Friends;

I would like to know if anyone has a list of Chaotropic and Lyotropic agents employed in LC. These agents are used alter the solubility of analytes in different solvents.


Thanks;

Benjamin

One can use practically all the agents listed in books on handling proteins. For instance the series from Holtzhauer, Biochemische Labormethoden, Springer, 1988:
NH4+, Rb+, K+, Cs+, Mg++, Ca++, Ba++

PO4---, SO4--, CH3COO-, Cl-, Br-, NO3-, ClO4-, F3CCOO-, I-, SCN-

These are considered lyotropic on the left and chaotropic on the right. Thus (NH4)2SO4 is strongly lyotropic and is used to precipitate proteins. We have used Na2SO4 to this effect in prechromatography. The widely used chaotropic agents are urea and guanidine-HCl, we have used urea (4M, usually 6M is recommended) in mobile phases. Also on the chaotropic side, NaSCN has been used in this lab (one should not acidify here!) for HPLC. One should not forget that even the most lyotropic salts are chaotropic at low concentrations (usually below an ionic strength of ~0.5).
Strangely, there seems to be no classification along these lines for detergents, PEG (etc), or organic solvents.

So far so good, but: how does one experimentally fit organic solvents, detergents, PEG, urea.... in with the ionic series?? (Seems I asked this before)

HW-please can you offer a definition of the terms "chaotropic" and "lyotrophic" as you understand them.

Many thanks.

David

Didnīt get to this yesterday, here then, a try at a conceptual explanation rather than a definition:

Chaotropic substances are those which are said to decrease the ordered structure of water. Among other things, they lower the cohesion between the water molecules. More on measurable terms: They lower the surface tension of water. With some inevitable discrepancies, the surface tension decreases as the chaotropy increases. Lyotropic substances are just the opposite of this: These increase the surface tension of water with increasing lyotropy. One view on relating this to chromatography is that it is more difficult to create a cavity in H2O for a macromolecule at higher surface tension, thus the macro is "squeezed" out and attaches more strongly to the columnīs surface (or precipitates when attaching to itself) at higher lyotropy.

Now, I would highly appreciate if someone could contribute how thermodynamics (free energy of hydration, entropy) fits into this.

Looking through this chain again it seems that a working definition could clarify things, after all:

Chaotropic substances increase the solubility of macromolecules in water.
Lyotropic substances decrease the solubility of macromolecules in water.