Computing excess functions of ionic solutions: the smaller-ion shell model versus the primitive model. 2. Ion-size
1Eltron Research & Development Inc., 4600 Nautilus Court South, Boulder, Colorado 80301-3241, United States.
Journal of Chemical Theory and Computation
|November 18, 2015
Summary
This study compares two models for ionic solutions. The Smaller-ion Shell (SiS) model provides more reliable ion-size parameters than the primitive model (PM), aligning better with experimental data.
Area of Science:
- Physical Chemistry
- Computational Chemistry
- Ionic Solutions
Background:
- A recent Monte Carlo simulation study of the primitive model (PM) proposed "recommended ionic radii" for ions in aqueous solutions.
- The PM model yielded larger radii for smaller cations, suggesting hydration, which contrasted with crystallographic radii.
Purpose of the Study:
- To compare the ion-size parameters (ISPs) derived from the unrestricted primitive model (MC-UPM) and the Smaller-ion Shell (SiS) treatment.
- To evaluate the reliability of ISPs from both models as physicochemical entities.
Main Methods:
- Comparison of model-experiment fits for mean ionic activity coefficients as a function of ionic concentration.
- Analysis of ion-size parameters (ISPs) obtained from "best fit" procedures in both MC-UPM and DH-SiS models.
Main Results:
- ISPs derived from the DH-SiS treatment closely match crystallographic or thermochemical ionic diameters for both cations and anions.
- DH-SiS derived ISPs are independent of the counterion, unlike those from the PM model.
- The PM model's "recommended ionic radii" were significantly larger than crystallographic values for smaller cations.
Conclusions:
- The DH-SiS treatment offers more reliable ion-size parameters compared to the PM model.
- DH-SiS derived ISPs are more consistent with established physical dimensions of ions.
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