A high resolution optical method was used to acquire contact angle-velocity data for hexadecane and octamethylcyclotetrasiloxane (OMCTS) advancing and receding on a smooth, virtually homogeneous fluorinated surface. The predictive abilities of the hydrodynamic, molecular-kinetic and combined molecular-hydrodynamic models of wetting dynamics were compared. In this context the behavior of hexadecane and OMCTS were distinctly different. Where application of the molecular-kinetic model is most appropriate (hexadecane advancing) the motion of the contact line does agree well with the molecular length. However when hydrodynamics play a role and the combined molecular-hydrodynamic model is required, then the characteristic lengths obtained greatly exceed molecular dimensions.

By analyzing the assumptions of the hydrodynamic, molecular-kinetic and molecular-hydrodynamic theories of wetting dynamics, we formulated qualitative criteria for their applicability on the basis of the wettingdewetting asymmetry of the dynamic contact angle/velocity dependence Θ/V presented on characteristic theoretical scales. The quantitative test concerns the physical plausibility of the values of the microscopic dynamic parameters obtained from the fit of the theoretical equations to experimental data. New Θ/V data were obtained and used to compare theory and experiment under well-defined conditions close to the assumptions of the theoretical models. The advancing and receding of methanol, ethanol, 1-butanol, and 1-octanol on the amorphous fluoropolymer AF 1600 were studied. The small AFM roughness (0.3-0.4 nm/ µm 2 ), static angle hysteresis between 3 and 7°, and reproducibility within (0.2°of the dynamic contact angles at different contact line positions demonstrate the good quality of the solid substrates. The use of pure liquids avoids complications with adsorption kinetics, and the low surface energy of the solid and the surface tension of the liquids minimize contamination effects. We found that the molecular-kinetic theory predicting a symmetry of advancing and receding branches of the cos Θ/Ca dependence works well at Ca ) µV/γ < (5 × 10 -4 but cannot explain the asymmetry at larger Ca values. The plot on the characteristic Θ 3 /Ca scale shows that two-region hydrodynamic solutions work above Ca ) (2 × 10 -3 but do not describe the observed wetting-dewetting asymmetry. The same holds for the three-region solutions introducing the slip law or/and geometry of the system as numerical constants. The molecular-hydrodynamic theory fits the data over the whole Ca range studied and represents the wetting-dewetting asymmetry of the Θ/Ca plots. However, it gives inadequately small values of the molecular oscillation frequency K 0 unless the viscous friction in the contact line vicinity is not taken into account. Unreasonably small slip lengths L S,max and distances λ between the potential wells of the liquid molecules that are too large were obtained with all of the theories. They indicate shortcuts of the theoretical models for systems with partial wetting. At the same time, literature data for complete wetting confirm the Θ 3 /Ca dependence and fluid interface shape predicted by the hydrodynamic theory.

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Langmuir monolayers of even n-fatty acids with increasing number N of carbon atoms in the hydrocarbon chain (N ) 11-23) were investigated on 1 × 10 -2 M HCl aqueous subsolutions by means of Brewster angle ellipsometry. Isotherms, relating the coefficient of ellipticity ∆F to the area per molecule F, were continuously recorded and interpreted on the basis of literature phase diagrams and in situ X-ray reflectivity and diffraction investigations. ∆F/N dependencies for the liquid-expanded (LE), super-liquid or solid (LS/S), and condensedsolid (CS) monolayer phases were extracted from the isotherms at constant areas per molecule: F LE ) 28 ( 1 A 2 , F LS/S ) 19.2 ( 0.3 A 2 , F CS ) 18.5 ( 0.3 A 2 . They show ∆F-discontinuities at the LE-LS and S-CS transitions but no change at the LS-S transition. The experimental ∆F/N data for the solid condensed monolayer were compared with calculated ∆F/N dependencies using the "one-layer" ellipsometric model with bulk isotropic or theoretical anisotropic n/N data. A very good agreement of d∆F exp /dN with the slope of the calculated "isotropic" dependence, and a significant difference from the "anisotropic" slope was observed. The calculated "isotropic" ∆F/N dependence was shifted to higher ∆F-values probably due to a different hydration state of the carboxyl groups in the bulk solid phase and Langmuir monolayer. The "one-layer" model was also used in the alternative calculation of the isotropic refractive indices of the LE, LS-S, and CS phases from our ∆F/N data. The obtained n/N dependencies were compared with bulk n/N data for n-fatty acids and n-alkanes which differ only by a terminal COOH group. No agreement was found between bulk and monolayer values. A similarity of the trends was observed for the RCOOH system; both the bulk and monolayer n/N dependencies show discontinuities at the liquid-solid, as well as the LE-LS, phase transition. In contrast, the n/N dependence of the bulk n-alkanes is smooth for N between 5 and 30. Such a behavior shows that the headgroups contribute specifically to the optical characteristics of Langmuir monolayers. A simple "heads-and-tails" model based on the Lorentz-Lorentz relationship was proposed to relate the ellipsometric signal of the monolayer to the polarizability and thickness of the headgroup and hydrocarbon chain regions and to the optical anisotropy of the latter. Application of this model to our ∆F/N data under some physical restrictions shows that the anisotropy of the methyl group in closely packed vertical chains is below 3.3% of the isotropic value. The ellipsometric dimension of the monolayer headgroups obtained implies that they have hydration shells approximately one water molecule thick. The model explains the observation reported in the literature 10 that Cd 2+ and Pb 2+ dissolved in the water substrate shift the ellipsometric chain length dependence to higher ∆F-values without affecting its slope. This effect seems to be due to the increased polarizability and decreased thickness (partial dehydration) of the headgroup regi...