Nanostructures across oppositely charged surfactant–polymer interfaces

Abstract

Oppositely charged surfactant and polymer systems have received increasing interest as they offer great versatility and control of highly ordered structures formed in solution for exploitation in various industrial and pharmaceutical applications. Early studies have focused on the phase behaviour of these systems as bulk mixtures or dispersions, however investigation across surfactant–polymer solution interfaces has been scarce. A novel approach was developed to study the kinetics of structure formation across such interfaces. Line scans were performed with synchrotron small angle X-ray scattering and Raman microscopy to obtain structural and compositional data spatially across SDS–polyDADMAC interfaces at various time points. Structures identified across these interfaces were compared with those formed in dispersions or bulk mixtures prepared at the same molar charge ratio to comment on their state of equilibrium. The release behaviour of model hydrophilic drug from a range of industrially and biologically relevant oppositely charged surfactant and polymer structured capsules to certain stimuli was also assessed. Results demonstrated the existence of both equilibrium and nonequilibrium nanostructures locally across SDS–polyDADMAC interfaces. The rate of structure formation across these interfaces and the extent to which equilibrium was reached was determined by the structure and concentration gradients existing at a given time and the mobility of the components within or from bulk regions, and across hexagonal and/or micellar phases. New insights gained into the slow mixing, equilibrium phase behaviour and structural attributes of oppositely charged surfactant and polymer systems can be applied in the development cost-effective formulations with the desired physicochemical properties and functionality, as well as the design of novel stimuli-responsive drug delivery systems.