Phase equilibria of ternary systems of carbon dioxide + ethanol, dimethyl sulfoxide, or N,N-dimethyl formamide + water at elevated pressures including near critical regions
Graphical abstract
Introduction
The phase equilibrium properties are essentially needed in the development of supercritical fluid techniques for supercritical fluid (SCF) extraction [1], reaction [2], fractionation [3], nano-particles formation [4], [5], [6], [7], etc. Carbon dioxide has been recognized as one of environmentally benign solvents and commonly used in a variety of SCF technologies, because it has mild critical conditions (Tc = 304.25 K, Pc = 7.38 MPa), inexpensive, nontoxic, nonflammable, and readily available. Ethanol, dimethyl sulfoxide (DMSO), and N,N-dimethyl formamide (DMF) are often used as solvents in supercritical anti-solvent (SAS) processing to generate ultra-fine particles for a variety of materials [4], [5], [6], [7]. Several investigators [6], [7], [8], [9], [10], [11] pointed out that the phase behavior of solvent + anti-solvent mixtures is a key factor to govern the morphology and the size of the resultant particles produced from the SAS process. In general, nanometric particles, micrometric particles, and dense films could be obtained when the SAS precipitation was conducted in supercritical, superheated vapor, and vapor-liquid coexistence phase regions [11], respectively. As a consequence, the vapor-liquid equilibrium (VLE) phase diagram of solvent + anti-solvent systems is fundamentally important for manipulating precipitation conditions to prepare particulate products having preferable size and morphology. The VLE phase boundaries near the critical region are especially of interest in developing the SCF micronization processes.
Regarding the preparation of pharmaceutically active ingredients for aerosol pulmonary delivery, the size distribution of particles is favorable within 1 μm–5 μm. As noted above, the precipitation condition of the SAS process is suggested to be in the superheated vapor region of the solvent + anti-solvent system. However, this operable window is rather narrow for typical carbon dioxide + organic solvent systems. Perez de Diego et al. [12], [13] claimed that the bubble points shift to higher pressures by introducing water into CO2 + DMSO. Therefore, the presence of water may provide a wider operable window, i.e., wider superheated vapor region, for particle formation by using SAS precipitation. Andreatta et al. [14] investigated the VLE behavior of CO2 + DMSO + water system including three different DMSO/water molar ratios. Among several others, the phase equilibrium data of ternary systems of CO2 + ethanol + water and CO2 + DMF + water are available in literature [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. Most of above mentioned literature data are in saturated liquid region (bubble points) and only a few data in saturated vapor region (dew points). Moreover, no available VLE data are found around critical regions. In the present study, a visual and volume-variable phase equilibrium analyzer (PEA) was employed to observe the phase transition boundaries of mixtures changing from single phase into vapor-liquid (or from two-liquid phase to vapor-liquid-liquid) coexistence region. Since the operation of the PEA is a synthetic method, it is especially applicable to determine the phase boundaries near critical region, in which analytic method often fails. The isothermal phase boundaries were measured for three ternary systems composed of CO2 + (DMSO/water = 9/1 M ratio), CO2 +(ethanol/water = 9/1 M ratio) and CO2 + (DMF/water = 7.5/2.5 M ratio) at temperatures from 298.2 K to 348.2 K over wide composition range, including near critical regions. Critical pressure and composition are estimated from each isothermal phase envelope.
In these three investigated ternary systems, there is a common binary pair, CO2-water, which may involve both physically and chemically interactions, simultaneously. Valtz et al. [26] found the Peng-Robinson (PR) equation of state [27] with Mathias and Copeman (MC) alpha function [28], [29] and the Wong-Sandler (WS) mixing rules [30] well represented the phase boundaries for this binary system. Similar model is taken in the present study. While the UNIFAC model [31] is used for estimating the excess Helmholtz free energy at infinite pressure, the binary interaction parameter in the calculation of mixtures' second virial coefficient is determined from the phase equilibrium data of the constituent binaries. The optimal values of the binary interaction parameters are adopted to predict the phase boundaries for the investigated ternary systems.
Section snippets
Materials
Carbon dioxide (purity of 0.995+ mass fraction) was supplied by Liu-Hsiang Co. (Taiwan). DMSO (0.999 mass fraction) was purchased from Arcos (USA), ethanol, HPLC grade (0.9999 mass fraction), from Fisher Scientific (USA), and DMF (0.9995 mass fraction) from Fluka (Germany). All the chemicals were used without further purification, except for degassing. Water was prepared by NANO pure-Ultra pure water system that was distilled and deionized with resistivity of 18.3 MΩ cm. The description of the
Experimental results and discussion
The reliability of the VLE data measured using the PEA has been checked elsewhere [33] by comparing with the literature data of CO2 + 1-octanol and CO2 + dimethyl sulfoxide (DMSO). In the present study, this apparatus was employed to measure the isothermal phase equilibrium boundaries for CO2 + ethanol + water, CO2 + DMSO + water, and CO2 + DMF + water in a temperature range of 298.2 K–348.2 K. Table 3, Table 4, Table 5 report the determined phase boundary data for these three ternary systems.
Correlation of VLE data
Firstly, we try to predict the VLE phase boundaries with the Peng-Robinson (PR) equation of state using the van der Waals one-fluid two-parameter mixing rule. As the binary interaction parameters were determined from the VLE data of the constituent binary systems, the deviations between calculated and experimental values are substantially large, especially near the critical regions. Graphical comparisons are shown in Fig. S1–S3 for the systems containing ethanol, DMSO, and DMF, respectively. To
Conclusions
The vapor-liquid (or vapor-liquid-liquid) phase transition boundaries have been determined experimentally for CO2 + ethanol + water, CO2 + DMSO + water, and CO2 + DMF + water in a temperature range of 298.2 K–348.2 K and pressures up to near critical values by using a visual and volume-variable phase equilibrium analyzer. The critical points have been determined by interpolation of the experimental phase boundary data. Introducing a certain amount of water into ethanol/CO2, DMSO/CO2, and DMF/CO2
Acknowledgment
The authors gratefully acknowledged the financial support from the Ministry of Science and Technology, Taiwan, through grant no. NSC95-2214-E-011-154-MY3.
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