The influence of solution environment on the nucleation kinetics and crystallisability of para-aminobenzoic acid.

The influence of solvent type on the solution thermodynamics, nucleation-kinetics and crystal growth of alpha para-aminobenzoic acid (PABA) crystallising from supersaturated ethanol, acetonitrile and water solutions, is examined using poly-thermal analysis of the metastable zone width. Application of a recently proposed model for analysis of crystallisation kinetics (J. Cryst. Growth, 2010, 312, 698-704) indicates a solvent and concentration dependence of the nucleation mechanism and key nucleation parameters for the alpha form of PABA. The mechanism of nucleation is found to change from instantaneous to progressive with decreasing concentration and also when changing the solvent from ethanol to acetonitrile to water. The dependence of the nucleation mechanism is correlated to the kinetic component of the nucleation rate through calculated values of instantaneously nucleated crystallites, which increase from 1.40 × 109 m-3 in ethanol to 1.08 × 1010 m-3 in acetonitrile to 2.58 × 1010 m-3 in water. This in combination with low calculated number concentrations of interfacial tension between 1.13 and 2.71 mJ m-2, supports the conclusion that the kinetic component of the nucleation rate is more limiting when crystallising PABA from ethanol solutions in comparison to water solutions. This finding is further supported by molecular dynamics simulations of the solvation free energy of PABA, which is found to be greatest in water, -42.4 kJ mol-1 and lowest in ethanol, -58.5 kJ mol-1.