Drying transition of water near hydrophobic surfaces

3 Drying transition of water near hydrophobic surfaces 

Drying transition may occur in a liquid phase upon heating along the liquid-vapor coexistence curve (see Section 2.1). This transition has drastic effect on the liquid-solid interface above the temperature Tj of a drying transition, the liquid is separated from the solid surface by a macroscopic vapor layer. However, even below Tj and out of the liquid-vapor equilibrium, distant etfect of the drying transition may noticeably affect the liquid density profile. Therefore, it is important to know the temperature of the drying transition of water and its sensitivity to the water-surface interaction. This allows description of the density profiles of liquid water near hydrophobic surfaces at various thermodynamic conditions. 

Various experimental methods were used to study density depletion of a liquid water near hydrophobic surfaces. In some experimental studies (ellipsometry [223, 224] and neutron reflectivity [225]), density depletion was not found [225]. In many other studies (neutron reflectivity [145, 226-228], X-ray reflectivity [229-231], ellipsometry [232], thermal conductivity [233], liquid water intrusion in hydrophobic pores [234]), noticeable depletion of the hquid water density near various hydrophobic surfaces was detected. Density depletion was found sensitive to the presence of dissolved gases [227], but no such sensitivity was observed in other studies [230, 231]. The available experimental methods do not give exphcit profiles of a liquid water density but rather allow estimation of a density deficit near the surface. For simplicity, this deficit may be attributed to the homogeneous vapor film (the so-called slab approximation), whose thickness D can be used as a measure of a liquid density depletion. If the realistic liquid density profile is close to a sigmoid-like function, D approximately corresponds to a distance between the inflection point of this function and the surface. 

In experimental studies, the effective thickness ) of a depletion layer is about 6 A for silica surface hydrophobized with the monolayer of trimethylsilil groups [234], For water near a octadecyltriethoxysiloxance monolayer, D is between 0.8 and 4.0 A [230]. Near octadecyltrichlorosi-lane monolayer, various values of D were reported 1.1 A [231], 2.5 A [233], and 5.2 A [227], The thickness of a depletion layer of liquid water near a film of polystyrene was estimated as 5 to 10 A by ellipsometry studies [232] and as 1.3 to 3.0 A by neutron reflectivity experiments [145]. These estimations of D clearly evidence the absence of a vapor layer at hydrophobic surfaces. Even if one attributes liquid density depletion to the vapor layer only, the thickness of this layer is just two to three molecular diameters, which is comparable to or less than the width of the liquid-vapor interface. 

a stable macroscopic vapor layer between a liquid and a solid or its formation via a surface phase transition (drying transition) was never observed experimentally for water and for any other fluid. This well agrees with the theoretical expectations [117, 118, 124] that a drying transition is suppressed by the long-range fluid-surface interaction up to the bulk liquid-vapor critical temperature Tc. Even for the extremely weak fluid-surface interface (liquid neon on cesium), formation of a 

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