Stationary phases solvation parameter model

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]. 

Retention of Rohrschneider-McReynolds standards of selected chiral alcohols and ketones was measured to determine the thermodynamic selectivity parameters of stationary phases containing (- -)-61 (M = Pr, Eu, Dy, Er, Yb, n = 3, R = Mef) dissolved in poly(dimethylsiloxane) . Separation of selected racemic alcohols and ketones was achieved and the determined values of thermodynamic enantioselectivity were correlated with the molecular structure of the solutes studied. The decrease of the ionic radius of lanthanides induces greater increase of complexation efficiency for the alcohols than for the ketone coordination complexes. The selectivity of the studied stationary phases follows a common trend which is rationalized in terms of opposing electronic and steric effects of the Lewis acid-base interactions between the selected alcohols, ketones and lanthanide chelates. The retention of over fifty solutes on five stationary phases containing 61 (M = Pr, Eu, Dy, Er, Yb, n = 3, R = Mef) dissolved in polydimethylsiloxane were later measured ". The initial motivation for this work was to explore the utility of a solvation parameter model proposed and developed by Abraham and coworkers for complexing stationary phases containing metal coordination centers. Linear solvation... 

Figure 1.3. Variation of the system constants of the solvation parameter model (section 1.4.3) with temperature for 37 % (v/v) propan-2-oI in water on the porous polymer PLRP-S stationary phase. The m constant reflects the difference in cohesion and dispersive interactions, r constant loan-pair electron interactions, s constant dipole-type interactions, a constant hydrogen-bond basicity and b constant hydrogen-bond acidity between the mobile and stationary phases. (From ref. [89] The Royal Society of Chemistry).

The master retention equation of the solvation parameter model relating the above processes to experimentally quantifiable contributions from all possible intermolecular interactions was presented in section 1.4.3. The system constants in the model (see Eq. 1.7 or 1.7a) convey all information of the ability of the stationary phase to participate in solute-solvent intermolecular interactions. The r constant refers to the ability of the stationary phase to interact with solute n- or jr-electron pairs. The s constant establishes the ability of the stationary phase to take part in dipole-type interactions. The a constant is a measure of stationary phase hydrogen-bond basicity and the b constant stationary phase hydrogen-bond acidity. The / constant incorporates contributions from stationary phase cavity formation and solute-solvent dispersion interactions. The system constants for some common packed column stationary phases are summarized in Table 2.6 [68,81,103,104,113]. Further values for non-ionic stationary phases [114,115], liquid organic salts [68,116], cyclodextrins [117], and lanthanide chelates dissolved in a poly(dimethylsiloxane) [118] are summarized elsewhere. 

System constants derived from the solvation parameter model for packed column stationary phases at 121°C... 

Figure 2.7. Principal component score plot with the system constants from the solvation parameter model as variables for 52 non hydrogen-bond acid stationary phases at 121°C. Loading for PC 1 0.996 a + 0.059 s -0.054 / - 0.024 c - 0.014 r. Loading for PC 2 0.940 s + 0.328 / + 0.080 r + 0.027 c - 0.037 a.

Figure 2.8. Nearest neighbor complete link cluster dendrogram for the stationary phases in Table 2.6. The system constants from the solvation parameter model were used as variables.

The system of stationary phase constants introduced by Rohrschneider [282,283] and later modified by McReynolds [284] was the first widely adopted approach for the systematic organization of stationary phases based on their selectivity for specific solute interactions. Virtually all-popular stationary phases have been characterized by this method and compilations of phase constants are readily available [28,30]. Subsequent studies have demonstrated that the method is unsuitable for ranking stationary phases by their selectivity for specific interactions [29,102,285-287]. The solvation parameter model is suggested for this purpose (section 2.3.5). A brief summary of the model is presented here because of its historical significance and the fact that it provides a useful approach for the prediction of isothermal retention indices. 

Figure 7.7. System constants of the solvation parameter model for retention on a porous polymer stationary phase with a binary mixture of carbon dioxide and 1,1,1,2-tetrafluoroethane as the mobile phase. Column 25 cm X 4.6 mm I.D. Jordi-Gel RP-C18 with a 5 pm average particle diameter. The total fluid flow rate was 1.0 ml/min, backpressure 200 bar and ternperamre I25°C.

An alternative for a more general measure of orthogonality should use specific parameters of the columns being considered, instead of analyte retention data. The solvation parameter model [55] characterizes column retention by using five parameters. These parameters define for each column a vector in a five-dimensional space. If d is the angle between two column vectors, cos 6 will be nearly 1 for very similar pair of stationary phases, while values of cos 6 close to 0 will correspond to column sets of high orthogonality [58]. 

In gas chromatography, a column is coated with a thin layer of a hquid stationary phase and a sample is separated by the relative abilities of its components to dissolve in it from the gas phase. The greater the interactions with the stationary phase, the slower the transit of the sample through the column. Combination of Abraham s solvation parameter model (Eq. (2.1)) [104] with GC measurements has been used to determine the nature of ionic liquid-solute interactions [105-107]. Given the experimental setup, this technique is likely to be particularly appropriate for understanding what ionic liquid-solute interactions could be important in SILP and other related surface-supported ionic liquid applications. 

The solvation parameter model intends to describe the free energy of transferring a solute from the mobile to the stationary phase as a series of product terms representing cavity formation (of hydrophobic solutes in the aqueous mobile phase) and dispersion interactions, plus dipole interactions, hydrogen bonding, and lone pair interactions between polar groups in the solute and polar groups on the solvated surface. 

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