Phase Diagram of Isobutyric Acid and Water in Dilute Silica Gel

Measurements of the isobutyric acid (IBA) concentration in a dilute silica gel, vs that in an IBA and water reservoir in equilibrium with the gel, show that first-order phase transitions occur at 15 and 19 ^∘{}C, but not at 23 ^∘{}C. The data show that the coexistence curve of this gel/mixture system is narrow, contained within that of the pure mixture, and lies below a broad region of apparent metastability. The existence of a metastable state above the critical temperature of the gel/mixture system is strikingly similar to the behavior observed for random-field magnets.