Intrinsic Depletion or Not: More on the Interfacial Wetting of Polystyrene by Water

 

The presence of a depletion layer of water along extended hydrophobic interfaces, and a possibly related formation of nanobubbles, is an ongoing discussion. The phenomenon was initially reported when we, years ago, chose thick films (~300-400Å) of polystyrene as cushions between a crystalline carrier and biomimetic membranes deposited thereupon and exposed to bulk water. While monitoring the sequential build-up of the sandwiched composite structure by continuous neutron reflectivity experiments the formation of an unexpected additional layer was detected (1). Located at the polystyrene surface in between he polymer cushion and bulk water the layer was attributed to water of reduced density and was called "depletion layer".  Impurities or preparative artefacts were excluded as its origin. Later on, the formation of nanobubbles from this vapour-like water phase was initiated by tipping the surface with an AFM (2).

The intuitive explanation for the depletion based on "hydrophobic mismatch" between the obviously hydrophilic bulk phase of water next to the hydrophobic polymer. It would thus be an intrinsic property of all interfaces between non-matching materials. The detailed physical interaction path giving rise to depletion layers, and the mechanisms and border conditions that control their presence and extension require still clarification.

Recently, careful systematic reflectivity experiments were re-done on the same system. No depletion layers were found, and it was conjectured that the whole observation might be due to artefacts. We thus decided to re-redo the experiments following the procedures described in (3) as much as possible. Moreover, we started to conduct a systematic variation of the hydrophilicity in the aqueous bulk phase by adding sugar into it. These new results will be presented and discussed in the framework of all known experiments so far.