What limits the number of chlorosilane molecules that could react with a surface

Question

What limits the number of chlorosilane molecules that could react with a surface? How many molecules 'thick' do you predict the surface layer would be? On the previous page we discussed the properties of the 'normal' SiO-2 surface. Companies that manufacture chromatographic stationary phase use reactions like the one you've written above to change the properties of the SiO_2 surface an d create a new type of surface. Before the reaction, the surface was polar/nonpolar. After the reaction, the surface is polar/nonpolar. As you did on the last page with the unmodified SiO_2 surface, now consider injection of an aliquot of a solution containing the two components ethanol and acetone down a column packed with SiO_2 particles whose surface had been modified by the surface reaction above. Which one of these solutes would have the greatest retention time? Why?

Explanation / Answer

1) The number of 'free' silanol groups present on silica surface will limit the number of chlorosilane molecules that would react. Utimately, it is the reaction between the available -OH group and Cl- resulting in a loss of HCl and formation of the --Si-O-Si-- linkage.

2) The surface layer would be one molecule thick.

3) When the silica surface is modified by the above process, it gets transformed from a polar to a non-polar stationar phase. This is because the polar -OH group has now been replaced by a substututent containing a long chain (C18) non-polar alkyl group. Out of the two components that are injected, ethanol is more polar that acetone. As a result, it will not interact much with the non-polar stationary phase and will get eluted first. While acetone being realtively non-polar will be reatined longer and get eluted later.

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