Changes of Silica Species in Sodium Chloride Solution by Fast Atom Bombardment Mass Spectrometry: Silicate Complex Substituted by Sodium Ion

Abstract

The types of chemical species of silicate complex ion exchanged with sodium ions (silicate–Na⁺ complex) with increasing concentration of sodium chloride (NaCl) and their numbers of chemical species of silicate–Na⁺ complexes formed were examined. Silicate–Na⁺ complexes in several concentrations of NaCl were identified by fast atom bombardment mass spectrometry (FAB-MS). The basic structures of silicate species are not complexes with Na⁺, such as the dimer, trimer, cyclic tetramer, linear tetramer, cyclic pentamer, linear pentamer, cyclic hexamer and linear hexamer, were identified. When the peak intensity ratios of silicate–Na⁺ complexes were plotted against the concentration of NaCl, the changes with increasing concentration of NaCl were mainly classified into two patterns. One is the linear silicate–Na⁺ complex pattern (dimer–Na⁺ complex, trimer–Na⁺ complex and linear tetramer–Na⁺ complex), and the other is the cyclic silicate–Na⁺ complex pattern (cyclic tetramer–Na⁺ complex and cyclic pentamer–Na⁺ complex). In the linear silicate–Na⁺ complex pattern, the peak intensity ratios of the linear silicate–Na⁺ for the basic structures vary largely between 0.001 and 0.1 mol dm⁻³ (M) NaCl, and they become constant at above 1 M NaCl. In the cyclic silicate–Na⁺ complex pattern, the peak intensity ratios of the basic structures of cyclic silicate–Na⁺ change slightly between 0.001 and 0.1 M NaCl and change considerably above 1 M NaCl. The basic structure of cyclic silicate has a stable structure and hydrophobic part. By substituting Na⁺, the hydrophobicity of the cyclic silicate complex increases. The linear silicate complex is not as stable as the cyclic silicate complex and it possesses a higher degree of hydration than the cyclic silicate. The hydrophobicity of the linear silicate also increases by the substitution of silanol groups by Na⁺. These changes of the hydrophobicity of the linear silicate due to the substitution of Na⁺ are larger than those for the cyclic silicate. The dissolution system of the silicate species in NaCl solution should be elucidated by considering the relationship between the stabilities of the basic structure and Na⁺-containing silicate complexes, and their hydration effect.

Keywords:

• Silica
• Silicate complex
• with Na⁺
• FAB-MS
• Salting-out effect
• Chemical species

Acknowledgments

This study was partly supported by the Kurita Water and Environment Foundation, the River Environment Fund (REF) of the Foundation of River and Watershed Environment Management (FOREM), Japan, Showa Shell Sekiyu for Promotion of Environmental Research, and The Japan Securities Scholarship Foundation. This study was also supported by a Grant-in Aid for Scientific Research (C)(2)(13640598) from the Japan Society for the Promotion of Science. We are very grateful to Yasuaki Esumi (Division of Molecular Characterization, RIKEN) for his assistance in performing FAB-MS.