New insights in static and dynamic properties of soluble monolayers

• Abstract In this paper we discuss selected equilibrium and dynamic properties of adsorp-tion layers of soluble surfactants. The surface state has been investigated by nonlinearoptical techniques based on second order χ(2)effects which exhibit a high surface specificityand suppress bulk contributions. The surface tension isotherm σ(c) of the homologousseries of n-alkyldimethylphosphine (n =8 − 12) can be described by Frumkin’s equation ofstate which yields the surface interaction parameter, surface coverage and the correspondingarea per molecule A. The comparison of the surface tension σ at a given area per moleculeA reveals a strong alternation within the homologous series. Odd C2n±1 layers show alower surface tension than the adjacent even members C2n of the homologous series. Thiseffect is also present at low surface coverage (A =1.4nm2) and cannot be attributed to adifferences in the chain-packing within a crystalline state. Infrared-Visible Sum-FrequencyGeneration Spectroscopy (SFGS) has been used to monitor the orientation and chain orderwithin the aliphatic tail. SFGS spectra have been recorded for different chain lengths andat different areas per molecule. The analysis of the spectra yields an order parameterG which is proportional to the number of gauche defects within the aliphatic tail. Theodd-even effect in the surface tension turned out to be accompanied by an odd-even effectin the order parameter G. The data suggest that an ordered structure has a bigger impacton the surface tension than an unordered structure. The odd-even effect is also observedin the orientation of the terminating methyl group as retrieved by polarization dependentSFGS measurements. The data shed some light in the relation between molecular andmacroscopic properties. Furthermore surface dilatational viscoelastic properties of afluorinated amphiphile have been measured by a novel version of the oscillating bubble.The oscillating bubble method generates a non-equilibrium state by a harmonic compressionand expansion of the surface layer formed at the tip of a capillary. The surface state ismonitored by Surface Second Harmonic Generation (SHG). This technique is highly surfacespecific and discriminates between monolayer and subsurface coverage. Our set-up allowsto measure the monolayer coverage under dynamic conditions and to relate this to surfacedilatational viscosity and elasticity. For a purely elastic surface layer the prediction of theLucassen van den Temple model (LvdT) are fulfilled.

Ämnesord

NATURVETENSKAP  -- Kemi -- Fysikalisk kemi (hsv//swe)

NATURAL SCIENCES  -- Chemical Sciences -- Physical Chemistry (hsv//eng)

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