Polymer-induced ordering in water-oil-surfactant mixtures

Small angle x-ray scattering (SAXS), freeze-fracture electron microscopy, and differential scanning microcalorimetry are used to investigate the temperature dependent microstructure of water-oil(isooctane)-surfactant(sodium di-2-ethylhexyl sulfosuccinate) mixtures containing variable concentrations of triblock copolymer (polyethyleneoxid-polyisoprene-polyethyleneoxid). Micrographs demonstrate that close to room temperature the microstructure consists of water droplets embedded in an oil matrix. Under increasing copolymer concentration a pronounced interaction peak shows up in SAXS spectra. This indicates a copolymer-induced transition from random towards locally ordered droplet arrangement. Under increasing temperature one crosses a phase boundary. This causes the position of the interaction peak to shift toward smaller Q values, and up to three further, equidistant peaks occur. Equal distances between peak positions implies a lamellar microstructure. It is shown by microcalorimetric measurements that this transition from a microemulsion into a lamellar phase is entropy driven, even when no copolymers are added. At low copolymer concentration the lamellar phase expels an excess oil phase. However, for sufficiently high copolymer concentration the system remains single phase. Elaborating on this point, we demonstrate that addition of a lipophilic polymer forms a method of stabilizing lamellar phases. Furthermore, the increasing number of peaks in the scattering spectra indicates that the order in the lamellar structure increases significantly by addition of copolymer.