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 ∼250 nm at some points along the interface. The range of this force is ∼100 times larger than the measured for gold (∼3 nm) and 10 times larger than the one for hydrophobic silicon surfaces (∼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 ε_int by the tip with a dielectric permittivity ε_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 ∼100 water molecules into larger clusters (∼100 nm). In the limit of small (less than 5 nm thick) water molecule cages we have modeled the static dielectric permittivity (ε) as the average response of those cages. In these regions the dielectric permittivity for water/air interfaces decreases monotonically from the bulk value ε ∼80 to ∼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 ∼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 ∼250 nm. Regions formed by various clusters result in lower values of the interfacial energy density.