Nonionic bilayers in dilute solutions: effect of additives

Abstract

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" phase and the L3 phase as function of additive concentration on the temperature scale.In the L" 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 2H 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 the 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 2H 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.

Effect of ionic surfactants on the iridescent color in lamellar liquid crystalline phase of a nonionic surfactant

“…In C 12 E 5 /SDS systems [18,[22][23][24] , the largest shift in Bragg peak is just around 30 nm, but in our system, the wavelength of the iridescent solution shifts from 724 to 400 nm, covering almost the whole range of visible light.…”

“…Small amounts of ionic surfactants suppress the Helfrich undulations, flatten the bilayers and decrease the interplanar distance. Jonströmer and Strey found that a blue shift in scattering spectrum when trace amounts of ionic surfactants were added to this nonionic surfactant system [18] . This blue shift in Bragg diffraction peak has been widely explained by the above mentioned Helfrich thermal undulation theory [19] , as shown in Eq.…”

“…Small amounts of ionic surfactants give a significant influence on the behavior of the dilute bilayer solutions of the nonionic agent [18] . Small amounts of ionic surfactants suppress the Helfrich undulations, flatten the bilayers and decrease the interplanar distance.…”

“…Taking the bilayer thickness δ and the volume fraction Ф to be 4 nm and 0.016, respectively, as a typical case in the normal nonionic iridescent systems, the blue shift in interplanar distance is about 20 nm [17,18] . This paper deals with an iridescent solution of a polymerizable nonionic surfactant, n-dodecyl glyceryl itaconate [DGI; n-C 12 H 25 OCOCH 2 C(=CH 2 )COOCH 2 CH(OH)CH 2 OH] [20] .…”

“…It has been reported that the nonionic surfactant systems of C 12 E 5 do not need any ionic surfactants to have the swollen bilayers showing the iridescent color [16][17][18] , although this type of nonionic surfactant may contain ionic impurities, in some cases owing to the oxidation of the terminal -COH group to ionizable -COOH group [29] . In our DGI systems, it is found that ionic surfactants are necessary to change from the lamellar liquid crystalline droplets of pure DGI having the spacing distance of several nm to the iridescent lamellar phases.…”