Dilute aqueous polystyrene suspensions are found to exhibit a novel vapor-liquid condensation. Upon de-ionization, weakly interacting homogeneous suspensions below a critical particle concentration condense into a concentrated phase with liquidlike order and a dilute vapor phase. This phenomenon strongly suggests net attraction between particles at interparticle separation several times the particle diameter. The present results are understood on the basis of an effective interparticle model potential.

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We report observations of stable bound pairs in very dilute deionized aqueous suspensions of highly charged polystyrene colloidal particles, with monovalent counterions, using a confocal laser scanning microscope. Through an analysis of several thousands of time series of confocal images recorded deep inside the bulk suspension, we find that the measured pair-potential, U(r) has a long-range attractive component with well depths larger than the thermal energy. These observations provide a direct and unequivocal evidence for the existence of longrange attraction in U(r) of like-charged colloidal particles.

In solutions or dispersions, solute distributions are considered to be more or less homogeneous and space-filling, particularly for concentrated ones. This is not experimentally the case, however, at low concentrations. Traverse photographs of a homogeneous dispersion of ionic latex particles (volume fraction φ = 0.05) taken by a Lang camera show the coexistence of ordered domains of particles (as studied by Kossel line analysis) and disordered regions. The video imagery study indicates the presence of at least two diffusion modes for particles in a “homogeneous” dispersion (φ = 0.02). The confocal laser scanning microscope (CLSM) study shows that negatively charged latex particles are positively adsorbed near likewise negatively charged glass interface. The ultra-small-angle X-ray scattering (USAXS) patterns of 4- or 6-fold symmetry are observed with five or four orders of Bragg diffraction from colloidal silica dispersions (φ = 0.0376), suggesting the formation of a bcc single crystal with a lattice constant of 0.3 μm. The two-dimensional USAXS study of the colloidal silica dispersion (φ = about 0.025) gives 22 scattering peaks below a scattering angle of 203‘ ‘, which uniquely prove that a single bcc crystal is formed, allowing us to accurately determine the lattice constant and direction of the crystal. The USAXS investigations again confirm the previously found fact that the closest interparticle distance was systematically smaller than the average distance expected from the overall particle concentration. For latices of poly(chlorostyrene−styrenesulfonate) copolymers, which allow concurrent scattering and microscopic studies, the inequality relation of the interparticle spacing is observed and the presence of void structures is visually confirmed at φ = 0.03 or below. The re-entrant phase transition is found when the net charge density of particles is increased. The bcc−fcc transition, void formation, and the re-entrant behavior can be accounted for by the Monte Carlo simulation with the Sogami potential containing a short-range repulsion and a long-range attraction.

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Detoxification of Cr(VI) under alkaline pH requires attention due to the alkaline nature of many effluents. An alkaliphilic gram-positive Bacillus subtilis isolated from tannery effluent contaminated soil was found to grow and reduce Cr(VI) up to 100% at an alkaline pH 9. Decrease in pH to acidic range with growth of the bacterium signified the role played by metabolites (organic acids) in chromium resistance and reduction mechanism. The XPS and FT-IR spectra confirmed the reduction of Cr(VI) by bacteria into +3 oxidation state. Chromate reductase assay indicated that the reduction was mediated by constitutive membrane bound enzymes. The kinetics of Cr(VI) reduction activity derived using the monod equation proved (K s = 0.00032) high affinity of the organism to the metal. This study thus helped to localize the reduction activity at subcellular level in a chromium resistant alkaliphilic Bacillus sp.

Monodisperse thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) microgel particles having a diameter of 520 nm were synthesized by free-radical precipitation polymerization and centrifuged to obtain a concentrated suspension. The centrifuged mother suspension was made to self-order into a crystalline state by repeated annealing beyond the volume phase transition (VPT) of the particles. We report here the three-dimensional (3D) real space structure, determined using a confocal laser scanning microscope, of PNIPAM microgel crystal samples prepared by two different recrystallized routes: (1) solidifying a shear melted colloidal liquid (referred as as-prepared sample) and (2) slow cooling of a colloidal liquid (referred as recrystallized sample). We have recorded images of several regions of the crystal with each region containing 15 horizontal crystal planes for determining the in-plane [two-dimensional (2D)] and 3D pair-correlation functions. The 2D pair-correlation function g(r) revealed hexagonal long-range order of particles in the layers with a lattice constant of 620 nm. The analysis of stacking sequence of layers recorded on as-prepared sample has revealed the existence of stacking disorder with an average stacking probability alpha approximately 0.42. This value of alpha together with the analysis of 3D pair-correlation function determined from particle positions revealed the structure of microgel crystals in the as-prepared sample to be random hexagonal close packing. We report the first observation of a split second peak in the 3D g(r) of the microgel crystals obtained from a shear melted liquid. Upon melting the sample above VPT and recrystallizing it the split second peak disappeared and the crystals are found to have a face centered cubic (fcc) structure with alpha approximately 0.95. From simulations, the split second peak is shown to arise from the displacement of some of the B-planes from the ideal hcp positions. The present results are discussed in light of those reported for charged and hard sphere colloidal crystals and plausible reasons for observing two different structures are also explained.

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.