Structural roles of CO_2 and [CO_3]^(2-) in fully polymerized sodium aluminosilicate melts and glasses

Abstract

Ab initio, molecular orbital calculations of the structures, energetics, and vibrational spectra of six T_2O_(br)-CO_2 clusters and five T-[CO_3]-T clusters (T = Al or Si, with and without Na^+ present) have been completed using a 3–21G^(∗∗) basis set and the Gaussian 92 program to evaluate possible configurations of CO_2 molecules and carbonate groups in melts and glasses on the NaAlO_2-SiO_2 join. Based on these calculations, we developed the following hypothesis for the solution mechanisms of CO_2 molecules and carbonate groups in fully polymerized melts and glasses on this join. Molecular CO2 is weakly bound to bridging oxygen atoms in T_2O_(br)-CO_2 clusters. Calculated energetics indicate that molecular CO_2 is more strongly bonded when associated with smaller angle T-O-T linkages, and thus may preferentially bond to linkages where at least one T = Al^(3+). [CO_3]^(2−) is most likely present in T-[CO_3]-T linkages. Si-[CO_3]-Al linkages probably form in melts toward the silica-rich end of the NaAlO_2-SiO_2 join; Si-[NaCO_3]-Al and/or Al-[CO_3]-Al become more significant with increasing NaAlO_2 content. The experimentally observed increase in [CO_3]^(2-)/CO_2 ratio accompanying higher NaAlO_2 compositions can be understood in terms of the predicted increasingly negative ΔG° for T_2O_(br)-CO_2 → T-[CO_3]-T reactions when Si-[NaCO_3]-Al and Al-[CO_3]-Al rather than Si-[CO_3]-Si and Si-[CO_3]-Al are the reaction products. In addition, a reaction pathway with a low activation energy was calculated for forming T-[CO_3]-T linkages from T_2O_(br)-CO_2 linkages. This model is consistent with available information on the vibrational and NMR spectra of C-bearing Na-aluminosilicate glasses, and on the relative proportions in these glasses of carbonate and molecular CO_2 and their dependence on pressure, temperature, and composition.