Solution complexation, versus solvent water

Figure 3.31 As (due to orientational response of aqueous solvent) versus e, calculated for ET in a large binuclear transition metal complex (D (Ru2+/3+) and A (Co2+/3+) sites bridged by a tetraproline moiety) molecular-level results obtained from a nonlocal polarization response theory (NRFT, solid lines) continuum results are given by dashed lines, referring to numerical solution of the Poisson equation with vdW (cont./vdW) and SAS (cont./SAS) cavities, or as the limit of the NRFT results when the full k-dependent structure factor (5(k)) is replaced by 5(0) 5(k) for bulk water was obtained from a fluid model based on polarizable dipolar spheres (s = 1.8 refers to ambient water (square)). For an alternative model based on TIP3 water (where, nominally, 6 = ), ambient water corresponds to the diamond. (Reprinted from A. A. Milishuk and D. V. Matyushov, Chem Phys., 324, 172. Copyright (2006), with permission from Elsevier).

Alkylbenzenes and alkylnaphthalenes were studied on a silica colunui using hexane, hexane/1-chlorobutane, hexane/l-bromobutane, or hexane/IPA as the mobile phase [619]. Modifier levels ranged fiom 0.005% to 10%. Capacity fiictors versus carbon number were plotted for each solvent mixture. Selectivity decreased for all solvent modifiers except 1-chlorobutane, for which selectivity increased as the level increased from 2% to 8%. The authors attribute diis to the formation of n-complexes between the 1-chlorobutane and the PAH solutes. Selectivity decreased, as expected, for the alkylnaphthalenes when I-bromobutane was used. Selectivity was lost rapidly as the level of IPA increased from 0.01%0.05% indicating that at low IPA concentrations IPA (or the water contained in the IPA) readily modifies or deactivates the silica support. 

Kiel et al. [1450], studied the effect of the presence and absence of an amine buffer and solvent pH on the retention and tailing of nortriptyline, desmethylnor-triptyline, and amitriptyline. A Cg column and a 50/50 acetonitrile/water (25 mM TEA) solvent gave baseline resolution, excellent peak shape, and elution within 4 rnin. Removal of the TEA led to significant peak tailing and an elution time of nearly 7 min. With no TEA, the mobile phase was buffered to different pH values (2.5-8) with 0.1M phosphate. A U-shaped plot of k versus pH resulted. This phenomenon was not because of a protonated-to-deprotonated form of the amine compounds, their piif, values are >8. Rather, it is due to surface silanol acidity functions and the complex interaction of the solutes with these sites. Therefore, it is important to consider the use of a basic mobile phase modifier when analyzing basic compounds. 

---