Solvent effects on nucleation of disodium guanosine 5′-monophosphate in anti-solvent/water mixtures

The influence of the solvent on the nucleation of disodium guanosine 5′-monophosphate (GMPNa₂) in eight methanol (ethanol)-water mixtures and at three supersaturation levels at 293.15 K was investigated. The interfacial free energy, the critical size for nucleation, and the number of molecules were calculated using equations re-derived from the classical nucleation theory. GMPNa₂ nucleation involves not only a primary nucleation process but also the conversion from a homogeneous to a heterogeneous process. The transformation experiment indicated that the nucleation of GMPNa₂ follows the two-step model. A surface entropy factor (f_b) was proposed to relate the solvent composition and interfacial free energy in order to explain the solvent effects on the nucleation process. Although the f_b values can be used to judge the crystal growth mode, nucleation and growth cannot be separated, since pure primary nucleation does not occur for this compound. The f_b values clearly increased with increasing solvent content and decreasing supersaturation. For the homogeneous process in a methanol-water system, regular crystals can be formed only when f_b > 5, regardless of the supersaturation, as was observed for solvent contents of 40 mol% and 30 mol% (S = 1.83 and 1.70, respectively). These results can help select a suitable solvent system and its composition during anti-solvent crystallization.