Water absorption clusters

Table 1-1. Nitrogen lone pair natural population (NP(lp)), nuclear shielding (with its paramagnetic and diamagnetic contributions) (ppm) and absorption energies (eV) for pyridazine and water—pyridazine clusters in vacuo (VAC) and in the presence of an external continuum (PCM). For 2 w clusters two values corresponding to the two nonequivalent nitrogens are reported. Calculations are at B3LYP/6-311++G(2d,2p) level for populations and nuclear shielding and at TDB3LYP/6-31+G(d,p) level for absorption energies...

In a another study (.8), we have analyzed the literature data associated with our SANS data and shown that the scattering data on perfluorosulfonated ionomer membranes are consistent with the scattering produced by a group of hard spheres dispersed in the polymeric matrix. The number of ions per cluster was found to change with the water absorption values, with the cation, and with equivalent weight. The occurence of the scattering maxima is due to interference effects between clusters. Further calculations have to be made to take into account a possible anisotropy of these ionic spheres. 

The effect of different cations on the SAXS scan is shown in Figure 13. Very little change in the position of the reflection is seen with changing cation. The intensity of the reflection decreases as the cation weight increases. This is a result of both the change in water absorption and a change in contrast for x-ray scattering between the ionic clusters and fluorocarbon matrix, since the electron density of the cluster will increase... 

In fluid solutions, the resolvation times can be in the subnanosecond time regime. For example, the rapid (<100 ps) relaxation of the excited-state absorption spectra of ruthenium polypyridyl complexes following metal-to-ligand charge-transfer (MLCT) excitation in aqueous solutions have been ascribed to diffusional resolvation of the MLCT excited state. Finally, Robinson and co-workers have provided evidence that the rate of ionization of the singlet excited state of 6-p-toluidine-2-naphthalenesulfonate is determined by the rate at which neighboring solvent fluctuations can form a 3-4 water molecule cluster capable of solvating the electron. 

The different transport mechanisms for cations and anions enable the permselectivity. However, it is insufficient for sulfonic acid fixed ions. The water absorption is too high and the diameter of clusters and especially of channels is too large. An improvement was achieved by decreasing the ion exchange capacity of the membrane material, i.e., with an expanded content of inactive PTFE material (increasing of number n at top of Fig. 2). Then less water is absorbed, the size of clusters and channels is diminished and their number is enlarged. Hence, the described mechanism of permselectivity operates more effectively. But above 80 % Na" ion permselectivity for a catholyte with 20 wt% NaOH is not attainable using this method. 

The deciding success was obtained by changing from sulfonic acid to carboxylic acid fixed ions (see upper part of Fig. 2). Then the water absorption is significantly decreased and obviously the dimensions of the cluster structure are advantageous for typically 98 % Na" ion permselectivity at 32 wt% caustic soda solution. 

Fig. 13 Clustering of hydrophilic domains in Nafion. At low temperatures, the sulfonic acid groups cluster to reduce the repulsive interactions with the PTFE matrix. Entropy drives the sulfonic acid groups to disperse at high temperatures. Water absorption increases the fraction of hydrophilic species inducing microphase separation...

Paul J B, Collier C P, Saykally R J, Scherer J J and O Keefe A 1997 Direct measurement of water cluster concentrations by infrared cavity ringdown laser absorption spectroscopy J. Phys. Chem. A 101 5211-14... 

Surfactants have a unique long-chain molecular structure composed of a hydrophilic head and hydrophobic tail. Based on the nature of the hydrophilic part surfactants are generally categorized as anionic, non-ionic, cationic, and zwitter-ionic. They all have a natural tendency to adsorb at surfaces and interfaces when added in low concentration in water. Surfactant absorption/desorption at the vapor-liquid interface alters the surface tension, which decreases continually with increasing concentrations until the critical micelle concentration (CMC), at which micelles (colloid-sized clusters or aggregates of monomers) start to form is reached (Manglik et al. 2001 Hetsroni et al. 2003c). 

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