Direct Measurement of Lateral Correlations under Controlled Nanoconfinement : The Long-range Attraction Between Hydrophobic Macroscopic Surfaces

Orateur : Patrick KEKICHEFF (Institut Charles Sadron, C.N.R.S., Strasbourg)

The origin and nature of the interaction between hydrophobic macroscopic substrates across water and aqueous solutions has been in debate for the last 30 years. Here, atomically smooth mica surfaces are rendered hydrophobic and charge neutral by immersion in cationic surfactant solutions at low concentrations. Lateral correlations along the hydrophobic surfaces whose separation can be varied continuously down to contact are measured by X-ray scattering using a modified Surface Force Apparatus coupled with synchrotron radiation, named SFAX. A weak isotropic diffuse scattering along the equatorial plane is revealed. The peak corresponds to a lateral surface correlation length of about 12 nm, without long-range order. Remarkably the scattering patterns remain stable for gap widths larger than the lateral period, but change as soon as the separation becomes smaller than the correlation length. This evolution codes for a redistribution of counterions (counterion release from antagonistic patches) and the associated new X-ray labeling of the patterns. The redistribution of counterions is the key mechanism to the long-range electrostatic attraction between similar, overall charge-neutral walls, measured directly by the Surface Force Apparatus [1]. Thus, the SFAX provides the ultimate experimental missing clue for explaining not only the origin of the long-range attraction between macroscopic hydrophobic substrates, but also confirms [2] the electrostatics nature of that interaction proposed earlier [1].

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