Amphiphilic block copolymers as efficiency boosters for microemulsions

We demonstrate that block copolymers of the poly(ethylenepropylene)-co-poly (ethylene oxide) (PEPPEO) type dramatically enhance the solubilization capa city of medium-chain surfactants in microemulsions, for example, in the ter nary system water-n-decane-C10E4. The effect exhibits itself in an enormous increase of the swelling of the middle phase with an associated increase i n the structural length scale of the microemulsion, while at the same time the (already ultralow) interfacial tension between water- and oil-rich phas es decreases even further. Typically, the surfactant mass fraction <(gamma) over tilde>(0) = 0.13 sufficient to form the balanced one-phase microemulsi on in the ternary system can be replaced by <(gamma)over tilde> = 0.035 of surfactant plus polymer. If delta is the polymer mass fraction in the surfa ctant/polymer mixture, the overall mass fraction of polymer in the microemu lsion amounts only to <(gamma)over tilde>delta = 0.004. Accordingly, in thi s example the polymer is f(B) = 24 times more efficient than the surfactant , where we define an efficiency boost factor by f(B) = (<(gamma)over tilde> (0) - <(gamma)over tilde>(1 - delta)/<(gamma)over tilde>delta. The magnitud e of the effect depends to some extent on the overall molar mass of the pol ymer but rather weakly on the relative size of the hydrophilic and hydropho bic blocks. Interestingly, the lamellar phase, which usually develops as su rfactants become more efficient, is suppressed.