The effect of lithium ions on the hydrophobic effect: does lithium affect hydrophobicity differently than other ions?

Ionic species have been shown to significantly perturb the interactions between non-polar solutes in aqueous solution. These perturbations are often analyzed in terms of the interactions existing between hydrophobic surfaces and ions. It has been known for some time, that ions with a high charge density are repelled from hydrophobic surfaces while ions with a low charge density tend to stick to these surfaces. Therefore, from a continuum model standpoint, small monovalent ions promote hydrophobicity by minimizing the exposed hydrophobic surface area, while "sticky" large monovalent ions interact with the hydrophobic surfaces and discourage aggregation. However, the charge-dense lithium ion often exhibits anomalous behaviour different from these predicted trends: instead of enhancing, the addition of lithium ions often seems to weaken the hydrophobic effect and on the contrary help dissolve hydrophobic molecules. This weakening of apparent hydrophobicity is considered to be one of the reasons for the protein denaturing properties of lithium salts. Recent theoretical and experimental results however have shown that lithium cations can interact with a variety of molecular functional groups. This suggests that this apparent lithium-induced lowering of hydrophobicity, that is often reported in the literature may be a result of specific interactions between these molecular functional groups and lithium, rather than weakening the interaction between hydrophobic surfaces. This work examines these possibilities by studying the effect of various cations on the simple hydrophobic interaction existing between methyl and phenyl contact-pairs and demonstrates that the effect of lithium cations on the hydrophobic effect follows the trend predicted by continuum models. In other words, the influence of an ion on the hydrophobic interaction between two non-polar surfaces is a function of the interaction of that ion and each non-polar surface.