Vibrational energy transfer : An angstrom molecular ruler in studies of ion pairing and clustering in aqueous solutions

The methodology and principle using vibrational energy transfer to measure molecular distances in liquids are introduced. The application of the method to the studies of ion pairing and clustering in strong electrolyte aqueous solutions is demonstrated with MSCN aqueous solutions where M = Li, Na, K, Cs, and NH₄. Experiments suggest that ions in the concentrated aqueous solutions can form substantial quantities of ion clusters in which both cations and anions are involved. More and larger clusters form in solutions that are relatively more concentrated and which include a larger cation. In KSCN solutions, the shortest anionic distance in the ion clusters is the same as that in the KSCN crystal. The rotational time of the anion and the nonresonant vibrational energy transfer time with a gap of 75 cm^-1 in the KSCN saturated solution are very similar to those in the KSCN crystal. However, the KSCN ion clusters are closer in structure to the melt. The clusters form an interconnected network with random ionic orientations. Because of ion clustering, the anion and water dynamics behave distinctly in the same solutions. At high concentrations, the anion rotation significantly slows down because of the increase in the size of the ion clusters, but the slowdown amplitude of water rotation is very modest because many of the water molecules still remain in the "bulk" state due to ion clustering. The rotational dynamics of both water and anions are slower in a solution with a smaller cation, primarily because a smaller cation has a stronger cation/anion interaction and a cation/water interaction that leads to more water molecules confined in the ion clusters. Adding ions or molecules into the KSCN solutions can perturb the ion clusters. Weakly hydrated anions can participate in clustering and form mixed ion clusters with KSCN, and strongly hydrated anions can reduce the effective water molecules solvating KSCN and facilitate the formation of more and larger KSCN ion clusters. Similarly, molecules which can strongly bind to SCN^- prefer to participate in the KSCN ion clusters. Molecules which are strongly hydrated prefer to remain hydrated and facilitate the ion clustering of KSCN.