Come Together: Molecular Details into the Synergistic Effects of Polymer-Surfactant Adsorption at the Oil/Water Interface

The synergistic adsorption of polymers with surfactants at the oil/water interface has applications that range from oil remediation to targeted drug delivery. However, the inherent inaccessibility of the buried oil/water interface has challenged the development of a molecular-level understanding of the structure-function relationship of these systems. This study uses vibrational sum frequency spectroscopy to examine the molecular structure, orientation, and electrostatic effects of synergistic adsorption of the surfactant cetrimonium bromide (CTAB) and polymer poly(acrylic acid) (PAA) at a planar oil/water interface. Results demonstrate that coadsorption leads to a high degree of interfacial ordering of both the polymer and the surfactant and a subsequent alteration of the interfacial water bonding and orientation. Complementary zeta potential measurements provide further information about how surface partitioning of a charged polymer and a surfactant relates to their aggregation behavior in a bulk solution. With the CTAB concentration fixed but the PAA concentration variable, hydrophobic interactions result in a modest synergic coadsorption when CTAB is in excess. However, when the PAA carboxylate monomer concentration approaches that of CTAB, the electrostatic interactions between the components change the structure and increase the amount of adsorbed PAA until the interfacial charge is neutralized. This work reveals that the synergic adsorption behavior of this model polyacid/surfactant system arises from a combination of concentration-dependent hydrophobic and electrostatic forces working in tandem.