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 capacity of medium-chain surfactants in microemulsions, for example, in the ternary system water-n-decane-C(10)E(4). The effect exhibits itself in an enormous increase of the swelling of the middle phase with an associated increase in the structural length scale of the microemulsion, while at the same time the (already ultralow) interfacial tension between water- and oil-rich phases decreases even further. Typically, the surfactant mass fraction <(gamma)over tilde>(0) = 0.13 sufficient to form the balanced one-phase microemulsion 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 surfactant/polymer mixture, the overall mass fraction of polymer in the microemulsion amounts only to <(gamma)over tilde>delta = 0.004. Accordingly, in this 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 magnitude of the effect depends to some extent on the overall molar mass of the polymer but rather weakly on the relative size of the hydrophilic and hydrophobic blocks. Interestingly, the lamellar phase, which usually develops as surfactants become more efficient, is suppressed.