Polarity!

[amaryl]

well i found two references both contradict.

which is more polar acetonitrile or methanol.

is it ACN 5.8 Methanol 5.1

or ACN 6.2 methanol 6.6

thanx

regards

amaryl

[Uwe Neue]

Mehtnaol is more polar than acetonitrile. I do not know what your numbers mean, or where they come from.

[amaryl]

Mehtnaol is more polar than acetonitrile. I do not know what your numbers mean, or where they come from.

well these figures are given in two literatures.

polarity index(P').

[Koen Hollebekkers]

Uwe is right, a mixture of MeOH is more polar compared than a mixture of equivalent concentration of ACN.
The values ACN 5.8 Methanol 5.1 are the right ones as far I know.
What that I also never figured out is that when you will calculate the polarity index for a mixture of 50/50 a ACN mixture will have a higher polarity index as MeOH. What is the opposite of the reality.
Who can explain?

[Uwe Neue]

OK I found the values in Snyder's book...

I never cared about these values, since they depend on the context.
In reversed-phase, acetonitrile its a stronger eluent than methanol, and THF is a stronger eluent than acetonitrile.

In normal phase chromatography, the opposite is true, and for all cases that I can think of, acetonitrile is a weaker eluent than methanol.

To go beyond this is probably voodoo, since there are many specific interactions between the sample and the solvent components that are more relevant than the general "solvent strength".

[HW Mueller]

According to a poster by Fisher on solvents (hanging in the lab) the dipole moment for ACN is 3.92, that for MeOH only 1.7 debye. However, MeOH easily does hydrogen bonding, that might be the explanation for some "erratic" behavior. (The dielectric constants were given as 35.94 and 32.66, the boiling point as 81.6° and 64.55°, the p1 as 5.8 and 5.1 for ACN and MeOH, respectively). Incidentally, the solvent strength nomograph given by Snyder, Glajch, Kirkland in Practical HPLC..., shows the eluting power of ACN and MeOH to be the same for 100% (no H2O) solvents, only with water present is ACN stronger (have seen the latter many times). It seems to be as complicated as the ion stuff discussed elsewhere.

[tom jupille]

AFAIK, the polarity index was defined in the context of liquid-solid adsorption chromatography (normal-phase). Given the different (solvophobic + whatever else) mechanism of reversed-phase, it shouldn't be surprising that RP solvent strength doesn't correlate perfectly with NP values.

The Syder & Kirkland nomogram is only an approximation, good to perhaps +/- 5%.

In any case, solvent strength predictions are much less important in RP than they are in NP, because the former is much more "robust". First of all, RP has access to a much narrower range of solvent polarities (not counting NARP, the range is from THF to water) than does NP. Furthermore, in RP, the relationship between k' and strong solvent composition is approximately log-normal. This means that you get about the same retention change when you change %B from 20-30% as you do when you change it from 70-80%, etc.. NP retention is closer to log-log, which means that you get about the same retention change when you go from 1-2% as you do when you go from 10-20%.