Dilute Aqueous Dispersions Research Articles

Rodlike silica and titania objects templated on extremely dilute aqueous dispersions of self-assembled sodium lithocholate nanotubes

Simple methods for preparation of mesoporous rodlike silica and titania nanoobjects, some with exceedingly high aspect ratios, are described. They involve hydrolytic sol–gel processes using nanotubes from aqueous assemblies of very dilute (0.1 wt%) sodium lithocholate as templates. Shearing of the lithocholate nanotubes results in aligned, templated silica rods. The relative rates of lithocholate self-assembly and of polymerization of the titania precursors, especially, appear to be important factors in templating efficiency.

Ultrathin hybrid films of clay minerals

Smectites or swelling clay minerals are naturally occurring nanomaterials that can be fully delaminated to elementary clay mineral platelets in dilute aqueous dispersion. This review article gives an overview of the recent progress on how the elementary clay mineral platelets can be reorganized in monolayered or multilayered hybrid nanofilms by layer-by-layer assembly or the Langmuir-Blodgett technique. In the latter case one hybrid layer consists of one layer of elementary clay mineral platelets with a theoretical thickness of 0.96 nm, covered on one side by amphiphilic cations. The organization of the elementary clay mineral platelets and that of the adsorbed amphiphilic cations in the nanofilms has been studied in great detail by ATR-FTIR, UV-Vis and fluorescence spectroscopy, XRD and AFM. The nanofilms carry functional properties, such as chirality, optical nonlinearity and magnetism, which are due to the nature of the amphiphilic cations and to the organization of both the amphiphilic molecules and the elementary clay mineral platelets.

Effect of pH on Rheological Properties of Microcrystalline Cellulose Dispersions

The size distribution and morphology of microcrystalline cellulose (MCC) particles and the effect of pH on the rheological properties of their dilute aqueous dispersion were studied. The 0.3 wt% aqueous dispersion of fine particles (89 nm) also showed solid‐like dynamic viscoelastic behavior at low pHs (below pH 3), in contrast to liquid‐like behavior at high pHs. These findings suggest that at lower pHs the electrostatic repulsion force between MCC particles decreases to induce the aggregation of particles into a three‐dimensional network structure. The minimum empty space in the network structure is expected to be 450 nm or more.

Density profiles of temperature-sensitive microgel particles

We have performed small angle neutron scattering measurements (SANS) on dilute aqueous dispersions of polymer microgel particles as a function of temperature, T . The microgel particles are spherical crosslinked assemblies of a loose gel network of a poly-N-isopropylacrylamide (NIPAM) polymer. When the temperature is raised beyond a critical temperature, T(lc) approximately 32 degrees C , the polymer becomes more strongly attracted to itself than the solvent, and the microgel particles contract. The measured form factor, F (q) , for dilute suspensions of uniform microgel particles exhibits many peaks that are characteristic of solid polymer nanospheres. The position and amplitude of the peaks as a function of wave number, q , provide insight into the density profile of poly-NIPAM within the microgels. These peaks can be described well over a wide range of temperature by a model of the polymer density within the particles that is constant up to an inner radius, R1 , and decreases linearly to zero at an outer radius, R2 .

Viscosity Effect on the Structural Compactness of Latex Flocs Formed under Weak Depletion Attractions

Dilute aqueous dispersions of colloidal polystyrene latex spheres were flocculated by adding a nonadsorbing polymer sample, poly(acrylic acid). The structural compactness of the flocs thus formed was characterized in terms of their mass fractal dimension using the small-angle static light scattering technique. It was found that with low poly(acrylic acid) concentrations and thus weak depletion attraction forces, the dispersion medium viscosity had a marked effect on the floc structure. An increase in the viscosity led to formation of denser flocs. This was revealed in three sets of depletion flocculation experiments: (a) adjusting the background electrolyte concentration at a fixed level of poly(acrylic acid), (b) using water and 30% (w/w) glycerol as the respective solvents, and (c) inducing latex flocculation with two poly(acrylic acids) of different molecular weights at the respective critical polyacid concentrations. Direct force measurements were made with atomic force microscopy to isolate the influence of viscosity on floc structure from that of interparticle interaction energies. We conclude that the formation of denser flocs with increasing medium viscosity can be attributed to the reduced diffusivity of particles in the solution. The latter resulted in an enhanced rate of floc restructuring (through relaxation of attached particles) relative to floc growth.

Light-Harvesting Supramolecular Hydrogels Assembled from Short-Legged Cationic L-Glutamate Derivatives and Anionic Fluorophores

Supramolecular hydrogels (see Figure for structure) are spontaneously formed by mixing aqueous dispersions of short-legged ammonium amphiphiles and fluorescent anions. Energy transfer between donor (naphthalene) and acceptor (anthracene) anions is shown to be more efficient in the hydrogels compared to that in dilute aqueous bilayer dispersions.

Surface Potential Driven Swelling of Polyelectrolyte Multilayers

The adsorption of polyelectrolytes to charged surfaces can be applied to build up multilayers of polyelectrolytes of alternating charge by layer-by-layer assembly. The hydration of such multilayers adsorbed to colloidal particles in dilute aqueous dispersions is investigated by NMR, namely, water 1H spin relaxation rates R2. For the system of alternating layers of poly(diallyl dimethylammonium chloride) (PDADMAC) and poly(sodium 4-styrenesulfate) (PSS) adsorbed to silica particles (PDADMAC/PSS), a linear increase of multilayer thickness and R2 with the number of layers is found, reflecting an increasing amount of hydration water. For the system (PAH)/PSS, involving the weak polyelectrolyte poly(allylamine hydrochloride) (PAH), it is found that while adsorption of a positive layer leads to an increase of R2, a decrease of R2 is observed after the adsorption of a negative layer. This reveals an increase of the net water mobility involving dehydration. Evidence is given that the reversible swelling behavior ...

Effect of Counterions on the Organized Structure of Cu2+-Coordinated Bilayer Membranes Formed by Monoalkyl Derivatives of Ethylenediamine

The complexation of monoalkyl derivatives of ethylenediamine, CnH2n+1NHCH2CH2NH2 (n = 8, 12, 14, 16, and 18), to CuX2 (X- = Cl- or ClO4-) in dilute aqueous dispersions can form stable synthetic bilayer membranes. The aggregate structure and properties of the bilayer membranes have been investigated thoroughly by small-angle X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, electronic reflectance spectroscopy, and differential scanning calorimetry (DSC). The results confirm a strong influence of the counterions on the organized structure of Cu2+-coordinated bilayer membranes. When X- = Cl-, the Cu2+-coordinated headgroups adopt a trans configuration, which gives a compressed tetrahedral CuN4 environment, and the two tails in the complex take an approximately diagonal position with tilted tail-to-tail packing fashion in bilayer assemblages. When X- = ClO4-, a cis configuration is adopted in the square plane of the Cu2+-coordinated headgroups, and the two tails are located at the same...

Gel−Fluid Transition in Dilute versus Concentrated DMPG Aqueous Dispersions

Aqueous dispersions of the anionic phospholipid dimyristoyl phosphatidylglycerol (DMPG) in 100 mM ionic strength, at pH values higher than 4, are known to exhibit a thermal behavior rather similar to that of the zwitterionic lipid dimyristoyl phosphatidylcholine (DMPC), which undergoes a gel-to-liquid crystalline phase transition at 23 °C. However, in aqueous medium of low ionic strength, DMPG was shown to present a large gel-fluid transition region, ranging from 18 to 35 °C. This intermediate phase is optically transparent and characterized by a continuous change in membrane packing. The present work shows that, below a certain lipid concentration, which we denote c′ (c′ ) 0.4 ( 0.2 mM), this anomalous behavior vanishes, being replaced by a sharper phase transition centered at a unique transition temperature Tm. Interestingly, Tm increases as the lipid concentration is further decreased, reaching a limiting value of about 41 °C. Below c′, opposite to the effect observed for DMPG at higher concentrations, the value of Tm decreases as the ionic strength is increased. Above 500 mM NaCl, the thermal behavior of DMPG aqueous dispersions is roughly independent of the lipid concentration. Furthermore, increasing sample pH significantly decreases Tm in the low concentration regime, whereas in concentrated DMPG dispersions, Tm is not affected by changes in pH above 6. The dependence of the thermal behavior of dilute DMPG aqueous dispersions on pH and ionic strength is discussed here in the light of the dependence of the headgroup protonation on DMPG concentration.

Antibacterial Cotton Blend Nonwovens

A new environmentally benign antibacterial agent (magnesium hydroperoxyacetate or MHPA, the reaction product of magnesium acetate tetrahydrate and hydrogen peroxide) was affixed to cotton/polyester webs of varying fiber content (10/90, 20/80 and 30/70 cotton/polyester) by spraying a dilute aqueous dispersion on the webs, drying and curing, then needlepunching the treated webs to produce antibacterial nonwovens. The composition of the MHPA was monitored by differential scanning calorimetry, and the peroxide content of the MHPA and of the modified nonwovens determined by iodometric titration. Resultant nonwovens had comparable physical properties to untreated and needlepunched nonwovens (air permeability, stiffness and bursting strength) and excellent antibacterial activity toward representative Grampositive and Gram-negative bacteria, even at the lowest level of cotton and MHPA in the blends. Several applications for the modified nonwovens are described.

The Electroviscous Behavior of Aqueous Dispersions of Amorphous Silica (Ludox)

The electroviscous effects in very dilute aqueous dispersions of amorphous silica (Ludox) were investigated at various levels of salt, pH, and volume fraction (<0.01) of solids. Viscosities were much higher than predicted from existing theories, which is ascribed to the formation of a thick, gel-like surface layer on the particle. The volume of a particle adjusts itself almost reversibly to the salt and pH levels of the liquid and can grow up to four times the dry volume. This explains the apparent discrepancy between published dry and wet particle sizes and also the reported anomalously large number of bound water layers around a particle. The existence of a gel layer leads to an abnormally large amount of surface conductance; this may explain the anomalities found in electrophoresis. The validity of the model is also supported by published results of the amount of nonbulk water as found with NMR.

Aggregate Structure in Dilute Aqueous Dispersions of Phospholipids, Fatty Acids, and Lysophospholipids

Cryo-transmission electron microscopy (cryo-TEM) was employed to investigate the aggregate structure in dilute aqueous dispersions of egg-phosphatidylcholine (EPC), oleic acid (OA), and the lysophospholipid monooleoylphosphatidylcholine (MOPC). At physiological pH and salt concentration, a relatively monodisperse population of unilamellar liposomes was detected in mixtures containing equimolar concentrations of the three components. Threadlike micelles constituted the dominant aggregate structure in samples containing high concentrations of MOPC. Excess fatty acid forced, on the other hand, the system toward structures with net negative curvature. In the absence of phospholipid, cryo-TEM revealed bilayer fragments in coexistence with threadlike micelles in mixtures containing the same molar amount of MOPC and OA. External addition of MOPC to preformed EPC liposomes gave rise to a concentration dependent evolution of intermediate structures, including open liposomes and bilayer fragments. The structural re...

The structure and strength of depletion force induced particle aggregates

Aggregating fine particulate matter is common practice in many industrial solid-liquid separation processes. Data obtained in this work on dilute aqueous dispersions of model colloidal polystyrene latex spheres indicate that depletion flocculation, which uses non-adsorbing polymer, can yield very compact aggregates. Flocculation of the negatively charged latex particles was induced by the addition of a poly(acrylic acid) at pH 10. The structural compactness of the latex flocs formed in the dilute dispersions was characterised using small-angle static light scattering in terms of mass fractal dimensions. Rheological measurements on the concentrated latex dispersions in the presence of the non-adsorbing polyacid showed Bingham yield stress behaviour. Both the compactness and strength of the latex flocs were found to be significantly dependent upon the level of the polyacid, as well as the concentration of the initial particles. In particular, as the level of the polyacid was raised the floc compactness decreased, whereas its strength increased. They were both seen to level off at high polymer concentrations. Atomic force microscopy measurements were made at varying concentrations of the polyacid to provide a qualitative explanation of the observed floc structural behaviour of the dilute dispersions. By combining the fractal dimension and the Bingham yield stress we were also able to estimate the energy required to separate the flocs into single units in the concentrated dispersions. It was concluded that the interparticle interaction energy is the key to understanding the dependence of both the floc structure and strength on the polymer concentration.

Relationship between interaction forces and the structural compactness of depletion flocculated colloids

The addition of non-adsorbing polymers to a colloidal dispersion can induce particle aggregation through depletion flocculation. Small-angle static light scattering has been used to show that the structure of aggregates formed upon the addition of non-adsorbing poly(acrylic acid) to a dilute aqueous dispersion of colloidal polystyrene latex particles exhibited power-law scattering behaviour. The observed floc mass fractal dimension, d F, was found to be dependent upon the polyacid concentration. Increasing the polyacid concentration resulted in lower fractal dimensions, with d F levelling off at high polymer concentrations. This structural behaviour was attributed to the deepening in the secondary potential energy well and hence higher particle sticking efficiencies upon collisions when the polymer concentration was raised. This is directly supported by atomic force microscopy data, which showed that the interaction force curves between charged surfaces for solutions of the non-adsorbing poly(acrylic acid) exhibited attractive secondary minima, the depths of which increased with increasing polymer concentration, saturating at high polyacid concentrations. In contrast to the much lower values obtained via salt-induced aggregation ( d F=1.78 to 2.20), significantly higher mass fractal dimensions and hence greater structural compactness were observed for these depletion-flocculated flocs.

A Small-Angle X-ray Scattering Study of the Structure of Aqueous Laponite Dispersions

Two Laponite clays, of general formula [Mn+]x/n [Lix Mg6-x Si8 O20 Fy (OH)4-y], were prepared with different Li+ levels (0.57 and 0.70% w/w) and F- levels (9.5 and 9.9% w/w). The clays were characterized by measurements of nitrogen adsorption isotherms, cation exchange capacity, and conductometric titration data. The clay with higher Li+ and F- levels had a higher (more negative) face charge and a lower (less negative) edge charge. Small-angle X-ray scattering (SAXS) data obtained from dilute aqueous dispersions indicate that both clays are of similar particle size (diameter ca. 40 nm and thickness ca. 3.5 nm). Experimental structure factors for concentrated Laponite dispersions were calculated using the ratio of scattered X-ray intensities from concentrated and dilute dispersions. Maxima in the experimental structure factors were observed at scattering angles of 0.14−0.2 nm-1 under favorable solution conditions (pH ≥ 9 and ionic strength ca. 5 × 10-3 mol dm-3) for one of the Laponite clays. These results are interpreted in terms of an ordered, aligned structure where the particles' face and edge electrical double layers are expanded and negative in sign. This is in contrast to a “house-of-cards” structure which results when particle aggregation is promoted by lower pH, higher ionic strength, or a lower particle edge charge.

Transitions between Ordered and Disordered Phases and Their Coexistence in Dilute Ionic Colloidal Dispersions

Order−disorder transitions in dilute aqueous dispersions of ionic colloids were studied experimentally in colloidal silica and polymer latex systems. A three-dimensional phase diagram of the silica system was determined by treating the particle density, the effective surface charge number, and the ionic strength as variables. On the basis of the phase diagram, the structures of the dispersions were studied by means of ultrasmall-angle X-ray scattering and confocal laser scanning microscopy. The results clearly revealed the presence of a reentrant order−disorder transition with increasing charge number, and a wide biphasic region where ordered and disordered phases coexist. The nearest interparticle distance 2Dexp was almost equal to that calculated assuming a homogeneous particle distribution, 2D0, in the ordered single-phase region, while 2Dexp was smaller than 2D0 under the biphasic condition. Further studies concerning the crystallization process of the dispersion in the biphasic region demonstrated the development of disordered liquidlike regions inside the space-filling crystalline grains which was induced by lattice contraction. The results suggest that the commonly held view based on a Yukawa-type potential, in which only repulsive interparticle interactions are considered, cannot explain the phase behavior of the dilute ionic colloids.

Single crystal of colloidal silica particles in a dilute aqueous dispersion as studied by a two-dimensional ultrasmall-angle x-ray scattering

A two-dimensional ultrasmall-angle x-ray-scattering apparatus was constructed with a full width at the half maximum of the intensity profile of the incident x-ray beam being about 17 s of arc. This apparatus enabled us to obtain the scattering intensity as a function of the scattering vector. The scattering profile of a colloidal crystal of silica particles in an aqueous dispersion was studied and analyzed with the aid of a method by Busing and Levy. The diffraction planes corresponding to 22 scattering peaks observed were uniquely identified; The lattice system was determined to be body-centered-cubic with a lattice constant of $3800\mathrm{\AA{}}$ and its [11\ifmmode\bar\else\textasciimacron\fi{}1] was parallel to the capillary axis.

Low-Shear Viscosities of (Semi-)Dilute, Aqueous Dispersions of Charged Boehmite Rods: Dynamic Scaling of Double Layer Effects

Low-shear viscosities were measured of dilute aqueous dispersions of charged boehmite rods with an average aspect ratio of 22.5 as a function of the Debye length κ-1 and the particle volume fraction Φ. It is found that the low-shear viscosity of the dispersions scales with the effective overlap concentration ν/ν1*, where ν1* is the minimum overlap concentration of rods with an effective length L* = L + 5κ-1. This dynamic scaling is also valid for low-shear viscosity data of FD-virus solutions (Graf et al. J. Chem. Phys. 1993, 98, 4920). The rescaled relative viscosity curves can be described by the (Maron-Pierce) equation ηr = (1 − (ν/ν1*)/(ν/ν1*)max)-2. The effective overlap concentration at which the low-shear viscosity diverges, (ν/ν1*)max, is much lower for the boehmite rods than for the semiflexible FD-virus and hard rods without electric double layers. Extrapolation of the reduced viscosity to ν/ν1* = 0 yields an unrealistic high intrinsic viscosity [η]. Possibly the relative viscosity of dilute dispersions at very low ionic strength shows a nonanalytical concentration dependence, which renders the definition of [η] for colloids with thick double layers questionable.

Control of the surface charge density of colloidal silica by sodium hydroxide in salt-freeand low-salt dispersions

Electrical conductivity measurements and conductometric titrations were performed on dilute salt-free aqueous dispersion of a colloidal silica (diameter: 0.11\ifmmode\pm\else\textpm\fi{}0.01\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}6}$nm) whose charge number can be varied with the quantity of coexisting sodium hydroxide. In the absence of sodium hydroxide, the silica particle had an effective (net) surface charge density ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{e}}$ of 8\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}8}$nC ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. Titrations by hydrochloric acid were performed in the presence of sodium hydroxide. When the NaOH concentration was sufficiently high, the titration curve could be divided into three regions with regard to slope values. These regions could be ascribed to titrations of excess sodium hydroxide, ionizable surface groups having Na ions as counterions, and excess hydrochloric acid. The analytical surface charge density ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{a}}$, estimated from the titration curves, increased with increasing [NaOH]. It was found that the concentrations of Na and OH ions in excess were negligible when the [NaOH] was smaller than 2.5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$nM and the volume fraction of the silica, \ensuremath{\varphi}, was larger than 2.6\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$. The value of ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{a}}$ at this threshold was 1.8\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$nC ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. Under these conditions we could control the ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{a}}$ value by varying [NaOH]. The present system provides larger possibilities in studying the influence of charge density on the physico-chemical properties of ionic colloidal systems. Viscosity measurements were performed for salt-free and low-salt dispersions at [NaOH]'s where its excess concentration was found to be negligible. The ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{a}}$ dependence of the viscosity was in good agreement with previous results obtained from ionic latices having various ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{a}}$'s. A relationship between the effective charge density at an infinite dilution, ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{e}\mathrm{|}\mathrm{\ensuremath{\varphi}}\mathrm{=}0}$, and ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{a}}$ was examined for latex systems. An empirical relation, ln ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{e}\mathrm{|}\mathrm{\ensuremath{\varphi}}\mathrm{=}0}$=0.49 ln ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{a}}$-1.0, was obtained by using ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{e}\mathrm{|}\mathrm{\ensuremath{\varphi}}\mathrm{=}0}$ values determined by the conductivity for latices with various ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{a}}$'s from 0.21\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}6}$ to 5.6\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}6}$nC ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. By assuming that this relation holds also for the silica system, viscosity data were analyzed. Satisfactory agreement was observed between the experimental value and Booth theoretical value on the first-order electroviscous effect.

The gelation of dialdehyde starch

The rheological behaviour of periodate oxidized potato starches has been investigated in both dilute and concentrated aqueous dispersions. On heating, low viscosity solutions were obtained at low concentrations and viscoelastic gels at high concentrations. During the gelation process, two types of gel structure were distinguished. First a particle gel is formed, and on continued heating and shearing, deformation and disruption of the granules is observed, resulting in the formation of a polymer network. The increasing stability of the particle gels with increasing degree of oxidation has been attributed to an increase in particle rigidity. The aldehyde groups in dialdehyde starch are supposed to reinforce the granular structures, as a result of intragranular hemiacetalization. The gel stability could be improved by adding native potato starch to the premix.