The Melting Process of Acetylsalicylic Acid Single Crystals

Crystallisation is generally regarded as a nucleation — growth mechanism of a solid phase and often studied using thermo chemical methods. The present work postulates an analogy to melting processes, looking at melting as nucleation — growth of a liquid phase. The melting process of acetylsalicylic acid single crystals was investigated by DSC measurements under isothermal conditions. The fraction of material molten after a certain time period, α(t), was calculated by integrating the DSC curves. The resulting kinetic curves were fitted using the Avrami-Erofeev equation: –ln(1–α)=kt ⁿ, where parameter n was analysed. According to established methods, functions I('2')=t('2')]/t('2')+t('3')]100% and I('3')=t('3')]/t('3')+t('2')]100% were introduced, where t('2') and t('3') is the absolute time of consumption two- and three-dimension nuclei growth, respectively. Applying correlation analysis, relationships between two- or three-dimensional growth and the independent variables describing the single crystals (for strictly definite trajectories into the space of sizes) were found. Particular correlations were:a) Two-dimensional growth is a function of the total surface area of the crystal, S, and of the surface area of the (ac)-face, S _ac; b) Three-dimensional growth is a function of S/M (where M is the mass of the single crystal). It is also a function of S _ac/M and of S. The obtained experimental data are explained by the ‘layer’ structure of crystals of acetylsalicylic acid. This revised version was published online in August 2006 with corrections to the Cover Date.