Calixarenes aqueous media

The ESIPT of 2-(2 -hydroxyphenyl)-4-methyloxazole (HPMO) (27) has been explored by Douhal and co-workers [166] for its probe characteristics in a variety of organized media which include cyclodextrin, calixarene, micelle, and HSA. The incorporation of HPMO into hydrophobic cavities in an aqueous medium involves the rupture of its intermolecular hydrogen bond to water and formation of an intramolecular hydrogen bond in the sequestered molecule. Upon excitation (280-330 nm) of this entity, a fast intramolecular proton-transfer reaction of the excited state produces a phototautomer (28), the fluorescence of which (Xm = 450 170 nm) shows a largely Stokes-shifted band. Because of the existence of a twisting motion around the C2—C bond of this phototautomer, the absorption and emission properties of the probe depend on the size of the host cav-... 

Cucurbit[n]urils (n = 1 and 8) (38) have also been used for the molecular recognition of zwitterionic species in aqueous medium (Figure 16). These systems show a hydrophobic cavity with the urea-type carbonyls pointing to the corresponding entrances. This peculiarity implies binding properties with some resemblance to other cavitands (CDs, calixarenes, or resorcinarenes). The size of the cucurbituril receptor can be controlled by the number of monomers present in the structure, and they are usually named as CBn (with n being the number of repeating units). 

Results obtained by ES/MS confirm that the stability of calixarene/cation complexes depends upon the medium. The calixarene in solution presents a strong affinity for cesium, whereas in the gas phase, it displays a stronger affinity for sodium. Moreover, the stability of calixarene/Na+ complexation in a solvent phase is increased by the presence of water in the dilution system (up to 40% in acetonitrile), whereas other alkali complexes are destabilized by the presence of water. Finally, affinity for sodium, which is weak in the solution for calixarenes bearing benzo moieties, considerably increases in the gas phase. These results confirm the interpretation of the MD simulations in an aqueous phase, which lead the authors to conclude that cesium-over-sodium selectivity is governed by the hydration of the sodium cation in the complex, and by the higher hydrophobicity of the complexation site leading to an enhancement of selectivity for cesium over sodium 49... 

Removal of cesium from medium-level radioactive wastes involves extraction of cesium from aqueous solutions which are 1 M in HNO3 and 4 M in NaNOa (8). In a preliminary study, extraction experiments with ligands 1-7 were performed by mixing equal volumes (5 to 7 mL) of aqueous and organic solutions (calixarene 10 M in 2-nitrophenyl hexyl ether or 2-NPHE) in sealed polypropylene tubes for one hour at room temperal re (25 1 C). The aqueous solutions contained either NaNO or CSNO3 (5.0x10 M) in HNO3 (1 M) to assess the selectivity toward cesium in the hypothetical presence of sodium. A measure for the selectivity was assumed to be the ratio of the distribution coefBcients obtained separately for both cations ... 

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