ABSTRACT
The formation of liposomes with low polydispersity index by application of ultrasounds was investigated considering methodology specifications such as sonication time and sonication power. Phosphatidylcholine (PC) liposomes were formed by the evaporation-hydration method. The vesicles were sonicated using several sonication conditions. The liposomes were then characterized by dynamic light scattering (DLS) and freeze-fracture electron microscopy (FFEM). Correlation functions from DLS were treated by cumulants method and GENDIST to obtain the mean radius and polydispersity index. These calculations allowed to fix an optimal sonication time (3000 s) and a useful interval of ultrasound power between 39 and 91 W. DLS and FFEM results confirmed that vesicle size, lamellarity and the polydispersity index decreased with the increase of sonication power. Thus, we propose a systematic method to form liposomes in which the physical characteristics of the vesicles may be controlled as a function of sonication time and power.
Subject(s)
Liposomes/chemistry , Phosphatidylcholines/chemistry , Ultrasonics , Chemical Phenomena , Chemistry, Physical , Microscopy, Electron , Models, Chemical , Particle Size , Scattering, Radiation , Sensitivity and Specificity , Solutions/chemistry , Sonication , Surface Properties , Time FactorsABSTRACT
Mini-emulsions have been formed in quaternary systems water/hexanol/sodium dodecyl sulfate/decane by dilution of a microemulsion with an excess of water. We have investigated systematically the effect of composition variables in the droplet size and Ostwald Ripening rate. This droplet size has been investigated by using dynamic light scattering of samples submitted to further dilution in water. According to the dynamic light scattering results, the initial droplet size depends on the initial microemulsion water content, the larger the initial water concentration, the smaller the initial droplet size. This is probably related to the structure of the initial phase. The rate of Ostwald Ripening depends on the final surfactant concentration as expected for differences in interfacial tension. At high surfactant and dispersed phase contents, instability due to flocculation has been observed. Other experiments in which a fifth component (a more hydrophobic oil) was added to slow down Ostwald Ripening showed an initial droplet size increasing comparable to the systems without additive but after a relatively long time of approximately 2 h, a decrease was observed.
ABSTRACT
The formation of closed-compact multilamellar vesicles (referred to in the literature as the "onion texture") obtained upon shearing lamellar phases is studied using small-angle light scattering and cross-polarized microscopy. By varying the shear rate gamma;, the gap cell D, and the smectic distance d, we show that: (i) the formation of this structure occurs homogeneously in the cell at a well-defined wave vector q(i), via a strain-controlled process, and (ii) the value of q(i) varies as (dgamma;/D)(1/3). These results strongly suggest that formation of multilamellar vesicles may be monitored by an undulation (buckling) instability of the membranes, as expected from theory.
Subject(s)
Liposomes/chemistry , Membranes, Artificial , Alkanes/chemistry , Light , Microscopy , Pentanols/chemistry , Scattering, Radiation , Sodium Dodecyl Sulfate/chemistry , Water/chemistryABSTRACT
We report the effect of shear flow on a phase-separated system composed of lyotropic lamellar (L(alpha)) and sponge (L3) phases in a mixture of brine, surfactant, and cosurfactant. Optical microscopy, small-angle light, and x-ray scattering measurements are consistent with the existence of a steady state made of multilamellar ribbon-like structures aligned in the flow direction. At high shear rates, these ribbon-like structures become unstable and break up into monodisperse droplets resulting in a shear-thickening transition.
Subject(s)
Biophysics , Membranes, Artificial , Biophysical Phenomena , Light , Microscopy , Salts , Scattering, Radiation , Stress, Mechanical , Temperature , X-RaysABSTRACT
Dynamic light scattering experiments on lyotropic lamellar phases of brine and surfactant subjected to a flow field have been realized. The obtained results reveal that the relaxation times measured depend strongly on the velocity of the flow. This dependence is indicative of an increase of the effective elasticity modulus K and a decrease of the effective compressibility modulus (-)B of the lamellar phase with the flow velocity. This leads to the conclusion that the shear can induce a suppression of the undulation fluctuations of the bilayers of the lamellar phase. Our results show also that the rigidity of the membranes decreases as the salt concentration of the sample increases.
Subject(s)
Biophysics/methods , Light , Salts/chemistry , Scattering, Radiation , Surface-Active Agents/chemistry , Biophysics/instrumentation , Salts/pharmacology , Stress, Mechanical , Time FactorsABSTRACT
We show the existence under shear flow of steady states in a two-phase region of a brine-surfactant system in which lyotropic dilute lamellar (non-Newtonian) and sponge (Newtonian) phases are coexisting. At high shear rates and low sponge phase-volume fractions, we report on the existence of a dynamic transition corresponding to the formation of a colloidal crystal of multilamellar vesicles (or "onions") immersed in the sponge matrix. As the sponge phase-volume fraction increases, this transition exhibits a hysteresis loop leading to a structural bistability of the two-phase flow. Contrary to single phase lamellar systems where it is always 100%, the onion volume fraction can be monitored continuously from 0 to 100 %.
