Water Interfaces Group – UIC Department of Physics

Ion Distributions at Water Interfaces

Ion distributions play a central role in various settings – from biology, where they mediate the electrostatic interactions between charged biomolecules in solution, to energy storage devices, where they influence the charging properties of supercapacitors. These distributions are determined by interactions dictated by the chemical properties of the ions and their environment as well as the long-range nature of the electrostatic force.

The distribution of ions at water interfaces underlies the equilibrium and dynamical properties of charged molecules and particles at interfaces, which are relevant to many scientific and industrial processes. These include electron and ion transfer at biomembranes and many practical applications in analytical chemistry, electrochemistry, and industrial processes. An example of an application of critical relevance to society is water treatment and purification because many natural and artificial contaminants found in water sources are charged.

Electrified Liquid-Liquid Interfaces

Interfaces between two immiscible electrolyte solutions are a convenient system to test fundamental ideas on the distribution of ions. This liquid-liquid interface is typically formed between an aqueous solution of hydrophilic ions and a solution of hydrophobic ions in a polar organic solvent. Electrodes inserted into the two bulk phases establish a tunable electric potential difference between them (as shown in Figure 1). Since the bulk phases are conductive most of the potential difference appears across the nanoscopically thin interfacial region. As a result, electric fields near the interface can be large, ~108 to 109 V m-1 for modest applied electric potential differences on the order of a few hundred millivolts. These fields have electrostatic energies as high as 15 to 20 kBT per monovalent ion at room T (or 38 to 50 kJ per mole), which can induce the movement of ions from the bulk phase to the interface, initiate redox reactions at the interface, distribute or direct the interfacial assembly of charged biomolecules or nanoparticles, and control interfacial transport. As a consequence, this model system can be used to investigate the distribution and role of ions in both high permittivity environments like water and in lower permittivity environments found in organic solvents. In addition to their use as model systems to study interfacial reaction kinetics and ion and electron transfer through interfaces, electrified liquid-liquid interfaces are used in many practical applications in analytical chemistry and electrochemistry.

ion distribution

Figure 1. Illustration of ions distributed at the liquid-liquid interface of two electrolyte solutions (represented as a colored continuum) under conditions for which the electric potential of the aqueous solution is positive with respect to that of the organic DCE (1,2-dichloroethane) solution. Alkali chloride hydrophilic ions (Na+ Cl-) are confined to the aqueous phase and the organic ions (BTPPA+ TPFB-) are confined to the organic phase.

Advances in Understanding Ion Distributions

The classic theories of ion distributions by Gouy, Chapman, Debye, and Hückel that were developed during the early twentieth century are still being used to analyze most experimental data, even though concerns were raised about the limitations of these theories soon after they were formulated. The early theories did not account for differences between ions of the same charge, say, Na+ and K+, did not include the effect of solvent structure, for example, the arrangement of water molecules, and did not account for correlations in the positions of solvent molecules and ions.

In this area, we have introduced new types of experiments, new methods of data analysis, and new connections between experimental results and modern theoretical ideas. Our most productive experiments in this area have been the result of combining the techniques of electrochemistry with new developments in the scattering of X-rays from the interface between two electrolyte solutions. Our theoretical developments have focused on using the potential of mean force.

In 2006, we showed how to use a single ion potential of mean force to calculate ion distributions (see lectures cited below for a tutorial on the subject of electrostatics at interfaces). This potential of mean force described the interaction of an ion with the liquid-liquid interfacial structure and was calculated either with molecular dynamics (MD) simulations [1] or by an analytic approximation [2]. By combining the MD simulations of the single ion potential of mean force (to describe the interfacial structure) with the effects of the mean electric field from all the other ions, we showed that our X-ray data could be explained without adjustable parameters [1]. Alternatively, we showed that we could extract the potential of mean force from the experimental data (see Figure 2) [2,3]. This allows a direct comparison to potentials of mean force determined by theoretical methods, which could be either MD simulations or other methods.


Figure 2. X-ray reflectivity data that determines ion potentials of mean force. (A) X-ray reflectivity normalized to the Fresnel reflectivity \(R(Q_z)/R_F(Q_z)\) for various electric potential differences \(\Delta \phi ^{w-o}\) as a function of wave vector transfer \(Q_z\) from liquid-liquid interfaces between a 10 mM LiCl aqueous solution and a 5 mM organic solution of BTPPATPFB (bis(triphenylphosphoranylidene)ammonium tetrakis(pentafluoro phenyl)borate) in DCE (1,2-dichloroethane). Curves are ordered from bottom to top according to increasing \(\Delta \phi ^{w-o}\), as marked, and are successively displaced upwards by +0.1 for clarity (note that \(R(Q_z)/R_F(Q_z) \rightarrow 1\) as \(Q_z \rightarrow 0\) for all measurements). Values at \(Q_z = 0 \) are measurements of the direct beam that passes, without reflection, through the aqueous phase just slightly above the interface. (B) Potentials of mean force for aqueous ions, as marked (the water phase is at z \(>\) 0 and the DCE phase is at z \(<\) 0) [3].

Spatial Correlations of Ions

Recent theoretical and computational studies by other scientists have explored the role of correlations between ions. This research has been motivated by observations of the reentrant condensation of DNA and proteins in solution, in which like-charged biomolecules aggregate in the presence of multivalent ions, and by reports of charge reversal in colloidal suspensions, in which the sign of the screened charge on a colloid can be changed by varying the surrounding electrolyte solution. Despite the broad interest in this subject, little is known about the role of ion–ion correlations in the structure of the electrical double layer from direct measurements.

In 2012, we provided a direct measurement of the molecular scale effect of ion correlations (Figure 3) by exploiting the properties of the electrified aqueous-organic interface to vary the coupling strength of ion–ion correlations from weak to strong while monitoring their influence on ion distributions at the nanometer scale with X-ray reflectivity [4]. Our data are in agreement with the predictions of a parameter-free density functional theory that includes ion–ion correlations and ion–solvent interactions over the entire range of experimentally tunable correlation coupling strengths. This study provided evidence for a sharply defined electrical double layer for large coupling strengths in contrast to the diffuse distributions predicted by mean field theory, thereby confirming a common prediction of many ion correlation models. Importantly, we also showed how to account for the interdependency between ion correlations and the interactions of ions with the molecular-scale structure of a soft interface.

reflectivity1 reflectivity2

Figure 3. (top) X-ray reflectivity data from the experimental system shown in Figure 1 is poorly described by the predictions of Poisson-Boltzmann (Gouy-Chapman) theory. (bottom) Red lines include the effect of ion-ion correlations and interfacial solvent structure, dashed lines include only the effect of the interfacial solvent structure. There are not any adjustable parameters in the theory [4].

For lectures on Electrostatics at Soft Interfaces presented at the 7th Summer School on Complex Fluids and Soft Solids held at the University of Massachusetts at Amherst during June 2015, see here.

For an introduction to X-ray scattering techniques used for studying liquid surfaces and interfaces, see this link.

For a more detailed description of these techniques, see the book described here.


[1] Ion Distributions Near a Liquid-Liquid Interface, Guangming Luo, Sarka Malkova, Jaesung Yoon, David G. Schultz, Binhua Lin, Mati Meron, Ilan Benjamin, Petr Vanysek, and Mark L. Schlossman, Science 311, 216-218 (2006).

[2] Ion Distributions at the Nitrobenzene-Water Interface Electrified by a Common Ion, Guangming Luo, Sarka Malkova, Jaesung Yoon, David G. Schultz, Binhua Lin, Mati Meron, Ilan Benjamin, Petr Vanysek, and Mark L. Schlossman, Journal of Electroanalytical Chemistry 593, 142-158 (2006), see also

[3] Ion Distributions at the Water/1,2-Dichloroethane Interface: Potential of Mean Force Approach to Analyzing X-Ray Reflectivity and Interfacial Tension Measurements, Binyang Hou, Nouamane Laanait, Hao Yu, Wei Bu, Jaesung Yoon, Binhua Lin, Mati Meron, Guangming Luo, Petr Vanysek, and Mark L. Schlossman, Journal of Physical Chemistry B, 117, 5365-5378 (2013).

[4] Tuning Ion Correlations at an Electrified Soft Interface, Nouamane Laanait, Miroslav Mihaylov, Binyang Hou, Hao Yu, Petr Vanysek, Mati Meron, Binhua Lin, Ilan Benjamin, and Mark L. Schlossman, Proceedings of the National Academy of Sciences (USA), 109, 20326-20331 (2012).