a thin layer of water. The problem is rather nontrivial since surface wettability can be influenced by other adsorbates (e.g., hydrocarbons leading to incomplete wetting), the optical properties of real films must be measured correctly [@Sve08], as well as the influence of surface roughness has to be taken into account [@Zwo08a; @Zwo08b]. The latter lead to uncertainties in the separation distance of real surfaces, and its contribution to the force has to be carefully scrutinized [@Zwo08b; @Zwo08c].

The paper is organized as follows. In Sec. \[Sec2\] we provide information on surface roughnesses and force measurements in the AFM experiment, and discuss the distance upon the contact deduced from the measured roughness. In Sec. \[Sec3.1\] the main definitions of the Lifshitz theory are given, and the roughness correction is related with the measured roughness profile. The dielectric function of water at the imaginary frequencies is described in Sec. \[Sec3.2\]. In Sec. \[Sec3.3\] we discuss optical data for gold in relation with the adsorbed water, and in Sec. \[Sec3.4\] the results of the dispersive force calculations are given for nonzero adsorbed water layer. Our conclusions are presented in Sec. \[Sec4\].

Experimental {#Sec2}
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The dispersive force is measured using the PicoForce AFM 