Humidity-Induced Structural Transformation in Self-Organized Polymer-Surfactant Multilayer Nanofilms

We have investigated the humidity-responsive structure of polymer-surfactant multilayer nanofilms composed of a hydrophilic maleic acid polymer and an amphoteric amine oxide surfactant (C12-AO). Neutron reflectometry (NR) revealed the presence of a smooth thin film comprising several surfactant bilayers intercalated with the polymer. Upon exposure to increased humidity, the nanostructured film reorganized into a vertically stratified multilayer structure with polymer chains located in the vicinity of hydrated surfactant headgroups, with further humidity increase leading to the formation of surfactant-rich domains, indicating a high mobility of the surfactant molecules in the polymer matrix. Off-specular neutron reflectometry and grazing-incidence X-ray scattering revealed the presence of surfactant nanocrystals in the as-prepared film, which diffused to form surfactant-rich domains upon exposure to humidity, thus providing a reservoir of surfactants. These findings help elucidate a mechanism of structural transformation in dried polymer-surfactant films, thus suggesting an avenue for engineering an antimicrobial coating with longevity and a humidity-activated release of active molecules using two-dimensional confinement effects.