Horizon formation by molten organic salts as a test of their fluidity

A simple but sensitive horizon test is described to determine whether the mesophases of molten organic salts exhibit any detectable elastic-plastic threshold, before they begin to flow under even very small applied shear stresses. By using this test with sodium isovalerate (mesophase range 188–280 °C) no detectable threshold is found above about 260 °C; above this temperature the melt thus behaves as completely fluid. However, between this temperature and the first melting point (188 °C) a definite limit stress appears to be required to start flow. Other properties of the mesophase also show changes around 260 °C with some analogies to fluid–glass transi­tions in other systems. Molten sodium n-butyrate, when tested in a similar way, fails to exhibit any distinctive elastic-plastic threshold. So far as can be detected by the horizon test, it remains completely fluid over the whole of its meso­phase range (252–324 °C).