Interfacial Structure and Electrostatics Related to Solute Activity in a Model Anionic-Surfactant/Polymer Self-Assembly

Polymer/surfactant composites are used in industry as an excipient for water-insoluble solutes. Such enhanced dissolution ability of composite media is related to the spontaneous formation of pre-micellar polymer surfactant aggregates (PS) at a magnitude of order lower than the surfactant critical micelle concentration in water. Combining electrochemical and spectroscopic studies, we investigate the micro-scopic interfacial structure (i.e., interface electrostatics and surface polarity) of PS formed in composite media. We establish that in a composite system, a mere change in the polymer concentration at a fixed surfactant concentration makes possible to regulate the counter-ion binding ability, surface potential, surface charge density, packing and surface polarity of the PS interface. Our study shows that the higher dissolution of water-insoluble nonionic solutes in composite media is driven by the depressing of surface charge density and polarity of the PS interface. A similar modulation of the PS interface acts as a barrier for the passive relocation of water-soluble charged solutes into the PS pseudo-phase. The time-resolved fluorescence anisotropy study allows us to underline the effect of surface charge modulation on the dynamical aspects of solutes at the PS interface.

Automated summary

Polymer/surfactant composites are used as excipients for water-insoluble solutes, with enhanced dissolution ability attributed to the formation of pre-micellar polymer surfactant aggregates (PS) at lower concentrations than the surfactant critical micelle concentration in water. This study investigates the microscopic interfacial structure of PS in composite media, including interface electrostatics and surface polarity. It is found that changing the polymer concentration in a composite system can regulate the counter-ion binding ability, surface potential, charge density, packing, and surface polarity of the PS interface, leading to increased solute dissolution. The modulation of the PS interface also acts as a barrier for the relocation of water-soluble charged solutes into the PS pseudo-phase, as shown by fluorescence anisotropy studies.