Microscope slides of soda-lime glass (26 mm × 76 mm × 1 mm, Thermo Fisher Scientific Gerhard Menzel B.V. & Co. KG, Germany) were hydrophobized by chemical vapor deposition with trichloro(1H,1H,2H,2H-perfluorooctyl)silane (PFOTS, Thermo Fisher Scientific)22 (link) or by coating with poly-dimethylsiloxane (PDMS) brushes by ‘grafting from’ using dimethyldichlorosilane (Sigma-Aldrich) as monomers.23 (link) The reaction time for the polymerization was 1800 s. According to previous work23 (link) a thickness of 7 ± 1 nm of the PDMS brush layer is to be expected. In the polymerisation reaction no catalyser or other initiator was added, it is a polyaddition reaction. Therefor no additional ionic components were added in the process which could influence the experiment.
Before being coated, the glass slides were first cleaned with acetone and ethanol in an ultrasonic bath, then treated with an oxygen plasma (Diener electronic FEMTO, Plasma-Surface-Technology, Germany, 6 cm3 min−1 oxygen flow rate, 120 W) for 600s to remove organic contaminations and to activate the surface. Using stock solutions of concentration 1 M (mol L−1) for NaOH, NaCl, CaCl2 (VWR, Germany), KNO3 (ROTH, Germany), and HCl (J T Baker Avantar, Germany) and deionized water (DI water, 18 MΩ cm, obtained from an Arium® Pro (Satorius, Germany)), solutions of concentrations of 0, 0.01, 0.1, 1, 10, 100 mM were prepared.
Before being coated, the glass slides were first cleaned with acetone and ethanol in an ultrasonic bath, then treated with an oxygen plasma (Diener electronic FEMTO, Plasma-Surface-Technology, Germany, 6 cm3 min−1 oxygen flow rate, 120 W) for 600s to remove organic contaminations and to activate the surface. Using stock solutions of concentration 1 M (mol L−1) for NaOH, NaCl, CaCl2 (VWR, Germany), KNO3 (ROTH, Germany), and HCl (J T Baker Avantar, Germany) and deionized water (DI water, 18 MΩ cm, obtained from an Arium® Pro (Satorius, Germany)), solutions of concentrations of 0, 0.01, 0.1, 1, 10, 100 mM were prepared.