Methanol vs Acetonitrile

[hajdaei]

Tom Thanks for the reply.

But is the access to silanols really that different for C1 and C4. It seems like there would need to be another reason for the noticeable difference in selectivity between these phases.

[Uwe Neue]

I do not believe that the silanol interaction of the C1 is the important factor. Yes, it is there, but we endcap all the longer-chain packings with the C1.

For one thing, I think that we need to keep in mind that a C1 is not really a C1, but a C3 phase: a trimethylsilane attached to the surface. TSecond, the coating level is high (this is why we use the trimethylsilyl group for endcapping). You can't get anywhere close to 4 micromoles/m^2 with a monofunctional C18, but it is easy to exceed such a value with the C1 silane. Third, all the carbon of the C1 bonding is available for interaction, while this may not be the case for a C18.

I am not surprised that there is so much hydrophobic interaction coming from a C1 packing.

[adam]

Ahhh. Now the lights come on.

If they are trimethyl silane groups then you have three carbon-silicon bonds for each ligand. These are pretty polar bonds and obviously would have different properties than carbon-carbon bonds. Plus, since carbon is more electronegative than silica you would have more of an electron cloud around the carbon atoms, than would be the case w/ C4 or C8, so there would be some van der Waals interactions with the analytes.

So, from what I'm gathering, this is why C1 columns behave fundamentally different from C4 columns or C8 columns. Because these columns have the carbons in a straight chain, where the properties are different

Plus the C1 column would allow some additional interaction with the base silica - as Tom said.

[HW Mueller]

adam, if you look at van der Waals interactions the H are foremost. There is a large difference between CH3 and CH2. The longer the chain the more CH2 dominate. The C in CH3 is generally more electronegative than that in CH2, polarities are diffrent, but there my "wisdom" ends regardng predictions of van der Waals interactions. Also, obviously, the methyl group is much smaller than anything else in the alkyl series. If I remember correctly it has about the size of an attached chlorine. Some steric effects have been mentioned above.