LANTHANUM THERMODYNAMICS IN THE CLAY-WATER SYSTEM: IDEAL BEHAVIOR IN A NON-IDEAL WORLD (Invited Presentation)

Raoult first defined an ideal mixture in the 1870’s, and since then most systems have been found to behave non-ideally. Most of us were warned as early as our first chemistry class to use activities in thermodynamic calculations instead of concentrations because of this non-ideality. Later, in physical chemistry, we may have learned that some rare organic liquid mixtures behaved ideally if all the liquids in the mixtures have little polarity and have similar structures. One might therefore be surprised to observe ideal behavior in a highly polar liquid like water. It might be even more surprising for a high charge ion like La³⁺ to behave ideally and when adsorbed onto a clay, where coordinating water molecules separate it from the basal surface. Nonetheless, there have been several reports of ideal solid phase behavior during ion-exchange reactions on montmorillonite. The present study specifically focuses on La³⁺ and Ba²⁺ exchange on montmorillonite, showing that Ba²⁺ and La³⁺ mix ideally on the solid phase. More remarkably, is that the Ba²⁺-La³⁺ exchange on montmorillonite occurs even in the presence of a third ion (NH₄⁺) does not mix ideally with either barium or lanthanum.

This study puts the ideal La³⁺ behavior on montmorillonite in context by examining other rare systems that behave ideally when water is present. These ideal systems have ions with a much lower charge density than water. Montmorillonite could be thought of as a low charge density ion. While the total charge per unit cell is high in montmorillonite, the charge per basal oxygen is lower than the partial charge on water’s oxygen. This may explain the common ideal behavior observed on montmorillonite, even when high charge density ions like La³⁺ are present.