On the metastable zone width of 1,3-dipalmitoyl-2-oleoylglycerol, tripalmitoylglycerol, and their mixtures in acetone solutions

Sangwal K., Smith K.W., On the metastable zone width of 1,3-dipalmitoyl-2-oleoylglycerol, tripalmitoylglycerol, and their mixtures in acetone solutions, Crystal Growth and Design, 2010, 10(2), 640–647

The extensive experimental data reported by Smith et al. (Eur. J. Lipid Sci. Technol. 2005, 107, 583−593) on the metastable zone width of 1,3-dipalmitoyl-2-oleoylglycerol, (POP) tripalmitoylglycerol (PPP), and their mixtures in acetone are analyzed using a novel approach based on classical three-dimensional nucleation theory (Sangwal, K. Cryst. Growth Des. 2009, 9, 942−950) and a self-consistent Nyvlt-like approach (Sangwal, K. Cryst. Res. Technol. 2009, 44, 231−247) to obtain quantitative information on the processes of nucleation of these systems. The relevant equations of the above approaches are (i) (T₀/ΔT_max) = F(1 − Z ln R) and (ii) ln(ΔT_max)/T₀) = Φ + β ln R, where T₀ is the saturation temperature and F and Φ are, respectively, the values of (T₀/ΔT_max) and ln(ΔT_max)/T₀) when ln R = 0, whereas Z is a parameter characteristic of the crystallizing system and the constant β is inverse of the apparent nucleation order m in the Nyvlt approach. It was found that (1) the value of parameters β and Z is relatively insensitive to saturation temperature T₀ but depends on the investigated compound and the technique used for the measurement of maximum supercooling ΔT_max and the presence of PPP impurity in POP solutions; (2) the values of β and Z for PPP and POP decrease with an increase in their solubility in acetone; and (3) the data of Φ and ln(F^1/2) as a function of saturation temperature T₀ follow the Arrhenius-type relation with an activation energy E_sat. Analysis of the results revealed that (1) the values of β and Z are directly connected with the solid−liquid interfacial energy γ; (2) the value of E_sat depends on the nature of ions/molecules participating in diffusion; (3) maximum supercooling ΔT_max measured by turbidimetry corresponds to a nucleation event alone, but that obtained from growth exotherm includes some contribution of growth of stable nuclei after nucleation; and (4) PPP impurity in POP solutions promotes crystallization by acting as growth centers for heterogeneous nucleation.