(514a) Hydrophobic, Electrostatic, and Time-Dependent Polymer Bridging Forces At Surfactant-Modified Silicone Surfaces

Rapp, M., University of California, Santa Barbara
Donaldson, S. H. Jr., University of California, Santa Barbara
Gebbie, M. A., University of California, Santa Barbara
Das, S., University of California, Santa Barbara
Kaufman, Y., Ben Gurion University
Israelachvili, J., University of California Santa Barbara

We have synthesized covalently grafted polydimethylsiloxane (PDMS) thin films on extended gold surfaces that behave as collapsed polymer brush surfaces in solution, with uniform thickness, surface coverage, and surface chemistry. The Surface Forces Apparatus (SFA) was used to measure the surface forces due to surfactant adsorption and self-assembly at these hydrophobic interfaces. We show that polymeric surfactants—containing PDMS anchoring domains and quarternary ammonium cationic headgroups—assemble at the PDMS surface, leading to both an effective charge reversal at the hydrophobic interface and strong adhesion energies to mica surfaces. Molecular structure and charge are shown to govern the assembly of surfactants on these surfaces:  PDMS bola-surfactants assemble to form fluctuating surface aggregates that extend into solution, leading to long-ranged and time-dependent attraction to mica surfaces, while other surfactants behave as an adsorbed layer of smeared-out charges and their interaction with mica can be described by the electric double layer theory of asymmetric surfaces. Additionally, specific Coulombic interactions between the surfactant headgroups and mica result in time-dependent polymer bridging forces as surfactants are pulled from the PDMS interface. Thus, surfactant self-assembly is presented as a method to tailor the complex and dynamic surface forces at hydrophobic interfaces, and can promote strong wet adhesion between hydrophobic and mineral surfaces.