Spontaneous formation of monodisperse vesicles near the cloud point of an aqueous amphiphilic system

Spontaneous formation of vesicles

his review highlights the relevant issues of spontaneous formation of vesicles. Both the common characteristics and the differences between liposomes and vesicles are given. The basic concept of the molecular packing parameter as a precondition of vesicles formation is discussed in terms of geometrical factors, including the volume and critical length of the amphiphile hydrocarbon chain. According to theoretical considerations, the formation of vesicles occurs in the systems with packing parameters between 1/2 and 1. Using common as well as new methods of vesicle preparation, a variety of structures is described, and their nomenclature is given. With respect to sizes, shapes and inner structures, vesicles structures can be formed as a result of self-organisation of curved bilayers into unilamellar and multilamellar closed soft particles. Small, large and giant uni-, oligo-, or multilamellar vesicles can be distinguished. Techniques for determination of the structure and properties of vesicles are described as visual observations by optical and electron microscopy as well as the scattering techniques, notably dynamic light scattering, small angle X-ray and neutron scattering. Some theoretical aspects are described in short, viz., the scattering and the inverse scattering problem, angular and time dependence of the scattering intensity, the principles of indirect Fourier transformation, and the determination of electron density of the system by deconvolution of p(r) function. Spontaneous formation of vesicles was mainly investigated in catanionic mixtures. A number of references are given in the review.

The origins of stability of spontaneous vesicles

Equilibrium unilamellar vesicles are stabilized by one of two distinct mechanisms depending on the value of the bending constant. Helfrich undulations ensure that the interbilayer potential is always repulsive when the bending constant, K, is of order k(B)T. When K k(B)T, unilamellar vesicles are stabilized by the spontaneous curvature that picks out a particular vesicle radius; other radii are disfavored energetically. We present measurements of the bilayer elastic constant and the spontaneous curvature, R(o), for three different systems of equilibrium vesicles by an analysis of the vesicle size distribution determined by cryo-transmission electron microscopy and small-angle neutron scattering. For cetyltrimethylammonium bromide (CTAB)/sodium octyl sulfonate catanionic vesicles, K =.7 k(B)T, suggesting that the unilamellar vesicles are stabilized by Helfrich-undulation repulsions. However, for CTAB and sodium perfluorooctanoate (FC(7)) vesicles, K = 6 k(B)T, suggesting stabilization by the energetic costs of deviations from the spontaneous curvature. Adding electrolyte to the sodium perfluorooctanoate/CTAB vesicles leads to vesicles with two bilayers; the attractive interactions between the bilayers can overcome the cost of small deviations from the spontaneous curvature to form two-layer vesicles, but larger deviations to form three and more layer vesicles are prohibited. Vesicles with a discrete numbers of bilayers at equilibrium are possible only for bilayers with a large bending modulus coupled with a spontaneous curvature.

Formation of vesicles in diluted aqueous solutions of surfactant investigated by direct analysis of light scattering

Formation of aggregates in the binary systems of double-tailed surfactant, sodium 4-(1-pentylheptyl)benzenesulfonate, and water in the dilute regime was supposed to occur within 5.0-8.5% of surfactant concentration. The size of particles was determined by light scattering. In addition, the samples were observed at room temperature using an Axiovert 35 Zeiss polarized light microscope operated with differential interference contrast optics. The observed aggregates could, in theory, belong a vesicle phase. All the histograms obtained by light scattering showed a bimodal distribution of particles. Weight factors including intensity, volume and number distribution indicate 97-100% of small aggregate sizes, since the peaks for the big sizes indicate only a small number of the aggregate population. Small aggregates have shown monodispersity with diameters of the aqueous core amounting to 38.94 and 54.94 nm relating to the surfactant concentration of 6.0 and 8.0%, respectively. Hydrodynamic radii determined by the Cumulant method, by the inverse Laplace transformation, and using a plot 1/tau vs. q2, showed values within the usual precision limits.