Ion-aggregate formation

The investigation of ion-aggregate formation based on conductivity studies can be extended to quadruple-ion formation [108-111], which is thought to be the reason for the conductivity maximum and the subsequent decrease of conductivity. 

The addition of alkali and alkaline earth metal perchlorates to AN causes in the region of the C-N stretching band of AN the appearance of a band produced by acetonitrile molecules in the solvation shells of the cations (fig. 1 for NaClOt solutions) a band of the originally triply degenerate antisymmetric Cl-O stretching vibration splits at increased salt concentration due to ion-aggregate formation (fig. 2, for a solution of LiClO ). The bands (a) and (b) in fig. 2 tire assigned to the free anions, band (1) is due to the vibration of a contact ion pair and band (2) to that of a solvent separated ion pair. Alkaline earth metal perchlorate solutions reveal in addition to the free anion bands (a) and (b) three bands due to [M + C10 )2Y,[M - C10 ]+ and [M + AN)C10 ]+,M + = Mg +,Ca +,Ba +. 

FIGURE 6 Ion-pair, triple-ion, and higher-ion aggregate formation exemplified by measurements of the equivalent conductivity of UBF4 in 1,2-dimethoxyethane solutions at 25°C and -45°C. [From Barthel, J., Gerber, R., and Gores, H.-J. (1984). Ber. Bun-senges. Phys. Chem. 88, 616-622.]... 

Once the concept of solubility is understood, students look at animations that show two solutions being mixed resulting in the exchange of ions and formation of an insoluble compound (see the screen captures in Figs. 6.2, 6.3 and 6.4). Students generally do not consider how precipitates are formed, so the animation showing how the ions (at the sub-microscopic level) attract each other and aggregate to form a precipitate (at the macroscopic level) will help them to understand the interactions of the ions involved. 

Ion-pair formation (or the formation of triplets, etc.) is a very simple kind of interaction between ions of opposite charge. As the electrolyte concentration increases and the mean distance between ions decreases, electrostatic forces are no longer the only interaction forces. Aggregates within which the ions are held together by chemical forces have certain special features (i.e., shorter interatomic distances and a higher degree of desolvation than found in ion pairs) and can form a common solvation sheath instead of the individual sheaths. These aggregates are seen distinctly in spectra, and in a number of cases their concentrations can be measured spectroscopically. 

The present author has developed a novel method called ion-association method. This is also a simple and versatile method for the preparation of ion-based organic dye nanoparticles in pure aqueous solution by the ion association approach [23]. It utilizes the control of hydrophilicity/hydrophobicity of the ionic material itself via ion-pair formation for example, addition of a cationic target dye solution into aqueous solution containing a certain kind of hydrophobic anions forms an electrically neutral ion-pair because of the strong electrostatic attraction, followed by aggregation of ion-pair species originated from van der Waals attractive interactions between them to produce nuclei and the subsequent nanoparticles (Fig. 3). In this case, hydrophobic but water-soluble anions, such as tetraphenyl-borate (TPB) or its derivatives (tetrakis(4-fluorophenyl)borate (TFPB), tetrakis [3,5-... 

The formation of solvent separated ion aggregates is largely determined... 

Main-group organometallic compounds are versatile tools in organic synthesis, but their structures are complicated by the involvement of the multicenter, two-electron bonds and ion-dipole interactions that are involved in aggregate formation (5). Electron deficiency or Lewis acidity of the metallic center and nucleophilicity or basicity of the substituents are important considerations in synthesis. The complexity of the structures and interactions is, however, the origin of much of the unique behavior of these organometallic compounds. 

Dye Fixation. On the dried fabric, the dye is only deposited on the fiber surface. It must penetrate into the fiber during a fixation step and be incorporated in the fiber by chemical reaction (reactive dyes), aggregation (vat, sulfur dyes), ion-pair formation (acid, cationic dyes), or in the form of a solid solution (disperse dyes). 

According to the MD simulations examined for cesium iodide solutions of 2.78 mol dm-3 (Csl H20 = 1 20) at 25 and 64°C and 5.56 mol dm-3 (Csl H20 = 1 10) at 64°C (34), higher ion pairs than 1 1 form in the 2.78 mol dm-3 solution, but formation is suppressed as the temperature is raised. Thus, ion-pair formation reactions between cesium and iodide, both of which are weakly hydrated, must be exothermic, although no thermodynamic data are available in the literature. The increase in concentration at a given temperature (64°C) enhances the formation of the ionic aggregates from raCsI = 1.07 to 1.80. 

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