

Modification of silane
The silanes used for surface modification without reaction mainly include the following:
If the contact angle of water is less than 30°, the surface is designated as hydrophilic because the interaction force between water and the surface is almost equal to the cohesive force of bulk water and water will not be expelled from the surface. A surface is usually designated superhydrophilic if water diffuses across the surface and the contact angle of the water diffusion front is less than 10° (provided that the surface does not absorb, dissolve in, or react with water).
On hydrophobic surfaces, water forms distinct droplets. As the hydrophobicity increases, the contact angle of the droplet with the surface increases. Surfaces with contact angles > 90° were designated as hydrophobic. The theoretical maximum contact angle of water on a smooth surface is 120°. Microtextured or micropatterned surfaces with hydrophobic roughness can exhibit apparent contact angles >150°, exhibiting the "lotus leaf effect".

1. Hydrophobic surface treatment
Factors that affect the ability of organosilanes to produce hydrophobic surfaces are their organic substitution, surface coverage, residual unreacted groups (from silane and surface), and the distribution of silanes on the surface
Aliphatic hydrocarbon substituents or fluorinated hydrocarbon substituents are hydrophobic functional groups that render silane-induced surfaces hydrophobic. In order to create a hydrophobic surface, the organic substitution of the silane must be non-polar.
Surfaces modified to be 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 interacting with water by creating apolar interfaces. The hydroxyl group is the most common site for hydrogen bonding, and the hydrogen on the hydroxyl group can be eliminated by forming an oxane bond with the organosilane. The effectiveness of the reaction of the silane with the hydroxyl group affects the hydrophobic behavior not only by eliminating the hydroxyl group as a water adsorption site, but also by eliminating The hydroxyl groups serve as water adsorption sites to achieve hydrophobicity. In general methyl-substituted alkylsilanes and fluorinated alkylsilanes provide better hydrophobic properties than linear alkylsilanes.

2. Superhydrophobic and oleophobic surface treatment
Hydrophobicity is often related to lipophilicity (a substance's affinity for oils) because non-polar organic substitutions are often hydrocarbons and have similar structures to many oils, allowing control of both hydrophobic and lipophilic interactions. Surfaces with a critical surface tension of 20-30 mN/m are wetted by hydrocarbon oils and are water repellent. At a critical surface tension < 20 mN/m, the hydrocarbon oil no longer diffuses and the surface is both hydrophobic and oleophobic. A superhydrophobic surface exhibits a contact angle > 120° and a completely hydrophobic surface (a contact angle of 180°). The most oleophobic silane finishes are fluorinated long chain alkyl silanes and methylated medium chain alkyl silanes.

3. Hydrophilic surface treatment
Most surfaces are hydrophilic. Water is ubiquitous in the environment, but the exact nature of water's interactions with specific surfaces is largely unknown. Water sorption may be homogeneous or isolated plates, and may be driven by many different physical and chemical processes. Other adsorbents present in the environment may hinder the adsorption of water to the surface. Hydrophilic surface treatments are used to control the nature and extent of water interaction with surfaces.

Hydrophilicity: non-polar < polar, no hydrogen bond < polar, hydrogen bond < hydroxyl < ion.
The choice of the type of hydrophilic substituent depends on the application. Polar aprotic silanes are preferred if water is to be distributed evenly over a surface to form a thin film that rinses and dries quickly without leaving "dry spots". If a coating that reduces non-specific binding of proteins or other biochromes is desired, polar hydrogen-bonding materials such as polyether-functional silanes are preferred. Polar non-hydroxyl materials can be used in thin-film proton-conducting electrolytes. Pigmented coatings are often hydroxyl-selective because they require a specific water-absorbed phase. Antistatic coatings are usually charged or charge induced. The combination of hydrophilicity and hydrophobicity can meet the requirements of selective adsorption applications in chromatography.

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