4 December 2003 New insights in static and dynamic properties of Gibbs monolayer
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Abstract
In this paper we discuss selected equilibrium and dynamic properties of adsorption layers of soluble surfactants. The surface state has been investigated by nonlinear optical techniques based on second order χ(2) effects which exhibit a high surface specificity and suppress bulk contributions. The surface tension isotherm σ(c )of the homologous series of n -alkyldimethylphosphine (n =8 −12) can be described by Frumki ’s equation of state which yields the surface interaction parameter, surface coverage and the corresponding area per molecule A . The comparison of the surface tension σ at a given area per molecule A reveals a strong alternation within the homologous series. Odd C2n±1 layers show a lower surface tension than the adjacent even members C2n of the homologous series. This effect is also present at low surface coverage (A =1.4nm2)and cannot be attributed to a differences in the chain-packing within a crystalline state. Infrared-Visible Sum-Frequency Generation Spectroscopy (SFGS)has been used to monitor the orientation and chain order within the aliphatic tail. SFGS spectra have been recorded for different chain lengths and at different areas per molecule. The analysis of the spectra yields an order parameter G which is proportional to the number of gauche defects within the aliphatic tail. The odd-even effect in the surface tension turned out to be accompanied by an odd-even effect in the order parameter G. The data suggest that an ordered structure has a bigger impact on the surface tension than an unordered structure. The odd-even effect is also observed in the orientation of the terminating methyl group as retrieved by polarization dependent SFGS measurements. The data shed some light in the relation between molecular and macroscopic properties. Furthermore surface dilatational viscoelastic properties of a fluorinated amphiphile have been measured by a novel version of the oscillating bubble. The oscillating bubble method generates a non-equilibrium state by a harmonic compression and expansion of the surface layer formed at the tip of a capillary. The surface state is monitored by Surface Second Harmonic Generation (SHG).This technique is highly surface specific and discriminates between monolayer and subsurface coverage. Our set-up allows to measure the monolayer coverage under dynamic conditions and to relate this to surface dilatational viscosity and elasticity. For a purely elastic surface layer the prediction of the Lucassen van den Temple model (LvdT)are fulfilled.
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Valentina Fazio, Valentina Fazio, Jonas Ortegren, Jonas Ortegren, P. Koelsch, P. Koelsch, A. Andersen, A. Andersen, Dieter Wantke, Dieter Wantke, Helmuth Mohwald, Helmuth Mohwald, Hubert Motschmann, Hubert Motschmann, } "New insights in static and dynamic properties of Gibbs monolayer", Proc. SPIE 5223, Physical Chemistry of Interfaces and Nanomaterials II, (4 December 2003); doi: 10.1117/12.509502; https://doi.org/10.1117/12.509502
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