Comparison of isoelectric points of single-crystal and polycrystalline α-Al2O3 and α-Fe2O3 surfaces

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The surface charging behavior as a function of pH and isoelectric points (IEPs) of single-crystal α-Al2O3 (0001) and (1102) and α-Fe2O3 (0001) was determined by streaming potential measurements using an electrokinetic analyzer. The IEPs of α-Al2O3 (0001) and (11-02)$(1\overline1 02)$ and α-Fe2O3 (0001) were found to be 4.5, 5.1, and 6.5, respectively. These IEP values for oriented single crystals of α-Al2O3 are in good agreement with literature values, whereas the new IEP value for α-Fe2O3 (0001) is significantly lower than four reported values (IEP = 8-8.5) for single-crystal α-Fe2O3 (0001) (Eggleston and Jordan 1998; Zarzycki et al. 2011; Chatman et al. 2013; Lützenkirchen et al. 2013) and significantly higher than one (IEP = 4) recently measured by Lützenkirchen et al. (2015) on a fresh α-Fe2O3 (0001) surface. Most of the single-crystal IEP values measured recently are lower than IEP values reported for polycrystalline α-Al2O3 and α-Fe2O3, which are generally in the pH range of 8 to 10. Calculations of the IEP values based on estimated Ka values of α-Fe2O3 and α-Al2O3 surfaces in contact with water as a function of defect type and concentration suggest that highly reactive surface defect sites (primarily singly coordinated aquo groups) on the Fe-and Al-oxide powders are possibly a major source of the surface charge differences between polycrystalline samples and their oriented single-crystal counterparts studied here. The results of this study provide a better understanding of the surface charging behavior of Fe and Al-oxides, which is essential for predicting complex processes such as metal-ion sorption occurring at mineral/water interfaces.


External organisations
  • Stanford University
Research areas and keywords

Subject classification (UKÄ) – MANDATORY

  • Geochemistry


  • defects, Fe-and Al-oxides, Isoelectric point (IEP), pH point of zero charge (pH), polycrystalline, single crystal, surface
Original languageEnglish
Pages (from-to)2248-2259
Number of pages12
JournalAmerican Mineralogist
Issue number10
Publication statusPublished - 2016 Oct 1
Publication categoryResearch