  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 C12H 25 chains
(PEO-2m) or addition of a PEO grafted at one extremity only with a C12H 25
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 C12H 25 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.
