June 2007

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.,

Most of the widely used organosilanes have one organic substituent and three hydrolyzable substituents. In the vast majority of surface treatment applications, the alkoxy groups of the trialkoxysilanes are hydrolyzed to form silanol-containing species. Reaction of these silanes involve four steps. Condensation to oligomers follows. The oligomers then hydrogen bond with OH groups of the substrate. Finally, during drying or curing, a covalent linkage is formed with the substrate with concomitant loss of water. Although described sequentially, these reactions can occur simultaneously after the initial hydrolysis step. At the interface, there is usually only one bond from each silicon of the organosilane to the substrate surface. The two remaing silanol groups are present either in condensed or free form. The R group remains available for covalent reaction of physical interaction with other phases.

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