IEP as a parameter characterizing the pH-dependent surface charging of materials other than metal oxides Marek Kosmulski ⁎ Department of Electrochemistry, Lublin University of Technology, Nadbystrzycka 38, PL-20618 Lublin, Poland Max-Planck-Institut für Kolloid-und Grenzflächenforschung, Wissenschaftspark Potsdam-Golm, Am Mühlenberg 1, OT Golm, 14476 Potsdam, Germany abstract article info Available online 27 January 2012 Keywords: Point of zero charge Isoelectric point Zeta potential Electrokinetic potential Mass titration The numerical values of points of zero charge (PZC, obtained by potentiometric titration) and of isoelectric points (IEP) of various materials reported in the literature have been analyzed. In sets of results reported for the same chemical compound (corresponding to certain chemical formula and crystallographic structure), the IEP are relatively consistent. In contrast, in materials other than metal oxides, the sets of PZC are incon- sistent. In view of the inconsistence in the sets of PZC and of the discrepancies between PZC and IEP reported for the same material, it seems that IEP is more suitable than PZC as the unique number characterizing the pH-dependent surface charging of materials other than metal oxides. The present approach is opposite to the usual approach, in which the PZC and IEP are considered as two equally important parameters character- izing the pH-dependent surface charging of materials other than metal oxides. © 2012 Elsevier B.V. All rights reserved. Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 1.1. The surface-charging of metal oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 1.2. Application of Parks' approach to materials other than metal oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 2. Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 2.1. Electrolyte . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 2.2. Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 2.3. Results intentionally ignored . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 3. Titration and CIP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 4. Case study: montmorillonite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 5. “Recommended” IEP of specific materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 5.1. Sparingly soluble (hydr)oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 5.2. Clay minerals, aluminosilicates, and phyllosilicates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 5.3. Mixed oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 5.4. Salts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 5.5. Glasses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 5.6. Carbon and carbon-rich materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 5.7. Other well-defined inorganic materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 5.8. Core–shell materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 5.9. Low-molecular organic compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 5.10. Polymers, macroscopic specimens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 5.11. Latexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 5.12. Natural high-molecular-mass organic compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 5.13. Microorganisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 5.14. Other materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 Advances in Colloid and Interface Science 171–172 (2012) 77–86 ⁎ Department of Electrochemistry, Lublin University of Technology, Nadbystrzycka 38, PL-20618 Lublin, Poland. E-mail address: mkosmuls@hektor.umcs.lublin.pl. 0001-8686/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cis.2012.01.005 Contents lists available at SciVerse ScienceDirect Advances in Colloid and Interface Science journal homepage: www.elsevier.com/locate/cis