Mechanical activation of trans-stilbene during wet grinding

Abstract

The influence of process temperature on product particle size and solubility has been studied for wet grinding of trans-stilbene in ethanol. A positive effect of lowering the process temperature with respect to minimum product particle size was observed. Remarkably, the smallest product particle sizes x50,3<1μm were found for the lowest process temperature (251K) at short process times (< 30min) and moderate stressing conditions. In contrast, for the same stressing conditions at room temperature (293K) particles of approximate size x50,3≈8μm are obtained.The product particle size is rather determined by the (temperature- and solvent-dependent) solid–liquid equilibrium, i.e. dissolution and precipitation phenomena, than by pure mechanical fracture. An increased solubility of stressed trans-stilbene with respect to the equilibrium solubility of the solid has been observed. By means of 1H NMR spectroscopy and thermodynamic considerations it is shown that the solubility increase is neither due to an isomerisation of the solid nor due to size effects. In fact, mechanical activation leads to an increase in solubility of the stressed solid which was clearly proven by means of solubility studies at different temperatures. The van’t Hoff enthalpy of dissolution of the stressed solid decreased remarkably in comparison to the enthalpy of the non-stressed solid as shown by evaluation of van’t Hoff plots.