Homogeneity of modifier ion distributions and the mixed alkaline earth effect in MgO–CaO–SiO2 silicate glasses using molecular dynamics

Abstract

Mountjoy (2017) Homogeneity of modifier ion distributions and the mixed alkaline earth effect in MgO-CaO-SiO2 silicate glasses using molecular dynamics.European AbstractOxide glasses containing a mixture of modifier ions have a wide range of applications, and it is often assumed that these modifier ions mix homogenously in the melt. Silicate glasses are known for their significant industrial importance and are often doped with magnesium and calcium modifier ions to utilise their properties for applications. This study investigated the structural and dynamical impact imposed on a glass by the presence of two different types of alkaline earth cations. This was achieved through increasingly substituting magnesium for calcium in xMgO-(50-x)CaO-50SiO 2 glasses which were modelled computationally using classical molecular dynamics simulation (MD). A mixed alkaline earth effect (MAEE) was found to cause the suppression of the modifier ion diffusivity. An absence of any significant deviation from the expected trends in the structural investigations confirmed that the MAEE is not a structural effect. This MAEE was therefore analogous to the mixed alkali effect (MAE), and was most prominent for roughly equimolar mixtures of calcium and magnesium ions. Interestingly, there was also evidence of the diffusivity being enhanced when only a small amount of a second alkaline earth ion was added.

Chemical durability of borosilicate pharmaceutical glasses: Mixed alkaline earth effect with varying [MgO]/[CaO] ratio

“…Among these three types, the low boron content type typically has higher fractions of modifier ions; thus, the MAEE is expected to be more severe. The impact of MAEE on thermal or mechanical properties have been reported in the literature, [20][21][22][23][24][25]26,27,28,29 but the report on chemical durability is rare. When mixing two alkaline earth oxides, the leaching dynamics of different network modifiers may change, which may affect the stability of the remaining silicate network.…”

“…The most widely used pharmaceutical glass is borosilicate with a mixture of different modifier ions (alkali or alkaline earth) 18,19 . Thus, a mixed modifier effect (MME), i.e., a nonlinear scaling of properties from the weighted average of two end‐member compositions, 20,21,22 may play a critical role in various aspects of glass properties. The mixed alkaline earth effect (MAEE) is one example of the MME and has been investigated in silicate and aluminosilicate glasses, but little attention has been given to borosilicate glasses used for pharmaceutical packaging 23,24,25 .…”