Electroneutrality breakdown and specific ion effects in nanoconfined aqueous electrolytes observed by NMR

ARTICLE Received 17 Sep 2014 | Accepted 23 Jan 2015 | Published 20 Feb 2015 DOI: 10.1038/ncomms7358 Electroneutrality breakdown and specific ion effects in nanoconfined aqueous electrolytes observed by NMR Zhi-Xiang Luo1, Yun-Zhao Xing2, Yan-Chun Ling2, Alfred Kleinhammes1 & Yue Wu1,2 Ion distribution in aqueous electrolytes near the interface plays a critical role in electrochemical, biological and colloidal systems, and is expected to be particularly significant inside nanoconfined regions. Electroneutrality of the total charge inside nanoconfined regions is commonly assumed a priori in solving ion distribution of aqueous electrolytes nanoconfined by uncharged hydrophobic surfaces with no direct experimental validation. Here, we use a quantitative nuclear magnetic resonance approach to investigate the properties of aqueous electrolytes nanoconfined in graphitic-like nanoporous carbon. Substantial electroneutrality breakdown in nanoconfined regions and very asymmetric responses of cations and anions to the charging of nanoconfining surfaces are observed. The electroneutrality breakdown is shown to depend strongly on the propensity of anions towards the water-carbon interface and such ion-specific response follows, generally, the anion ranking of the Hofmeister series. The experimental observations are further supported by numerical evaluation using the generalized Poisson–Boltzmann equation. 1 Department of Physics and Astronomy, University of North Carolina, Chapel Hill, North Carolina 27599-3255, USA. 2 Department of Applied Physical Sciences, University of North Carolina, Chapel Hill, North Carolina 27599-3216, USA. Correspondence and requests for materials should be addressed to Y.W. (email: ). NATURE COMMUNICATIONS | 6:6358 | DOI: 10.1038/ncomms7358 | www.nature.com/naturecommunications & 2015 Macmillan Publishers Limited. All rights reserved. 1 ARTICLE E NATURE COMMUNICATIONS | DOI: 10.1038/ncomms7358 lectric double layer near the interface is of fundamental importance in various applications ranging from electrochemistry1 and electrophoresis2, to colloidal particles assembly3 and nanofluidics4. The neutrality of the total charge is an important condition in deriving the ion distribution near the interface in the electric double layer theory. For an uncharged hydrophobic surface such as the water/air interface, positive and negative ions can still be separated in the interfacial region (B10 Å) due to different propensities towards the interface between the cations and anions5–8; such effect is called specific ion effect9–13 since it is driven by non-electrostatic interactions that vary significantly between different ions even for ions with the same electrovalency (for example, F  and I  ). In the scenario of aqueous electrolytes confined by hydrophobic surfaces where the pore size is comparable in size to the interfacial region determined by the specific ion effect, a natural question raised is how the tendency of charge separation near the interface reconciles with electroneutrality inside nanoconfined regions. Could electroneutrality of the total charge in fact be violated substantially inside nanoconfined regions driven by the specific ion effect? Theoretical studies nearly always take the total charge neutrality inside nanoconfined regions for granted and experimental evaluation of electroneutrality inside nanoconfined regions is lacking. Such evaluation could contribute significantly to our understanding of some very important processes such as energy storage in supercapacitor14, ion transport through nanochannels15 and ionic processes in proteins9. Nanoporous carbon with graphitic-like internal surfaces provides an ideal model system for investigating the electroneutrality in nanoconfined aqueous electrolytes using nuclear magnetic resonance (NMR). Previous studies showed that fluid inside carbon nanopores exhibits a different NMR chemical shift from that outside the nanopores due to the ring current effect, which gives rise to a nucleus-independent chemical shift16–20. This shift provides a clear NMR marker for selectively and quantitatively monitoring the electrolyte inside nanometer-sized regions confined by hydrophobic graphitic-like carbon surfaces. It provides an excellent tool for determining quantitatively the cation and anion concentrations inside nanopores. Ions, especially anions, can be ordered by their influence on a vast variety of specific ion effects, called the Hofmeister series9,12,21. A typical ranking is SO24  oF  oCl  oBr  oNO3 oI  oBF4 oClO4 for some anions with increasing protein solubility in aqueous electrolytes to the right side (often referred to as the chaotropic side)12. Evaluating the electroneutrality with systematic change of anions according to the Hofmeister series provides another avenue for revealing the potential electroneutrality breakdown caused by the ion-specific interfacial effect. Here we report such a quantitative NMR study of the ion concentrations in nanoconfined aqueous electrolytes. Hydrophobic graphitic-like porous carbon is used as a model system to provide the nanoconfinement. Direct experimental evidence is observed for a significant electroneutrality breakdown of the total charge inside nanometer-sized regions even when the carbon material is uncharged. Interfacial specific ion effects and ion–ion correlations are shown to play crucial roles in determining the degree of electroneutrality breakdown inside nanopores. The importance of the specific ion interfacial effect is further revealed by the asymmetric and nonlinear responses of cation and anion concentrations to the external charging of the nanoconfining carbon walls. Such information is obtained using a charge-controlling device built into the NMR probe. The experimental results are further validated by a numerical calculation using the generalized Poisson–Boltzmann equation in nanopores, demonstrating that specific ion interfacial effect can indeed dominate the electrostatic 2 interactions leading to the breakdown of electroneutrality inside nanoconfined regions. Results Electroneutrality breakdown in nanoconfinement. A highquality nanoporous carbon derived from polymer polyetheretherketone (PEEK)22,23 is used to provide the hydrophobic nanoconfinement in this work (see Methods). The activated carbon sample is designated as P-40 and the average pore size is 0.9 nm from wall surface to wall surface assuming a slit-shaped pore (1.2 nm from carbon centre to carbon centre) according to the previous study17. Unless specified, all results discussed here refer to that obtained using P-40. However, activated carbon with pore size of 1.9 nm (carbon to carbon centres), labelled P-92, was also used in the current study and will be mentioned as well. The capability of NMR approach to selectively and quantitatively study nanoconfined fluids is demonstrated in Fig. 1a where the 1H, 19F and 23Na static NMR spectra of NaBF electrolyte 4 injected into P-40 are shown (see Methods) (...truncated)