Constant Phase

In a series of papers published throughout the 1980s, Colin Poole and his co-workers investigated the solvation properties of a wide range of alkylammonium and, to a lesser extent, phosphonium salts. Parameters such as McReynolds phase constants were calculated by using the ionic liquids as stationary phases for gas chromatography and analysis of the retention of a variety of probe compounds. However, these analyses were found to be unsatisfactory and were abandoned in favour of an analysis that used Abraham s solvation parameter model [5]. 

The HcReynolds system of phase constants has become the most widely used systematic approach to solvent selectivity characterisation and virtually all pedlar phases have been characterized by this method. In spite of its popularity the approach is fundamentally flawed and the phase constants are an unreliable indication of i ase properties. The basic approach, however, has influenced the development of other methods of selectivity characterization, and although these methods have inherited many of the deficiencies of their parent, a brief description of the HcReynolds approach is worthwhile to. idicate the general limitations of methods based on retention index differences. 

Solvent selectivity is a measure of the relative capacity of a solvent to enter into specific solute-solvent interactions, characterized as dispersion, induction, orientation and coaplexation interactions, unfortunately, fundamental aiq>roaches have not advanced to the point where an exact model can be put forward to describe the principal intermolecular forces between complex molecules. Chromatograidters, therefore, have come to rely on empirical models to estimate the solvent selectivity of stationary phases. The Rohrschneider/McReynolds system of phase constants [6,15,318,327,328,380,397,401-403], solubility... 

The value of the phase constant a does not affect the size or shape of the orbit, but it does affect the position of the orbit as a function of time. If at time t = t number of different identical oscillators have q = 0 but values of q lying between v and — v the representative points will lie on a straight line coincident with the p axis. In time, these points will describe their individual orbits but remain collinear. 

Equation 4.1 describes the Rohrschneider-McReynolds system in terms of the five probes and their corresponding phase constants namely, benzene (X ), butanol (Y ), 2-pentanone (Z ), nitropropane (U ), and pyridine (S ) with the overall difference in the Kovats retention index (AI). 

The value of each phase constant (i.e., X, Y, Z, U, and S ) is determined by subtracting the retention index of the probe on a squalane stationary phase (Isq) from the retention index of the probe on the stationary phase being characterized (Ijp). For example, the phase constant of benzene (X ) would be calculated as shown in Equation 4.2. 

To complete the determination of all constants, a value of 1 is assigned to each of the test solutes. In the case of benzene, X, its coefficient from Equation 4.1 would have the value of a = 1, whereas the remaining coefficients would be set equal to zero (i.e., b = 0, c = 0, d = 0, and e = 0). This process is repeated for the remaining solutes. The magnitude of each phase constant indicates the importance of the interaction in solute retention. Additionally, the overall polarity of the stationary phase can be determined by taking the average of all five phase constants. 

In the derivatives with respect to the number of moles of each component of the system, the numbers of moles of all other components (j / i) are held constant. To simplify the notation while keeping it useful for treating phase transformation processes, the same species in different phases will be designated by different subscripts. We first apply this equation to a system with a constant number of moles (all dni = 0). With the number of moles of all components (including the same component in different phases) constant, material equilibrium is not an issue and we can use Eq. (20) of Chapter 4 ... 

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