Quantitative Characterization of Ion Pairing and Cluster Formation in Strong 1:1 Electrolytes

Abstract

Aqueous solutions of 1:1 strong electrolytes are considered to be the prototype for complete ionic dissociation. Nonetheless, clustering of strong 1:1 electrolytes has been widely reported in all atom molecular dynamics simulations, and their presence is indirectly implicated in a diverse range of experimental results. Is there a physical basis for nonidealities such as ion pairing and cluster formation in aqueous solutions of strong 1:1 electrolytes? We attempt to answer this question by direct comparison of results from detailed molecular dynamics simulations to experimentally observed properties of 1:1 electrolytes. We report the analysis of a series of lengthy molecular dynamics simulations of alkali-halide solutions carried out over a wide range of physiologically relevant concentrations using explicit representations of water molecules. We find evidence for pronounced nonideal behavior of ions at all concentrations in the form of ion pairs and clusters which are in rapid equilibrium with dissociated ions. The phenomenology for ion pairing seen in these simulations is congruent with the multistep scheme proposed by Eigen and Tamm based on data from ultrasonic absorption experiments. For a given electrolyte, we show that the dependence of cluster populations on concentration can be described through a single set of equilibrium constants. We assess the accuracy of calculated ion pairing constants by favorable comparison to estimates obtained by Fuoss and co-workers and based on conductometric experiments. Ion pairs and clusters form on length scales where the size of individual water molecules is as important as the hard core radius of ions. Ion pairing results as a balance between the favorable Coulomb interactions and the unfavorable partial desolvation of ions needed to form a pair.

Cation Effects on Rotational Dynamics of Anions and Water Molecules in Alkali (Li+, Na+, K+, Cs+) Thiocyanate (SCN–) Aqueous Solutions

“…20,27 The effects of ion/ion interactions were almost completely ignored, though in the studies the ion concentrations were typically high (>2 M) in the solutions where substantial amounts of ion pairing and clustering are anticipated from the predictions of MD simulations. 28,29 Recently, by applying the ultrafast vibrational energy exchange method, 30,31 we found that ions form significant amounts of clusters in MSCN (M = K, NH 4 , Na, Cs, and Li) aqueous solutions (≥1 M). 32,33 Combining vibrational energy exchange measurements with ultrafast anisotropy decay measurements, we also found that in the KSCN aqueous solutions the structural inhomogeneity because of ion clustering leads to distinct rotational dynamics of water molecules and SCN − anions.…”

Spontaneous Formation of KCl Aggregates in Biomolecular Simulations: A Force Field Issue?

“…A large ensemble of MD studies of aqueous ionic solutions using various parameter sets and particle mesh Ewald (PME) summation methods for the treatment of long-range electrostatic interactions have been published, including simulations of LiF, 27 LiCl, [33][34][35] NaCl, 7,[36][37][38][39][40][41][42][43][44][45] KCl, 15,40,45,46 RbCl, 35,45 CsCl, 45 NaBr, KBr, RbBr, CsBr, 45 and CsI. 35 Polarizable force-fields have also been used in other force-fields.…”

“…As reported here, no ion aggregation has been reported in simulations using the Dang parameters even under high and supersaturated salt conditions. 38,42,43 AMBER parameters are rarely used in simulations of ionic aqueous solutions, 7,45 while they are recurrently used in MD simulations of biomolecular systems. [9][10][11][12][13] In one study, a comparison of calculated properties of a ∼1.0 M NaCl aqueous solution generated by using six different parameter sets revealed some level of aggregation for various force-fields including AMBER and GROMOS, while as expected, the Dang parameters did not lead to any detectable formation of ion clusters during 2 ns MD simulations.…”

“…[9][10][11][12][13] In one study, a comparison of calculated properties of a ∼1.0 M NaCl aqueous solution generated by using six different parameter sets revealed some level of aggregation for various force-fields including AMBER and GROMOS, while as expected, the Dang parameters did not lead to any detectable formation of ion clusters during 2 ns MD simulations. 7 In another study, 45 the GROMACS program 51 was used to simulate NaCl to CsCl and NaBr to CsBr aqueous salts at various concentrations ranging from 0.10 to 1.0 M. The Åqvist parameters were used for the Na + , K + , Rb + , and Cs + cations and different parameters were used for the Cl -52 and Br -53 anions. The authors reported the formation of ion clusters for all salts at 1.0 M, but not at 0.10 M, in agreement with our own data.…”

“…But we suspect that these parameters suffer from the same flaws because they have been parametrized in a similar manner. 45 A reevaluation of the performances of all available parameters in the context of three component electrolyte solutions should be undertaken with a special emphasis on this aggregation issue in the framework of a recent proposal devoted to create a set of descriptive parameters and measures allowing us to judge the "quality" and reliability of MD simulations. 64 In conclusion, the combination of more precise experimental and theoretical studies will lead to a generation of force-fields free from such imbalanced interatomic potential terms.…”