Ion Hydration and Association in Aqueous Potassium Phosphate Solutions

Abstract

Ionic hydration and ion association in aqueous solutions of KH2PO4, K2HPO4, and K3PO4 at 25 °C up to high concentrations have been investigated using dielectric relaxation spectroscopy (DRS). The three phosphate anions were found to be extensively hydrated, with total hydration numbers at infinite dilution of ~11 (for H2PO4(-)), ~20 (HPO4(2-)), and ~39 (PO4(3-)). These values are indicative of the existence of a second hydration shell around HPO4(2-) and especially PO4(3-). Two types of hydrating water molecules could be quantified: irrotationally bound (ib, H2O molecules essentially "frozen" on the DRS time scale) and "slow" (loosely bound water molecules with identifiably slower dynamics than bulk water). For H2PO4(-) over the entire concentration range and for HPO4(2-) and PO4(3-) at concentrations c ≲ 1 mol L(-1), only "slow" H2O was detected; however, at higher concentrations of the latter two anions, an increasing fraction of ib water appears, making up ~50% of the total hydration number close to the saturation limit of K2HPO4. Contrary to common belief, all three salts showed significant ion pair formation, with standard association constants of the 1:1 species increasing in the order: KH2PO4(0)(aq) < KHPO4(-)(aq) < KPO4(2-)(aq). The main type of ion pair in solution shifted from solvent-shared ion pairs (SIPs) to double-solvent-separated ion pairs (2SIPs) in the same sequence.

Microglobule formation and a microscopic order parameter monitoring the phase transition of aqueous poly( N -isopropylacrylamide) solution

“…To obtain deeper insights into the hydration state of the pNIPAm chains, we evaluate effective hydration numbers per monomer unit using the generalized Cavell equation [43][44][45],…”

Effect of Phosphate Salt Concentration and Solution pH on the Aqueous‐Phase Homo and Copolymerization of N‐Vinyl Pyrrolidone

“…Water molecules tend to aggregate around salts in aqueous solution to hydrate them. In fact, phosphates are known to produce strong structure‐forming ions in water, having a coordination number reported as 39 . This could lead to a decrease in the number of water molecules available for interaction with NVP molecules resulting in an increase of unfavorable monomer/monomer interactions and a further decrease in the propagation rate constant and in the rate of polymerization.…”

Nanometer patterning of water by tetraanionic ferrocyanide stabilized in aqueous nanodrops

“…For example, recent dielectric relaxation studies on hydrated phosphates indicate interactions extend into a full second solvation shell. 33 Although previous neutron diffraction studies have revealed extended ion–water interactions, 36 experiments with hydrated phosphate salts show no evidence for ionic influences beyond direct coordination to the ion. 30 The dynamic nature of water molecules makes probing weak ion-induced structural effects beyond a rigid inner shell a challenging endeavor.…”

“…The distance to which ion-induced solvent orientation can extend has been studied in solution using multiple techniques, including diffraction, 29,30 spectroscopy 31–33 and theory. 34,35 Although ion-induced structuring of water molecules within the coordination shell is unambiguous, there have been conflicting interpretations amongst these results as to whether ion–water interactions can extend beyond the first solvation shell.…”