Phase behavior and 3D structure of strongly attractive microsphere-nanoparticle mixtures

We investigate the phase behavior and 3D structure of strongly attractive mixtures of silica microspheres and polystyrene nanoparticles. These binary mixtures are electrostatically tuned to promote a repulsion between like-charged (microsphere-microsphere and nanoparticle-nanoparticle) species and a strong attraction between oppositely charged (microsphere-nanoparticle) species. Using confocal fluorescence scanning microscopy, we directly observe the 3D structure of colloidal phases assembled from these mixtures as a function of varying composition. In the absence of nanoparticle additions, the charged-stabilized microspheres assemble into a polycrystalline array upon sedimentation. With increasing nanoparticle volume fraction, nanoparticle bridges form between microspheres, inducing their flocculation. At even higher nanoparticle volume fractions, the microspheres become well coated with nanoparticles, leading to their charge reversal and subsequent restabilization. We demonstrate how this fluid-gel-fluid transition can be utilized to control the morphology of the colloidal phases formed under gravity-driven sedimentation.     

Nanoparticle-mediated epitaxial assembly of colloidal crystals on patterned substrates

We have studied the assembly of 3-D colloidal crystals from binary mixtures of colloidal microspheres and highly charged nanoparticles on flat and epitaxially patterned substrates created by focused ion beam milling. The microspheres were settled onto these substrates from dilute binary mixtures. Laser scanning confocal microscopy was used to directly observe microsphere structural evolution during sedimentation, nanoparticle gelation, and subsequent drying. After microsphere settling, the nanoparticle solution surrounding the colloidal crystal was gelled in situ by introducing ammonia vapor, which increased the pH and enabled drying with minimal microsphere rearrangement. By infilling the dried colloidal crystals with an index-matched fluorescent dye solution, we generated full 3-D reconstructions of their structure including defects as a function of initial suspension composition and pitch of the patterned features. Through proper control over these important parameters, 3-D colloidal crystals were created with low defect densities suitable for use as templates for photonic crystals and photonic band gap materials.     

Modeling the flow of fluid/particle mixtures in microchannels: encapsulating nanoparticles within monodisperse droplets

We develop a hybrid computational approach for simulating mixtures of binary fluids and mobile, submicron particles. The model couples a lattice Boltzmann method for the binary fluid with a Brownian dynamics model for the particles. The particles can exhibit preferential wetting interactions with the different components of the fluid. As an illustration of the method, we carry out simulations in two dimensions to compare the spinodal decomposition of a pure binary fluid with the phase separation of binary blends that contain either fixed or mobile particles. We then isolate conditions where the flow of a binary fluid/particle mixture past surfaces with well-defined asperities leads to the formation of monodisperse droplets, which encapsulate the nanoparticles. The findings provide guidelines for creating multiphase emulsions with well-controlled morphologies.     

Electrostatically tuned interactions in silica microsphere-polystyrene nanoparticle mixtures

We explore the generality of nanoparticle haloing as a novel colloidal stabilization mechanism in binary mixtures of silica microspheres and polystyrene nanoparticles. By selectively tuning their electrostatic interactions, both the initial microsphere stability and the role of nanoparticle additions are varied. Adsorption isotherm and zeta potential measurements indicate that highly charged nanoparticles exhibit a weak (haloing) association with negligibly charged microspheres, whereas they either strongly adsorb onto oppositely charged or are repelled by like-charged microsphere surfaces, respectively. Bulk sedimentation and confocal scanning fluorescence microscopy reveal that important differences in system stability emerge depending on whether the added nanoparticles serve as haloing, bridging, or depletant species.     

DNA-induced size-selective separation of mixtures of gold nanoparticles

We present a novel method for size-selectively separating mixtures of nanoparticles in aqueous media utilizing the inherent chemical recognition properties of DNA and the cooperative binding properties of DNA-functionalized gold nanoparticles. We have determined that the melting temperatures (T(m)s) of aggregates formed from nanoparticles interconnected by duplex DNA are dependent upon particle size. This effect is proposed to derive from larger contact areas between the larger particles and therefore increased cooperativity, leading to higher T(m)s. The separation protocol involves taking two aliquots of a mixture of particles that vary in size and functionalizing them with complementary DNA. These aliquots are mixed at a temperature above the T(m) for aggregates formed from the smaller particles but below the T(m) for aggregates formed from the larger particles. Therefore, the aggregates that form consist almost exclusively of the larger particles and can be easily separated by sedimentation and centrifugation from the smaller dispersed particles. This unusual size-dependent behavior and separation protocol are demonstrated for three binary mixtures of particles and one ternary mixture.     

Partial specific adsorption of organic molecules in binary mixtures of adsorbents: 2,2'-bipyridine on kaolinite and hematite and 1,10-phenanthroline on montmorillonite and hydroxy-Al montmorillonite

The adsorption of 2,2'-bipyridyl in binary mixtures of sodium kaolinite and hematite at pH 6.6 and of 1,10-phenanthroline in binary mixtures of sodium montmorillonite and hydroxy-Al montmorillonite at pH 7.5 was studied as a function of the equilibrium concentration and composition of the mixture. A model is presented for determining the partial specific adsorption of two adsorbents by each component of the mixture, based on the graphic method for determining partial quantities. The partial specific adsorption values obtained for binary mixtures of Na-kaolinite with hematite and Na-montmorillonite with hydroxy-Al montmorillonite are strongly influenced by the concentration of adsorbates and adsorbents in the mixtures.     

Nematic nanocomposites with enhanced optical birefringence

The proper performance of electro-optical devices utilising liquid crystals (LCs) requires materials with high diffraction efficiency, i.e. with high optical/dielectric anisotropy, low threshold voltage and fast switching. One can achieve increase of dielectric anisotropy by using chemical synthesis or mixing LC materials. However, in most cases, this causes an increase in the threshold voltage and switching times. Therefore obtaining materials with high dielectric anisotropy and keeping threshold voltage and switching times low is a challenging task. We achieved promising results by making binary mixtures of a polar nematic LC 4'-hexyl-4-biphenylcarbonitrile (HBPCN) with low percentage (1–10% by weight) gold nanoparticles. We report that for the mixtures with 1% and 2% gold the dielectric anisotropy increases by 100% and the birefringence by about 50% of their values for pure nematic. We also report that the increase of the dielectric anisotropy in the mixtures only slightly affects threshold voltage and switching times. We propose that this increase is caused by cluster formation in the mixtures.     

Phase diagrams of binary mixtures of oppositely charged colloids

Phase diagrams of binary mixtures of oppositely charged colloids are calculated theoretically. The proposed mean-field-like formalism interpolates between the limits of a hard-sphere system at high temperatures and the colloidal crystals which minimize Madelung-like energy sums at low temperatures. Comparison with computer simulations of an equimolar mixture of oppositely charged, equally sized spheres indicate semiquantitative accuracy of the proposed formalism. We calculate global phase diagrams of binary mixtures of equally sized spheres with opposite charges and equal charge magnitude in terms of temperature, pressure, and composition. The influence of the screening of the Coulomb interaction upon the topology of the phase diagram is discussed. Insight into the topology of the global phase diagram as a function of the system parameters leads to predictions on the preparation conditions for specific binary colloidal crystals.     

Shape tunable synthesis of anisotropic gold nanostructures through binary surfactant mixtures

In recent years, much effort has been made to produce gold (Au) nanorods of different sizes through the use of binary surfactant mixtures via a seed-mediated growth approach. However, how the ratio of two different surfactants influences the shape of the resulting Au nanoparticles remains to be elucidated. Here, we report the shape-controlled synthesis of Au nanoparticles using a binary surfactant mixture of CTAB (cetyltrimethylammonium bromide) and DDAB (didodecyldimethylammonium bromide) via a silver-assisted seed-mediated growth approach. Decreasing the CTAB/DDAB ratio results in a shape transition from Au nanorods to elongated tetrahexahedra and finally to Au bipyramids. The results showed significant improvement in the yield of Au bipyramidal type nanoparticles in different sizes (nm to μm) by using binary surfactant mixtures without any need for shape selection procedure. By varying the pH and concentration of ascorbic acid, we can control the shape and size of Au nanoparticles (i.e., truncated bipyramids, dogbones, and nanodumbbells) at fixed CTAB/DDAB ratios. A preliminary growth mechanism has been proposed based on the change in the mixed micelle soft-template induced by the increasing concentration of DDAB and reaction parameters (i.e., pH, concentration of ascorbic acid). These results constitute the advances in the understanding for synthesizing anisotropic Au nanoparticles of tunable optical properties via engineering the design of a soft-template. These anisotropic Au nanoparticles, especially, bipyramids of different morphologies and sizes are potential candidates for the enhancement of the optical response and developing label-free biosensing devices.     

Correlation between excess adsorption data measured for solvent mixture/adsorbent systems and RM values obtained for different solutes chromatographed in binary mobile phases

This paper concerns the application of excess adsorption isotherms, measured for solvent mixture/adsorbent systems, to the characterization of TLC data. For this purpose the excess adsorption isotherms for three liquid mixtures: cyclohexane/ benzene, benzene/acetone, and carbon tetrachloride/ethyl acetate on silica gel at 20°C have been measured. These mixtures have been used as binary mobile phases in TLC measurements. It has been shown for a given solute in binary mobile phase that the quantity R_M is a simple function of the excess adsorption. Parameters of this function have been used to characterize chromatographic systems with binary mobile phases.     

高压下二元系超额体积的拟合

利用F函数改进的立方状态方程—FRKS方程,并在二元互作用函数中引入压力项因子,对高压下二元系的超额体积进行拟合。压力项因子的引入,使得该方法能够适用于温度、压力变化的体系。对十个高压下的二元系的拟合结果表明,该方法能给出较好的拟合结果。     

Determination and measurement of solid–liquid equilibrium for binary fatty acid mixtures based on NRTL and UNIQUAC activity models

The estimation of solid–liquid phase equilibrium is important for the design, development, and operation of many industrial processes because of application in many manufacturing fields such as cosmetic, pharmaceutic, and biotechnology industries. In this work, we measured solid–liquid phase equilibrium of six fatty acid binary mixtures using the DSC technique and developed thermodynamic approaches for binary fatty acid mixtures to estimate melting temperatures as a function of mole fraction in solid–liquid phase equilibrium. Derivation of NRTL and UNIQUAC activity models was developed to predict melting temperatures and latent heat to achieve eutectic points of undecylic acid, pentadecylic acid, margaric acid, and stearic acid six pairwise binary mixtures. The fatty acids eutectic mixtures are appropriate for heat water systems, phase clothes, concrete, and other similar applications. The results showed low deviations from experimental data measured in this study.

     

Size ratio effects on interparticle interactions and phase behavior of microsphere-nanoparticle mixtures

We investigate the interparticle interactions and phase behavior of microsphere-nanoparticle mixtures of high charge asymmetry and varying size ratio. In the absence of nanoparticles, negligibly charged microspheres flocculate as a result of van der Waals interactions. Upon addition of a lower critical nanoparticle volume fraction, the microspheres are stabilized by the formation of nanoparticle halos around each microsphere. , A weak attraction between the two species leads to a pronounced enhancement of the effective nanoparticle concentration near the microsphere surface relative to the bulk solution. Above an upper critical nanoparticle volume fraction, the microspheres undergo reentrant gelation. Binary mixtures, in which the effective nanoparticle size is reduced at a fixed microsphere diameter, exhibit a narrow window of stability that ultimately disappears with increasing ionic strength. By contrast, binary mixtures of varying microsphere diameter are stabilized at similar nanoparticle volume fractions and exhibit a broader window of stability with decreasing size ratio. This unexpected observation may arise from the reduced attraction between smaller microspheres because negligible differences in nanoparticle halo formation are observed in these mixtures.     

An approach to calculate solid–liquid phase equilibrium for binary mixtures

An approach is presented to calculate solid–liquid phase equilibrium for binary mixtures, using expressions for the temperature as a function of the molar fraction. For Margules model the expression gives explicitly the temperature, while for other liquid phase activity models an iterative procedure is required to calculate the temperature. The method is very easy to apply and it can be used for mixtures that have peritectic and eutectic points, or just a eutectic point. The approach was applied to five case studies with binary mixtures of fatty acids and triglycerides. The results were in good agreement with experimental data.     

Analysis of laser-induced breakdown images measuring the sizes of mixed aquatic nanoparticles

Sizes of aquatic colloidal nanoparticles are determined by utilizing the laser-induced breakdown detection technique. Specifically, the number of breakdown events is measured as a function of area in laser-induced plasma images, generated from a mixture of two different size nanoparticles in aqueous solution, by minimizing the energy of the incident laser irradiation to generate the plasma. We find that the accuracy of measuring the sizes of nanoparticles in mixtures is greatly improved when selecting the plasma images produced only within one-half of the Rayleigh range of the focused Gaussian beam where the irradiation is most uniformly distributed.     

Self‐Assembly of ZnO Nanocrystals in Colloidal Solutions

The self‐organization in solution of ZnO nanocrystals into superlattices is monitored by dynamic light scattering. When long‐alkyl‐chain amines or carboxylic acids are used as stabilizing ligands, no organization is observed. In contrast, when binary mixtures of long‐alkyl‐chain amines and carboxylic acids are used, the presence of a thermodynamic equilibrium between free and organized ZnO nanoparticles is detected in THF or toluene. The superlattices of organized ZnO nanoparticles are independently observed by TEM and SEM. The coordination mode of the ligands at the surface of the ZnO nanoparticles is evidenced by NMR studies. The presence of ion‐paired ammonium carboxylate surrounding the surface of ZnO nanoparticles appears to be a necessary requirement to govern this reversible organization. This is substantiated by the absence of organization of ZnO nanoparticles when either a solvent of high dielectric constant, such as acetone, or a strong hydrogen‐bond acceptor is used.     

Quantitative measurement of nanoparticle halo formation around colloidal microspheres in binary mixtures

A new colloidal stabilization mechanism, known as nanoparticle "haloing" (Tohver, V.; Smay, J. E.; Braem, A.; Braun, P. V.; Lewis, J. A. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, (16), 8950-8954), has been predicted theoretically and inferred experimentally in microsphere-nanoparticle mixtures that possess high charge and size asymmetry. The term "halo" implies the existence of a nonzero separation distance between the highly charged nanoparticles and the negligibly charged microspheres that they surround. By means of ultrasmall-angle X-ray scattering, we have quantified the microsphere-nanoparticle separation distance as well as the number of nanoparticles and their lateral separation distance within the self-organized halos that form in these binary mixtures.     

Re-entrant behavior in dynamics of binary mixtures of soft hybrid nanocolloids and homopolymers

We present results of measurements of temperature and wavevector dependent dynamics in binary mixtures of soft polymer grafted nanoparticles and linear homopolymers. We find evidence of melting of the dynamically arrested state of the soft nanocolloids with addition of linear polymers followed by a re-entrant slowing down of the dynamics with further increase in polymer density, depending on the size ratio, δ, of the polymers and the nanocolloids. For higher δ the re-entrant behavior is not observed, even for the highest added polymer density, explored here. Possible explanation of the observed dynamics in terms of the presence of a double-glass phase is provided.     

Mixed films of poly(n-hexyl isocyanate) and poly(vinyl acetate) at the air-water interface

We study interfacial properties of rigid-rod-like poly(n-hexyl isocyanate) (PHIC), flexible poly(vinyl acetate) (PVAc), and mixed films of PHIC and PVAc spread at the air-water interface as a function of the molar fraction of PHIC by surface pressure measurements and fluorescence microscopy. From the plots of the experimental mean area of the mixed polymer films at a constant surface pressure as a function of the molar fraction of PHIC in the mixed films, the binary mixtures of PHIC/PVAc were concluded to be compatible at the air-water interface. This means that the hydrophobic hexyl group of PHIC takes a horizontal orientation to the air-water interface rather than a perpendicular one, leading to PHIC and PVAc having the same interfacial orientation. Compatibility of the binary mixtures of PHIC/PVAc at the air-water interface is also confirmed by their fluorescence microscopic images, since PHIC proves to be inhomogeneous and PVAc is homogeneous with the aid of a fluorescence probe, respectively.     

Separating nanoparticles by surface charge group using pH-controlled passivated gel electrophoresis

Because ionically stabilized colloids in aqueous dispersions have net surface charges that depend on pH, it is potentially possible to separate mixtures of nanospheres having identical radii, yet different types of stabilizing surface charge groups, efficiently using passivated gel electrophoresis (gel-EP). To demonstrate this, we separate a binary dispersion of polystyrene nanospheres that have nearly identical radii and surface group densities, yet different types of anionic stabilizing surface charge groups: sulfate and carboxylate. We achieve an efficient separation by adjusting the pH of the running buffer to lie between the pK_a values of these charge groups, resulting in significantly different protonation and, consequently, different electrophoretic propagation velocities of the nanospheres. The measured steady-state propagation velocities of both types of anionic nanoparticles as a function of pH can be fit well by an equilibrium model of pH-dependent protonation of anionic surface charge groups. Thus, pH-controlled passivated gel-EP opens a route for separating similarly sized charged colloidal objects that are stabilized by a variety of different surface charge groups.