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I used a 50% wt mixture of Tetraethylorthosilicate + Kerosene for silanization of some glass plates with different treatment times (from 1 sec to 35 sec). However, the contact angle(CA) that I got does not seem consistent as it does not always increase as I increase the treatment time (when treatment time increases, the CA fluctuates).

What might be the reason? and whether I should change the method or the solution(s)?

P.S. I cleaned the glass with acetone in a sonicator. Then I put the glass into solution and after specific time put it into water to clean the solution from surface and then clean the remaining oil (from kerosene) by rubbing a napkin to the glass plate. Then I measured the CA.

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Rubbing is a very inconsistent process, and will introduce a lot of variation. I would suggest using a volatile non-polar solvent such as acetone, hexanes, or ethyl acetate to remove residual kerosene and allowing to air dry.

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  • $\begingroup$ So, do you mean the main issue that may cause this fluctuations in CA could be this rubbing that I've done with a napkin?(I mean do you think that if I perform the process correctly, I should basically get the lowest CA in 1 sec treatment and the highest CA in 35 sec. Is that right?) $\endgroup$
    – vorujak
    Dec 26, 2015 at 4:31
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    $\begingroup$ In theory there should be a correlation, but tetraethoxysilane (TEOS) is not a good chemical for silanization, as the ethoxy group can easily hydrate leaving you with a hydrophilic $\ce{Si}-\ce{O}-\ce{H}$ surface. I did assume everything else you did was the same, but factors such as humidity and exposure time to air can affect results due the the nature of TEOS $\endgroup$
    – A.K.
    Dec 26, 2015 at 4:39
  • $\begingroup$ I have the following three more solutions available. Based on your above comment that TEOS is not very appropriate for silanization, do you think which solution is the more suitable one for creating different levels of hydrophobicity? (please also let me know if there are any instructions for silanizing that you think I should know) 1) ChloroTrimethylSilan 2) OctaDecylTrichloroSilan 3) DichloroDimethylSilan $\endgroup$
    – vorujak
    Dec 26, 2015 at 5:23
  • $\begingroup$ I like chlorotrimethylsilane because it has only one chloro group, and octadecyltrichlorosilane because it is sterically bulky. The latter should work best from the steric, but the former gives complete surface reaction, so I cannot say with certainty. $\endgroup$
    – A.K.
    Dec 26, 2015 at 13:53
  • $\begingroup$ One other thing to consider. Are you using a consistent amount of water? This can affect your contact angle. $\endgroup$
    – A.K.
    Dec 27, 2015 at 17:34

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