Potential of mean force for tetramethylammonium binding to cagelike oligosilicates in aqueous solution

We have carried out free energy calculations to compute the potential of mean force for the cagelike silicate polyion-TMA+ cation ion pair interaction in aqueous solution. We also have studied solvent reorganization-related entropic effects. We conclude that the organocations, as opposed to, for example, alkali-metal ions, play a pivotal role in reorganizing the solvent around the cagelike silicates in a manner conducive to the formation of heteronetwork clathrates that are stable both thermodynamically and kinetically. In the case of stable cagelike polysilicate anions, this solvent reorganization correlates with entropic losses. We also infer that transient cagelike polysilicate species, that may indeed form but participate in floppy clathrates, eventually have to give way to cagelike polysilicates that lead to more rigid structures.