A comparison of DMPC membranes mixed with melittin or C12E5

Spontaneous formation of nanovesicles in mixtures of nonionic and dialkyl chain cationic surfactants studied by surface tension and SANS

Surface tension and small-angle neutron scattering have been used to characterize the surface properties and the structure of the aggregates formed in the dilute part of the ternary system didodecyldimethyl ammonium bromide, DDAB/tetraethylene glycol monododecyl ether, C12E4/D2O. For surfactant molar ratios, Rn, between 0.3 and 1 (pure DDAB), the surface tension measurements show the existence of two break points at concentrations of around 10(-5) and 10(-3) mol/L, respectively. The SANS measurements have shown that the first break point corresponds to a critical micellar concentration (cmc) and the second one corresponds to a critical vesicle concentration (cvc). In the intermediate composition range, Rn = 0.3-0.8, very small unilamellar vesicles (nanovesicles) are formed with the inner radius varying between 28 and 85 A and a bilayer thickness of approximately 23 A. At Rn = 0.8, we observed a transition from small vesicles (V) to large bilamellar or multilamellar vesicles (BLV, MLV) with a relatively large lamellar periodicity of around 1000 A. In the nonionic-rich region below Rn = 0.3, more classical surface tension behavior was observed, with only one break point corresponding to the onset of formation of small globular micelles.

Self-assembly in mixed dialkyl chain cationic-nonionic surfactant mixtures: dihexadecyldimethyl ammonium bromide-monododecyl hexaethylene glycol (monododecyl dodecaethylene glycol) mixtures

The self-assembly of dialkyl chain cationic surfactant dihexadecyldimethyl ammonium bromide, DHDAB, and nonionic surfactants monododecyl hexaethylene glycol, C(12)E(6), and monododecyl dodecaethylene glycol, C(12)E(12), mixtures has been studied using predominantly small-angle neutron scattering, SANS. The scattering data have been used to produce a detailed phase diagram for the two surfactant mixtures and to quantify the microstructure in the different regions of the phase diagram. For cationic-surfactant-rich compositions, the microstructure is in the form of bilamellar, blv, or multilamellar, mlv, vesicles at low surfactant concentrations and is in an L(beta) lamellar phase at higher surfactant concentrations. For nonionic-rich compositions, the microstructure is predominantly in the form of relatively small globular mixed surfactant micelles, L(1). At intermediate compositions, there is an extensive mixed (blv/mlv) L(beta)/L(1) region. Although broadly similar, in detail there are significant differences in the phase behavior of DHDAB/C(12)E(6) and DHDAB/C(12)E(12) as a result of the increasing curvature associated with C(12)E(12) aggregates compared to that of C 12E 6 aggregates. For the DHDAB/C(12)E(12) mixture, the mixed (blv/mlv) L(beta)/L(1) phase region is more extensive. Furthermore, C(12)E(12) has a greater impact upon the rigidity of the bilayer in the blv, mlv, and L(beta) regions than is the case for C(12)E(6). The general features of the phase behavior are also reminiscent of that observed in phospholipid/surfactant mixtures and other related systems.

Role of dislocation loops on the elastic constants of lyotropic lamellar phases

We study the role of dislocation loops defects on the elasticity of lamellar phases by investigating the variation of the lamellar elastic constants, B and K, induced by the proliferation of these defects. We focus our interest on one particular lamellar phase made up of a mixture of C(12)E(5) and DMPC in water, which is already well-characterised. This lamellar phase undergoes a second-order (or weakly first-order) lamellar-to-nematic phase transition at about 19 degrees C and dislocation loops are seen to proliferate within the lamellar structure when temperature is decreased below 30 degrees C. The values of both elastic constants of this given lamellar phase are measured as a function of temperature, approaching the lamellar-to-nematic transition, with the help of Quasi-Elastic Light Scattering (QELS) on oriented lamellar phases. Very surprisingly we observe a strong and rapid increase in both Band K as the lamellar-to-nematic transition temperature is approached. These increases are seen to start as soon as dislocation loops can be observed in the lamellar phase. We interpret our results as being the consequence of the appearance and proliferation of dislocation loops within the lamellar structure. According to a simple model we developed we show that B and K are proportional to the density of dislocation loops in the lamellar phase.