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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

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  • $\begingroup$ 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. $\endgroup$
    – Ivan Neretin
    Dec 15, 2015 at 21:01

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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.

If I had to do this I would make various binary mixtures of TEOS, chlorotrimetyhlsilane, and dichlorodimethylsilane and treat the surface with them under constant conditions and see what spectrum of hydrophilicity you achieve. For the most hydrophobic surfaces I would use mixtures of octadecyltrichlorosilane and chlorotrimetyhlsilane.

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  • $\begingroup$ There is a very nice comparison in answer above in terms of how well each solution can hydrophobize the glass; and that's exactly what I want to do as I need one solution to give me the highest hydrophobicity and then choose a few more solutions such that each one gives me a lower level of hydrophobicity so that in general I can create glass surfaces with different contact angles (e.g. varying from let say 140 deg to 40 deg.). Do you think, the 4 solutions that I've chosen are able to do this for me? or I can have a better selection (especially for lower hydrophobicities). Thanks $\endgroup$
    – vorujak
    Dec 15, 2015 at 23:50
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    $\begingroup$ I believe that the chemicals you have are sufficient. The real devil is in the that tenant of materials science no one talks about-processing. You can do the varying mixtures that I suggested, also you can dilute your chemicals is a non-protic solvent like kerosene to vary the extent of reaction, and lastly you can vary the time of the treatments. I believe you have everything you need, but some experimentation will be required. $\endgroup$
    – A.K.
    Dec 16, 2015 at 0:13
  • $\begingroup$ I have two more questions and thanks in advance for your answers! 1) my solutions are mostly 98% or 99% concentrated. Should I deep coat particles into dense solution or should I dilute them and how much? 2) For varying the time, could you please give me an order of magnitude for the time required for a full treatment and the time steps that I should decrease from that in each new experiment (I need first treat the flat glass slides, measure the CA on them and use the same treatment for glass beads, supposing that it will give me the same CA at the same treatment time) $\endgroup$
    – vorujak
    Dec 16, 2015 at 2:09
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    $\begingroup$ I would Dilute in kerosene. Your solutions will react quickly so time on the order of 10's of seconds should suffice. $\endgroup$
    – A.K.
    Dec 16, 2015 at 2:15

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