Mean Residence Times of Solvent Molecules Near Ions

1 Mean Residence Times of Solvent Molecules Near Ions 

The mean residence times, MRT, of water molecules in the immediate vicinity of ions were studied extensively by means of these quantum-mechanical combined with molecular-mechanical computer simulations as reviewed at the time by Hofer et al. [70], The computational program employed has evolved over the years as was the minimal time t, above which a molecule is deemed to have left its position in the immediate vicinity of an ion, from 2ps in the earlier studies to 0.5 ps used in the later ones. The MRT of water molecules in the bulk solvent, r =1.7 ps, is only one-tenth of the time it takes the molecule to diffuse completely away. The relative mean residence times of water molecules in the second hydration shell to that in bulk water, RMRT = /t w (in %) at 25°C, are shown in Table 5.4. The MRT of water in the first hydration shells of multivalent ions are longer than could be studied by the computations. The RMRTs of water molecules near the ions are roughly proportional to the surface density of the charge on the ions, o. RMRT=0.22+l.l4(oJC mrr ), but exceptions are noted. 

The relative mean residence times, RMRT, of water molecules in the vicinity of ions indicate whether the ions are water structure making if they are 100% or structure breaking if they are 100% [91]. Note that the RMRTs for large univalent ions, both cations and anions, are 100%. This means that around such ions, the water molecules are more free to move than those bound in the hydrogen-bonded network 

The RMRT of water molecnles in the second hydration shell of cations (the first for univalent ones) are compared in Table 5.4. with log(A /s), the (logarithm of the) experimental (mainly from NMR measurements) rate constant of the first-order reaction of water molecules leaving the hydration shells of cations in exchange for incoming molecnles. The larger the RMRT of the water molecules, the slower is the exchange as measnred by log(A /s), but a definite proportionality or linear dependence could not be established. 

Similar stndies have been made on ions in liquid ammonia (at 240K). The mean residence times of ammonia molecules in the second solvation shells of the ions stndied are longer than for water molecules 12.7ps compared to 2.6ps for Ag [116], 28.5 ps compared with 6.5ps for Co [117], but shorter in the case of Cu 3.2ps [118] compared with 7.7ps for water [91]. Molecular dynamics computer simulations of solutions of ions in liquid ammonia [119] yielded the self-diffusion coefficients of ammonia molecules, D/10 m s , in the solvation shells of 6.1 and of R 7.4, shorter than the value for ammonia molecules in the bulk liquid, 11.5 1.5. These studies thus indicate that K and R are structure breakers and Ag and Co are structure makers regarding the inherent structure of liquid anunonia. 

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