What can vibrational spectroscopy tell about the structure of dissolved sodium silicates?

Aqueous solutions of various sodium silicates are frequent ingredients for the synthesis of zeolites and mesoporous solids but their structure is poorly understood. This paper aims to explore the ability of FTIR and Raman spectroscopy for obtaining such structural information. It is demonstrated that both the average molweight and the dissociation level are functions of both the Na/Si ratios and the concentration of dissolved silicates. In contrast to a frequent belief only some of the widely used commercial solutions contain mainly monomers at pH >11 and the dissociation into Na+ and silicate anions remains between 30% and 90% in the practically important 0.2–3 mol/L concentration range. The published Qn connectivity assignments of νas Si–O–Si stretching vibrations (≳≳800 cm−1) measured by FTIR spectroscopy were found not to be applicable for these silicates but the corresponding Raman assignments might be valid except that the spectrum of monomer metasilicate does not support the Q0 assignment of ∼870 cm−1 Raman band. With the exception of dilute N-silicate which is the most frequently used zeolite synthesis ingredient, these Raman data suggest that all studied silicate structures must mainly be composed of Q2 type [SiO4] tetrahedra with a small number of terminal Q1 type units. The absence of Q3 connectivity contradicts many potential structures identified by Si29 NMR and also the possible presence of double 3, 4, 5, or 6 member ring structures which could serve as secondary building units for zeolite synthesis. Beside a universal O–Si–O related 420–450 cm−1 band, only Raman shows substantial deformation vibrations at <800 cm−1. Some of them can be associated with published vibrations of small siloxane rings.