In the typical Hofmeister series for ex F->Cl->Br->I-(Ionic radius 0.133,0.181,0.196,0.220 respectively), The ions on the left of Cl- is a Kosmotrope, whereas the ions on the right are chaotropes. Now as the ionic radius decreases, the solvation increases. I do understand the ordering of the chaotropes, but why is F- termed as a Kosmotropic ion?
Small ions interact more strongly with the solvent molecules as ion-dipole interaction energy is proportional to 1/distance^2. It is believed that there is a distinct difference between the structure of the hydration shell of the small F- (very, very ordered) and Cl- to I- (not so ordered). The distinction is empirical, based on various experimental data, so there is not much logic in it.
I personally don't like the distinction and I think it is misleading. Every time I tried it, it lead me nowhere. The boundary between "kosmotropes" and "chaotropes" varies with the experiment you look at (e.g., to my experience, it is better to put it between Cl and Br for ion-surface interactions). There is little correlation with measured ion-solvent correlation functions which are pretty similar for all the four ions - i.e., the explanation is questionable. Anyway, it is good for systematization of the observations.
The terms kosmotrope and chaotrope come from rather old literature and they can be rather misleading. In the middle of the 20th century it was thought that chaotropes destroy the structure of water and kosmotropes promote it, at least as the structure was understood at the time. More recent spectroscopic work has revealed that there is rather little perturbation in the water structure beyond the first solvation shell. See for example work by the group of Bakker in the Netherlands (Vibrational Spectroscopy as a Probe of Structure and Dynamics in Liquid Water, Chem. Rev. 110, 1498 (2010) or Structural Dynamics of Aqueous Salt Solutions, Chem. Rev. 108, 1456 (2008)). There is a gradual increase of solvation free energy towards kosmotropes, which suggests a stronger interaction with water and/or better fitting into the solution structure. So this is a gradual effect and no artificial boundaries should be invoked. In particular, in many interfacial systems besides water and the ions we must also consider the nature of the surface itself, which can invert the Hofmeister series (as demonstrated by the group of R. Netz, Reversed Anionic Hofmeister Series: The Interplay of Surface Charge and Surface Polarity, Langmuir 26, 7370 (2010)). I would expect that every interacting interface would shift the artificial boundary between chaotropes and kosmotropes. In my own work with lipid monolayers (DPPC liquid-expanded monolayers as model systems to understand the anionic Hofmeister series.2. Ion partitioning is mostly a matter of size”, J. Phys. Chem. B 113(5), 1460-1467 (2009)) I found that the dividing line between kosmo- and chaotropes is at Cl-, but when we used DMPE as the lipid we found that Br- was the neutral ion. Bear in mind as well that the Hofmeister line must probably be broken in more than two sections. Very chaotropic ions start disrupting soft matter, while very hydrophilic may in fact act as osmolytes.
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