Water Film Stability

Of course the hydrophobic surface state must not only be described in terms of the elemental surface composition and structure but also must be described in terms of the interfacial water structure; in fact the instability of the interfacial water film accounts for bubble attachment at a hydrophobic surface. The characteristic features of interfacial water and its instability at a hydrophobic surface have not been so well described until recently. Now with the use of atomic force microscopy, surface spectroscopy, and a laser optical cavity technique, these features of interfacial water have been revealed in greater detail.

Direct force measurements during the 1980s and 1990s have revealed that attractive hydrophobic forces are usually 10 to 100 times larger than those expected from van der Waals interactions. These forces extend to distances of as much as 100-200 nm from the surface. The extent of attraction between hydrophobic surfaces is related to the degree of hydrophobicity but seems to be also independently effected by discrete features of the surface like roughness and heterogeneity.

At the same time, during the 1990s, in situ surface spectroscopy (sum frequency generation (SFG) and Fourier transform infrared/internal reflection spectroscopy (FTIR/IRS)) of water at hydrophobic surfaces has revealed important characteristics of interfacial water. The SFG spectral information clearly shows a distinction between water at a hydrophobic surface and water at a hydrophilic surface. Interfacial water at a hydrophobic surface is distinguished by a stronger absorption band at 3600 cm-1 characteristic of a dangling free OH bond. In contrast, interfacial water at a hydrophilic surface is distinguished by a diminished absorption band at 3600 cm"1 and a stronger signal at 3200 cm"1 characteristic of an ice-like structure with complete tetrahedral coordination. Based on these surface spectroscopy studies, it appears that interfacial water at a hydrophilic surface can be viewed as organized dipoles in tetrahedral coordination and oriented with respect to the polarity of the hydrophilic surface, whereas interfacial water mol ecules at a hydrophobic surface are not so well organized at the surface and have incomplete tetrahedral coordination with dangling free OH bonds.

It might be assumed that this in situ spectral data can then be used to account for film instability at a hydrophobic surface. Unfortunately, it seems that the phenomenon is not that simple. It is expected that the interfacial water structure will extend only a distance of a few molecular diameters, not more than a few nanometers or so. On the other hand, the hydrophobic attractive forces can extend to 100 nm, and even more. Thus it would seem that film instability at a hydrophobic surface involves more than just the hydrogen bonding characteristics of interfacial water.

Some researchers have attributed film instability to cavitation phenomena. The presence of nanobubbles or defects in the interfacial water region at a hydro-phobic surface has been reported based on experimental results using a laser optical cavity technique. Also it should be noted that surface force measurements reveal that the range of the attractive hydro-phobic force is significantly greater in gas-saturated solution then in degassed solution. It is expected that slight perturbations in the pressure field would cause these nanobubbles to coalesce and form cavities which upon further coalescence would lead to cavita-tion and failure of the water film at a hydrophobic surface as shown in Figure 4. In some cases, discontinuities during force measurements were observed which may be attributed to the phase transition (cavity formation) between approaching surfaces. Finally, recent FTIR/IRS spectroscopic evidence, indeed, shows that dissolved gas is accommodated at a hydro-phobic surface but not so at a hydrophilic surface. Thus the presence of nanobubbles in the interfacial water region of a hydrophobic surface is supported by these spectroscopic results.

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Solar Panel Basics

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