| The goal of this work was to obtain an improved fundamental understanding of the formation of organic and polymeric particles by the rapid expansion of supercritical solutions into either air (RESS) or liquid solvents (RESOLV).; Rapid expansion of a fluorinated tetraphenylporphyrin, 5,10,15,20-tetrakis(3,5-bis(trifluoromethyl)phenyl)porphyrin (TBTPP), from supercritical solutions of carbon dioxide was investigated. Surprisingly, true nanoparticles (38 +/- 9 nm) were produced by RESS, independent of porphyrin concentration, degree of saturation, and pre-expansion pressure. Particle size increased in a well-behaved manner with pre-expansion temperature (Tpre). RESOLV of TBTPP-CO2 solutions was investigated both for minimizing particle growth in the free jet and for preventing particle agglomeration. Rapid expansion into an aqueous solution of 0.025 wt % Pluronic F68 produced stable nanoparticle (28 +/- 9 nm) suspensions, independent of the processing conditions selected. The fact that nanoparticles were produced by RESS of TBTPP, vs. the microparticles reported with other organics, can be explained in terms of Friedlander's collision-coalescence theory and the solid-state diffusion coefficient D, which is low for this system because of the high melting point (Tm = 388°C) of TBTPP.; The effect of D (which is ∝ exp(-Tm/Tpre)) on RESS was further investigated by using a polymer as the solute, as T m can be varied via the molecular weight without changes in chemical properties. RESS experiments on poly(L-lactide) (PLLA) with two different melting points (Tm = 121°C and Mw = 1340; T m = 162°C and Mw = 6050) in CO2-THF mixtures were performed. Typical PLLA products consisted of both nanoparticles (30--100 nm) and larger (submicron- and micron-sized) particles. The presence of individual nanoparticles suggests that the initially formed precipitates are nano-sized, and that larger particles are obtained because of coalescence effects in the free jet. As with TBTPP, results for PLLA can be explained in terms of D. For example, at a given Tpre, the size of the larger particles decreased from 10 to 1 mum as the Tm of the PLLA polymer increased from 121 to 162°C. Furthermore, for a given Tm, particle fusion in the agglomerates decreased with decreasing Tpre. |
