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 thermochemical methods. The present work postulates an analogy to melting processes, looking at melting as nucl eation - growth of a liquid phase. The melting process of acetylsalicylic a cid single crystals was investigated by DSC measurements under isothermal c onditions. The fraction of material molten after a certain time period, alp ha (t), was calculated by integrating the DSC curves. The resulting kinetic curves were fitted using the Avrami-Erofeev equation: -ln(1-alpha)=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 consumpt ion two- and three-dimension nuclei growth, respectively. Applying correlat ion analysis, relationships between two- or three-dimensional growth and th e independent variables describing the single crystals (for strictly defini te trajectories into the space of sizes) were found. Particular correlation s were: a) Two-dimensional growth is a function of the total surface area o f 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 experiment al data are explained by the 'layer' structure of crystals of acetylsalicyl ic acid.