Graphite exfoliation in cellulose solutions.

Shaking graphite powder dispersed in an aqueous alkaline cellulose solution produces stable dispersions of hydrophilic, thin graphite sheets with lateral dimensions reaching many micrometers. The X-ray diffractogram and Raman spectrum of the exfoliated graphite differ from the well-known graphite patterns. Analytical transmission electron micrographs show cellulose bound to the surface of thin lamellae and this is confirmed by scanning probe micrographs. The dispersant properties of dissolved cellulose are assigned to its adsorption on graphite by juxtaposition of the hydrophobic planes on both substances, forming hydrophilic particles. This method uses only simple and easily accessible chemicals, processed under mild conditions. The resulting nanographite-cellulose dispersions are suitable for making conductive lignocellulosic nanocomposites and coatings.

Detection of charge distributions in insulator surfaces.

Charge distribution in insulators has received considerable attention but still poses great scientific challenges, largely due to a current lack of firm knowledge about the nature and speciation of charges. Recent studies using analytical microscopies have shown that insulators contain domains with excess fixed ions forming various kinds of potential distribution patterns, which are also imaged by potential mapping using scanning electric probe microscopy. Results from the authors' laboratory show that solid insulators are seldom electroneutral, as opposed to a widespread current assumption. Excess charges can derive from a host of charging mechanisms: excess local ion concentration, radiochemical and tribochemical reactions added to the partition of hydroxonium and hydronium ions derived from atmospheric water. The last factor has been largely overlooked in the literature, but recent experimental evidence suggests that it plays a decisive role in insulator charging. Progress along this line is expected to help solve problems related to unwanted electrostatic discharges, while creating new possibilities for energy storage and handling as well as new electrostatic devices.

Synthesis of biocompatible poly[2-(methacryloyloxy)ethyl phosphorylcholine]-coated magnetite nanoparticles.

A well-defined, double-hydrophilic diblock copolymer comprising poly[2-(methacryloyloxy)ethyl phosphorylcholine]-block-(glycerol monomethacrylate) (PMPC30-PGMA30, where the numbers represent the average degrees of polymerization for each block) was evaluated for the synthesis of colloidally stable ultrafine magnetite sols. Sterically stabilized paramagnetic sols were prepared in aqueous solution by chemical coprecipitation of ferric and ferrous salts in the presence of this block copolymer. The PMPC30-PGMA30-stabilized magnetite sol had a mean transmission electron microscopy (TEM) diameter of 9.4 +/- 1.7 nm and a mean hydrodynamic diameter of 34 nm. This sol exhibited improved colloidal stability with respect to long-term storage and pH variation compared with magnetite sols prepared in the presence of alternative water-soluble homopolymers and diblock copolymers. Fourier transform infrared (FT-IR) spectroscopy, thermogravimetry, electron spectroscopy imaging (ESI), and zeta potential studies indicate that the PMPC30-PGMA30 diblock copolymer was adsorbed onto the surface of the sol via the PGMA30 block, with the PMPC30 chains acting as the stabilizing block. Such sterically stabilized sols are expected to be improved contrast agents for magnetic resonance imaging (MRI) applications.

Characterization of the nanomorphology of polymer-silica colloidal nanocomposites using electron spectroscopy imaging.

The internal nanomorphologies of two types of vinyl polymer-silica colloidal nanocomposites were assessed using electron spectroscopy imaging (ESI). This technique enables the spatial location and concentration of the ultrafine silica sol within the nanocomposite particles to be determined. The ESI data confirmed that the ultrafine silica sol was distributed uniformly throughout the poly(4-vinylpyridine)/silica nanocomposite particles, which is consistent with the "currant bun" morphology previously used to describe this system. In contrast, the polystyrene/silica particles had a pronounced "core-shell" morphology, with the silica sol forming a well-defined monolayer surrounding the nanocomposite cores. Thus these ESI results provide direct verification of the two types of nanocomposite morphologies that were previously only inferred on the basis of X-ray photoelectron spectroscopy and aqueous electrophoresis studies. Moreover, ESI also allows the unambiguous identification of a minor population of polystyrene/silica nanocomposite particles that are not encapsulated by silica shells. The existence of this second morphology was hitherto unsuspected, but it is understandable given the conditions employed to synthesize these nanocomposites. It appears that ESI is a powerful technique for the characterization of colloidal nanocomposite particles.

Latex Fractionation by Sedimentation and Colloidal Crystallization: The Case of Poly(styrene-co-acrylamide).

Poly(styrene-co-acrylamide) (PS-AAM) latex was prepared, fractionated by sedimentation under gravity, and characterized by PCS, infrared spectra, secondary and backscattered electron imaging in the scanning electron microscope, and electron spectroscopy imaging in an analytical transmission electron microscope. Three latex fractions were obtained. The lower fraction was opalescent and its particles were the more uniform, concerning size, chemical composition, and topochemical features. This lower fraction was still further fractionated by zonal centrifugation in a density gradient, yielding two fractions with similar macrocrystal-forming abilities but different sizes and chemical compositions. These results confirm those previously obtained for the PS-HEMA latex. Copyright 2000 Academic Press.

Latex Surface and Bulk Coagulation Induced by Solvent Vapors.

Latex exposure to solvent vapors leads to highly specific changes in latex stability as well as on the morphologies of the particle association products, depending on the latex and solvent used. Examples of solvent vapor-induced aggregation are given: surface films are obtained on two PS latexes; in one case, the film surface is mirror-reflective and very flat, as evidenced by AFM. Another PS latex coagulates under exposure to acetone vapors, and the morphologies of the coagula are highly sensitive to the exposure conditions. This latex yields a highly porous foam-like structure, in which particles are strongly coalesced but form percolating patches around the pores. The same latex but under other conditions produces a coagulum of large numbers of aggregated particles with a raspberry-like morphology. Density centrifugation experiments show that the effect of solvents on different latex fractions is not uniform, and some fractions show larger density changes than others, thus evidencing a variability in their swelling ability. Copyright 2000 Academic Press.

Aluminum Polyphosphate Nanoparticles: Preparation, Particle Size Determination, and Microchemistry.

Admixture of aluminum nitrate, sodium polyphosphate, and ammonium hydroxide solutions yields stable dispersions of hydrated aluminum polyphosphate particles within a broad reagent concentration range. These particles are formed by liquid-liquid phase separation, for which a phase diagram was calculated using suitable models for concentrated electrolyte solutions. Particle effective diameters range from a few nanometers to many hundreds and are fractionated by centrifugation. Particle electrophoretic mobility is very low and the hydration degree is high ( approximately 80% v/v). Dry nanoparticles (1- to 15-nm diameter as observed by TEM) as well as particle aggregates are obtained by lyophilization. Element (P, Al, and Na) mapping by ESI-TEM shows that particle aggregates have a core-and-shell morphology, with a higher content of P in the aggregate core and a higher Na content at the outer shell. Copyright 1999 Academic Press.

Polymer Latex Stability Modification by Exposure to Hydrophobic Solvents.

The stability of latex particles toward coagulation in the presence of salt is modified by swelling the latex with toluene and chloroform vapors. Short-term stability was determined by turbidimetric titrations, and the long-term stability was evaluated by adding latex and salt solutions, allowing the mixture to age for 24 or 48 h and determining the characteristics of the supernatant and of the sediment. Nine different latexes were examined, with variable results: in some cases, both apolar solvents stabilize the latex; in other cases, increased stability is induced by only one of the solvents, either toluene or chloroform. There is also coherence, but not a strict correlation, between the solvent effects on short- and long-term stability. For instance, in the case of a core-and-shell styrene-butyl methacrylate latex, chloroform has a small stabilizing effect in the titration experiment, but it prevents the formation of a coagulated latex sediment even 48 h after mixing latex and salt. Two hypotheses are discussed to account for these observations: (i) swelling solvents decrease the particles ability to dissipate the collision kinetic energy, so that particles collide but without joining each other; (ii) the solvents induce the release of trapped charged groups from the particle interior to the interface, enhancing the usual (electrostatic, steric, hydration) stability factors. Copyright 1998 Academic Press.

Sedimentation coefficient and minimum molecular weight of extracellular hemoglobin of Glossoscolex paulistus (Oligochaeta).

A new procedure for the determination of the sedimentation coefficient in the 50-1000 S range was designed and tested. The characteristics of this protocol are: the use of density gradients self-generated by osmosedimentation and the use of a low-speed centrifuge and of visual monitoring of the sedimenting zone. This procedure was used to determine the sedimentation coefficient of erythrocruorin from Glossoscolex paulistus. The value obtained, S20, omega = 58 S, corresponds to a MW of 3.1 x 10(6) Daltons. The minimum MW (heme), determined by the pyridine-hemochromogen method, was 25,250 Da.

The osmosedimentation effect: its application to chemical and biochemical separation.

1. The fundamentals of the osmosedimentation effect are reviewed and its applications to chemical and biochemical separation problems are presented. 2. The osmosedimentation effect (solute sedimentation enhanced by osmotic and reverse-osmotic solvent mass currents spontaneously generated within a vertical dialysis cell) may be described by using standard treatments of non-equilibrium thermodynamics. 3. This effect allows many hitherto unfeasible or very difficult experiments to be performed: polymer and particle MW determination, under gravity or low-speed (1000-5000 rpm) centrifugation; dissolved protein concentration, by low-speed centrifugation; density gradient generation, at low centrifugation speeds; fast separation of fine particles; particle concentration and fractionation, under gravity and low speed centrifugation. 4. Osmocentrifugation methods may be easily implemented in any laboratory, as their requirements are: the availability of standard centrifuges and rotors, inexpensive dialysis cells and cellulose acetate membranes, which can be prepared in the laboratory.

The osmosedimentation effect: its application to chemical and biochemical separation

1. The fundamentals of the osmosedimentation effect are reviewed and its applications to chemical and biochemical separation problems are presented. 2. The osmosedimentation effect (solute sedimentation enhanced by osmotic and reverse-osmotic solvent mass spontaneously generated within a vertical dialysis cell) may be described by using standard treatment of non-equilibrium thermodynamics. 3. This allows many hitherto unfeasible or very difficult experiments to be performed: polymer and particle MW determination, under gravity or low-speed (1000-5000 rpm) centrifugation; dissolved protein concentration, by low-speed centrifugation; density gradient generation, at low centrifugation speeds; fast separation of line particles, particle concentration and fractionation, under gravity and low speed centrifugation. 4. Osmocentrifugation methods may be easily implemented in any laboratory, as their requirents are: the availability of standard centrifuges and rotors, inexpensive dialysis cells and cellulose acetate membranes, which can be prepared in the laboratory

Percoll and Ficoll self-generated density gradients by low-speed osmocentrifugation.

Percoll and Ficoll self-generated density gradients can be obtained by low-speed centrifugation of their solutions within dialysis cells. Useful Percoll density gradients can be obtained after 10-30 min centrifugation at 220-2010g, within dialysis cells. Ficoll density gradients, which are more difficult to self-generate, can be obtained by the same technique. Red cell band formation in a Percoll density gradient can be done in a single step by using dialysis cells as the centrifugation solution container.

Kinetics of the hydrolysis of synthetic substrates by horse urinary kallikrein and trypsin.

The kinetic constants for horse urinary kallikrein and trypsin hydrolysis of BAEE, TAME, bradykinin methyl ester and bradykinyl-Ser-Val-Gin-Val-Ser were determined. The values of the ratio kcat/Km show that (1) kallikrein is catalytically less efficient than trypsin for all the substrates (2) the three esters are equally good substrates for trypsin while horse urinary kallikrein is 100-fold more effective on bradykinin methyl ester than on the other substrates (3) for both enzymes the ester of bradykinin is a better substrate than the tetradecapeptide.