ABSTRACT
In this, a review on the phase behavior and physical properties that are relevant to particle formation processes with CO2 is given. As a practical example, the polymer poly(ethylene glycol), or PEG, with many applications in food and pharmaceutical product processing, used with supercritical CO2 for particle formation and encapsulation is described. Vapor-liquid equilibrium (VLE) and solid-liquid equilibrium (SLE) data is necessary because particle formation processes begin by dissolution of CO2 into the PEG solution and
then are formed through solidification of atomized droplets and ex-solution of CO2. CO2 solubility in PEG for given temperatures (333 and 353 K) is not distinctly different for molecular weights from 400 to 20,000 g/mol and varies from 3 wt% CO2 at 2 MPa to 25 wt% CO2at 28 MPa. The density of PEG solutions (PEG 400 and 1600) initially decreases by about 1% when pressurized with CO2 to around 10-15 MPa but then shows trends to increase at higher pressures. When using CO2 to reduce the viscosity of PEG solution, the viscosity reduction for PEG solutions (PEG 200-12,000) is above 40% in the presence of 10 wt% of dissolved CO2. The viscosity reduction ratios can be calculated with theory that uses equations of state. 5.1 Introduction
Supercritical fluid technologies have been applied to extraction, material synthesis, separation, and energy processes [1-3] and analytical methods [4]. Many researchers have focused on supercritical water and carbon dioxide (CO2) because they are benign for the environment. Particle formation processes using supercritical water and CO2 have been developed for producing small fine particles and elimination of residual solvents in the products, especially in applications of pharmaceutical and food processing [5-8]. Polymers in these applications are used as substrates and encapsulating materials. The polymers should be biodegradable and biocompatible for which poly(ethylene glycol) (PEG) and poly(lactic acid) (PLA) are some of the most popular choices. PEG polymers belong to the family of poly(oxyethylene) (POE) that have molecular weights (Mw) below 20,000 g/mol. PEGs that have molecular weights greater than 20,000 g/mol are referred to as poly(ethylene oxide) [9]. The characteristic properties of PEGs are (i) high solubility in both aqueous and organic solvents, (ii) no known immunogenicity, antigenicity, and toxicity effects, and (iii) high flexibility and hydration of the polymer chain [10]. PEGs are established reference polymers for pharmaceutical and biomedical applications, and they have been used as additives in food and cosmetic products [10-12]. Supercritical CO2 has been applied to the processing of PEG particles through rapid expansion of supercritical solutions with a nonsolvent (RESS-N) [13], particles from a gas-saturated
solution (PGSS), aerosol solvent extraction system (ASES) [14], and supercritical assisted atomization (SAA) [15]. In these processes, particles are formed through solidification of atomized droplets dissolved with CO2, so that the phase behavior of a CO2-PEG mixture is necessary for understanding the particle formation mechanism and for developing practical applications. The size of the atomized droplets strongly depends on the fluid flow characteristics of the solutions [16] and on the fundamental physical properties of the droplet such as viscosity, h(Pa.s); surface tension, s(J/m2); and density, r(kg/m3) [17]. These characteristics and properties affect mass transport of CO2 into and out of the droplet and control particle size and morphology (Fig. 5.1).
