Water Density in the Double Layer

Abstract

A recently proposed approach to the problem of ion hydration in solutions is extended to water at a charged metallic electrode. Water in the closest proximity of the electrode is compressed due to electrostriction and the dipole moments of H 2 O molecules become completely ordered by extremely strong electric field created by the surface charge. It is estimated that for common values of the charge density of the order of 10 -1 C m -2 the effect of striction corresponds to that of pressures of the order of gigapascals. A consistency between the present approach and recent X-ray measurements of the density of water at a silver electrode is discussed.

Water Density in the Electric Double Layer at the Insulator/Electrolyte Solution Interface

“…I note that the value of F within the accuracy of the experiment is equal to the density of water under normal conditions, F w ) 3.34 × 10 29 e -/m 3 . 27 The following equation, similar to eq 4, relates the volume content of silica in the low-density layer, f 2 , 1, with its electron density F 2 :…”

“…25 I note that eq 4 does not correctly take into account the polarization of the interface and the changes of the dielectric properties of the media in the transition region. Second, the concentration of particles in layer 3, c 3 -, significantly exceeds that in the bulk concentration, c b :…”

“…In fact, this estimation of δF/F is considerably lower than the values calculated earlier for the density of water in the first layer of molecules at the electrode's surface with a similar surface charge. 3 A comparison with the calculations of I. DanielewiczFerchmin and A. R. Ferchmin would not be valid if the charge in the compact layer were screened by a diffuse layer of OHlocated near the surface with hexane. However, this situation is unlikely: there is a huge deficit of free hydroxyl ions in the solution that would prevent the buildup of any significant countercharge in layer 1.…”

“…Concentrated suspensions of colloidal silica particles in water (obtained from Dupont), of approximately 70 Å in diameter, D, at pH ≈ 10 were stabilized with alkali. The resulting homogeneous solution of NaOH and solid silica particles in water (30% SiO 2 and 0.5% Na by weight) had a specific gravity, ς, of 1.177 ( 0.003 g/cm 3 . The molar concentration of free hydroxyl ions in the sol bulk is extremely low, c -≈ 10 -4 mol/L, compared with the concentration of sodium ions, c + ) f Na ς/M Na ≈ 2.4 × 10 -1 mol/L (M Na ≈ 23 g/mol is the atomic weight of Na, and f Na is the weight fraction of sodium in the suspension), because of the adsorption of OHions at the silica surface.…”

“…A field of such strength is impossible to obtain in a electrolytic capacitor, but is common in the first hydration shell of a small inorganic ion (for instance, Na + , Cu 2+ , Al 3+ ). 3 Here, I report the findings on the water density in the electric double layer, which was explored by combining the techniques of X-ray reflectivity and grazing incidence small-angle scattering.…”