# How to create different levels of hydrophobicity on glass beads surface?

I have some glass beads and I need to create different levels of hydrophobicity on their surfaces (various contact angles). I know that this is possible by 'silanization'. However, I need to know about the following questions:

1. Whether there is any other reasonable method to do that.
2. In silanization, how can I change the level of hydrophobicity that my glass beads achieve? (e.g. by changing coating time, using different solutions or concentrations, etc.)

PS. I have the following solutions for silan. Please let me know if anybody has any comments on them or knows a more suitable solution for this purpose. - CHLOROTRIMETHYLSILANE - DICHLORODIMETHYLSILANE - TETRAETHYL ORTHOSILICATE - OCTADECYLTRICHLOROSILANE

• Sure, there are plenty of methods. For example, you may cover them in cooking oil and carefully heat in a stove. However, I'd go with silanization, for it is clean and controllable. – Ivan Neretin Dec 15 '15 at 21:01

Lets assume first that the surface $\ce{Si-O}$ bonds of teh glass are terminated by $\ce{Si-OH}$ groups. Hydrophilicity will depend greatly on the ration of $\ce{Si-OH}$ to $\ce{Si-R}$ groups on the surface of the glass. Chlorotrimethylsilane is great for a completely hydrophobic surface as it will cap all of the $\ce{Si-OH}$ groups to $\ce{Si-O-Si(CH3)3}$ groups allowing no polar interaction of the surface with the water. dichlorodimethylsilane is not as good leaving you with $\ce{-Si-O-(Si(CH3)2)-O-Si{-}}$ and $\ce{-Si-O-Si(CH3)2OH}$ groups at the surface which affords a bit more interaction. Octadecyltrichlorosilane does provide even more polar groups than the last two, but because of the large octadecyl group this will likely make a more hydrophobic surface than the chlortrimethylsilane. The tetraethyl orthosilicate or rather tetraethoxy silane (TEOS) will hydrolze to $\ce{Si(OH)4}$ in water which you can use to help make the surface more hydrophilic.