(79d) Surfactant Adsorption Thermodynamics at Fluid/Fluid Interfaces Studied with Diblock Copolymers

Surfactant adsorption thermodynamics at fluid/fluid interfaces is based on the concept of a Gibbsian surface excess mass arising from a singlet density distribution across the interface. Unfortunately, the small length scale of classical aqueous surfactants does not permit experimental measure of this density distribution. Here we investigate the equilibrium concentration profiles of an AB diblock co-polymer (i.e., the surfactant) at an A polymer/ B polymer interface. We create thin polymer films containing two surfactant-bearing polymeric interfaces and follow the concentration profiles of the diblock copolymer by secondary-ion mass spectroscopy (SIMS). Henry adsorption is found at low concentrations followed by asymptotic approach to a monolayer at the binodal where an AB-rich phase forms. Phase equilibrium of the AB copolymer between the A and B polymer bulk phases obeys a simple Nernst distribution law. Measured AB concentration profiles and the Gibbsian adsorption isotherms are in excellent agreement with self-consistent field theory (SCFT), where all necessary parameters were determined independently from SANS and gel-permeation-chromatography measurements. AB copolymers prove an excellent model system to study surfactant behavior at fluid/fluid interfaces.