Diblock copolymer membranes investigated by single-particle tracking

We report a study on particle diffusion in membranes formed from polystyrene-block-poly(2-(dimethylamino)ethyl methacrylate) (PS-b-PDMAEMA) diblock copolymers. The membranes were investigated by scanning electron microscopy and by single-particle tracking employing carboxy-functionalized polystyrene beads loaded with a fluorophore as spectroscopic probes. From the diffusion trajectories we extracted the domain size distribution of the membranes and the local diffusion coefficient of the beads as a function of the size of the beads. The single-particle tracking data revealed that the effective domain sizes of the membranes are reduced with respect to the domain sizes obtained from scanning electron microscopy, reflecting the confined diffusion of the probe particles due to interactions with the domain walls. This is corroborated by a clear correlation between the diffusion coefficient of an individual polystyrene bead and the size of the actual domain to which it is confined.     

Electrocapillary phenomena at oil-water interfaces

Summary In order to discuss the structure of the electrical double layer at the oil-water interface, we measured the electrocapillary curves for various inorganic electrolyte aqueous solutions (aq. phase) in contact with the oil phase containing surface active agents (oil phase). When the aqueous phase contained the potassium halide, the depression of the interfacial tension over the cathodic polarization range due to the adsorption of the surface active agent, i. e. cetyl pyridinium chloride or cytyltrimethylammonium chloride, was suppressed strongly. The suppression was larger for the anion of larger crystal radius. This phenomenon indicated that the adsorbed surface active ion was neutralised by the binding of counter-ions. When the oil phase contained sodium dodecylbenzenesulphonate, the interfacial tension was depressed over the anodic polarization region. This depression was again suppressed by the counter-ion binding. The order of this suppression for various divalent cations agreed with that of the binding ability to chondroitin sulphate. Moreover, high valent cations had strong binding ability. These counter-ion binding support the idea of the penetration of polar groups of orientated surface active agents into the aqueous phase at the oil-water interface.

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Zusammenfassung Um Aussagen über die Struktur der elektrischen Doppelschicht an Öl-Wasser-Grenzflächen zu erhalten, wurden die Elektrokapillaritätskurven von wässerigen Lösungen verschiedener anorganischer Salze in Kontakt mit der Ölphase, die grenzflächenaktive Stoffe enthielt, gemessen.Die durch Cetylpyridiniumchlorid oder cetyltrimethylammoniumchlorid in der Ölphase bewirkte Erniedrigung der Grenzflächenspannung wurde im Bereich der kationischen Polarisation durch Kaliumhalogenide in der wässerigen Phase aufgehoben. Die Wirkung des anorganischen Salzes wurde mit zunehmendem Anionenradius stärker.Die durch Natriumdodecylbenzolsulfonat bewirkte Erniedrigung der Grenzflächenspannung wurde durch die Kaliumhalogenide im Bereich der anodischen Polarisation aufgehoben. Die Wirkung zweiwertiger Kationen lief parallel mit der Bindungsfestigkeit dieser Ionen an Chondroitinsulfat.Diese Ergebnisse weisen darauf hin, daß die adsorbierten grenzflächenaktiven Ionen in ihrer Wirkung durch die Bindung von Gegenionen neutralisiert werden und daß die polaren Gruppen der an der Öl-Wasser-Grenzfläche orientierten grenzflächenaktiven Molekeln in die wässerige Phase eindringen.

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With 7 figures and 1 table     

Electrocapillary phenomena at oil-water interfaces

Summary The electrocapillary phenomena at oil-water interfaces, i. e. the change in the interfacial tension with an applied potential difference, take place over a moderate polarization range of ca. 20 to –20 V, when the systems have sufficiently high electric conductance and contain surface active agents. Experiments were carried out in the presence of potassium chloride in the aqueous phase and ionic surface active agents in the oil phase, respectively. It was found that the interfacial tension was suppressed over the cathodic or anodic polarization range, depending on whether the surface active agent used was cationic or anionic, respectively. Here the sign of polarization was conventionally taken as that of the water phase with respect to the oil phase. It was then concluded by the use of theLippmann-Helmholtz equation that the water side of the interface is charged positively or negatively in the respective case mentioned above. It appears that this charge is due to the counter ion layer which is formed corresponding to the adsorption of ionic surface active agents on the oil side.

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Zusammenfassung Die Elektrokapillarität von Öl-Wasser-Grenzflächen mit Kaliumchlorid in den wässerigen und grenzflächenaktiven Ionen in der Ölphase wurde bei geringer Polarisation im Bereich von +20 V bis –20 V untersucht. Dabei wurde beobachtet, daß die Grenzflächenspannung durch kationische bzw. anionische Polarisation erniedrigt wurde, je nachdem, ob die grenzflächenaktiven Stoffe kationisch oder anionisch waren. Die Ergebnisse wurden mit Hilfe derLippmann-Helmholtz-Gleichung gedeutet. Es bildet sich eine elektrische Doppelschicht mit den grenzflächenaktiven Ionen an der Ölschicht und den anorganischen Gegenionen an der wässerigen Seite der Grenzfläche.

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With 10 figures and 2 tables     

Electrocapillary phenomena at oil-water interfaces

Summary The interfacial tension, between the oil phase containing an ionic surface active agent and the aqueous phase containing an inorganic electrolyte, is a function of the potential difference applied to the two phases from an outer circuit. This phenomenon, the electrocapillarity at oil-water interfaces, was used by the present authors to study the structure of electrical double layers and the interaction between surface active agent ions and counterions. In the present study an amphoteric material, the lecithin, was used as the surface active agent, and its adsorption and dissociation behaviours as well as its interaction with counterions were examined. It was found that the lecithin dissolved in the oil phase behaved as the cationic or anionic surface active agent according as the p_H of the aqueous phase was acidic or alkaline. This showed that the ionic groups of the lecithin molecule are immersed into the aqueous side of the interface. Since the interfacial tension was independent of the applied potential at the interfacial isoelectric point, this point could be measured with a high sensitivity. The ionic species of the inorganic electrolyte in the aqueous phase had a marked influence on the isoelectric point. The reversal of charge spectra thus obtained were found to agree fairly well with that obtained by other authors using electrophoretic measurement.With 12 figures     

Particle tracking microrheology of lyotropic liquid crystals

We present comprehensive results on the microrheological study of lyotropic liquid crystalline phases of various space groups constituted by water-monoglyceride (Dimodan) mixtures. In order to explore the viscoelastic properties of these systems, we use particle tracking of probe colloidal particles suitably dispersed in the liquid crystals and monitored by diffusing wave spectroscopy. The identification of the various liquid crystalline phases was separately carried out by small-angle X-ray scattering. The restricted motion of the particles was monitored and identified by the decay time of intensity autocorrelation function and the corresponding time-dependent mean square displacement (MSD), which revealed space group-dependent behavior. The characteristic time extracted by the intersection of the slopes of the MSD at short and long time scales, provided a characteristic time which could be directly compared with the relaxation time obtained by microrheology. Further direct comparison of microrheology and bulk rheology measurements was gained via the Laplace transform of the generalized time-dependent MSD, yielding the microrheology storage and loss moduli, G'(ω) and G'(ω), in the frequency domain ω. The general picture emerging from the microrheology data is that all liquid crystals exhibit viscoelastic properties in line with results from bulk rheology and the transition regime (elastic to viscous) differs according to the specific liquid crystal considered. In the case of the lamellar phase, a plastic fluid is measured by bulk rheology, while microrheology indicates viscoelastic behavior. Although we generally find good qualitative agreement between the two techniques, all liquid crystalline systems are found to relax faster when studied with microrheology. The most plausible explanation for this difference is due to the different length scales probed by the two techniques: that is, microscopical relaxation on these structured fluids, is likely to occur at shorter time scales which are more suitably probed by microrheology, whereas bulk, macroscopic relaxations occurring at longer time scales can only be probed by bulk rheology.     

Dynamics of charged microparticles at oil-water interfaces

Solid-stabilized emulsions have been used as a model system to investigate the dynamics of charged microparticles with diameters of 1.1 microm at oil-water interfaces. Using confocal microscopy, we investigated the influences of interfacial curvature, cluster size, and temperature on the diffusion of solid particles. Our work suggests that a highly curved emulsion interface slows the motion of solid particles. This qualitatively supports the theoretical work by Danov et al. (Danov, K. D.; Dimova, R.; Pouligny, B. Phys. Fluids 2000, 12, 2711); however, the interfacial curvature effect decreases with increasing oil-phase viscosity. The diffusion of multiparticle clusters at oil-water interfaces is a strong function of cluster size and oil-phase viscosity and can be quantitatively related to fractal dimension. Finally, we report the influence of temperature and quantify the diffusion activation energy and friction factor of the particles at the investigated oil-water interfaces.     

Interfacial microrheology: Particle tracking and related techniques

Microrheology offers several advantages over traditional macroscopic rheology: the use of very small samples, the possibility of studying heterogeneous samples and the broad range of frequency that can be explored. In this paper the study of the microrheology of fluid interfaces is reviewed, with special emphasis on particle tracking techniques. We comment the main results and the assumptions of the different approaches for describing the hydrodynamics of a particle trapped at a surfactant or polymer monolayer.     

Adsorbed and spread beta-casein monolayers at oil-water interfaces

A previous study (Langmuir 2003, 19, 8436) used a Langmuir type pendant drop film balance to form beta-casein monolayers at the air-water interface. The present paper reports the application of that technique to the formation of protein monolayers at liquid interfaces. This technique allows a direct comparison between spread and adsorbed beta-casein interfacial behaviors that is presented in terms of their pi-A isotherms and static elasticity moduli. Pi-A isotherms of adsorbed and spread protein have been compared and found to be fairly similar in shape, stability, and also hysteresis phenomena. Examination of the elasticity moduli of both layers shows a similar analogy although slight differences arise and are interpreted in terms of the protein unfolding extent attained by both procedures at the oil interface.     

Sandwich structures at oil-water interfaces under alkaline conditions

Equilibrium liquid crystal (LC) layer on an interface between crude oils and water was observed at high pH. This layer is composed mainly of sodium naphthenates produced in situ at the water/oil interface. Transient LC layer was also evolved at the interface of aqueous phase of sodium hydroxide solutions and oleic phase of naphthenic acid (NA) solutions as result of a chemical reaction between NaOH and NA. This chemical reaction causes transport process resulting in a disturbance of the interface. Optical observation of this interface disturbance reviled that the interface covered with LC shows considerably lower flexibility as compared to LC free interface. The LC layer eventually dissolves in the water phase at low oil-to-water ratio, while at high oil-to-water ratio it can form an equilibrium phase, which spreads spontaneously at the oil-water interface.     

Dissection the endocytic routes of viral capsid proteins-coated upconversion nanoparticles by single-particle tracking

Real-time exploring the cellular endocytic pathway of viral capsid proteins (VCPs) functionalized nanocargos at the single-particle level can provide deep insight into the kinetic information involved in virus infection. In this work, porcine circovirus type 2 (PCV2) VCPs with different functions are modified onto the surface of upconversion nanoparticles (VCPs-UCNPs) to investigate the cellular internalization process in real-time. Clathrin-mediated endocytosis is found to be the essential uptake mechanism for these VCPs-UCNPs. Besides, it is verified that P₁-UCNPs (PCV2 VCPs with nuclear localization signal, namely P₁) can be easily assembled close to the perinuclear area, which is different from that of P₂-UCNPs (PCV2 VCPs without nuclear localization signal, namely P₂). Interestingly, multistep entry processes are observed. Particularly, confined diffusion is observed during the transmembrane process. The intracellular transport of VCPs-UCNPs is dependent on microtubules toward the cell interior. During this process, P₁-UCNPs display increased velocities with active transport, while diffusion much faster around the perinuclear area. But for P₂-UCNPs, there are only two phases involved in their endocytosis process. This study presents distinct dynamic mechanisms for the nanocargos with different functions, which would make a useful contribution to the development of robust drug delivery systems.     

Diffusive dynamics of vesicles tethered to a fluid supported bilayer by single-particle tracking

We recently introduced a method to tether intact phospholipid vesicles onto a fluid supported lipid bilayer using DNA hybridization (Yoshina-Ishii, C.; Miller, G. P.; Kraft, M. L; Kool, E. T.; Boxer, S. G. J. Am. Chem. Soc. 2005, 127, 1356-1357). Once tethered, the vesicles can diffuse in two dimensions parallel to the supported membrane surface. The average diffusion coefficient, D, is typically 0.2 microm(2)/s; this is 3-5 times smaller than for individual lipid or DNA-lipid conjugate diffusion in supported bilayers. In this article, we investigate the origin of this difference in the diffusive dynamics of tethered vesicles by single-particle tracking under collision-free conditions. D is insensitive to tethered vesicle size from 30 to 200 nm, as well as a 3-fold change in the viscosity of the bulk medium. The addition of macromolecules such as poly(ethylene glycol) reversibly stops the motion of tethered vesicles without causing the exchange of lipids between the tethered vesicle and supported bilayer. This is explained as a depletion effect at the interface between tethered vesicles and the supported bilayer. Ca ions lead to transient vesicle-vesicle interactions when tethered vesicles contain negatively charged lipids, and vesicle diffusion is greatly reduced upon Ca ion addition when negatively charged lipids are present both in the supported bilayer and tethered vesicles. Both effects are interesting in their own right, and they also suggest that tethered vesicle-supported bilayer interactions are possible; this may be the origin of the reduction in D for tethered vesicles. In addition, the effects of surface defects that reversibly trap diffusing vesicles are modeled by Monte Carlo simulations. This shows that a significant reduction in D can be observed while maintaining normal diffusion behavior on the time scale of our experiments.     

Three-dimensional real-time tracking of nanoparticles at an oil-water interface

Single-particle tracking with real-time feedback control can be used to study three-dimensional nanoparticle transport dynamics. We apply the method to study the behavior of adsorbed nanoparticles at a silicone oil-water interface in a microemulsion system over a range of particles sizes from 24 nm to 2000 nm. The diffusion coefficient of large particles (>200 nm) scales inversely with particle size, while smaller particles exhibit an unexpected increase in drag force at the interface. The technique can be applied in the future to study three-dimensional dynamics in a range of systems, including complex fluids, gels, biological cells, and geological media.     

SANS and SAXS analysis of charged nanoparticle adsorption at oil-water interfaces

A systematic study of the adsorption of charged nanoparticles at dispersed oil-in-water emulsion interfaces is presented. The interaction potentials for negatively charged hexadecane droplets with anionic polystyrene latex particles or cationic gold particles are calculated using DLVO theory. Calculations demonstrate that increased ionic strength decreases the decay length of the electrostatic repulsion leading to enhanced particle adsorption. For the case of anionic PS latex particles, the energy barrier for particle adsorption is also reduced when the surface charge is neutralized through changes in pH. Complementary small-angle scattering experiments show that the highest particle adsorption for PS latex occurs at moderate ionic strength and low pH. For cationic gold particles, simple DLVO calculations also explain scattering results showing that the highest particle adsorption occurs at neutral pH due to the electrostatic attraction between oppositely charged surfaces. This work demonstrates that surface charges of particles and oil droplets are critical parameters to consider when engineering particle-stabilized emulsions.     

Bidirectional nanoparticle crossing of oil-water interfaces induced by different stimuli: insight into phase transfer

Swap transactions: Bidirectional spontaneous transfer of gold nanoparticles coated with stimuli-responsive polymer brushes across oil-water interfaces has been implemented. The water-to-oil transfer of the gold nanoparticles is dictated by the ionic strength in water, while the nanoparticle oil-to-water transfer occurs only when the environmental temperature is reduced below 5?°C.     

Mobility and in situ aggregation of charged microparticles at oil-water interfaces

Particle mobility, aggregate structure, and the mechanism of aggregate growth at the two-dimensional level have been of long-standing interest. Here, we use solid-stabilized emulsions as a model system to investigate the mobility of charged microparticles at poly(dimethylsiloxane) (oil)-water interfaces using confocal laser scanning microscopy. Remarkably, the rate of diffusion of the charged colloidal-sized polystyrene particles at the oil-water interface is only moderately slower than that in the bulk water phase. The ambient diffusion constant of solid particles is significantly reduced from 1.1 x 10(-9) cm2/s to 2.1 x 10(-11) cm2/s when the viscosity of the oil phase increases from 5 cSt to 350 cSt. In addition, we successfully observe the in situ structural formation of solid particles at the oil-water interface.     

Quantitative characterization of changes in dynamical behavior for single-particle tracking studies

Single-particle tracking experiments have been used widely to study the heterogeneity of a sample. Segments with dissimilar diffusive behaviors are associated with different intermediate states, usually by visual inspection of the tracking trace. A likelihood-based, systematic approach is presented to remove this incertitude. Maximum likelihood estimators are derived for the determination of diffusion coefficients. A likelihood ratio test is applied to the localization of the changes in them. Simulations suggest that the proposed procedure is statistically robust and is able to quantitatively recover time-dependent changes in diffusion coefficients even in the presence of large measurement noise.     

水-油界面上聚合物单链的动态性质及疏水相互作用

通过荧光关联光谱研究了不相容的水-油界面上聚乙二醇单链的横向扩散运动,系统地研究了PEO单链的扩散运动速率随着水溶液中电解质NaCl浓度的升高的变化规律,发现随着盐浓度的升高,扩散系数的变化主要取决于油相的黏度,说明NaCl浓度的升高增强了聚合物链与烷烃油相的疏水相互作用.