Swollen micelles plus hydrophobically modified hydrosoluble polymers in aqueous solutions: Decoration versus bridging. A small angle neutron scattering study

In this paper we examine the effective interactions introduced between the droplets of an oil in water microemulsion upon progressive addition of hydrophobically modified water soluble poly(ethylene oxide) (PEO) using essentially small angle neutron scattering. To discuss the relative importance of decoration and bridging of the droplets, we compare analogous samples with addition of a PEO grafted at both extremities with hydrophobic C12H25 chains (PEO-2m) or addition of a PEO grafted at one extremity only with a C12H25 chain (PEO-m). PEO-m or PEO-2m adsorb onto the droplets via their hydrophobic extremities, and the droplets are found to retain their form and size upon addition of up to 40 hydrophobic C12H25 chains per droplet. When the volume fraction of droplets is less than about 10%, the effective interactions introduced by PEO-m or PEO-2m are found to be very different: PEO-m introduces a repulsive interaction, while PEO-2m introduces an effective attractive interaction. This attractive interaction leads to an associative phase separation in the range of low volume fraction when a sufficient amount of PEO-2m is added. At higher volume fraction the contribution of PEO-m or PEO-2m to the interaction is a small repulsive contribution. We discuss qualitatively the experimental results in terms of decoration or bridging of the droplets by the PEO polymeric chains.