Water molecule clusters measured at water/air interfaces using atomic force microscopy

Abstract

During the tip approach to hydrophobic surfaces like the water/air interface, the measured interaction force reveals a strong attraction with a range of approximately 250 nm at some points along the interface. The range of this force is approximately 100 times larger than the measured for gold (approximately 3 nm) and 10 times larger than the one for hydrophobic silicon surfaces (approximately 25 nm). At other points the interface exerts a medium range repulsive force growing stepwise as the tip approaches the interface plane, consequently the hydrophobic force is a strong function of position. To explain these results we propose a model where the force on the tip is associated with the exchange of a small volume of the interface with a dielectric permittivity epsilon(int) by the tip with a dielectric permittivity epsilon(tip). By assuming a oscillatory spatial dependence for the dielectric permittivity it is possible to fit the measured force profiles. This dielectric spatial variation was associated with the orientation of the water molecules arrangement in the interfacial region. Small nanosized hydrogen-bond connected cages of water molecules present in bulk water at the interface are oriented by the interfacial electric field generated by the water molecules broken bonds, one broken hydrogen bond out of every four. This interfacial field orients small clusters formed by approximately 100 water molecules into larger clusters (approximately 100 nm). In the limit of small (less than 5 nm thick) water molecule cages we have modeled the static dielectric permittivity (epsilon) as the average response of those cages. In these regions the dielectric permittivity for water/air interfaces decreases monotonically from the bulk value epsilon approximately 80 to approximately 2 at the interface. For regions filled with medium size cages, the tip senses the structure of each cage and the static dielectric permittivity is matched to the geometrical features of these cages sized approximately 25 to 40 nm. Interfacial electric energy density values were calculated using the electric field intensity and the dielectric permittivity obtained by the fitting of the experimental points. The integration of the electric energy density along the interfacial region gives a value of 0.072 J m(-2) for interfacial energy density for points where the hydrophobic force has a range of approximately 250 nm. Regions formed by various clusters result in lower values of the interfacial energy density.

Ice nucleation on hydrophilic silicon

“…Experiments probing the interface between water and hydrophobic silicon have shown that the structure of water is perturbed ͑relative to the bulk liquid͒ 10-15 nm away from the interface. 13 It is in this region that we propose the ice embryos form. The growth of an ice embryo near the interface is hindered by the influence of the silicon; the spectra of both the thin films and droplets show water at the interface which is depleted in the icelike component relative to bulk water or ice at the same temperature, which is indicative of a liquidlike hydrogen bonding network.…”

Polarizable contributions to the surface tension of liquid water

“…Moreover, the nearly unchanged form of the distribution lead us to believe that surface effects on the orientation will continue much further from the interface than we can probe in these simulations. These long-range effects on molecular orientation have recently been studied experimentally by Teschke and de Souza 29 in their atomic force microscopy ͑AFM͒ study of the gas-liquid interface. They show that AFM tips detect attractive forces at distances as far as 250 nm and attributed the phenomena to the broken hydrogen bonds at the interface.…”

Theoretical evidence of maximum intracellular currents versus frequency in an Escherichia coli cell submitted to AC voltage

“…Next, the cytoplasm was likened to a uniform medium with an electrical conductivity of s ¼ 0.22 S/m [Bai et al, 2006]. Regarding the appropriate value to be defined for its relative permittivity, it was set at an intermediate value of e r of 40, which is between the bulk state value of free water (e r-bulk % 80) and its bound state one (e r-bound % 2), usually considered very close to the cell wall [Teschke and De Souza, 2005]. The bacterium was considered immersed in a physiological medium (s ¼ 1.3 S/m, e r-bulk ¼ 80).…”