Aggregation of Ions

There is apparently some aggregation of ions as a result of the formation of OH bridges to give species such as [(H20)4A1(0H)2A1(H20)4]4+. In the vast majority of complexes, aluminum has a coordination number of 6, but the number can also be 4 as in LiAlH4. 

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-... 

Keeping in mind the above "generalisation" organic molecules may be considered, at least formally, as aggregates of ions (and will be eventually, represented with formal charges on the atoms). 

Colloids are similar to solutions in that they consist of one phase uniformly dispersed in a second phase. There are many common examples of colloids, including milk, blood, paint, and jelly. However, colloids are not true solutions because the particles in the dispersed phase are not the size of molecules or ions. The particles in a colloid range in size from 10 to 200 nm. The dispersed particles might be supersized molecules (e.g., proteins) or aggregates of ions. While these particles are typically too small to be discerned, even by a microscope, they are much larger than, say, a sodium ion, which has a diameter of about 0.1 nm. 

Alkali metal derivatives of organic compounds exist as aggregates of ion pairs, namely, dimers, trimers, and tetramers, in solvents of low polarity. 

Exchange between ions and ion pairs of different activities has been analyzed in anionic systems and the small broadening of polydispersities was used for the evaluation of the dynamics of the exchange [2,19]. When exchange becomes slower, as in the case of aggregates of ion pairs, then polydispersities increase and distribution may become bimodal [20]. 

It has been pointed out that the formation of a precipitate can often be considered the find stage in the formation of polynuclear complexes. Aggregates of ions that form the building stones in the lattice are produced in the solution, and these aggregates combine with other ions to form neutral compounds. 

NMR techniques.116 Finally, the possible aggregation of ion pairs to form species such as ion quadruples, hextuples, and higher-order aggregates has also been investigated.101,105,121-125 The main conclusion seems to be that, at the concentrations typically used in olefin polymerizations, catalyst ion pairs are unlikely to be present as higher aggregates.105,123,124 These aspects have been summarized in pertinent reviews.41,116... 

Aggregation of ion-pairs has been demonstrated in the polymerization of ethylne oxide (... -CH206 K in THF solvent) apparently cyclic trimers of ion pairs dominate, formed with the equilibrium constants equal approx, to 10 107 l2.mol"2. This value was determined from the analysis of the kinetics of polymerization (30). Polymerization of e-caprolactone with Na as counterion in THF solvent also shows the 1/3 dependence of the rate of polymerization on the total concentration of active species... 

Added salts can encourage the aggregation of ions to micelles, salting out . This is of considerable importance in biological electrolytes, such as bile salts and phospholipids. 

The very low dielectric constant of dioxane of about 2.2, comparable to that of hydrocarbons, may induce some aggregation of ion-pairs in this solvent. Such an aggrega-... 

It seems that the aggregation of ion-pairs is less pronounced in the potassium system than in the sodium, while their direct dissociation into free ions, as well as their capacity to form triple ions, is enhanced. 

Of course, other equilibria may occur in a given system of interest, requiring a more complex treatment, and it is worthwhile at least to mention some of the possibilities. Beyond simple ion pairs, species formed by aggregation of ions and ion pairs may exist, especially when ion solvation is weak, the solvent-phase salt concentration is high, and the anion has amphiphilic properties. For example, ion pairs may aggregate ... 

Amorphous calcium carbonate (ACC) is a metastable precursor to crystalline CaCOa phases that precipitates by aggregation of ion pairs and prenucleation clusters. Ca solid-state NMR spectroscopy was used to probe the local structure and transformation of ACC synthesized from seawater-like solutions with and without... 

A colloid is a dispersion containing particles between 1 nm and 1000 nm. Colloid particles are usually aggregates of ions or molecules. 

Association colloids consist of special aggregates of ions and molecules called micelles. To understand how these arise, consider sodium stearate, a typical soap... 

It will be pointed out later in this chapter that the description of compounds as aggregates of ions is an approximation. The electronic structure of molecules and crystals usually described as ionic involves only a partial transfer of electrons from the metal atoms to the nonmetal atoms. Nevertheless, the discussion of ionic valence in relation to the argononic electron configurations, as given above, is an important and useful part of-chemical theory. 

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