Hofmeister effects at low salt concentration due to surface charge transfer |
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| Institution: | 1. School of Engineering and Information Technology, Murdoch University, 90 South St, Murdoch, WA 6150, Australia;2. Department of Chemical and Geological Sciences, University of Cagliari-CSGI and CNBS Cittadella Universitaria, S.S. 554 Bivio Sestu, 09042 Monserrato (CA), Italy;1. Research Institute of Applied Earth Sciences, University of Miskolc, Miskolc, Egyetemváros, H-3515, Hungary;2. F.D. Ovcharenko Institute of Biocolloidal Chemistry, NAS of Ukraine, 42, Vernadsky av., Kyiv, Ukraine;3. MTA-ME Materials Science Research Group, University of Miskolc, Hungary;4. The Transcarpathian II. Ferenc Rakoczi Hungarian Institute, 6, Kossuth sq., Beregovo 90200, Ukraine;1. Biocolloid and Fluid Physics Group, Department of Applied Physics, University of Granada, Av. Fuentenueva S/N, 18071 Granada, Spain;2. CNRS, Centre de Recherche Paul Pascal (CRPP), UPR 8641, F3300 Pessac, France, Université de Bordeaux, CRPP, UPR 8641, F-33600 Pessac, France;3. ICMAB-CSIC, Campus de la UAB, E-08193 Bellaterra, Barcelona, Spain;1. Faculty of Health Sciences, University of Ljubljana, Ljubljana SI-1000, Slovenia;2. Department of Physics, North Dakota State University, Fargo, ND 58108-6050, USA |
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| Abstract: | We present a theoretical comparison of the surface forces between two graphite-like surfaces at salt concentrations below 10 mM with surfaces charged by various mechanisms. Surface forces include a surface charging or chemisorption contribution to the total free energy. Surfaces are charged by charge regulation (H+ binding), site competition (H+ and cation binding) and redox charging with electrodes coupled to a countercell. Constant surface charge is also considered. Surface parameters are calibrated to give the same potential when isolated. Nonelectrostatic physisorption energies of the potential determining ions provide a specific and significant contribution to the charging energy. Consequently ion specificity is found in the surface forces at concentrations of 1–10 mM, which is not observed under constant charge conditions. The force between redox electrodes continues to show Hofmeister effects at 0.01 mM. We refer to this low concentration Hofmeister effect as “Hofmeister charging”, and suggest that the more common high concentration ion specific effects may be known as “Hofmeister screening”. Hofmeister series are considered over LiCl, NaCl, KCl and NaNO3, NaClO4, NaSCN with the cations (or H+) being the potential determining ions. A K+ anomaly is attributed to the small size of the weakly hydrated chaotropic K+ ion, with Li+ and Na+ explicitly modelled as strongly hydrated cosmotropes. |
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