From microhydration to bulk hydration of Rb+ metal ion: DFT, MP2 and AIMD simulation study

Role of Counterions in the Adsorption and Micellization Behavior of 1:1 Ionic Surfactants at Fluid Interfaces─Demonstrated by the Standard Amphiphile System of Alkali Perfluoro-n-octanoates

“…Furthermore, there are also works contradicting this conclusion by stating the inverse, namely, that the effective (stable) hydration number of the lithium ion will be rather 4 instead of 6 11,43,50,63 or that those of Rb + and Cs + are 8 and 10 35,41 or 7 for the Rb + -ion's first hydration shell. 64 The most disappointing conclusion of ours is that obviously it has, so far, not been possible to clearly establish whether there will be a discrete effective, stable hydration shell at all, or whether there will be an additional second one, the nature of which has to be discriminated from the former. 35,36,38,62 These studies strive to find out the thermodynamically most stable hydration number.…”

“…Estimating these results on the background of the voluminous investigations on the hydration of alkali and alkali earth metal ions in aqueous solution, − ,,− we have to realize that their results have so far been dissatisfied. Although modern methods are applied such as nuclear magnetic resonance spectroscopy (NMR), ,− X-ray diffraction, , ionization spectroscopy, Raman, infrared, and mass spectroscopies, and calorimetric, cation-exchange, ,, as well as electro-catalysis studies, we do not get concrete, reliable information on the real nature of the hydration sphere.…”

“…The endeavor to investigate the nature of the cations’ hydration is accompanied by quite a few theoretical ones on molecular dynamics simulations applying various options. ,,,,− ,− These approaches may simulate the nature of hydration by applying quite a lot of molecular properties such as the hydration structure, partition function ratio, dipole moment, residence time, self-diffusion coefficients and orientation of water molecules, ,, electrostatic effects, angle of dipoles, distribution sphere around the cation, M–O bond distance, , and flexibility within the hydration shell . In most cases, communications provide the average number of hydrate water molecules around the cation.…”

“…Usually, the strongest hydrated cation Li + is assumed to possess six or seven water molecules, whereas the less hydrated ones Rb + and Cs + are estimated to have only two to four. Furthermore, there are also works contradicting this conclusion by stating the inverse, namely, that the effective (stable) hydration number of the lithium ion will be rather 4 instead of 6 ,,, or that those of Rb + and Cs + are 8 and 10 , or 7 for the Rb + -ion’s first hydration shell …”

“…However, this was apparently limited to the three lightest alkali cations of lithium, sodium, and potassium because the two heaviest ions of rubidium and cesium were seemingly of equal size, namely, like that of potassium. The resulting discrepancy remained chemically inconsistent insofar as the surfactant adsorption of the lightest counterions would thus be exclusively determined by the counterion, whereas it would become negligible for the heavier counterions. The results of modern methods of investigating the hydration of alkali and alkali earth metal ions in aqueous solution − ,,− have so far been dissatisfying. They do not provide concrete, reliable information on the real nature of the hydration sphere.…”