Dear Manan Vishvas Desai, 

A publication posted few days ago describes in detail the two terms and illustrating what ions are included in each term. However, you should be careful of such grouping since the ion can behave differently in different environments.

Herein some important paragraphs taken from the publication:

Definitions of kosmotropes and chaotropes

The terms 'kosmotrope' (order-maker) and 'chaotrope' (disorder-maker) originally denoted solutes that stabilized, or destabilized respectively, proteins and membranes; thus chaotropes unfold proteins, destabilize hydrophobic aggregates and increase the solubility of hydrophobes whereas kosmotropes stabilize proteins and hydrophobic aggregates in solution and reduce the solubility of hydrophobes. A recent review makes a strong case for reverting to these meanings [2361], however most present literature uses these terms to refer to the apparently correlating property of increasing, or decreasing respectively, the structuring of water (a review of structure-making and structure-breaking ions has been presented [1567]). Recently, the terms kosmotropes and chaotropes have been quantified in terms of hydrophobicity and hydrophilicity [2212]. Although useful, the terminology may sometimes be misleading as such properties may vary dependent on the circumstances, method of determination or the solvation shell(s) investigated. For example a solute may not always act in the same way at different concentrations or in the presence of macromolecules or gels [276]. Also some solutes with less well-defined properties (for example, urea) are sometimes classified as kosmotropes [276] and sometimes as chaotropes [283]. An alternative term used for kosmotrope is 'compensatory solute' as they have been found to compensate for the deleterious effects of high salt, and/or high chaotrope contents [2513], (which destroy the natural hydrogen bonded network of water) in osmotically stressed cells, but again behavior as a kosmotrope in one system does not mean that a solute may behave as a 'compensatory solute' in another or even that it will stabilize the structuring of water in a third. Both the extent and strength of hydrogen bonding may be changed independently by the solute but either of these may be, and has been, used as measures of order making. It is, however, the effects on the extent of quality hydrogen bonding that is of overriding importance as true kosmotropes shift the local equilibrium

 less dense water (for example, ES)= more dense water (for example, CS)

 to the left and chaotropes shift it to the right. The ordering effects of kosmotropes may be confused by their diffusional rotation, which creates more extensive disorganized junction zones of greater disorder with the surrounding bulk water than less hydrated chaotropes. It seems clear that most kosmotropes do not cause a large scale net structuring in water [595].

 Temperature and pressure both have effects on the kosmotropic/chaotropic status with the effects disappearing at high temperatures, particularly at high concentrations [1170]. For example, at very high pressures (0.6 GPa) Na+ ions change from being weak kosmotropes into weak chaotropes as their links to water molecules are preferably broken [1170]. [Back to Top  ]

Ionic kosmotropes and chaotropes

Ionic kosmotropes a should be treated differently from non-ionic kosmotropes, due mainly to the directed and polarized arrangements of the surrounding water molecules. They are best not described as 'structure-makers' or structure-breakers' in terms of their effects on the properties of water outside their immediate solvation shells [1389].

 For viewing the figure, please use the following link:

http://www1.lsbu.ac.uk/water/kosmotropes_chaotropes.html

Generally, ionic behavior parallels the Hofmeister series. Large singly charged ions, with low charge density (for example, SCN-, H2PO4-, HSO4-, HCO3-, I-, Cl-, NO3-, NH4+, Cs+, K+, (NH2)3C+ (guanidinium) and (CH3)4N+ (tetramethylammonium) ions; exhibiting weaker interactions with water than water with itself and thus interfering little in the hydrogen bonding of the surrounding water), are chaotropes whereas small or multiply-charged ions, with high charge density, are kosmotropes (for example, SO42-, HPO42-, Mg2+, Ca2+, Li+, Na+, H+, OH- and HPO42-, exhibiting stronger interactions with water molecules than water with itself and therefore capable of breaking water-water hydrogen bonds). b The Jones-Dole coefficient has also been used to classify ionic kosmotropes and chaotropes with negative B coefficients indicating chaotropes and positive B coefficients indicating kosmotropes. Kosmotropes remain hydrated near the water surface, while the chaotropes lose their hydration sheath [1663]. The radii of singly charged chaotropic ions are greater than 1.06 Å for cations and greater than 1.78 Å for anions [284]. Thus the hydrogen bonding between water molecules is more broken in the immediate vicinity of ionic kosmotropes than ionic chaotropes [2046]. Reinforcing this conclusion, a Raman spectroscopic study of the hydrogen-bonded structure of water around the halide ions F-, Cl-, Br- and I- indicates that the total extent of aqueous hydrogen bonding increases with increasing ionic size [685] and an IR study in HDO:D2O showed slow hydrogen bond reorientation around these halide ions getting slower with respect to increasing size [895]. It is not unreasonable that a solute may strengthen some of the hydrogen bonds surrounding it (structure making; for example, kosmotropic cations will strengthen the hydrogen bonds donated by the inner shell water molecules) whilst at the same time breaking some other hydrogen bonds (structure breaker; for example, kosmotropic cations will weaken the hydrogen bonds accepted by the inner shell water molecules) [274], so adding to the confusion in nomenclature. Kosmotropic ions such as Na+ reduce the average diffusion of water by slightly less than that expected if the hydrated water was not diffusionally active [2001].

 Much local organization around a solute is entropically compensated by reduced organization between the water molecules further away [2087], such that K+ and Cl- (and Ar) have almost identical hydration entropies [1495]. Other factors being equal, water molecules are held more strongly by molecules with a net charge than by molecules with no net charge; as shown by the difference between zwitterionic and cationic amino acids [532].

 Rather surprisingly, chaootropic ions such as K+ slightly the average diffusion of water at moderate to high concentrations (0.25 - 2 M) [2001], showing that they have a net destructive effect on the structuring in water at these concentrations. Weakly hydrated ions (chaotropes, K+, Rb+, Cs+, Br-, I-, guanidinium+) may be 'pushed' on to weakly hydrated surfaces by strong water-water interactions with the transition from strong ionic hydration to weak ionic hydration occurring where the strength of the ion-water hydration approximately equals the strength of water-water interactions in bulk solution (with Na+ being borderline on the strong side and Cl- being borderline on the weak side) [284]. Strongly hydrated surfaces, where the water molecules are prevented from forming ES-like expanded water clusters, favors strongly hydrated kosmotropic ions and effectively repels weakly hydrated chaotropic ions [1726]. Neutron diffraction studies on two important chaotropes (guanidinium and thiocyanate ions) show their very poor hydration, supporting the suggestion that they preferentially interact with the protein rather than the water [488]. In contract to the kosmotropes, there is little significant difference between the properties of ionic and nonionic chaotropes due to the low charge density of the former.

 Optimum stabilization of biological macromolecule by salt requires a mixture of a kosmotropic anion with a chaotropic cation. As ionic kosmotropes primarily achieve their increased structuring solely within their hydration shell, they partition into the more dense (CS) water where they can obtain this hydration water more readily, whereas the ionic chaotropes, by avoiding interference with water's hydrogen-bonded network, tend to clathrate formation within the less dense (ES) environment. Thus there is agreement with the defining characteristic of an ionic chaotrope in that it partitions selectively into low-density water whereas a kosmotrope partitions selectively into high-density water [276]. The stabilizing of structured low-density water (by ionic chaotropes) in turn stabilizes the hydration shell around lower molecular masshydrophobes, as seen in the promoted association of polyene antibiotics by ionic kosmotropes and the stabilization of their solution by ionic chaotropes [1030]. 

For more details, please use the following link:

http://www1.lsbu.ac.uk/water/kosmotropes_chaotropes.html

There are other two publications which may helpful in understanding the concept of the topic of concern:

http://pubs.rsc.org/en/content/articlehtml/2014/cs/c4cs00144c

http://www.ncbi.nlm.nih.gov/pubmed/8994593

Hoping this will be helpful,

Rafik

Thank you Rafik for the URLs. I have already referred to link from which the you have provided the explanation. However, in some articles Ca 2+ and Mg 2+ are referred as chaotropic. " However, to some extent the chaotropic effects of Mg2+ and Ca2+ can be counteracted by the presence of kosmotropic ions (Williams and Hallsworth, 2009)."  That where the confusion is . Is it that These divalent cations have a tendency to shuffle from being komsmotrpoic and chaotropic?

Dear Manan,

Williams and Hallsworth in their publication "Limits of life in hostile environments: no barriers to biosphere function" explain their findings based on the shuffle of some divalent cations between komsmotrpoic and chaotropic.

Therefore, it might be that calcium and magnesium cations have the tendency to shuffle between being komsmotrpoic or chaotropic.

Rafik

Dear Manan,

Did you ever figure this out? I have the exact same question. And the link for lsbu is confusing, because on some pages the divalent cations are listed as kosmotropes, and on others (in the same publication) as chaotropes. 

Thanks,

Alex.

The mechanistic answer you're looking for is in the following publication, though it will likely create more questions than answers: https://europepmc.org/article/med/18183287

James Langan thank you