Ionic Strength Effects on Silicic Acid (H4SiO4) Sorption and Oligomerization on an Iron Oxide Surface: An Interesting Interplay between Electrostatic and Chemical Forces

• Published in: Langmuir 2011-11, Vol.27 (21), p.12930-12937
• Main Authors: Dol Hamid, Rossuriati, Swedlund, Peter J, Song, Yantao, Miskelly, Gordon M
• Format: Article
• Language: English
• Subjects: Chemistry; Exact sciences and technology; General and physical chemistry; Interfaces: Adsorption, Reactions, Films, Forces; Surface physical chemistry
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/la201775c

The effect of ionic strength on reactions at aqueous interfaces can provide insights into the nature of the chemistry involved. The adsorption of H4SiO4 on iron oxides at low surface silicate concentration (ΓSi) forms monomeric silicate complexes with Fe–O–Si linkages, but as ΓSi increases silicate oligomers with Si–O–Si linkages become increasingly prevalent. In this paper, the effect of ionic strength (I) on both ΓSi and the extent of silicate oligomerization on the ferrihydrite surface is determined at pH 4, 7, and 10, where the surface is, respectively, positive, nearly neutral, and negatively charged. At pH 4, an increase in ionic strength causes ΓSi to decrease at a given H4SiO4 solution concentration, while the proportion of oligomers on the surface at a given ΓSi increases. At pH 10, the opposite is observed; ΓSi increases as I increases, while the proportion of surface oligomers at a given ΓSi decreases. Ionic strength has only a small effect on the surface chemistry of H4SiO4 at pH 7, but at low ΓSi this effect is in the direction observed at pH 4 while at high ΓSi the effect is in the direction observed at pH 10. The pH where the surface has zero charge decreases from ≈8 to 6 as ΓSi increases so that the surface potential (Ψ) is positive at pH 4 for all ΓSi and at pH 7 with low ΓSi. Likewise, Ψ < 0 at pH 10 for all ΓSi and at pH 7 with high ΓSi. The diffuse layer model is used to unravel the complex and subtle interactions between surface potential (Ψ) and chemical parameters that influence interfacial silicate chemistry. This analysis reveals that the decrease in the absolute value of Ψ as I increases causes ΓSi to decrease or increase where Ψ is, respectively, positive or negative. Therefore, at a given ΓSi, the solution H4SiO4 concentration changes with I, and because oligomerization has a higher H4SiO4 stoichiometry coefficient than monomer adsorption, this results in the observed dependence of the extent of silicate oligomerization on I.