The origins of stability of spontaneous vesicles

Jung, Hee‐Tae; Coldren, B.; Zasadzinski, J. A.; Iampietro, DJ; Kaler, E. W.

doi:10.1073/pnas.98.4.1353

Cited by 281 publications

(375 citation statements)

References 38 publications

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“…Above the chain-melting temperature, with or without excess salt, diluted unilamellar vesicles are the most stable state, and in-plane mixing of the anionic and cationic components is ensured (12)(13)(14). In this molten state, rigidity of the bilayer is low (Ϸ1-10 k B T); and, thus, small unilamellar vesicles Ϸ50 nm in diameter can be formed without too much bending energy (15). Stability of micelles (at low temperature, when chains are crystallized) was discussed a long time ago by Fromherz (16).…”

Section: Model Mixed Molecular System Investigatedmentioning

confidence: 99%

Shape control through molecular segregation in giant surfactant aggregates

Dubois

Lizunov

Meister

et al. 2004

Proc. Natl. Acad. Sci. U.S.A.

132

228

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Mixtures of cationic and anionic surfactants crystallized at various ratios in the absence of added salt form micrometer-sized colloids. Here, we propose and test a general mechanism explaining how this ratio controls the shape of the resulting colloidal structure, which can vary from nanodiscs to punctured planes; during cocrystallization, excess (nonstoichiometric) surfactant accumulates on edges or pores rather than being incorporated into crystalline bilayers. Molecular segregation then produces a sequence of shapes controlled by the initial mole ratio only. Using freezefracture electron microscopy, we identified three of these states and their corresponding coexistence regimes. Fluorescence confocal microscopy directly showed the segregation of anionic and cationic components within the aggregate. The observed shapes are consistently reproduced upon thermal cycling, demonstrating that the icosahedral shape corresponds to the existence of a local minimum of bending energy for facetted icosahedra when the optimal amount of excess segregated material is present.C ontrolling both size and shape of colloidal particles is a major challenge to the predictable formulation of mixed systems consisting of surfactants, polymers, and inorganic solids. Successful control of size and shape requires simultaneous knowledge of the mixture's equilibrium phase behavior and the mechanism of formation. The aim of this article is to describe and test hypotheses based on bending energy to control a general sequence of colloidal shapes, from large discs to punctured planes.If the elemental building blocks of a complex colloidal aggregate are amphiphilic molecules, the basic concept used to rationalize self-assembly is the concept of spontaneous curvature originating from the surface-to-volume ratio of the surfactant film (1). Common single-chain ionic amphiphiles have a spontaneous radius of curvature equivalent to one surfactant length (2) and therefore form globular micelles. Decreasing monolayer curvature obtained by mixing surfactants first produces giant cylindrical and finally locally flat bilayers.When the elementary building block is a fragment of a bilayer, line tension of pores, or rims of discs, and elastic energy associated to dihedral angles on the contact line between adjacent facets need to be considered (3). The crystallization͞ segregation should be consistent with the sequence of shapes observed with strongly interacting charged colloids in the absence of salt. Molecular segregation is demonstrated by specificity of labeling with a dye and direct observation, and we show finally that the resulting shapes correspond to local minima of energy of formation. Mechanism of Shape Control Through Molecular SegregationConsider an initial state of the dispersion with unilamellar vesicles in the fluid state. In the fluid state of mixed vesicles, the two components exhibit in-plane miscibility.Y Upon cooling, nucleation and growth of planar crystals occur in the form of polygonal frozen bilayers, which can only form at a fixe...

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Section: Model Mixed Molecular System Investigatedmentioning

confidence: 99%

Shape control through molecular segregation in giant surfactant aggregates

Dubois

Lizunov

Meister

et al. 2004

Proc. Natl. Acad. Sci. U.S.A.

132

228

View full text Add to dashboard Cite

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“…41 In this case, the salt induces strong attractive interactions and the deformation stems directly from the vesicle clustering, i.e., close vesicle-vesicle interactions. 41 This type of mechanism where the role of salt could be replaced with the polycation does not take place here, since indiVidual faceted vesicles are observed in various samples at different temperatures above T m .…”

Section: Mechanisms For Polymer-induced Changes In Vesiclementioning

confidence: 99%

Mechanisms behind the Faceting of Catanionic Vesicles by Polycations: Chain Crystallization and Segregation

et al. 2006

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Vesicles composed of an anionic and a cationic surfactant, with a net negative charge, associate strongly with a hydrophobically modified polycation (LM200) and with an unmodified polycation with higher charge density (JR400), forming viscoelastic gel-like structures. Calorimetric results show that in these gels, LM200 induces a rise of the chain melting temperature (T m ) of the vesicles, whereas JR400 has the opposite effect. For both polymer-vesicle systems, the shear viscosity exhibits an inflection point at T m , and for the LM200 system the measured relaxation times are significantly higher below T m . The neat vesicles and the polycationbound vesicles have a polygonal-like faceted shape when the surfactant chains in the bilayer are crystallized, as probed by cryo-transmission electron microscopy. Above T m , the neat and the LM200-bound vesicles regain a spheroidal shape, whereas those in the JR400 system remain with a deformed faceted shape even above T m . These shape changes are interpreted in terms of different mechanisms for the polymer-vesicle interaction, which seem to be highly dependent on polymer architecture, namely charge density and hydrophobic modification. A crystallization-segregation mechanism is proposed for the LM200-vesicle system, while, for the JR400-vesicle one, charge polarization-lateral segregation effects induced by the polycation in the catanionic bilayer are envisaged.

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“…It has been shown previously that vesicles formed by catanionic surfactant mixtures of unequal chain length are typically polydisperse. 63 In a study by Coldren et al, 51 it has been shown that the stiffer (higher bending constant, K) the bilayer the less polydisperse the vesicle sample; these authors determined the sum of the Helfrich bilayer elastic parameters and the spontaneous curvature radius from the size distribution obtained from the cryo-TEM images for three different systems. Relating their results to ours, we can infer that the vesicle system composed of ALA and SOS probably has higher bilayer stiffness than that composed of ALA and SCS, resulting in a less polydisperse size distribution as can be seen in Figure 5b.…”

Section: Discussionmentioning

confidence: 99%

Spontaneous Formation of Vesicles and Dispersed Cubic and Hexagonal Particles in Amino Acid-Based Catanionic Surfactant Systems

et al. 2006

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Mixed catanionic surfactant systems based on amino acids were investigated with respect to the formation of liquid crystal dispersions and the stability of the dispersions. The surfactants used were arginine-N-lauroyl amide dihydrochloride (ALA) and N R -lauroyl-arginine-methyl ester hydrochloride (LAM), which are arginine-based cationic surfactants; sodium hydrogenated tallow glutamate (HS), a glutamic-based anionic surfactant; and the anionic surfactants sodium octyl sulfate (SOS) and sodium cetyl sulfate (SCS). It is demonstrated that in certain ranges of composition there is a spontaneous formation of vesicular, cubic, and hexagonal structures. The solutions were characterized with respect to internal structure and size by cryogenic transmission electron microscopy (cryo-TEM), dynamic light scattering (DLS), and turbidity measurements. Vesicles formed spontaneously and were found for all systems studied; their size distribution is presented for the systems ALA/SCS/W and ALA/SOS/W; they are all markedly polydisperse. The aging process for the system ALA/SOS/W was monitored both by turbidity and by cryo-TEM imaging; the size distribution profile for the system becomes narrower and the number average radius decreases with time. The presence of dispersed particles with internal cubic structure (cubosomes) and internal hexagonal structure (hexosomes) was documented for the systems containing ALA and HS. The particles formed spontaneously and remained stably dispersed in solution; no stabilizer was required. (Cubosome and hexosome are USPTO registered trademarks of Camurus AB, Sweden.) The spontaneous formation of particles and their stability, together with favorable biological responses, suggests a number of applications.

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The origins of stability of spontaneous vesicles

Cited by 281 publications

References 38 publications

Shape control through molecular segregation in giant surfactant aggregates

Shape control through molecular segregation in giant surfactant aggregates

Mechanisms behind the Faceting of Catanionic Vesicles by Polycations: Chain Crystallization and Segregation

Spontaneous Formation of Vesicles and Dispersed Cubic and Hexagonal Particles in Amino Acid-Based Catanionic Surfactant Systems

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