ratio $h/\lambda$, which is not very small. This frequency range corresponds to the interband absorption of gold where the dielectric function $\varepsilon_2$ cannot be predicted. For this reason we cannot determine $h$ because we have no reliable value to compare with $\langle \varepsilon\rangle-\delta\varepsilon$.

We can conclude that the dielectric function measured ellipsometrically is a good approximation for the dielectric function of pure gold even if there is a layer of adsorbed water. Information on the thickness of the water layer $h\sim 1\;nm$ cannot be extracted from the data since it is below the noise level.

The dispersive force for nonzero water layer {#Sec3.4}
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![The force between a flat sphere ($R=50\;\mu
m$) and a plate in the range of small separations between gold surfaces. The curves correspond to different thicknesses $h$ of the water layer.[]{data-label="fig5"}](fig5.eps){width="45.00000%"}

In the evaluation of the force it will be assumed that water forms a continuous film on the gold surface. In general it can be not the case because water can wet metal surface incompletely due to presence of hydrocarbons and other chemicals on the surface. In the case of incomplete wetting one can use the approach of effective dielectric functions [@Asp82], which reduces the 