Asymmetric Electrostatic and Hydrophobic–Hydrophilic Interaction Forces between Mica Surfaces and Silicone Polymer Thin Films

Abstract

We have synthesized model hydrophobic silicone thin films on gold surfaces by a two-step covalent grafting procedure. An amino-functionalized gold surface reacts with monoepoxy-terminated polydimethylsiloxane (PDMS) via a click reaction, resulting in a covalently attached nanoscale thin film of PDMS, and the click chemistry synthesis route provides great selectivity, reproducibility, and stability in the resulting model hydrophobic silicone thin films. The asymmetric interaction forces between the PDMS thin films and mica surfaces were measured with the surface forces apparatus in aqueous sodium chloride solutions. At an acidic pH of 3, attractive interactions are measured, resulting in instabilities during both approach (jump-in) and separation (jump-out from adhesive contact). Quantitative analysis of the results indicates that the DerjaguinÀLandauÀVerweyÀOverbeek theory alone, i.e., the combination of electrostatic repulsion and van der Waals attraction, cannot fully describe the measured forces and that the additional measured adhesion is likely due to hydrophobic interactions. The surface interactions are highly pH-dependent, and a basic pH of 10 results in fully repulsive interactions at all distances, due to repulsive electrostatic and steric-hydration interactions, indicating that the PDMS is negatively charged at high pH. We describe an interaction potential with a parameter, known as the Hydra parameter, that can account for the extra attraction (low pH) due to hydrophobicity as well as the extra repulsion (high pH) due to hydrophilic (steric-hydration) interactions. The interaction potential is general and provides a quantitative measure of interfacial hydrophobicity/hydrophilicity for any set of interacting surfaces in aqueous solution.

Measuring Forces and Spatiotemporal Evolution of Thin Water Films between an Air Bubble and Solid Surfaces of Different Hydrophobicity

“…27,35,38 The decay length D H has been measured to be about 1 nm for solid hydrophobic surfaces, and a shorter decay length of ∼0.3 nm was also measured between hydrophobic oil droplets very recently. 30,45 For the asymmetric interaction between a planar airÀwater interface and a hydrophobized mica surface, a similar exponential form for the interaction free energy per unit area: W H (h) = ÀC exp(Àh/D H ) can be posited.…”

“…It is evident from our results that the characteristic decay length of hydrophobic interaction is influenced by the surface hydrophobicity that affects the structure and orientation of water molecules near the surface. 38,39 The asymmetric bubble-mica hydrophobic interaction free energy per unit area between the air bubble and the hydrophobic substrate can be described by…”

Influence of Molecular Dipole Orientations on Long-Range Exponential Interaction Forces at Hydrophobic Contacts in Aqueous Solutions

“…25,27 This increased decay length may indicate that the interfacial dipole alignment (i.e., the alignment of water molecules with the surface dipole of the M9 SAM surface) may lead to an entropically unfavorable longer ranged ordering of water. Hence a dipole-induced solvent layering at the interface may mediate the longer ranged force ; a striking conceptual similarity to purely hydrophobic forces.…”

“…The small inlet shows the histogram plots of the measured adhesion forces. 10875 that can be well fitted using the Hydra model 12,25,28 (see the methods section) with a decay length of λ Hy = 2 nm. As discussed above, this may be due to the preferential orientation of water dipoles in line with the surface dipoles of the SAM molecules.…”

“…Using a distance-dependent Hamaker constant A H (D) takes into account that long-range VDW forces are dominated by the Au|mica dispersion, while short-range VDW forces are dominated by the adsorbed organic thin films (SAM and lipid monolayer). For an asymmetric system A H (D) can be approximated using combining rules 25 as follows:…”