Editorial pubs.acs.org/JPCB
Ion Pairing: From Water Clusters to the Aqueous Bulk
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Methodologically, considerable progress in establishing quantitatively the binding strength and geometry of individual ion pairs has been achieved by refining computational methods with the assistance of structural and spectroscopic experimental techniques. Molecular dynamics simulations with simple force fields tend to overestimate ion pairing, and the results are very sensitive to the interaction parameters. The situation can be improved significantly by careful force field parametrization, e.g., effectively including electronic polarization effects, or (at significantly larger computational costs) by switching to ab initio molecular dynamics. The present Virtual Issue (http://pubs.acs.org/page/vi/ion_ pairing.html), which is a compilation of recent articles from JPCA, JPCB, and JPCL, thus exemplifies current progress in experimental and computational approaches to ion pairing in cluster and bulk aqueous systems with important practical implications, in particular, for biological problems.7−29
he concept of ion pairing goes back to Bjerrum who suggested in the 1920s that oppositely charged ions in solution should be treated either as associated or free depending on whether their mutual distance is smaller or larger than a given value.1 Despite the fact that the later developed Pitzer theory was able to provide activity coefficients without explicitly invoking ion pairing,2 this concept has strong support both in experiments and simulations.3−5 As a matter of fact, modern molecular tools allow one to fine-grain the modeling of multistage ion pairing ranging from contact ion pairs to solvent shared and solvent separated pairs (see Figure 1), as originally suggested by Eigen.6
Pavel Jungwirth, Senior Editor, J. Phys. Chem. A/B/C
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Academy of Sciences of the Czech Republic
AUTHOR INFORMATION
Notes
Views expressed in this editorial are those of the author and not necessarily the views of the ACS.
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Figure 1. Free energy profile of association of Li+ and F− ions in water from ab initio molecular dynamics. Representative structures of the contact and solvent shared ion pairs as well as the transition state separating them are depicted in ball-and-stick representation. The figure is based on calculations from ref 28.
(1) Bjerrum, N. Untersuchem uber Ionenassoziation I. K. Dan. Vidensk. Selsk. 1926, 7, 1−48. (2) Pitzer, K. S.; Mayorga, G. Thermodynamics of Electrolytes. 2. Activity and Osmotic Coefficients for Strong Electrolytes with One or Both Ions Univalent. J. Phys. Chem. 1973, 77, 2300−2308. (3) Marcus, Y.; Hefter, G. Ion Pairing. Chem. Rev. 2006, 106, 4585− 4621. (4) Buchner, R.; Chen, T.; Hefter, G. Complexity in “Simple” Electrolyte Solutions: Ion Pairing in MgSO4(aq). J. Phys. Chem. B 2004, 108, 2365−2375. (5) Fennell, C. J.; Bizjak, A.; Vlachy, V.; Dill, K. A. Ion Pairing in Molecular Simulations of Aqueous Alkali Halide Solutions. J. Phys. Chem. B 2009, 113, 6782−6791. (6) Eigen, M.; Tamm, K. Schallabsorption in Elektrolytlosungen als Folge Chemischer Relaxation. 1. Relaxationstheorie der Mehstufigen Dissoziation. Z. Elektrochem. 1962, 66, 93−107. (7) Jin, T.; Zhang, B.; Song, J.; Jiang, L.; Qiu, Y.; Zhuang, W. Infrared Signature of the Early Stage Microsolvation in the NaSO4−(H2O)1−5 Clusters: A Simulation Study. J. Phys. Chem. A 2014, DOI: 10.1021/ jp5028299. (8) Baer, M. D.; Fulton, J. L.; Balasubramanian, M.; Schenter, G. K.; Mundy, C. J. Persistent Ion Pairing in Aqueous Hydrochloric Acid. J. Phys. Chem. B 2014, 118, 7211−7220. (9) Aragones, J. L.; Rovere, M.; Vega, C.; Gallo, P. Computer Simulation Study of the Structure of LiCl Aqueous Solutions: Test of Non-Standard Mixing Rules in the Ion Interaction. J. Phys. Chem. B 2014, 118, 7680−7691.
Ion pairing is typically weak in dilute bulk solutions of monovalent salt ions due to the high value of the dielectric constant of water. Nevertheless, there are several ways to crank up the ion pairing effect and make it more pronounced. One way is to move to multivalent ions, which tend to pair more efficiently in water due to their higher charge densities. Another possibility is to lower the effective dielectric constant of the solvent. Within aqueous systems (nonaqueous solvents being beyond the scope of this Virtual Issue), this can be achieved by lowering the number of available solvent molecules upon moving from aqueous bulk to small cluster systems. The archetypal systems for ion pairing have been traditionally aqueous alkali halides. These simple salt solutions continue to keep researchers busy; nevertheless, more and more emphasis has been directed to nonspherical molecular ions and charged groups recently. On one side, these are solutions of molecular ions of more complex salts, bases, and acids. On the other side, considerable interest has been dedicated to elucidating the strengths of salt bridges in proteins and peptides, which are among the most important ion pairs in biological molecules, as well as to underlying mechanisms of the Hofmeister series of ions. © 2014 American Chemical Society
REFERENCES
Published: August 14, 2014 10333
dx.doi.org/10.1021/jp507964q | J. Phys. Chem. B 2014, 118, 10333−10334
The Journal of Physical Chemistry B
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Editorial
NOTE ADDED AFTER ASAP PUBLICATION This paper was published ASAP on August 14, 2014. A duplicate reference was removed, and the reference numbering was adjusted. The corrected version was reposted on August 20, 2014.
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