Effect of bilayer thickness on membrane bending rigidity

The bending rigidity k(c) of bilayer vesicles self-assembled from amphiphilic diblock copolymers has been measured using single- and dual-micropipet techniques. These copolymers are nearly a factor of 5 greater in hydrophobic membrane thickness d than their lipid counterparts and an order of magnitude larger in molecular weight M(n). The macromolecular structure of these amphiphiles lends insight into and extends relationships for traditional surfactant behavior. We find the scaling of k(c) with thickness to be nearly quadratic, in good agreement with existing theories for bilayer membranes. The results here are key to understanding and designing soft interfaces such as biomembrane mimetics.     

Particles trapped at the droplet interface in water-in-water emulsions

Water-in-water emulsions were formed by mixing incompatible aqueous solutions of dextran and poly(ethylene oxide) (PEO) in the presence of latex or protein particles. It was found that particles with a radius as small as 0.1 μm become trapped at the interface between the PEO- and dextran-rich phases with interfacial tensions down to 10(-6) N/m. The particles were visualized at the interface of the emulsion droplets using confocal laser scanning microscopy (CLSM) allowing determination of the contact angle. Various degrees of coverage with particles could be observed. On densely covered droplets, the particles had a hexagonal crystalline order. At intermediate coverage, transient clustering of the particles was observed. The diffusion coefficient of the particles at the interface was determined using multiparticle tracking. Fusion of droplets was observed in all cases leading eventually to macroscopic phase separation.     

Dynamics of encapsulation and controlled release systems based on water-in-water emulsions: negligible surface rheology

A nonequilibrium thermodynamic model based on the interfacial transport phenomena (ITP) formalism was used to study deformation-relaxation behavior of water-in-water emulsions. The ITP formalism allows us to describe all water-in-water emulsions with one single theory. Phase-separated biopolymer solutions, hydrogel beads, liposomes, polymersomes, colloidosomes, and aqueous polymer microcapsules are all limiting cases of this general theory with respect to rheological behavior of the bulk phases and interfaces. Here we have studied two limiting cases of the general theory, with negligible surface rheology: phase-separated biopolymer solutions and hydrogel beads. We have determined the longest relaxation time for a small perturbation of the interfaces in these systems. Parameter maps were calculated which can be used to determine when surface tension, bending rigidity, permeability, and bulk viscoelasticity dominate the response of a droplet or gel bead. In phase-separated biopolymer solutions and dispersions of hydrogel beads six different scaling regimes can be identified for the relaxation time of a deformation. Hydrogel beads may also have a damped oscillatory response to a deformation. The results presented here provide new insight into the complex dynamics of water-in-water emulsions and also suggest new experiments that can be used to characterize the interfacial properties of these systems.     

Viscous behavior of concentrated emulsions of two immiscible Newtonian fluids with interfacial tension

A new equation for the relative viscosity of infinitely dilute emulsions of noncolloidal droplets is proposed using the analogy between shear modulus and shear viscosity. In the limit of capillary number -->0, the proposed equation reduces to the well-known Taylor viscosity law for infinitely dilute emulsions. Starting from the proposed equation for an infinitely dilute emulsion, new viscosity equations for concentrated emulsions are then developed using a differential scheme. The proposed equations for concentrated emulsions are evaluated in light of a large body of published experimental data on the viscosity of emulsions.     

Influence of interfacial rheology on the viscosity of concentrated emulsions

New models are developed for the viscosity of concentrated emulsions taking into consideration the effects of interfacial rheology and Marangoni phenomenon. The interface is assumed to be viscous with non-zero surface-shear and surface-dilational viscosities. The Marangoni effect is accounted for through non-zero Gibbs elasticity of the interface. The experimental viscosity data for a number of emulsion systems are interpreted in terms of the proposed models.     

Effect of bending and torsion rigidity on self-diffusion in polymer melts: a molecular-dynamics study

Extensive molecular-dynamics simulations have been performed to study the effect of chain conformational rigidity, controlled by bending and torsion potentials, on self-diffusion in polymer melts. The polymer model employs a novel torsion potential that avoids computational singularities without the need to impose rigid constraints on the bending angles. Two power laws are traditionally used to characterize the dependence of the self-diffusion coefficient on polymer length: D proportional to N(-nu) with nu=1 for NNe (reptation regime), Ne being the entanglement length. Our simulations, at constant temperature and density, up to N=250 reveal that, as the chain rigidity increases, the exponent nu gradually increases towards nu=2.0 for NNe. The value of Ne is slightly increased from 70 for flexible chains, up to the point where the crossover becomes undefined. This behavior is confirmed also by an analysis of the bead mean-square displacement. Subsequent investigations of the Rouse modes, dynamical structure factor, and chain trajectories indicate that the pre-reptation regime, for short stiff chains, is a modified Rouse regime rather than reptation.     

Recent developments in the field of bending rigidity measurements on membranes

This review gives a brief overview of experimental approaches used to assess the bending rigidity of membranes. Emphasis is placed on techniques based on the use of giant unilamellar vesicles. We summarize the effect on the bending rigidity of membranes as a function of membrane composition, presence of various inclusions in the bilayer and molecules and ions in the bathing solutions. Examples for the impact of temperature, cholesterol, some peptides and proteins, sugars and salts are provided and the literature data are discussed critically. Future directions, open questions and possible developments in this research field are also included.     

The aqueous surroundings alter the bending rigidity of lipid membranes

The bending elasticity is the mechanical property that characterizes the deformability of lipid bilayers. In the present study the bending elasticity of phosphatidylcholine lipid membranes is reported in aqueous media with various chemical composition and pH. The bending modulus is obtained from analysis of the thermal shape fluctuations, performed on nearly spherical giant lipid vesicles. Lower bending rigidity of phosphatidylcholine bilayers is measured in aqueous media, containing potassium or sodium chlorides, compared to its value in water without salts. The results reported here for the membrane bending elasticity at three acidic values of pH are compared with the literature data from micromanipulation measurements of giant unilamellar vesicles from the same lipid. In accordance with previous results, further evidences are provided for the softening of lipid bilayers in the presence of sucrose in the aqueous surroundings.     

The effect of interfacial microstructure on the lipid oxidation stability of oil-in-water emulsions

Novel rattle-type magnetic mesoporous carbon spheres are successfully prepared using composite spheres with Fe(3)O(4) as core and mesoporous SiO(2) as shell plus solid SiO(2) as a middle layer as templates. These rattle-type spheres possess the magnetization strength of as high as 37.5 emu/g, high and tunable specific surface areas (382-512.6 m(2)/g) due to mesoporous carbon shells. This magnetic rattle-type structure and the readily accessible mesoporous shell are very favoring for the fast adsorption and release of guest objects triggered by external stimulus, for example, the spheres showed very good adsorptive property to dye.     

Opposing effects of cation binding and hydration on the bending rigidity of anionic lipid bilayers

We correlate the molecularly realistic self-consistent field predictions for the mean bending modulus kc of charged lipid vesicles with experimental observations of the size R of corresponding vesicles that are produced by the freeze-thaw method. We elaborate on the Ansatz that the bending modulus is related to the membrane persistence length and that this length scale sets the radius of the vesicles. Alkali cations have a remarkable effect on the mean bending modulus and thus on the equilibrium radius of negatively charged entropically stabilized dioleoylphosphatidylglycerol (DOPG) vesicles. Where cation hydration typically results in thicker and thus stiffer membranes, specific adsorption to the bilayer surface results in a decrease of the surface charge density and the thickness of the membrane-associated electric double layer. As a result of these opposing effects on kc and R, the largest DOPG vesicles are found in the presence of K+, which combines an intermediate hydration enthalpy and PG-binding affinity.     

Influence of interfacial characteristics on Ostwald ripening in hydrocarbon oil-in-water emulsions

The influence of the nature of the interfacial membrane on the kinetics of droplet growth in hydrocarbon oil-in-water emulsions was investigated. Droplet growth rates were determined by measuring changes in the droplet size distribution of 1 wt % n-tetradecane or n-octadecane oil-in-water emulsions using laser diffraction. The interfacial properties of the droplets were manipulated by coating them with either an SDS layer or with an SDS-chitosan layer using an electrostatic deposition method. The emulsion containing SDS-coated octadecane droplets did not exhibit droplet growth during storage for 400 h, which showed that it was stable to Ostwald ripening because of this oils extremely low water-solubility. The emulsion containing SDS-coated n-tetradecane droplets showed a considerable increase in mean droplet size with time, which was attributed to Ostwald ripening associated with this oils appreciable water-solubility. On the other hand, an emulsion containing SDS-chitosan coated n-tetradecane droplets was stable to droplet growth, which was attributed to the ability of the interfacial membrane to resist deformation because of its elastic modulus and thickness. This study shows that the stability of emulsion droplets to Ostwald ripening can be improved by using an electrostatic deposition method to form thick elastic membranes around the droplets.     

Investigation of the interfacial properties of water-in-diluted-bitumen emulsions using micropipette techniques

The interfacial properties of water-in-diluted bitumen emulsions were studied using micropipette techniques. It was observed that, as bitumen concentration in the bulk phase (C0) increased, the interfacial tension on the water droplet surfaces decreased. In addition, there was a small effect on the interfacial tension when different solvent mixtures were used. Mixtures of toluene and heptane in different ratios were used as solvents for bitumen dilution. Crumpling of the interface was influenced by bitumen concentration and type of solvent. No crumpling was found for bitumen content less than 0.01% for all solvents used. Crumpling was observed at higher bitumen concentrations when deionized water (pH 5.4-5.6) was used. Setting "heptol[A]" to be the mixture of toluene and heptane, with the volume percent of toluene being A, the following were concluded. Crumpling disappeared at C0 > 1% and when heptol[100] was used, and also at C0 > 10% and when heptol[30] was used. Crumpling was strongly affected by the water pH. In the case of heptol[50], at a higher pH, the crumpling region that normally occurred at C0 > 0.01% disappeared. The micropipette technique proved to be useful in studying the interfacial properties of micrometer-sized emulsion drops.     

Effect of interfacial tension on the dynamic behavior of droplet formation during microchannel emulsification

Microchannel (MC) emulsification is a novel technique for producing monodisperse emulsions. In this study, we investigated the effect of interfacial tension on the dynamic behavior of droplet formation with various surfactant concentrations. Interfacial tension did not affect the resultant droplet diameter in lower flow velocity ranges, but it did affect the time-scale parameters. These results were interpreted using the droplet formation mechanism reported in our previous study. At surfactant concentrations below 0.3%, the emulsification behavior was differed from that at higher surfactant concentrations. An analysis of diffusional transfer indicated that dynamic interfacial tension affects the emulsification behavior at lower surfactant concentrations. Dynamic interfacial tension that exceeded the equilibrium value led to a shorter detachment time. This resulted in stable droplet formation of monodispersed emulsions by spontaneous transformation, even at flow velocities above the predicted critical flow velocity. A previous study predicted that the droplet formation would become unstable and polydispersed larger droplets would form over critical flow velocity. Wetting of the MC with the dispersed phase at lower surfactant concentrations induced formation of larger polydispersed droplets at high flow velocities.     

On the viscoelastic behavior of multiple emulsions

The dynamic viscoelastic behavior of multiple emulsions is investigated. A modified Palierne model is used to predict the storage and loss moduli of multiple emulsions. The multiple emulsions exhibit two relaxation domains due to relaxation of two different interfaces--internal and external. The internal interface is the interface between internal droplets and their suspending medium (primary emulsion continuous-phase). The external interface is the interface between multiple-emulsion droplets and their suspending medium (external continuous-phase). The factors affecting the dynamic viscoelastic behavior of multiple emulsions are discussed.     

Formation and rupture mechanisms of visco-elastic interfacial films in polymer-stabilized emulsions

Considering the need for low oil price, polymer flooding has been demonstrated to be vitally important for enhanced oil recovery (EOR) in the oil industry. However, polymer-stabilized emulsions form during the displacement process, causing severe challenges in oilfield surface production, including separation performance, high operation and maintenance costs, pollution of facilities, and human health and environmental threats. In this paper, the formation and rupture of visco-elastic interfacial films are described. The emulsification structure of the polymer-stabilized emulsions is emphasized, and the film thickness is both measured and calculated. Furthermore, the thinning behavior of the interfacial films is presented based on the established destabilization process with a pulsed electric field. The emulsion stability theory is in good agreement with experimental results. The concentration of back-produced polymer is responsible for the increase in the elastic modulus of the interfacial films and dominates the formation and stability of the interfacial films. The rupture mechanism of the films and their ability to overcome droplet coalescence primarily depends on the thinning characteristics of the films in polymer-stabilized emulsions. By understanding the destabilization process, an improved thinning rate can be achieved for visco-elastic interfacial films, and the rupturing rate of high-strength films can be promoted.     

Stability of water/crude oil emulsions based on interfacial dilatational rheology

The dilatational viscoelasticity behaviors of water/oil interfaces formed with a crude oil and its distilled fractions diluted in cyclohexane were investigated by means of an oscillating drop tensiometer. The rheological study of the w/o interfaces at different frequencies has shown that the stable w/o emulsions systematically correspond to interfaces which present the rheological characteristics of a 2D gel near its gelation point. The stability of emulsions was found to increase with both the gel strength and the glass transition temperature of the gel. As expected, the indigenous natural surfactants responsible for the formation of the interfacial critical gel have been identified as the heaviest amphiphilic components present in the crude oil; i.e., asphaltenes and resins. Nevertheless, we have shown that such a gel can also form in the absence of asphaltene in the oil phase.     

Influence of interfacial properties on Ostwald ripening in crosslinked multilayered oil-in-water emulsions

The influence of interfacial crosslinking, layer thickness and layer density on the kinetics of Ostwald ripening in multilayered emulsions at different temperatures was investigated. Growth rates of droplets were measured by monitoring changes in the droplet size distributions of 0.5% (w/w) n-octane, n-decane, and n-dodecane oil-in-water emulsions using static light scattering. Lifshitz-Slyozov-Wagner theory was used to calculate Ostwald ripening rates. A sequential two step process, based on electrostatic deposition of sugar beet pectin onto fish gelatin or whey protein isolate (WPI) interfacial membranes, was used to manipulate the interfacial properties of the oil droplets. Laccase was added to the fish gelatin-beet pectin emulsions to promote crosslinking of adsorbed pectin molecules via ferulic acid groups, whereas heat was induced to promote crosslinking of WPI and helix coil transitions of fish gelatin. Ripening rates of single-layered, double-layered and crosslinked emulsions increased as the chain length of the n-alkanes decreased. Emulsions containing crosslinked fish gelatin-beet pectin coated droplets had lower droplet growth rates (3.1 ± 0.3 × 10⁻²⁶ m³/s) than fish gelatin-stabilized droplets (7.3 ± 0.2 × 10⁻²⁶ m³/s), which was attributed to the formation of a protective network. Results suggest that physical or enzymatic biopolymer-crosslinking of interfaces may reduce the molecular transport of alkanes between the droplets in the continuous phase.     

Experimental evidence of the electrostatic contribution to the bending rigidity of charged membranes

We address the issue of the origin of the bending rigidity of a charged membrane formed from amphiphilic molecules. Electrostatic effects are investigated by direct measurement of the force necessary to deform a catanionic membrane as function of the ionic strength of the medium by means of an atomic force microscope (AFM). Using continuum mechanical modeling of membrane deformation, we derive the bending rigidity of the catanionic membranes and monitor for the first time its decrease in response to increasing salt concentration.     

Stabilisation of emulsions by interfacial polymerisation of poloxamer surfactant derivatives

A method of increasing the stability ofo/w emulsions and providing a potential additional barrier to drug release from the oil droplets is described. Diacryloyl derivatives of non-ionic block copolymers (poloxamers) have been used to stabilise isopropyl myristate in water emulsions. Cross-linking of these stabilisers at the oil-water interface produces a polymeric region which increases the stabilityo/w emulsions to centrifugation and of thew/o emulsions to creaming.     

Effect of working temperature on the interfacial behavior of overmolded hybrid fiber reinforced polypropylene composites

In the present study, the interfacial behavior of overmolded hybrid fiber reinforced polypropylene composites (hybrid composites) in the working temperature range from 23 °C to 90 °C was studied by experimental and constitutive methods. Monotonic and cycle loading-unloading single-lap-shear tests were employed to determine the interfacial properties of hybrid composites. The experimental results show that both interfacial shear strength and shear stiffness decrease with increasing working temperature. A regression function was adopted to evaluate the decaying degree of interfacial properties with increasing working temperature. The shear stress-displacement relationship under monotonic loading exhibits nonlinear behavior after an initial elastic region. The envelope lines of shear stress-displacement of hybrid composites under cyclic loading indicate that the nonlinearity in the curve is caused by the plastic deformation of polypropylene in the interphase region. A constitutive model was built to describe the nonlinear shear stress-displacement relation of hybrid composites at different working temperatures. A full suite of temperature-dependent plastic parameters in the model was obtained from cyclic loading-unloading tensile tests. The predicted shear stress–displacement curves agreed well with experimental results from different working temperatures. In addition, the failure mode of hybrid composites varied with working temperature.