Frequency dependence of the dielectric and electro-optic response in suspensions of charged rod-like colloidal particles

We have performed an experimental investigation on the electrokinetic properties of charged rod-like fluorinated latex colloids. Systematic measurements of electrophoretic mobility, dielectric constant and electric birefringence have been performed as a function of the concentration of added nonionic surfactant and salt. In the investigated range of parameters, the zeta potential is a strongly decreasing function of the concentration of nonionic surfactant, while it is basically independent from ionic strength. We have obtained the frequency dependence of dielectric constant and Kerr constant as a function of zeta-potential and ionic strength. We observe the transition from a low frequency behavior, where both the dielectric constant and the Kerr constant are enhanced by the presence of the double layer, to a high frequency behavior, where both quantities take the value expected for unchanged particles in an insulating medium. The shape of the frequency dispersion of the Kerr constant coincides with that of the dielectric constant, but the cut-off frequencies are the same only when the zeta-potential of the particles is low.     

The rheology and microstructure of charged colloidal suspensions

The effects of electric charge interation and particle correlations on suspension rheology are examined. A one-component fluid analysis using a Smoluchowski equation for the equilibrium structure is applied to charged suspensions of spherical colloids under shear. The frequency dependent modulus and viscosity, predicted as functions of particle and added salt concentrations, are compared with published rheological measurements on model suspensions. Recent improvements in the statistical mechanical theories for the equilibrium microstructure, its nonequilibrium deformation, and the bulk shear stresses are included. The direct electrostatic interaction is found to drive the divergence in the shear viscosity near the liquid-solid phase transition. Extensions of the theory predict the elastic modulus of binary mixtures of charged colloids. Estimates of the primary electroviscous effect, hydrodynamic interactions, and errors in the Yukawa limiting form for the potential and applications of asymptotic theories are presented. Predictions for the rheology based on effective hard-sphere models are found to be reasonable when using a parameter fit from the equilibrium phase behavior. Mean-field mode coupling theories predict larger relaxation times than calculated from the Smoluchowski equation (=SE). A study of binary mixing effects on elasticity shows non-ideal behavior. It is noted that equilibrium structural information can be used to resolve discrepancies between the theoretical predictions and the measured rheology.     

Gas-solid coexistence in highly charged colloidal suspensions

Aqueous suspensions of highly charged polystyrene particles with different volume fractions have been investigated for structural ordering and phase behavior using static light scattering (SLS) and confocal laser scanning microscope (CLSM). Under deionized conditions, suspensions of high-charge-density colloidal particles remained disordered whereas suspensions of relatively low charge density showed crystallization by exhibiting iridescence for the visible light. Though for the unaided eye crystallized suspensions appeared homogeneous, SLS measurements and CLSM observations have revealed their inhomogeneous nature in the form of the coexistence of voids with dense ordered regions. CLSM investigations on disordered suspensions showed their inhomogeneous nature in the form coexistence of voids with dense disordered (amorphous) regions. Our studies on highly charged colloids confirm the occurrence of gas-solid transition and are in accordance with predictions of Monte Carlo simulations using a pair-potential having a long-range attractive term [Mohanty, P. S.; Tata, B. V. R. J. Colloid Interface Sci. 2003, 264, 101]. On the basis of our experimental and simulation results, we argue that the reported reentrant disordered state [Yamanaka et al. Phys. Rev. Lett. 1998, 80, 5806 and Toyotama et al. Langmuir 2003, 19, 3236] in charged colloids observed at high charge densities is a gas-solid coexistence state.     

Effect of quenched size polydispersity on the fluid-solid transition in charged colloidal suspensions

We study the effect of quenched size polydispersity on the phase behavior of charged colloidal suspensions using free-energy calculations in Monte Carlo simulations. The colloids are assumed to interact with a hard-core repulsive Yukawa (screened-Coulomb) interaction with constant surface potential, so that the particles are polydisperse both in size and charge. In addition, we take the size distribution to be fixed in both the fluid and crystal phase (no size fractionation is allowed). We study the fluid-solid transition for various screening lengths and surface potentials, finding that upon increasing the size polydispersity the freezing transition shifts toward higher packing fractions and the density discontinuity between the two coexisting phases diminishes. Our results provide support for a terminal polydispersity above which the freezing transition disappears.     

Equation of state of colloidal dispersions

We present a comparison of experimentally and theoretically determined osmotic pressures for various colloidal dispersions. Experimental data is collected from several different silica and polystyrene dispersions. The theoretical pressure determinations are based on the primitive model combined with the cell model, and the physical quantities are calculated exactly using Monte Carlo simulations in the canonical and grand canonical ensemble. The input to the simulations in terms of colloidal particle size, surface charge density, and so forth are taken directly from experiments, and the approach does not contain any adjustable parameters. The agreement between theory and experiment is very good without any fitting parameters, showing that the simplifications behind the primitive model and the cell model are physically sound. The results reveal a surprising correspondence between the equations of state in spherical and planar geometries, indicating that the particle shape is of secondary importance in dispersions dominated by repulsive interactions. For one of the silica dispersions, we have also investigated how various monovalent counterions influence the swelling properties. Within experimental error, we are unable to detect any ion specificity, which is further support for the theoretical models used.     

Lyotropic smectic B phase formed in suspensions of charged colloidal platelets

Here, we present the first observation of a smectic B (Sm(B)) phase in a system of charged colloidal gibbsite platelets suspended in dimethyl sulfoxide (DMSO). The use of DMSO, a polar aprotic solvent, leads to a long range of the electrostatic Coulomb repulsion between platelets. We believe this to be responsible for the formation of the layered liquid crystalline phase consisting of hexagonally ordered particles, that is, the Sm(B) phase. We support our finding by high-resolution X-ray scattering experiments, which additionally indicate a high degree of ordering in the Sm(B) phase.     

Secondary minimum coagulation in charged colloidal suspensions from statistical mechanics methods

A statistical mechanics approach is applied to predict the critical parameters of coagulation in the secondary minimum for charged colloidal suspensions. This method is based on the solution of the reference hypernetted chain (RHNC) integral equation, and it is intended to estimate only the locus of the critical point instead of the full computation of the "gas-liquid" coexistence. We have used an extrapolation procedure due to the lack of solution of the integral equation in the vicinity of the critical point. Knowing that the osmotic isothermal compressibility of the colloidal system should ideally diverge in the critical point, we work out the critical salt concentration for which the inverse of the compressibility should be zero. This extrapolation procedure is more rapid than that previously proposed by Morales and co-workers [Morales, V.; Anta, J. A.; Lago, S. Langmuir 2003, 19, 475], and it is shown to give equivalent results. We also present experimental results about secondary minimum coagulation for polystyrene latexes and use our method to reproduce the experimental trends. The comparison between theory and experiment is quite good for all colloidal diameters studied.     

Temperature dependence of thermodiffusion in aqueous suspensions of charged nanoparticles

Measurements of particle flows driven by temperature gradients are conducted as a function of temperature on aqueous suspensions of polystyrene nanoparticles and proteins of T4 lysozyme and mutant variants of T4 lysozyme. The thermodiffusion coefficients are measured using a microfluidic beam deflection technique on suspensions with particle concentrations on the order of 1 vol %. At T < or ~ 20 degrees C, all of the nanoparticles studied migrate to the hot regions of the fluid; i.e., the thermodiffusion coefficient is negative. At higher temperature, T > or ~ 50 degrees C, the thermodiffusion coefficient is positive with a value consistent with the predictions of a theoretical model originally proposed by Derjaguin that is based on the enthalpy changes due to polarization of water molecules in the double layer.     

Aggregation in colloidal suspensions and its influence on the suspension viscosity

Influence of aggregate formation on rheological properties of concentrated suspensions is discussed and reviewed. Methods of Stokesian and Brownian dynamic simulations of aggregation are discussed and the results of simulations for both Brownian and non-Brownian particles are presented.     

Drude-type conductivity of charged sphere colloidal crystals: density and temperature dependence

We report on extensive measurements in the low-frequency limit of the ac conductivity of colloidal fluids and crystals formed from charged colloidal spheres suspended in de-ionized water. Temperature was varied in a range of 5 degrees C < Theta < 35 degrees C and the particle number density n between 0.2 and 25 microm(-3) for the larger, respectively, 2.75 and 210 microm(-3) for the smaller of two investigated species. At fixed Theta the conductivity increased linearly with increasing n without any significant change at the fluid-solid phase boundary. At fixed n it increased with increasing Theta and the increase was more pronounced for larger n. Lacking a rigorous electrohydrodynamic treatment for counterion-dominated systems we describe our data with a simple model relating to Drude's theory of metal conductivity. The key parameter is an effectively transported particle charge or valence Z(*). All temperature dependencies other than that of Z(*) were taken from literature. Within experimental resolution Z(*) was found to be independent of n irrespective of the suspension structure. Interestingly, Z(*) decreases with temperature in near quantitative agreement with numerical calculations.     

Video microscopy of colloidal suspensions and colloidal crystals

Colloidal suspensions are simple model systems for the study of phase transitions. Video microscopy is capable of directly imaging the structure and dynamics of colloidal suspensions in different phases. Recent results related to crystallization, glasses, and 2D systems complement and extend previous theoretical and experimental studies. Moreover, new techniques allow the details of interactions between individual colloidal particles to be carefully measured. Understanding these details will be crucial for designing novel colloidal phases and new materials, and for manipulating colloidal suspensions for industrial uses.     

Excess enthalpy of the system chloroform + carbon tetrachloride and a thermodynamic evaluation of its state dependence

Siddiq, M.A. and Lucas, K., 1984. Excess enthalpy of the system chloroform + carbon tetrachloride and a thermodynamic evaluation of its state dependence. Fluid Phase Equilibria, 16: 87–98.Excess molar enthalpies of the system chloroform + carbon tetrachloride have been measured over the entire concentration range at 283.15, 293.15, 303.15, 313.15 and 323.15 K using an isothermal high-pressure flow calorimeter. The effect of pressure was also studied by measuring excess enthalpies at 15 and 30 MPa along the 293.15, 313.15 and 323.15 K isotherms. The temperature dependence of the excess enthalpies was used to calculate vapour-liquid equilibria as a function of temperature. The results are excellent. Further evaluation of the temperature and pressure dependences of the excess enthalpy is discussed.     

A macroion correlation effect on the structure of charged stabilized colloidal suspensions: a self-consistent integral equation study

Using the mean field Poisson–Boltzmann (PB) and the Ornstein–Zernike (OZ) integral equation theories, we have determined the macroions effective interactions and the structure of charged stabilized colloidal suspension for a large charge range of macroion and screening parameter values. The renormalized parameters are calculated by solving the PB equation written in the framework of the modified Jellium model. The structures have been determined by solving the OZ equation coupled with a self-consistent integral equation, which is related to the Verlet’s modified closure. Our results of the effective parameters are in good agreement with the experimental data, also the structure presents acceptable improvement compared to the Monte Carlo simulation data, against the HNC structure results, PACS: 61.20 Gy, 82.70 Dd, and 82.70 Kj.     

Polarizability and complex conductivity of dilute suspensions of spherical colloidal particles with charged (polyelectrolyte) coatings

The dielectric relaxation of polyelectrolyte-coated colloidal particles is examined via "exact" numerical solutions of the governing electrokinetic equations. The charged polymer coatings are characterized by a nominal charge density, thickness, and permeability. Brush-like segment density profiles are considered here, but more sophisticated segment and charge density profiles are accommodated by the model. The role of added counterions and nonspecific adsorption is considered briefly before examining how the experimentally measured conductivity and dielectric constant increments reflect the frequency of the applied electric field, the strength of the electrolyte, and characteristics of the polymer coatings, namely the charge, charge density, and permeability. Finally, a strategy is suggested by which dielectric spectroscopy and electrophoresis can be used to characterize polymer-coated particles. This approach complements experiments where electroviscous effects such as dynamic light scattering and sedimentation are weak.     

Conventional optical microscopy of colloidal suspensions

Optical microscopy can resolve detail at the larger end of the colloidal length scale, and to image suspensions at an individual particle level of resolution would allow the investigation of local behaviour in a way denied to the established scattering techniques. However, to achieve high-contrast single-particle resolution in dense suspensions that are thick enough to show behaviour the same as would be expected in the bulk is not a trivial exercise. We build on established advanced techniques of the conventional (i.e. non-confocal) light microscopy of phase objects to develop a suitable experimental protocol. Furthermore, we demonstrate the effectiveness of this protocol by means of an 'atlas' of the hard-sphere crystalline solid (where random stacking results in many complex facets), which should serve as a compendium for future study.     

Macroscopic analysis of the behavior of colloidal suspensions

Three distinct and separate scales of discourse relevant to systems such as porous media and colloidal suspensions are identified: 1) the molecular scale; 2) the microscopic or Navier-Stokes scale; and 3) the macroscopic or Darcy scale. This article reviews the macroscopic mode of analysis which had its origins in problems of flow in porous media and which has been generalized and extended to apply to the behavior of colloidal suspensions. Applied to suspensions, the central concepts concern the energetics and hydrodynamics of the liquid phase.The approach leads to a non-linear Fokker-Planck equation, which reduces to a non-linear diffusion equation in some contexts. Simple experiments yield two functions characterizing the suspension. These functions express the dependence on particle volume fraction (or, equivalently, on ?, the ‘liquid ratio’) of ψ, the relevant chemical potential of the liquid, and κ, the permeability for flow relative to the particles. With these functions known, the analysis enables quantitative prediction of the time course of various flow, filtration, and sedimentation processes undergone by colloidal suspensions.The great generality and adaptability of the approach is emphasized. The necessary and sufficient condition for its application is simply that the characterizing functions exist and are measurable. The approach has been found to apply to a range of colloidal suspensions occurring in practical contexts in nature and industry. The analysis is free of the abstractions, simplifications, and assumptions required for quantitative theoretical studies on the molecular and microscopic scales. We may remain completely agnostic about particle shape, size distribution, and interactions. All information required is embodied in the ψ(?) and κ(?) functions.     

Dielectric spectroscopy of bidisperse colloidal suspensions

Tuning the luminescence intensity of fluorophores using nanoparticles has shown great potential for the detection of inorganic metal ions, viruses, and proteins. The enhancement or quenching of a dye's fluorescence intensity is strongly dependent on the spatial separation of the dye from the nanoparticle surface. To extend luminescence probing from the solution platform to the solid-state platform, we explored and performed dye quenching assessment using an array format in this study. We report the distance-dependent fluorescence behavior of Au-DNA conjugates prepared by equilibrating phosphine-stabilized gold nanoparticles (AuNPs) of 10-nm size with the designed spacer ds-DNA consisting of thiol-modified target and Cy3-labeled complementary probe of different lengths (5-20 nm). The Cy3-labeled products were immobilized onto MPTMS (3-mercaptopropyltrimethoxysilane)-modified glass substrates and then excited with a 532-nm laser source. Quenching efficiency of AuNPs with increasing Au-to-dye distance was assessed using ligand exchange of the thiolated oligonucleotide by 2-mercaptoethanol (ME) to obtain free Cy3-DNA probe, thus eliminating nanoparticle effect on the dye's luminescence intensity. Effective exchange, revealed by UV-vis absorption and fluorescence profiles, was achieved in a few minutes. It was observed that fluorescence quenching of Au-DNA-Cy3 assessed using the array format was consistent with the result in solution phase for the conjugates with up to 10-nm Au-to-Cy3 separation distance.