Retention physico-chemical bases

The physical theories for electric charges have been incorporated in colloid and surface chemistry for many years. In the treatment presented here, these theories have been selected, adapted, and applied to describe the retention of ionic solutes. The approximations made in these models are well known and have limitations. Here, they are chosen from four requirements they should have a meaningful physico-chemical interpretation, be easy to use, be easy to understand, and give practical and useful results. This implies that the presented models are useful starting points for describing and understanding the retention properties for the type of systems that are discussed here. When the experimental results deviate from the model, it may be possible to extend it within its framework. In other situations, empirically based functions may complement the model, or it may be necessary to resort to more sophisticated models. 

In conclusion, it is easier to develop useful theories for the retention caused by the Coulombic interaction than for the other types of intermolecular interactions. Furthermore, the strength of the Coulombic interaction implies that it usually determines the physico-chemical properties of ions. Theories based on the solution of the P-B equation with appropriate boundary conditions are, therefore, a good starting point for the retention theories of ionic solutes. 

The LC methods discussed before were based mainly on physico-chemical interactions between the solute on the one hand and the two chromatographic phases on the other. Although we have seen that in RPLC the degree of ionization of weakly acidic or basic solutes may be a major factor in the control of retention and selectivity, the ionic species themselves were not exploited purposefully to realize or enhance the separation. In fact, in a typical RPLC system all fully ionized solutes will show little retention and therefore little resolution can be achieved between different ions. The methods described in this section make positive use of the ionic character of solutes to create a chromatographically selective system. 

Similar equations but based on other solute descriptors were proposed in literature with the aim of better chromatographic data [Abraham, Ibrahim et al, 2004]. In particular, five solute descriptors, here called LafFort solute descriptors (Table LI), were defined by Laffort et al. using GLC retention data on five stationary phases for 240 compounds [Laffort and Patte, 1976 Patte, Etcheto et al., 1982[. These solute descriptors were used to fit a number of physico-chemical and biochemical properties. Note that in the first paper [Laffort and Patte, 1976[, the five solute descriptors were obtained by Principal Component Analysis on the data obtained from 25 stationary phases for 75 compounds, thus their numerical values differ from those obtained in the later paper. 

Methods based on experimental physico-chemical properties such as partition coefficients, chromatographic retention, boiling point, and molecular volume. 

The equations that describe the retention on hybrid micellar mobile phases were first derived on a pure empirical basis. A further concern was to find an interpretation of these equations, based on physico-chemical properties. This permitted the improvement of the descriptive models, and the evaluation of the parameters of interaction between the three environments involved in MLC stationary phase, bulk water, and micelles, according to equilibria 8.1 and 8.2 [ 18]. The coefficients in eq. 8.26 were related to several parameters of retention. From the reciprocal of this equation ... 

A different approach to the systematic characterization of stationary phases is the correlation of analyte structure and retention in a given chromatographic system with the help of quantitative structure retention relationships (QSRR), a distinct discipline of linear free energy relationships (LFER). In QSRR, the total retention of an analyte is separated into individual contributions such as dipole-dipole, K-n, acid-base, and hydrophobic interactions. This approach enables strict interrelations on the basis of fundamental mechanistic aspects in order to improve the physico-chemical understanding of chromatographic retention. 

The most simple and efficient approach is based on gelation which is a simple method that allow a good compromise between the retention of the IL and its fluidity inside the polymeric network. These so called ion gels are simpler than solid polymer electrolytes and exhibit improved conductivities. In fact ion gels hold both the processability and mechanical properties of polymers, but with added physico-chemical properties and were primary developed as replacements for current solid-state polyelectrolytes in energy devices, such as dye-sensitized solar cells, supercapacitors, lithium ion batteries, and fuel cells. (Fernicola et al., 2006 Galinski et al., 2006 Le Bideau et al., 2011 Lu et al., 2002 Mazille et al., 2005 Stephan, 2006)... 

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