Hydrophobic, Electrostatic, and Dynamic Polymer Forces at Silicone Surfaces Modified with Long-Chain Bolaform Surfactants

Surfactant self-assembly on surfaces is an effective way to tailor the complex forces at and between hydrophobic-water interfaces. Here, the range of structures and forces that are possible at surfactant-adsorbed hydrophobic surfaces are demonstrated: certain long-chain bolaform surfactants - containing a polydimethylsiloxane (PDMS) mid-block domain and two cationic α, ω-quarternary ammonium end-groups - readily adsorb onto thin PDMS films and form dynamically fluctuating nanostructures. Through measurements with the surface forces apparatus (SFA), it is found that these soft protruding nanostructures display polymer-like exploration behavior at the PDMS surface and give rise to a long-ranged, temperature- and rate-dependent attractive bridging force (not due to viscous forces) on approach to a hydrophilic bare mica surface. Coulombic interactions between the cationic surfactant end-groups and negatively-charged mica result in a rate-dependent polymer bridging force during separation as the hydrophobic surfactant mid-blocks are pulled out from the PDMS interface, yielding strong adhesion energies. Thus, (i) the versatile array of surfactant structures that may form at hydrophobic surfaces is highlighted, (ii) the need to consider the interaction dynamics of such self-assembled polymer layers is emphasized, and (iii) it is shown that long-chain surfactants can promote robust adhesion in aqueous solutions. Direct force measurements reveal that long-chain (polydimethylsiloxane) bolaform surfactants self-assemble on silicone surfaces to form dynamically fluctuating nanostructures that protrude into solution. Sparingly low concentrations of these surfactants mediate remarkably strong adhesion between a polymer and a mineral surface in aqueous solution, via both specific Coulombic bonds and non-specific hydrophobic interactions.