Volume Fraction Of Vesicles Research Articles

Interactions, Diffusion, and Membrane Fluctuations in Concentrated Unilamellar Lipid Vesicle Solutions

Lipid vesicles are widely used as models for cell membranes, hosts for membrane protein studies, and containers for hydrophilic molecules. The vesicle solutions in these applications are usually prepared at a specific lipid concentration; however, because vesicles are solvent-filled structures, the corresponding volume fraction of vesicles is at least a factor of three times higher than the corresponding lipid volume fraction and critically depends on the vesicle radii. Here we show that these higher than may be expected vesicle volume fractions result in measurable interactions between the vesicles as well as affect the vesicle diffusion. We show that vesicle solutions prepared with lipid mass fractions, mL, as low as ≈ 0.004, which correspond to a lipid concentration of ≈ 4 mg/mL or 5 mmol/L (mM), not only have a measurable apparent structure factor (S′(q)) in small angle neutron scattering (SANS) experiments, but that this repulsive structure factor also affects the measured diffusion coefficient at small scattering vectors (q) such as those probed with dynamic light scattering (DLS). The measured diffusion coefficients are further affected by indirect solvent mediated interactions described by a hydrodynamic factor (H(q)). Accounting for the concentration-dependence of the vesicle diffusion shows that the lipid concentration dependence measured in neutron spin echo (NSE) spectroscopy is due to differences in the effective vesicle diffusion coefficients and not the membrane fluctuation dynamics. The results have practical implications for static and dynamic scattering experiments as well as provide interesting insights into the interactions between soft lipid vesicles.

Structure and hydrodynamic factors in lipid vesicle solutions

Lipid vesicles are widely used as models for cell membranes and extracellular vesicles, hosts for membrane protein studies, and containers for hydrophilic biotherapeutic molecules. Vesicle solutions are usually prepared at a specific lipid concentration; however, because vesicles are solvent-filled structures, the corresponding volume fraction of vesicles is at least a factor or 3 times higher than the corresponding lipid volume fraction and critically depends on the vesicle radii. Seemingly low lipid concentrations correspond to significantly higher vesicle volume fractions and closer face-to-face distances between their surfaces than may be expected.

Phase separation and collapse in almost density matched depletion induced colloidal gels in presence and absence of air bubbles: An MRI imaging study

HypothesisVesicle-polymer dispersions are found in drug-delivery systems and consumer products but undergo phase separation. Previous studies of phase separation have focussed on systems with high density differences between continuous and vesicular phases. In this study, we investigate phase separation in vesicle-polymer mixtures with very small density differences, in the presence and absence of air bubbles. ExperimentsMagnetic resonance (MR) imaging, X-ray Computed Tomography and rheological measurements are reported which characterise the properties and stability of vesicle suspensions composed of the cationic surfactant, diethylesterdimethyl ammonium chloride, mixed with non-adsorbing polymer. 1H T2 MR relaxation images are employed to observe phase separation, for a range of vesicle-polymer mixtures, which are analysed using Moran’s I spatial autocorrelation to quantify the extent and rate of phase separation. FindingsIt was found that in presence of air bubbles, phase separation follows a compression/collapse mechanism, typical of colloidal gels with large density differences between the phases. Without air bubbles, phase separation develops through the formation of tiny cracks and fractures in the samples. MRI enabled visualisation of the evolution of phase separation inside highly turbid samples. The rate of phase separation was found to generally increase with increasing polymer concentration and decrease with increasing vesicle volume fraction.

Dynamics and rheology of vesicle suspensions in wall-bounded shear flow

The dynamics and rheology of suspensions of fluid vesicles or red blood cells is investigated by a combination of molecular dynamics and mesoscale hydrodynamics simulations in two dimensions. The vesicle suspension is confined between two no-slip walls, which are driven externally to generate a shear flow with shear rate . The flow behavior is studied as a function of , the volume fraction of vesicles, and the viscosity contrast between inside and outside fluids. Results are obtained for the encounter and interactions of two vesicles, the intrinsic viscosity of the suspension, and the cell-free layer near the walls.

Poroelastic Consolidation in the Phase Separation of Vesicle−Polymer Suspensions

The addition of the polymer poly(diallyl dimethyl ammonium chloride) (polyDADMAC) to multilamellar vesicle dispersions of ditallowethylester dimethyl ammonium chloride (DDAC) causes the vesicle particles to aggregate and form a space-spanning network. These networks collapse under their own buoyant weight. The time evolution of the vesicle network height is investigated as a function of polymer concentration, vesicle volume fraction, and initial sample height. A poroelastic consolidation model accurately relates the time scales of the network collapse to the balance of gravitational, viscous, and elastic forces. Network formation is consistent with an arrested spinodal decomposition of the colloid−polymer mixture; however, while such networks usually lead to nonequilibrium gels, the buoyant stresses are sufficient to drive the phase separation of this colloid−polymer mixture to completion.

Beta‐Hydroxy Acids from a Formulation Point of View

The difference from a formulation point of view between a di‐carboxylic acid and its two related hydroxy acids with one and two hydroxy groups was investigated by comparing the structure of their emulsions with a simple nonionic surfactant as stabilizer and the changes taking place during evaporation. The results showed that the introduction of a hydroxy group into the structures has radical impact on the structure of the emulsion, especially the changes during evaporation. An evaluation of the volume fraction of vesicles in the emulsion showed these to occupy a larger volume than is intuitively assumed.

Gels of Catanionic Vesicles and Hydrophobically Modified Poly(ethylene glycol)

The association between hydrophobically modified polyethylene glycol (HM‐PEG) and sodium dodecyl sulphate/didodecyldimethylammonium bromide (SDS/DDAB) catanionic vesicles is studied by rheology, 1H‐NMR, and cryo‐TEM techniques. Phase behavior studies were performed as a function of polymer weight fraction and vesicle volume fraction. At sufficiently high polymer weight fraction and/or vesicle volume fraction, a gel phase is detected, indicating the presence of a three‐dimensional network. Phase separation was also identified, with a dilute solution coexisting with a bluish precipitate (gel fraction). Vesicle‐polymer binding is expected to occur via the insertion of the polymer hydrophobes into the vesicle bilayer. Each vesicle can act as a cross‐linking point in the network structure. The size of the vesicles is found to be important in terms of the storage modulus and viscosity of the polymer‐vesicle mixed solutions. On the other hand, changes in the vesicle total net charge do not influence the association and the rheological properties. We also address the question of how the length of the hydrophobic modification can influence the rheology of the system and, as expected, larger hydrophobic groups increase the cross‐linking and the relaxation time.

Shear-induced formation of vesicles in membrane phases: kinetics and size selection mechanisms, elasticity versus surface tension.

Multilamellar vesicles can be formed upon shearing lamellar phases (L(alpha)) and phase-separated lamellar-sponge (L(alpha)/L(3)) mixtures. In the first case, the vesicle volume fraction is always 100% and the vesicle size is monitored by elasticity ("onion textures"). In the second system the vesicle volume fraction can be tuned from 0 to 100% and the mean size results from a balance between capillary and viscous forces ("Taylor droplets"). However, despite these differences, in both systems we show that the formation of vesicles is a strain-controlled process monitored by a universal primary buckling instability of the lamellae.

Diffusion of water in multilamellar vesicles of dialkyl and dialkyl ester ammonium surfactants

The NMR self-diffusion technique is used for measuring diffusion of water in highly concentrated multilamellar vesicle solutions. The signal intensity of water, i.e. the water echo-decay, is monitored down to very small intensities thus providing an accurate measure of how water is diffusing in the solution. It is noted that a large curvature is dominating the functional form of the echo-decay indicating the presence of a large number of multilamellar vesicles. It is also concluded that in order to measure accurately the volume fraction of water inside and outside vesicles the experimental time scale can be changed. From a multiexponential fit to the echo-decay the fraction fast and the sum of all slow components can be extracted. When the apparent fraction “vesicle water”, Pvwapp, is plotted versus the experimental time scale the graph produced is a good representation of the difference in how fast water diffuses over the different vesicle membranes. From an extrapolation to “zero time” the true fraction of trapped water can be extracted, i.e. a quantitative measure of the volume fraction of vesicles at a certain concentration of surfactant.

Osmophoresis in a dilute suspension of spherical vesicles

The osmophoretic motion of two spherical vesicles in a constant gradient of solute concentration is analyzed using a method of reflections. The vesicles are oriented arbitrarily with respect to the gradient, and they are allowed to have different semipermeable membranes, hold arbitrary solutes, and possess unequal sizes. The method of reflections is based on an analysis of the solute concentration and fluid velocity disturbances produced by a single spherical vesicle placed in an arbitrarily varying concentration field. The quasisteady results for two-vesicle interactions are correct to O( r −7 12), where r 12 is the distance between the vesicle centers. Our analytical results are found to be in good agreement with the numerical solution obtained using spherical bipolar coordinates. Based on a microscopic model the results for two-vesicle interactions are used to determine the effect of the volume fraction of vesicles of each type on the average osmophoretic velocity in a bounded suspension. For a suspension of identical vesicles, this average velocity is increased as the vesicle volume fraction is increased.

Vesicle layering in solidified intrusive magma bodies: a newly recognized type of igneous structure

We report a novel type of layering structure in igneous rocks. The layering structure in the Ogi picrite sill in Sado Island, Japan, is spatially periodic, and appears to be caused by the variation in vesicle volume fraction. The gas phase forming the vesicles apparently exsolved from the interstitial melt at the final stage of solidification of the magma body. We call this type of layering caused by periodic vesiculation in the solidifying magma body "vesicle layering." The presence of vesicle layering in other basic igneous bodies (pillow lava at Ogi and dolerite sill at Atsumi, Japan) implies that it may be a fairly common igneous feature. The width of individual layers slightly, but regularly, increases with distance from the upper contact. The layering plane is perpendicular to the long axes of columnar joints, regardless of gravitational direction, suggesting that the formation of vesicles is mainly controlled by the temperature distribution in the cooling magma body. We propose a model of formation of vesicle layering which is basically the same as that for Liesegang rings. The interplay between the diffusion of heat and magmatic volatiles in melt, and the sudden vesiculation upon supersaturation, both play important roles.

Particle interactions in osmophoresis

An exact analytical study is presented for the osmophoretic motion of two spherical vesicles in a constant solute concentration gradient arbitrarily oriented with respect to the line of vesicle centers. The vesicles may be formed from different semipermeable membranes, contain arbitrary solutes and have unequal radii. The appropriate equations of conservation of solute species and fluid momentum are solved in the quasisteady limit using spherical bipolar coordinates and the translational and angular velocities of the vesicles are calculated for various cases. The interaction between vesicles can be strong and peculiar when the surface-to-surface spacing gets close to zero. The influence of the interaction, in general, is stronger on the smaller vesicle than on the larger one. For the osmophoresis of two identical vesicles along their line of centers, both migrate slower than the velocity they would possess if isolated. On the contrary, for the case of two identical vesicles undergoing osmophoresis normal to their line of centers, they migrate faster than their undisturbed velocity except when the two vesicles are very close together. A comparison between our exact results for osmophoretic velocities and those evaluated from asymptotic formulas obtained using a method of reflections is made for a case of two identical vesicles. The asymptotic formulas for the vesicle velocities up to O(r 12 −6) , where r 12 is the center-to-center distance between the vesicles, are found to underestimate (for the axisymmetric osmophoresis) or to overestimate (for the transverse motion) the effect of particle interactions; the error can be significant when the vesicle surfaces are less than half the vesicle radius apart. Our numerical results for the interaction between two vesicles are also used to find the effect of the volume fraction of vesicles on the average osmophoretic velocity in a bounded dispersion.

Magma vesiculation and pyroclastic volcanism on Venus

Theoretical consideration of the magma vesiculation process under observed and inferred venusian surface conditions suggests that vesicles should form in basaltic melts, especially if CO 2 is the primary magmatic volatile. However, the high surface atmospheric pressure ((∼90 bars) and density on Venus retard bubble coalescence and disruption sufficiently to make explosive volcanism unlikely. The products of explosive volcanism (fire fountains, convecting eruption clouds, pyroclastic flows, and topography-mantling deposits of ash, spatter, and scoria) should be rare on Venus, and effusive eruptions should dominate. The volume fraction of vesicles in basaltic rocks on Venus are predicted to be less than in chemically similar rocks on Earth. Detection of pyroclastic landforms or eruption products on Venus would indicate either abnormally high volatile contents of Venus magmas (2.5–4 wt%) or different environmental conditions (e.g., lower atmospheric pressure) in previous geologic history.