On the Electrostatic Component of the Adhesional Interaction between Solid Substrates and Langmuir Monolayers

The work of adhesion between Langmuir monolayers and solid surfaces in a hydrophilic Langmuir‐Blodgett configuration has been measured as function of the surface charge densities σ of the contacting surfaces. It was derived from the equilibrium meniscus height which was measured with the transfer fluorescence microscope. The surface charge density of the contacting surfaces was varied by changing the subphase pH. With both surfaces negatively charged (dimyristoylphosphatidylethanolamine onto SiO2‐substrates) and low charge densities (σ < 1 e−/100 Å2)) we observe a decrease of the adhesion with increasing charge density. At high charge densities (dimyristoylphosphatidic acid onto SiO2‐substrates, σ < 1 e−/100 Å2) the work of adhesion is hardly affected by charge density variations, as theoretically predicted for electrostatic double layer forces. Systems with only one surface of varying negative charge density (dipalmitoylphosphatidylcholine onto SiO2) unfold the predicted decrease of the work of adhesion with increasing charge density. Control experiments with varying pH and uncharged surfaces (dipalmitoylphosphatidylcholine onto polymethylmethacrylate surfaces) show a pH‐independent work of adhesion and thus corroborate that the measured behaviour is caused by the electrostatic double layer interaction.