Ion clustering in aqueous solutions probed with vibrational energy transfer

Abstract

Despite prolonged scientific efforts to unravel the hydration structures of ions in water, many open questions remain, in particular concerning the existences and structures of ion clusters in 1∶1 strong electrolyte aqueous solutions. A combined ultrafast 2D IR and pump/probe study through vibrational energy transfers directly observes ion clustering in aqueous solutions of LiSCN, NaSCN, KSCN and CsSCN. In a near saturated KSCN aqueous solution (water/KSCN molar ratio ¼ 2.4∕1), 95% of the anions form ion clusters. Diluting the solution results in fewer, smaller, and tighter clusters. Cations have significant effects on cluster formation. A small cation results in smaller and fewer clusters. The vibrational energy transfer method holds promise for studying a wide variety of other fast short-range molecular interactions.T he solution properties of ions in water are relevant to a wide range of systems, including electrochemistry, biological environments, and atmospheric aerosols (1, 2). For more than 100 yr, tremendous scientific efforts have been devoted to unravel the hydration structures of ions in water (1-11). However, many fundamental questions remain open, in particular concerning the existence, concentration, and structure of ion clusters in 1∶1 strong electrolyte aqueous solutions. Whether strong 1∶1 electrolytes (especially salts of Na þ and K þ ) form ion pairs or clusters in water has been considered a key issue for understanding many important problems, e.g., the excess ionic activity in 1∶1 electrolytes (12), ion dependent conformational and binding equilibria of nucleic acids (13), the concentration difference between Na þ and K þ in living cells, protein denaturation by salts (14, 15), and ion concentration dependent properties of ion channels (16).The properties of aqueous solutions of 1∶1 strong electrolytes deviate from the ideal dilute solution at extremely low concentrations (<10 −5 M). The deviations were generally believed to be caused by the attraction between ions of opposite charge and the repulsion of ions of the same charge, leading to the development of the Debye-Hückel theory (17, 18). However, this theory begins to fail at a very low concentration (∼10 −3 M), as the assumptions upon which the theory was based become invalid. The formation of ion pairs containing two ions of opposite charge has been proposed to be primarily responsible for this failure (1, 2). Recently, calculations from molecular dynamics (MD) simulations, suggested that, clusters with more than one ion of the same charge which are traditionally viewed as unlikely, could be a major factor contributing to the nonideality of solutions at medium or high concentrations (12,19). However, these predicted ion clusters cannot be investigated by the usual tools for probing molecular structures and particle sizes in liquids, e.g., X-ray or neutron diffraction (20), or the dynamic light scattering (19,21), because the contribution of ion-ion correlations to the total scattering pattern is too small compared to the contributions ...

Cation Effects on Rotational Dynamics of Anions and Water Molecules in Alkali (Li+, Na+, K+, Cs+) Thiocyanate (SCN–) Aqueous Solutions

“…24 In other words, the dynamic segregation is the natural result of ion clustering which was experimentally observed with the vibrational energy exchange method. 24,30,33 By applying the vibrational energy exchange method, we also found that in other MSCN aqueous solutions substantial amounts of ions form ion clusters. It is conceivable that the dynamic segregation observed in other MSCN solutions can also be explained in terms of ion clustering in a way similar to that for the KSCN solutions.…”

“…As we have measured, in MSCN (M = Li, Na, K, Cs, NH 4 ) aqueous solutions of high concentrations, significant amounts of ion pairs and clusters form. 30,33 At higher concentrations, the sizes and concentrations of ion clusters become larger. 30,33 Therefore, the local environments of SCN − anions in the aqueous solutions at higher concentrations should be closer to those of the molten salts.…”

“…30,33 At higher concentrations, the sizes and concentrations of ion clusters become larger. 30,33 Therefore, the local environments of SCN − anions in the aqueous solutions at higher concentrations should be closer to those of the molten salts. Accordingly, the CN stretch frequencies of SCN − in solutions of higher concentrations should become more similar to those in the molten salts.…”

“…20,27 The effects of ion/ion interactions were almost completely ignored, though in the studies the ion concentrations were typically high (>2 M) in the solutions where substantial amounts of ion pairing and clustering are anticipated from the predictions of MD simulations. 28,29 Recently, by applying the ultrafast vibrational energy exchange method, 30,31 we found that ions form significant amounts of clusters in MSCN (M = K, NH 4 , Na, Cs, and Li) aqueous solutions (≥1 M). 32,33 Combining vibrational energy exchange measurements with ultrafast anisotropy decay measurements, we also found that in the KSCN aqueous solutions the structural inhomogeneity because of ion clustering leads to distinct rotational dynamics of water molecules and SCN − anions.…”

“…4 Å is the shortest anion distance in KSCN crystal determined with XRD measurements 37 and also the average anion distance in the saturated KSCN aqueous solutions estimated from the vibrational energy transfer methods described in our previous publications. 30,31,33 Ion clusters with different cations have slightly different critical distances as discussed in the following energy-transfer part. The definition of ion clusters does not exclude the situations that some water molecules or cations are buried inside the clusters.…”