The effect of ball mass on the mechanochemical transformation of a single-component organic system: anhydrous caffeine

Abstract

ABSTARCTMechanochemical methodologies, particularly ball milling, have become commonplace in many laboratories. In the present work, we examine the effects of milling ball mass on the polymorphic conversion of anhydrous caffeine. By investigating a single-phase system, the rate-limiting step of particle-particle contact formation is eliminated. It is found that larger milling balls lead to considerably faster conversion rates. Modelling of the transformation rate suggests that a single, time-independent rate constant is insufficient to describe the transformation. Instead, a convolution of at least two rate-determining processes is required to correctly describe the transformation. This suggests that the early stages of the transformation are governed only by the number of particle-ball collisions. As the reaction proceeds, these collisions less frequently involve reactant, and the rate becomes limited by mass transport, or mixing, even in originally single-phase systems, which become multi-phase as the product is formed. Larger milling balls are less hindered by poorly mixed material. This likely results from a combination of higher impact energies and higher surface areas associated with the larger milling balls. Such insight is important for the selective and targeted design of mechanochemical processes.

Ball size or ball mass – what matters in organic mechanochemical synthesis?

“…Quantitative analysis of a single-phase transformation suggested that increasing the size of a milling ball was more intimately related to an increase in its surface area, rather than the mass itself. 19 That said, this previous study was also restricted by simultaneous variation in ball mass and surface area. It was therefore not possible to distinguish unambiguously between the two effects without specially designed experiments.…”

“…In mechanochemistry, this excess energy is a function of mechanical impact, and is intimately connected to the mass and velocity of the impacting milling ball through the classical mechanical equations of motion. [16][17][18][19][20][21][22][23][24] Generally, reports which discuss the effects of the ball mass on mechanochemical transformations involve variation of the milling ball size (and hence mass). 19,25 Alternatively, authors have varied the quantity of sample powder and thus the ball to sample mass ratio.…”

Direct Access by Mechanochemistry or Sonochemistry to Protonated Merocyanines: Components of a Four‐State Molecular Switch

“…In the present work, we directly synthesized the protonated merocyanine forms of nitro‐ and bromo‐substituted SP (MEH + , X=NO 2 , Br) by mechano‐ and sonosynthesis. Mechanosynthesis consists in the use of mechanical energy to trigger chemical reactions between solids . Solid‐state reactions are therefore performed in mixer mills .…”

“…Mechanosynthesis consists in the use of mechanical energy to trigger chemical reactions between solids. [25][26][27] Solid-state reactions are therefore performed in mixer mills. [28] Among the advantages of mechanosynthesis are:l arge quantitieso fs tarting materials can be used, timesaving properties, and the possibility to reduce side reactions, af actor that can lead to higher yields and better conversions.…”

Preface

“…The next two papers of this collection deal with such in situ studies. Thus, Michalchuk et al [5] examine the effect of different milling parameters in the mechanically induced polymorphic transformation of anhydrous caffeine, whereas Akhmetova et al use the same technique to monitor reaction pathways in the synthesis of two Mn(II) phosphonates, with different N-containing organic ligands [6]. As oxide particles are hard and brittle, mechanochemical reactions in oxide systems require longer milling times and more powerful milling devices making it very difficult to carry out real-time monitoring experiments.…”