To clarify the origin of the long-range attraction between hydrophobic surfaces in water, the interaction between the surfaces silanated by the popular method (type I) and that between the surfaces silanated without exposing to air (type II) were examined using an atomic force microscope (AFM) and their characteristics were compared. The interaction between type I surfaces was long-ranged, and a discontinuous step appeared in the approaching and separating force curves, respectively, whereas the interaction between type II surfaces was short-ranged and no step was found. Once type II surfaces were exposed to air, however, the similar interaction to that for type I surfaces appeared. As for type I surfaces, the force curves depended on the local property of the surface, and the interaction in the first cycle of force measurements differed from those in the later cycles. These findings enabled us to estimate the following mechanism for the long-range attraction. When surfaces are hydrophobized, they are usually exposed to air during the hydrophobizing reaction or in the drying process. Then they are immersed in water to measure the interaction without removing microscopic bubbles on the surfaces completely. These bubbles coalesce before the surfaces contact and generate a strong long-range interaction. Hence, this interaction is not the genuine hydrophobic attraction.
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