From solvated ions to ion-pairing: a THz study of lanthanum(iii) hydration

Abstract

Ion radius and charge density are important parameters that determine the solvation behavior in aqueous electrolyte solutions. Here, we report on high precision THz absorption measurements of solvated LaCl3 and LaBr3 using narrow-band (75-90 cm(-1)) p-Ge laser and wideband (30-350 cm(-1)) Fourier transform spectroscopy. The concentration dependent absorption up to 3.3 M shows a prominent nonlinearity indicating ion pair formation with increasing electrolyte concentration. A more detailed analysis in terms of a chemical equilibrium model allowed us to separate the ion and ion pair contributions from bulk and solvation water. Thus we were able to characterize anion and cation solvation independently. The center frequencies of the Cl(-) and Br(-) rattling modes are in agreement with those found in aqueous alkali and earth alkali halide solutions. The coupling between anion and cation hydration is found to be small. Based upon our detailed analysis we propose increasing formation of solvent shared ion pairs with increasing solute concentration. The well defined ion resonances imply that in spite of its high charge density La(3+) acts locally on the water structure. Terahertz absorption spectroscopy is found here to be an experimental tool which allows us to directly observe solute hydration shells as well as ion pair formation.

A THz/FTIR fingerprint of the solvated proton: evidence for Eigen structure and Zundel dynamics

“…For HCl, the resonance around 180 cm À1 is attributed to a Cl À rattling mode which has been observed before in measurements of Cl À containing salts. [46][47][48][49] Furthermore we observe resonances for both, HCl and HBr, at E130 cm À1 and 340 cm À1 .…”

“…a eff ion contains contributions from the ions and ion-associates as well as any change in the water absorption induced by the solvated ions compared to bulk water. 48 For an ideal bi-component (cation(aq) + anion(aq)) electrolyte solution with negligible ion association a ion eff is strictly proportional to the salt concentration. Any nonlinear changes will therefore become apparent when calculating the molar effective ionic extinction e eff ion ¼…”

Water Dynamics from THz Spectroscopy Reveal the Locus of a Liquid–Liquid Binodal Limit in Aqueous CaCO3 Solutions

“…All absorption changes are typical of the formation of neutral species rather than highly charged ionic complexes, which would give rise to more dramatic differences in the absorption, Da. [20][21][22][23] Importantly, the turnover in THz absorption is not due to the induction times associated with the formation of solid CaCO 3 . Experiments in which the addition of CaCl 2 was stopped at defined IAPs show that the induction time after the addition of 0.8 mL CaCl 2 (10 mm, minimum in Figure 2 B) still exceeds 60 min; however, solid CaCO 3 has likely nucleated during the THz analysis from the sample drawn from the titration assay after addition of 1.0 mL CaCl 2 solution (Figure S5 A).…”

“…[19] THz spectroscopy has been shown to be a powerful tool to assess ion hydration and ion pair formation in aqueous environments. [20][21][22][23] Aside from THz spectroscopic investigations of the nucleation stages of additive-free aqueous CaCO 3 systems, the effect of poly(aspartic acid) (PAsp) and poly(acrylic acid) (PAA) was explored, which are known to stabilize PILPs. [24,6] Combined with a quantitative titration assay, [25] this approach reveals the impact of water dynamics on the early stages of CaCO 3 precipitation, that is, the binodal limit according to the PNC pathway, [9] and the influence of polycarboxylates.…”

2H and 139La NMR Spectroscopy in Aqueous Solutions at Geochemical Pressures

“…At the time when the Lee et al paper [5] was published (1974), there was no firm evidence for complexation of La 3+ and Cl À although the authors clearly knew of this possibility.T here have since been careful NMR, [10e,16] X-ray, [17] vibrational, [18] and other spectroscopic [19] studies of the reactions given in Equations (3) and (4 and these are supported by theoretical studies.…”