the dispersive force is considerably masked by large error in the separation upon contact $d_0$. On the other hand, for the distances $d<10\; nm$ the force measurements are rather limited due to strong jump-up-to contact by formation of capillary bridges [@Zwo08d]. In addition for these distances not only water layer but also roughness is a significant factor, which increases the force up to five times with respect to flat surfaces as our former studies [@Zwo08a] indicated. All this lead to a rather complex situation at $d<10\; nm$.

Conclusions {#Sec4}
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In conclusion, we investigated the influence of an ultra thin water layer on the dispersive force between $Au$ surfaces. Evaluation of the force in terms of the Lifshitz theory predicts a strong influence of the water layer on the force, especially for small separations $d<10\; nm$, where a higher force is obtained because the effective distance between the $Au$ surfaces decreases. Furthermore, the theoretical predictions are compared to the experimental measurements for distances $d\geq 13\; nm$ (limited due to strong jump-up-to contact by formation of capillary bridges). It is shown that although the water layer increases the force, it falls within the error margins of the measured force. The errors 