X-ray reflectivity measurements reveal a significant dewetting of a large hydrophobic paraffin surface floating on water. The dewetting phenomenon extends less than 15 A into the bulk water phase and results in an integrated density deficit of about one water molecule per 25-30 A(2) of water in contact with the paraffin surface. The results are supported by molecular dynamics simulations and related to the hydrophobic effect.

This paper provides the first direct structural evidence describing conjugated polymer self-assembly at the air-water interface. Grazing-incidence X-ray diffraction (GIXD) and X-ray reflectivity measurements on a number of derivatives of amphiphilic regioregular polythiophenes (e.g., poly(3′-dodecyl-3-(2,5,8trioxanonyl)-2′,5-bithiophene), polymer 1) show that these conjugated polymers self-assemble as 2-dimensional polycrystalline monolayers at the air/water interface with the amphiphilic polymers preorganized into rigid boards standing edge-on on the water surface. The monolayer consists of highly ordered (∼70% crystalline) domains, with a centered rectangular unit cell having the polymer backbone along the a-axis and the thiophene π-stack along the b axis with a distance of 3.85-3.94 Å depending on the applied surface pressure. These domains are connected by soft, more disordered boundaries. This is evidenced by the macroscopic compressibility of the entire LB film (C macro ≈ 4-7 m/N) being one order of magnitude larger than the microscopic compressibility (C micro ≈ 0.6 m/N) of the polycrystalline domains. The alkyl chains in the 3-position of the thiophene rings are in a crystallographically disordered state due to their cross-sectional mismatch with the packing of the thiophenes. The importance of having the side chains coupled in a regioregular fashion to the 3-position of the thiophene rings is evidenced by a dramatic increase in the coherence length of the crystalline domains for highly regioregular samples (>95% head-tail couplings) as compared to less regioregularly coupled polymers (∼80% head-tail couplings). Transfer to solid support by the Langmuir-Blodgett technique induces an overall orientation of the domains in the film, giving rise to a dichroic ratio of up to 4. Reflectionabsorption infrared spectroscopy (RAIRS) shows that the alkyl chains of transferred films are in an all-trans conformation with a locally ordered environment, having only few gauche defects.

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