sample 3 from Ref. [@Sve00a]. The results are presented in Fig. \[fig3\] by the red line. One can see that the force without any water layer agrees reasonably well with the experimental data. The question is what restriction can be derived from this agreement on the thickness of the water layer on the gold surface?

Dielectric function of water {#Sec3.2}
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In order to understand how the water layer will contribute to the force, we have to know first the dielectric function of water at imaginary frequencies $\varepsilon_1(i\zeta)$. Water is a well investigated medium, and there are a few works where the dielectric function was calculated. Parsegian and Weiss [@Per81] fitted $\varepsilon_1(\omega)$ by a number of Lorentzian oscillators as it is traditionally used by spectroscopists. Then the function $\varepsilon_1(i\zeta)$ can be found by analytic continuation. This method, however, does not always give sufficient precision. For example, refitting of the same input data used in [@Per81] gave considerably different result [@Rot96]. Recently a new analytical model for the dielectric function was proposed [@Shu07].

A more reliable approach can be based on the direct use of the optical data of water. It was realized in Ref. [@Dag00] where $\varepsilon_1(i\zeta)$ was found from 