Scope:

ASTM C813 detects layers of organic molecules by observing the contact angle measurements. Organic molecules change the contact angle of glass. The contact angle is the angle where a liquid-vapor interface meets a solid surface. It quantifies the wettability of a solid surface by a liquid via the Young equation. 

The data from the contact angle test is used to evaluate processes that remove hydrophobic contaminants. It is used to detect glass that harbors hydrophobic contaminants when exposed to ambient conditions. For this, a surface free of organic films is exposed to the ambient conditions and is later tested.

Test Procedure:  

In ASTM C813 testing, drops of permanganate-distilled water are deposited on the glass surface using a mounted hypodermic syringe to measure the advancing contact angle. The contact angle is measured by either (1) viewing a sessile drop through a comparator microscope with a goniometer scale; or (2) photographing a sessile drop and measuring the angle with a protractor. 

Organic contamination on surfaces leads to contact angles higher than the near-zero angles seen on clean surfaces. The sensitivity of this test decreases if the surface is rough or porous, or if the surface has hydrophilic substances. 

Data: 

The contact angle measured on the glass surface is reported. As per ASTM C813, the surface is free of hydrophobic contaminants if the advancing angle is not greater than 5°. 

Conclusion:

ASTM C813 detects hydrophobic contamination on glass surfaces by measuring the contact angle.