If you're having trouble viewing this email, you may see it online.
|Silanes and Surface Modification|
A Silane (not to be confused with a Saline)
is pyrophoric (i.e., a gas which will ignite spontaneously without an
external ignitor -- at room temperature at least) and a silicon analog
of an alkane hydrocarbon. Silanes react with inorganic solids (such as
glass and metals) to form stable covalent bonds which alter the surface
properties of the solid. Among these properties are charge conduction,
absorption, dielectric, and release. Two additional and important
property modifications are: hydrophobicity and hydrophilicity. Silanes
that are used as coupling agents react with with the substrate while
silanes used to alter the surface energy do not. Some classes of silanes
such as Methyl, various Alkyl, Aryl, and Dipodal are used to increase
hydrophobicity while other classes, such as polar, hyroxylic, ionic, and
masked will increase hydrophilicity.
The term hydrophobic refers to a surface that does not absorb water well -- i.e., one with poor wettability. The contact angle of liquid water will be higher on hydrophobic surfaces. The opposite is true with a hydrophilic surface: lower contact angle, greater wettability, and greater solid surface free energy. Silanes are employed to alter the wettability of a surface with an impressive level of precision.
Since water is often used to measure contact angle and thus wettability and surface energy, it's important to recognize that surface interactions with water are also affected by other factors: van der Walls forces, dipole interactions, hydrogen bonding and proton exchange.
In the case of large contact angles, say over 150°, the surface is considered superhydrophobic. This phenomenon is also often referred to as the lotus effect after the lotus leaf (see image below) which has unusually low wettability. Alas this surface is not flat and so some percentage of the lack of wettability is attributable not to the chemical makeup of the surface but rather the physical topography.
The following chart outlines the relative level of hydrophobicity and hydrophilicity.
For fairness we work with only flat surfaces when modifying their surface energy with silanes. Thus we exclude superhydrophobic examples. To modify a surface using a silane treatment, it's beneficial to understand how this occurs. According to a world leader in the production silanes, Gelest, Inc.,
For further study of this topic, we highly recommend the above referenced technical paper by Gelest.
The following table outlines some of the common silanes used in surface modification.
Lastly, if this is a topic of interest to you, we should point out that this week in Cincinnati, Ohio, the Sixth International Symposium on Silanes and other Coupling Agents meets, directly after the Symposium on Polymer Surface Modification. Barry Arkles and Youlin Pan of Gelest, Inc. will present their work on this topic.
Hydrophobicity, Hydrophilicity and Silane Surface Modification, 2006, Gelest, Inc. Available in PDF format on their website at www.gelest.com.
MST Conferences: www.mstconf.com