NONIONIC BILAYERS IN DILUTE-SOLUTIONS - EFFECT OF ADDITIVES

We have investigated the effect of alcohols and other additives on the binary phase diagram water-C12E5. In particular, we have located the dilute lamellar L(alpha) phase and the L3 phase as function of additive concentration on the temperature scale. In the L(alpha) phase undulations stabilize bilayer membranes at distances on the order of hundred nanometers. We have determined their repeat distance quantitatively by light scattering. By addition of medium-chain alcohols, the projected area of the undulating membranes was found to increase drastically. We quantitatively determined the fraction of alcohol molecules dissolved in the membranes by H-2 NMR using deuterated alcohols. The longer the hydrocarbon chain of the alcohols, the stronger the partitioning of the alcohols into the membranes. From our experiments we conclude that the bilayers act as a two-dimensional phase into which thc medium-chain alcohols preferentially dissolve, apparently because of the favorable polarity gradient of the films. This latter conclusion arises from the surfactant-like order parameter profile observed by H-2 NMR for the alcohols. The effect of adding salt, increasing the hydrocarbon chain length of either surfactant or alcohol, as well as reducing the number of ethylene oxide units of the surfactant is, in each case, to render the bilayers more hydrophobic. We discuss the effects in terms of the geometric properties of the molecules and bending properties of the membranes.