Fluorescence of KCl Aqueous Solution: A Possible Spectroscopic Signature of Nucleation

Abstract

Ion pairing in water solutions alters both the water hydrogen-bond network and ion solvation, modifying the dynamics and properties of electrolyte water solutions. Here, we report an anomalous intrinsic fluorescence of KCl aqueous solution at room temperature and show that its intensity increases with the salt concentration. From the ab initio density functional theory (DFT) and time-dependent DFT modeling, we propose that the fluorescence emission could originate from the stiffening of the hydrogen bond network in the hydration shell of solvated ion-pairs that suppresses the fast nonradiative decay and allows the slower radiative channel to become a possible decay pathway. Because computations suggest that the fluorophores are the local ion-water structures present in the prenucleation phase, this band could be the signature of the incoming salt precipitation.

Figure 1

(A) Excitation and emission spectra of 4 M KCl in water. Excitation spectrum with emission centered at 310 nm (blue line) and at 430 nm (red line); emission spectrum with excitation centered at 227 nm (dotted blue line) and at 240 nm (dotted red line). (B) Fluorescence emission spectra of 4 M KCl aqueous solutions at different temperatures, from 25 to 95 °C; excitation at 227 nm. (C,D) Fluorescence emission spectra of KCl aqueous solutions at different concentrations from 1.28 mM to 4 M; (C) excitation at 227 nm, (D) excitation at 240 nm. Conditions: spectrofluorometer, Varian Cary Eclipse; photomultiplier gain, 900 V; excitation slit, 5 nm; emission slit, 5 nm.

Figure 2

Molecular orbital energy diagram for (KCl) (H₂O)_n clusters with the decrease in KCl/water ratio (n = 1, 4, 8, 16, and 32). The HOMO/LUMO gap is reported in eV, and the horizontal bars represent the values of the calculated MO energy eigenvalues. Different bar colors have been used to represent the atomic localization of HOMO and LUMO as a function of KCl/water ratio. Green: HOMO localized on Cl^– 3p; blue: HOMO localized on O^– 2p; and red: LUMO localized on K⁺ 3s. The structures reported at the top of the panel are the n = 1 and n = 32 minimum forms identified in this investigation. As can be seen in the figure, for low n, corresponding to concentrated solutions, HOMO is localized on the Cl^– 3p anion (green bars). Going from lower to higher n, that is, diluting the solution, the energy of the MO localized on Cl^– decreases and its stabilization increases. When n = 32, the HOMO corresponds to the oxygen 2p MO (blue bar). In contrast, the LUMO is localized on potassium 3s (red bars) for all the values of n.

Figure 3

KCl(H₂O)₅₄ and (KCl)₂(H₂O)₁₀₆ buried ion models. (A–C) KCl(H₂O)₅₄ models simulating CIP, SSIP, and SIP conformations. (D) (KCl)₂(H₂O)₁₀₆ model that mimics a prenucleation cluster structure. Ball atoms are ions. Thicker stick models are used to put in evidence water molecules that are involved in the ion-pair interactions. Distance in Å.

Figure 4

Proposed model for S₁ ion dynamics of KCl solution at low concentration (left) and high concentration (right). At a low concentration (left), where separated hydrated free ions are the most populated ion structural arrangements, S₁ excitation dynamics is not hindered by electrostatic interaction and allows S₁ nonradiative decay; at a higher concentration (right), solvent shared ions or CIP structures are more frequent. In this case, S₁ excitation dynamics is strongly hindered by ion pairing electrostatic interaction and allows emissive S₁ decay.