Phase classification schemes

When we study a solid that does not have the characteristic lustrous appearance of a metal, we find that the conductivity is extremely low. This includes the solids we have called ionic solids sodium chloride, sodium nitrate, silver nitrate, and silver chloride. It includes, as well, the molecular crystals, such as ice. This solid, shown in Figure 5-3, is made up of molecules (such as exist in the gas phase) regularly packed in an orderly array. These poor conductors differ widely from the metals in almost every property. Thus electrical conductivity furnishes the key to one of the most fundamental classification schemes for substances. 

Data for the hydrogen sulfide-water and the methane-n-hexane binary systems were considered. The first is a type III system in the binary phase diagram classification scheme of van Konynenburg and Scott. Experimental data from Selleck et al. (1952) were used. Carroll and Mather (1989a b) presented a new interpretation of these data and also new three phase data. In this work, only those VLE data from Selleck et al. (1952) that are consistent with the new data were used. Data for the methane-n-hexane system are available from Poston and McKetta (1966) and Lin et al. (1977). This is a type V system. 

Another classification scheme is based on the size of the dispersed particles within the dispersion medium (Table 2). The particles of the dispersed phase may vary considerably in size, from large particles visible to the naked eye, down to particles in the colloidal size range, and particles of atomic... 

Table 1 Classification Scheme of Disperse Systems on the Basis of the Physical State of the Dispersed Phase and the Dispersion Medium... 

The mobile phase can be either a gas or a liquid, while the stationary phase can be either a liquid or solid. One classification scheme is based on the nature of the two phases. All techniques which utilize a... 

Each of these solid phases can be described in terms of their mineralogy. This classification scheme is based on crystal structure and chemical composition. The most common minerals found in marine sediments are listed in Table 13.2. Most are silicates in which Si and O form a repeating tetrahedral base unit. Other minerals common to marine sediments are carbonates, sulfates, and oxyhydroxides. Less common are the hydrogenous minerals as they form only in restricted settings. These include the evap-orite minerals (halides, borates, and sulfates), hydrothermal minerals (sulfides, oxides, and native elements, such as gold), and phosphorites. 

To differentiate between the variety of phase equilibria that occur, Ehrenfest proposed a classification of phase transitions based upon the behavior of the chemical potential of the system as it passed through the phase transition. He introduced the notion of an th order transition as one in which the nth derivative of the chemical potential with respect to T or p showed a discontinuity at the transition temperature. While modern theories of phase transitions have shown that the classification scheme fails at orders higher than one, Ehrenfest s nomenclature is still widely used by many scientists. We will review it here and give a brief account of its limitations. 

The membrane classification scheme described above works fairly well. However, a major membrane preparation technique, phase separation, also known as phase inversion, is used to make both isotropic and anisotropic membranes. This technique is covered under anisotropic membranes. 

Approximately 70 chiral stationary phases (CSPs) have been marketed since 1981 [256]. A classification scheme has been proposed for the numerous commercially available CSPs which takes into account chiral recognition mechanism and chemical structure (Figure 5.6). 

The most popular classification scheme stems from the manner in which the analyte interacts with the stationary phase. With this approach, chromatography may be divided into five separation mechanisms adsorption, partition, size exclusion, affinity, and ion exchange, as illustrated in Figure 1.1. 

The second classification scheme is less common than the first but is found in the literature. It is based on the operating method, or the mechanism by which the sample is removed from the column, and is therefore dependent on the nature of the mobile phase. This classification, which was introduced by Tiselius21 in 1940, includes elution development, displacement development, and frontal analysis, as shown in Figure 1.2.22 In practice, only elution and to a lesser extent displacement development are commonly used. 

In chromatography, one phase is held immobile or stationary, and the other one is passed over it (the mobile phase). The designations GC and LC refer to the physical state of the mobile phase. Further classifications can be made by naming the mobile and stationary phases thus we have gas-solid (GS), gas-liquid (GL), liquid-liquid (LL), and liquid-solid (LS) chromatography. More recently, supercritical fluids have been used as mobile phases, and these techniques have been named supercritical fluid chromatography (SFC) irrespective of the state of the stationary phase. Other names have also become popular, and Table 1 shows a complete classification scheme. Included in the classification scheme are not only the states of the two phases but also the configuration of the chromato-... 

The data plots of Fig. 15b (silica) are differentiated for the use of methyl- -butyl ether (MTBE) or acetonitrile (ACN) as localizing solvent C in the mobile phase. It is seen that for some solute pairs (Fig. 15a and c) the open squares (MTBE) fall on a different curve than the closed squares (ACN). This implies that the constant in Eq. (31a) is solvent-specific, rather than being constant for all solvents (as first-order theory would predict). A similar behavior is observed for alumina as well. Figure 15a plots data for 18 different polar solvents B or C, and some scatter of these plots of log a versus m is observed here, as in Fig. 15b for silica. The variation of Q with the localizing solvent C used for the mobile phase has been shown (18) to correlate with the relative basicity of the solvent, or its placement in the solvent classification scheme of Refs. 40 and 41. Thus, for relatively less basic solvents (groups VI or VII in Refs. 40 and 4/),... 

Understanding three-phase flow dynamics is necessary for design of these reactors. A major aid is the classification scheme of Fan, depending on stream direction and degree of fluidization, expanded bed or transported bed. For example, a continuous liquid phase reactor, with gas and liquid inlets at the bottom and solids at the bottom, is classified as Mode E-I-a-1 if it operates as an expanded bed and Mode T-I-a-1 if all phases are flowing together, called transport. 

Within the last several years HPLC separations have been optimized in terms of the most appropriate mobile phase composition for a particular set of solutes by exploring the whole plane of solvent selectivities using this solvent classification scheme with a minimal number of measurements in statistically-designed experiments. For reversed phase HPLC systems, the selectivity triangle is often defined by methanol, acetonitrile, and tetrahydrofuran with water as the diluent (37). 

---