Ionic anion/cation structure

Figure 2.101 General anion/cation structure of ionic liquid phases, 1,9-di(3-vinyl-imidazolium) nonane bis(trifluoromethyl) sulfonyl imidate phase of the SLB-IL100 column (Supelco).

Since hydrofluoride synthesis is based on thermal treatment at relatively high temperatures, the possibility of obtaining certain fluorotantalates can be predicted according to thermal stability of the compounds. In the case of compounds whose crystal structure is made up of an octahedral complex of ions, the most important parameter is the anion-cation ratio. Therefore, it is very important to take in to account the ionic radius of the second cation in relation to the ionic radius of tantalum. Large cations, are not included in the... 

For many 1 1 ionic crystais such as NaCi, the most stabie arrangement is a face-centered cubic array of anions with the cations packed into the hoies between the anions. This structure appears in Figure 11-32. In addition to... 

In an early study, Mauritz et al. investigated anion—cation interactions within Nation sulfonate membranes versus degree of hydration using FTIR/ ATR and solid state NMR (SSNMR) spectroscopies. An understanding of the dynamic ionic—hydrate molecular structures within and between the sulfonate clusters is essential for a fundamental understanding of the action of these membranes in ion transport. This information can be directly related to the equilibrium water swelling that, in turn, influences molecular migration. 

Below the CMC, the surfactant mixing in monolayers composed of similarly structured surfactants approximately obeys ideal solution theory. This means that the total surfactant concentration required to attain a specified surface tension for a mixture is intermediate between those concentrations for the pure surfactants involved. For mixtures of ionic/nonionic or anionic/cationic surfactants, below the CMC, the surfactant mixing in the monolayer exhibits negative deviation from ideality (i.e., the surfactant concentration required to attain a specified surface tension is less than that predicted from ideal solution theory). The same guidelines already discussed to select surfactant mixtures which have low monomer concentrations when micelles are present would also apply to the selection of surfactants which would reduce surface tension below the CMC. 

Our data, to date, show that molecular interaction between two surfactants, both in mixed monolayers at the aqueous solution/air interface and in mixed micelles in aqueous solution, increases in the order POE nonionic-POE-nonionic < POE nonionic-betaine < betaine-cationic < POE nonionic-ionic (cationic, anionic) betaine-anionic cationic-anionic. The greatest probability of synergism exists, therefore, in cationic-anionic mixtures, followed by betaine-anionic mixtures. Synergism can exist in POE nonionic-ionic mixtures only if the surfactants involved have the proper structures. 

A common observation with all the solutes studied is that, no matter how well-defined are the solute-cation and solute-anion interactions, the ionic liquid cation-anion structure is relatively unaffected—cations are still surrounded by a bulk-like arrangement of anions, and vice versa. 

Figure 10.1 Comparison of the structures of an ionic liquid and an organic solvent. In the ionic liquid, cations are solvated by anions and vice versa. ...

Neutral. A bis(ethylenediamine) structure has been incorporated into the surfactant molecule -Ci6H33C(H)[CON(H)(CH2)2NH2]2 in older to incorporate metal ions in an LB film structure via coordination instead of ionic complexation as occurs for anionic/cationic amphiphiles (14). Also, films of n-octadecylacetoacetate containing Cu2+ have been prepared, and exposure to H2S has resulted in the formation of a copper sulfide (39). Ditetradecyl-A-[4- [6-(A, N, W -trimethyl-ethylenediamino)-hexyl]oxy]benzoyl]-L-glutamate (DTG), which also contains the ethylenediamine unit, was used to make self-assembled films containing Cd2+ (40). 

AB2 structures. Fluorides and oxides of the formula AB2, which are distinctly ionic, crystallize in structures determined by size considerations. As in the case of AB structures, it is the coordination geometry of anions around the cation that determines the structural arrangement. The coordination may be 8-, 6-, or 4-fold, fixing the corresponding anion coordination numbers to 4,3 or 2. We thus have the following structures for ionic AB2 compounds fluorite (8 4), rutile (6 3) and silica (4 2) and these structures are shown in Fig. 1.7. 

When pyridoxamine with a dipolar ionic ring structure (Fig. 14-9) and an absorption peak at 30,700 cm-1 (326 ran) is irradiated, fluorescence emission is observed at 25,000 cm 1 (400 ran). When basic pyridoxamine with an anionic ring structure and an absorption peak at 32,500 cm 1 (308 nm) is irradiated, fluorescence is observed at 27,000 cm-1 (370 nm), again shifted 5500 cm 1 from the absorption peak. However, when the same molecule is irradiated in acidic solution, where the absorption peak is at 34,000 cm 1 (294 nm), the luminescent emission at 25,000 cm 1 is the same as from the neutral dipolar ionic form and abnormally far shifted (9000 cm ) from the 34,000 cm-1 absorption peak.185186 The phenomenon, which is observed for most phenols, results from rapid dissociation of a proton from the phenolic group in the photoexcited state. A phenolic group is more acidic in the excited state than in the ground state, and the excited pyridoxamine cation in acid solution is rapidly converted to a dipolar ion. 

MacFarlane et al. [129] and Watanabe et al. [24a, 114] discussed the difference in diffusivity of component ions. Reported diffusion coefficients of ILs are shown in Table 3.19 together with viscosity and ionic conductivity. From that table, it is easy to see that lower viscosity ILs show larger diffusion coefficients and higher ionic conductivity. Cations generally have larger diffusion coefficient values than do anions in ILs. This means that the cation diffuses more easily than the anion. However, the transference numbers of onium cation (t+) in ILs calculated from the results of PFG-NMR is in the range 0.5 to 0.6 and their contribution to the ionic conductivity is mostly the same, irrespective of the ion species. In the case of [bpy][BF4], the BF4- shows a larger diffusion coefficient than that of bpy+, and therefore t+ is below 0.5 [24a], Thus, as well as thermal and electrochemical properties, the diffusion behavior of component ions is dependent on their structure. 

Adsorption can be measured by direct or indirect methods. Direct methods include surface microtome method [46], foam generation method [47] and radio-labelled surfactant adsorption method [48]. These direct methods have several disadvantages. Hence, the amount of surfactant adsorbed per unit area of interface (T) at surface saturation is mostly determined by indirect methods namely surface and interfacial tension measurements along with the application of Gibbs adsorption equations (see Section 2.2.3 and Figure 2.1). Surfactant structure, presence of electrolyte, nature of non-polar liquid and temperature significantly affect the T value. The T values and the area occupied per surfactant molecule at water-air and water-hydrocarbon interfaces for several anionic, cationic, non-ionic and amphoteric surfactants can be found in Chapter 2 of [2]. 

Few studies exist for ionic silicone surfactants. Several trisiloxane anionic, cationic and zwitterionic surfactants have been found to form micelles, vesicles and lamellar liquid crystals. As would be expected, salt shifts the aggregates toward smaller curvature structures [40]. 

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