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June 2007

 
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.


Amplified view of water drop on Lotus Leaf

The following chart outlines the relative level of hydrophobicity and hydrophilicity.

Superhydrophobic 150° and greater
Hydrophobic 90° to 120°* 
Normal 30° to 90°* 
Hydrophilic below 30° 
**about 120° is considered to be the theoretical maximum contact angle that can be achieved on a flat surface

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.

Solid Silane
Liquid Crystal Display Octadecyl ammonium chloride  
Gold Sulfur
Tin Amine

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

Environmental Chamber
For over 30 years, the ramé-hart Environmental Chamber (p/n 100-07) has been one of the most popular modular accessories. This option is easily mounts on the standard ramé-hart leveling stage and is clamped down for security.


ramé-hart Environmental Chamber p/n 100-07 shown with
Optional Rotating Support

The chamber includes high density heaters capable of heating the chamber up to 300° C with the optional Proportional Temperature Controller (p/n 100-50). Additional coolant ports allow for rapid cooling. Windows are premium instrument-grade quartz for minimal distortion and filtering. The chamber is also designed to accept the ramé-hart quartz cell (p/n 100-07-50). The lid can be quickly removed for cleaning or locating the substrate sample. An optional cover with stage (p/n 100-09-10) is available. Dispensing is accomplished via the quick-release lid and can be used with either our manual microsyringe or Automated Dispensing System. All parts are anodized 6061 aluminum for long-life and easy cleaning and long life. Spare parts including o-rings, windows, and thermocouple are kept in stock. The Environmental Chamber can also be used in conjunction with our Elevated Temperature Syringe (p/n 100-11).  If you would like pricing or more product information on this part or any other accessory, please contact us.  

 


Regards,

Carl Clegg
Director of Sales
Phone 973-448-0305
www.ramehart.com
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