Colloidal Size Range Research Articles

Improvement of the Low-Temperature Stability of an Aqueous Colloidal Ethylcellulose Dispersion, Aquacoat® ECD, and Preparation/Characterization of a Redispersible Aquacoat® ECD Powder

Aquacoat® ECD, an aqueous ethylcellulose dispersion, is susceptible to low-temperature storage, resulting in irreversible coagulation as indicated by a strong increase in viscosity and particle size. This destabilization of the ethylcellulose dispersion is caused by the anionic surfactant sodium dodecyl sulfate, which precipitates at low temperatures because of its low Krafft point. This problem could be solved by using the more hydrophilic, ethoxylated, sodium dodecyl ether sulfate, which was an effective stabilizer at low temperatures. A redispersible ethylcellulose powder was prepared by freeze- or spray-drying of the original polymer dispersion (Aquacoat ECD). The pH of the dispersion medium had a strong influence on the redispersibility of the dried ethylcellulose particles because of the dissociation behavior. At a pH>7, polymer dispersions in the colloidal size range were obtained. At lower pH values, the dried ethylcellulose agglomerates could not be redispersed. Drug release studies from pellets coated with the redispersed and the original dispersion showed a comparable release pattern when using alkalized aqueous dispersion systems. Higher plasticizer concentration and curing of the coated pellets improved the film formation of the redispersed polymer particles.

Solid lipid nanoparticles (SLN) as ocular delivery system for tobramycin

Aim of this study was to evaluate solid lipid nanoparticles (SLN) as carriers for topical ocular delivery of tobramycin (TOB). The SLN were in the colloidal size range (average diameter below 100 nm; polydispersity index below 0.2) and contained 2.5% TOB as ion-pair complex with hexadecyl phosphate. The preocular retention of SLN in rabbit eyes was tested using drug-free, fluorescent SLN (F-SLN): these were retained for longer times on the corneal surface and in the conjunctival sac when compared with an aqueous fluorescent solution. A suspension of TOB-loaded SLN (TOB-SLN) containing 0.3% w/v TOB was administered topically to rabbits, and the aqueous humour concentration of TOB was determined up to six hours. When compared with an equal dose of TOB administered by standard commercial eyedrops, TOB-SLN produced a significantly higher TOB bioavailability in the aqueous humour.

Semisolid SLN™ dispersions for topical application: influence of formulation and production parameters on viscoelastic properties

Aqueous solid lipid nanoparticle (SLN) dispersions with a high lipid content up to 35% and viscous to semisolid consistency were produced by a high pressure homogenization process. Despite their high lipid content and viscosity these dispersions preserved their colloidal size range. The SLN dispersions were compared to nanoemulsions and microparticle dispersions with regard to particle size, viscoelastic properties and formation of a semisolid gel structure. Viscoelastic measurements including oscillation stress sweep tests and oscillation frequency sweep tests demonstrated that the existence of a solid particle matrix with a particle size in the nanometer range is a prerequisite to form a semisolid dispersion having the appropriate consistency for topical application. Striking differences were observed between solid lipid micro- and nanodispersions of the same composition. Particle size reduction resulted in an 80-fold increase of the elastic modulus. Particle size distribution, the physical state of the dispersed lipid phase and the emulsifier concentration have been identified as further key factors for the viscoelastic properties and gel structure of the lipid nanodispersions. By conducting oscillation measurements it was possible to relate the stability of lipid dispersions to specific rheological parameters therefore providing a sensitive tool in stability assessment. Changing the production process from a 40 ml batch to a 2 l batch turned out to have an influence on the colloidal structures of semisolid SLN dispersions. Consistency increased but particle size and ratio of elastic to viscous properties stayed in the same range.

Sorption irreversibility and coagulation behavior of 234Th with marine organic matter

Abstract The partitioning of 234 Th to natural organic matter (NOM) in the colloidal size range (1 kDa–0.1 μm) was evaluated in order to examine the sorption and coagulation behavior of marine colloidal organic matter. Colloids were isolated using large volume cross-flow ultrafiltration and the partitioning of 234 Th was quantified using stirred cell ultrafiltration and radioactive assay. The uptake of 234 Th by NOM is irreversible over a period of 5 days, implying that over the mean life of 234 Th, very little release of 234 Th would occur after binding to NOM. Furthermore, the Th–NOM complex is stronger than the Th–EDTA complex, as EDTA was unable to displace the 234 Th from its association with NOM. The extent of the initial partitioning of 234 Th to suspended matter and colloids is similar and independent of pH in the range from 3 to 9. Coagulation experiments show that 234 Th complexed with low molecular weight (1–10 kDa) colloids is transferred to larger (>0.1 μm) filter retained fractions. However, 234 Th is transferred to a greater extent than is organic matter and this results in greater partitioning coefficients for 234 Th onto particle phases with time. The final equilibrium between 0.1 μm filter-retained and filter-passing 234 Th activity is the same regardless of whether the Th was tagged initially to colloidal or suspended matter fractions. The coagulation of colloidal organic matter, COM, consists of both fast and slow steps, with kinetic rate constants on the order of 0.02–0.03 and 0.003–0.007 h −1 , respectively. The stickiness (or collision efficiency) factor, α , for COM was experimentally determined to be 0.7(−) for seawater conditions. Using the colloidal pumping model of Honeyman and Santschi [J. Mar. Res. 47 (1989): 951], the ‘predicted’ “fast-phase” coagulation rate coefficient is 0.03 h −1 in our coagulation experiments when the measured α value and the experimental conditions are used for model inputs. These experiments demonstrate that coagulation is the dominant step in the transport of 234 Th to the particulate phase.

Non-biological colloids as a model for biological colloids in membrane filtration Nichtbiologische Kolloide als Modell für biologische Kolloide bei der Membranfiltration Colloïdes non-biologiques comme modèles de colloïdes biologiques en filtration membranaire Coloides no biológicos como modelos para coloides biológicos en filtración membrana

Non-biological (polymeric/inorganic) colloids are widely used to simulate microorganisms, which are biological or living colloids. This is because of their similarity in size and charge; i.e. microorganisms are within the colloidal size range of 1 nm-1 μm and are generally negatively charged. The results of the presented work indicate that these two categories of colloids may behave in an opposing manner. The addition of E. coli bacteria to a virus suspension provides sites for the adsorption of viruses onto the surface of bacteria, resulting in the higher retention of poliovirus in a microfiltration process. However the addition of large latex particles to gold sol suspension reduces the retention of small gold particles by a microfiltration membrane. This is because of interference by the latex particles, which inhibits the formation of the self-rejecting cake layer by the fine gold particles. The main reason for the different behaviour of biological and non-biological colloids is down to the nature of the two types of colloids. Biological colloids have adsorptive surfaces and produce extracellular polymers, allowing interactions between microorganisms, while non-biological colloids are hard spheres, with relatively smooth and non-adsorptive surfaces.

Filtration artifacts caused by overloading membrane filters.

The conventional practice of using 0.45 or 0.40 microm membranes to distinguish between the particulate and dissolved phases in natural waters neglects the importance of colloids. Many of the colloids in natural waters pass through 0.45 or 0.40 microm membranes, but a significant fraction at the upper end of the colloidal particle size range is retained. Membrane clogging during filtration decreases the effective pore size and can cause the retention of increasing amounts of colloids. This filtration artifact can cause serious errors in sampling and in assigning trace metals to various particle size classes. We evaluated the effect of membrane loading for two common membrane types (0.45 microm Millipore Durapore and 0.40 microm Nuclepore) on the retention of colloidal Fe, Al, Mn, and OM in three Connecticut rivers. In addition, we used a 1.0 microm Nuclepore membrane to estimate the amount of colloids in the 0.40-1.0 microm size fraction that are retained by membranes during conventional filtration. All samples were collected with clean techniques, and all filtrations were carried out in a class 100 clean room. A peristaltic pump, set at an initial flow rate of 120 mL/min, was used to pump samples through 47 mm diameter inline Teflon filter holders. Back pressure and flow rate were monitored during filtration, and both are good indicators for the onset of membrane clogging. The results show a consistent correlation between increasing back pressure and decreasing concentration of colloidal Fe and sometimes Al, Mn, and OM in the filtrate for all membrane types. Although the shape of the loading-retention curves varied dramatically by site and by membrane type, the essential relationship between back pressure, flow rate, and filtration artifacts during membrane clogging remained the same.

Soluble and Colloidal Iron in the Oligotrophic North Atlantic and North Pacific

In the oligotrophic North Atlantic and North Pacific, ultrafiltration studies show that concentrations of soluble iron and soluble iron-binding organic ligands are much lower than previously presumed "dissolved" concentrations, which were operationally defined as that passing through a 0.4-micrometer pore filter. Our studies indicate that substantial portions of the previously presumed "dissolved" iron (and probably also iron-binding ligands) are present in colloidal size range. The soluble iron and iron-binding organic ligands are depleted at the surface and enriched at depth, similar to distributions of major nutrients. By contrast, colloidal iron shows a maximum at the surface and a minimum in the upper nutricline. Our results suggest that "dissolved" iron may be less bioavailable to phytoplankton than previously thought and that iron removal through colloid aggregation and settling should be considered in models of the oceanic iron cycle.

Development of sedimentation field-flow fractionation-inductively coupled plasma mass-spectrometry for the characterization of environmental colloids

A relatively new hyphenated technique for the simultaneous size separation and elemental analysis of colloids has been further developed and applied to the characterization of soil colloids. Sedimentation field-flow fractionation (SdFFF) was directly interfaced to an inductively coupled plasma-mass spectrometer (ICP-MS) to provide high-resolution sizing and elemental analysis of colloids in the range 0.05–1.0 μm. For this work our existing SdFFF instrument was modified by addition of an upgraded motor and software for centrifuge speed control and data collection. Analytical techniques were developed for the calibration and drift correction of the ICP-MS data collected during on-line SdFFF-ICP-MS analyses. Software was developed to allow off-line computation of drift-corrected, elemental concentrations across the colloid size range. SdFFF-ICP-MS examination of two colloid samples isolated from surface soil horizons showed significant enrichment in iron-containing phases in both the smaller and larger colloids relative to intermediate particle sizes (≈0.3 μm). Element ratios obtained by SdFFF-ICP-MS indicated that the relative amounts of kaolinite, vermiculite, and illite vary across the colloid size range, with kaolinite less concentrated in the larger sizes (>0.3 μm). These results demonstrate the utility of SdFFF-ICP-MS for examination of soil chemistry and mineralogy and suggests the technique will have application to other environmental and geochemical studies.

The complexation of `dissolved' Cu, Zn, Cd and Pb by soluble and colloidal organic matter in Narragansett Bay, RI

It is widely accepted that the speciation of most bioactive metals in seawater is regulated by natural organic ligands, but the nature of these molecules has remained a mystery. We used a combination of physical and chemical separation schemes to better characterize organic molecules complexing Cu, Zn, Cd, and Pb in Narragansett Bay, RI. Conventionally filtered ( 1 kDa), with the bulk of it found in the 1–8 kDa colloidal size range. Of the three Cu-binding ligand classes measured, the strongest class occurred mainly in the soluble fraction while the weaker Cu-binding ligand classes were predominantly colloidal (>1 kDa). Approximately 40% of chelated Pb was colloidal but, in contrast to Cu, the bulk of these ligands resided in the larger colloidal size range (8 kDa–0.2 μm). Thus, a continuum of metal complexing ligand size exists, spanning from truly soluble to colloidal, the nature of which differs for individual bioactive metals. These findings support the hypothesis that metal complexation in seawater is dominated by distinct, metal-specific ligand molecules. A central question that now emerges is whether these ligand molecules function predominantly to buffer metal ion activities in seawater, thereby decreasing metal sorption to particulates, or to facilitate metal removal by sweeping organically bound metals into particulate phases via colloid aggregation.

Potential for the formation and migration of colloidal material from a near-surface waste disposal site

Organic material typically constitutes a substantial volume (∼ 90%) of the low-level radioactive wastes (LLRW) intended for near-surface disposal at Chalk River Laboratories (CRL), Ontario, Canada. These wastes can contain a large variety of organic materials, including paper, cardboard, plastic bags, used clothing, and mop heads. After emplacement in a disposal facility, leaching of the LLRW by water can mobilize inorganic and organic substances, ranging from small molecules such as acetic acid to unidentifiable material of colloidal size range. This study determined the potential for colloid formation produced by LLRW degradation, because colloid-facilitated transport of contaminants could affect the safety performance of a disposal facility. The decomposition of compacted LLRW was simulated by recirculating water in a closed system over several compacted bales of waste to determine the potential composition and colloid content of leachates. Size fractionation of organic matter was performed on leachate samples that had been aged for 18 months to simulate the microbial degradation of organic matter within leachates during migration out of the LLRW disposal facility. The aged leachates contained high concentrations of dissolved organic matter, ranging between 74 and 5074 mg/l as C. In most of the leachates, volatile fatty acids accounted for a significant fraction (up to 81%) of the dissolved organic carbon. Although 5–110 mg/l of organic colloids were observed in leachates, in most cases, the organic colloids made up a very small fraction of the total leached organic carbon. Therefore, since the complexation properties of dissolved and colloidal organics are probably similar, contaminants complexed to organics are most likely to be dissolved and not affected by colloid transport. The leachates also contained significant quantities of Fe and Al, which could potentially precipitate Fe and Al as colloids after oxidation. Although a significant portion of the dissolved Fe may have been produced by the corrosion of the ☐es used to contain the bales, the high Fe concentrations could be representative of leachates from LLRW that contain metallic Fe components. If Fe and Al colloids are stable, stable concentrations in LLRW leachates could be high enough to affect contaminant transport. Therefore, the Fe and Al content of LLRW should be minimized. The concentrations of natural colloids in sandy aquifers, such as those found at CRL are too low to affect contaminant migration significantly.

Relative shear stability of mini‐ and macroemulsion latexes

The shear stability of mini- and macroemulsion latexes is compared and quantitatively evaluated with respect to their particle-size distributions. The effect of a few externally added large particles on the shear stability of these two types of latexes was also investigated. All the latexes selected were in the colloidal size range (less than 1 micron). The original particle sizes for the macroemulsion latexes ranged from 141 to 241 nm, and those for the miniemulsion latexes ranged from 96 to 209 nm. The miniemulsion latexes were found to be more shear stable than were their macroemulsion latex counterparts over the particle-size range investigated. This trend was repeated even in the presence of a few large particles. Additionally, seeding experiments suggest that mini- and macroemulsion latexes incur different levels of shear aggregation due to inherent differences in their particle-size distributions. The shear rate used along with the particle size and number were quantitatively shown to significantly influence the aggregation process. Finally, a quantitative method for evaluating relative shear stability in emulsion polymerization was demonstrated, which, although not very rigorous, could serve as a starting point for further quantitative isolation and investigation of the various parameters that affect the shear aggregation process.

The lifetime of a colloid-sized gas bubble in water and the cause of the hydrophobic attraction

When considered to be open in the thermodynamic sense with respect to the enclosed gas, bubbles that are present in a liquid bulk phase are unstable in all respects and tend to dissolve. Conversely, full mechanical (and physicochemical) stability is guaranteed when a bubble is closed with respect to the gaseous component. By assuming the diffusion of dissolved gas molecules away from a spherical gas bubble to determine the shrinkage rate, we calculate the lifetime of a bubble as a function of its size. Gas bubbles in the colloidal size range, with radii between 10 and 100 nm, have surprisingly short lifetimes, between about 1 and 100 μs, whereas a nm-sized bubble can persist for months. Our results (which confirm and partwise extend the old calculations of Epstein and Plesset [5] [J. Chem. Phys., 18 (1950) 1505], indicate that the bridging bubble/cavity mechanism proposed earlier, can hardly provide the proper explanation of the long-ranged attraction force observed between hydrophobic surfaces immersed in water.

A classification scheme for marine organic colloids in the Adriatic Sea: colloid speciation by transmission electron microscopy

Organic colloids from the Adriatic Sea, covering the full colloid size range, were characterized on a per-colloid basis by transmission electron microscopy applied to ultrathin sections of embedded colloids. To detect, assess, and minimize artifacts, a multimethod correlative approach to analysis was used. A colloid classification scheme was developed from the characterizations, with individual colloid species defined in the broad sense established by environmental analytical aquatic chemists. The basic scheme is related to derivative classification schemes in which abundant colloid species are listed according to activities known from the literature, with a focus on activities related to carbon transformations and transport. Morphological criteria show that abundant colloid species occupy broad size ranges, such that colloid fractionation protocols based on sizing alone are likely to be of questionable scientific utility. Image analysis revealed that small colloids, designated as humic substances, represented 91% of the total organic colloid burden of the bulk water during a summer period of high biological productivity. At this time, fibrils were rare in the bulk water while representing the most abundant colloid in marine snow.

Particle removal in different filtration systems for tertiary wastewater treatment - a comparison

In seven wastewater installations in the area of Zürich, Switzerland, four different filtration systems for tertiary wastewater treatment (deep bed, cloth drum, continuous up-flow and cell filters) were investigated with special attention to particle size distributions in the in- and outflow of the flocculation and the filtration step. Flocculation and filter performance was characterised by the shift of particle number curves. For each filter, particle number, surface and volume removal as a function of particle size was determined and revealed to be an excellent tool to evaluate the filter plants under practical conditions. The number removal efficiency for small particles in the colloidal size range proved to be the most sensitive parameter and could be related to operating parameters such as flocculant dose and filtration rate. The results of the particle analysis were compared to quality parameters related to small particles such as micro-organism and heavy metal concentrations. A good agreement between the removal of small size particles (< 8 μm) and bacteria counts as well as copper was found in the different filter systems. The filters were finally classified according to their particle removal performance.

Investigation of Cd, Cu, Ni and 234Th in the colloidal size range in the Gulf of Maine

Size-fractionated seawater samples were collected from the Gulf of Maine to determine the fraction (fc/d) of total dissolved ( ∼ 1–10 mg L−1), such as in near-shore and estuarine waters.

Electrokinetic and Direct Force Measurements between Silica and Mica Surfaces in Dilute Electrolyte Solutions

An atomic force microscope has been used to study the forces between a silica sphere in the colloidal size range and silica or mica flat surfaces as a function of distance of separation. At low ionic strength, independent electrokinetic measurements (ζ potentials) of both the spheres (by electrophoresis) and flat surfaces (by streaming potential) under the same conditions show excellent agreement with the diffuse double layer potentials derived from the force data using conventional DLVO theory. At higher ionic strength, the electrokinetically derived potentials were found to deviate from those derived from the fitted atomic force microscopy data, and a short range steric type repulsion was observed between the surfaces, the magnitude of which increased with decreasing pH.

Particle removal in different filtration systems for tertiary wastewater treatment - A comparison

In seven wastewater installations in the area of Zürich, Switzerland, four different filtration systems for tertiary wastewater treatment (deep bed, cloth drum, continuous up-flow and cell filters) were investigated with special attention to particle size distributions in the in- and outflow of the flocculation and the filtration step. Flocculation and filter performance was characterised by the shift of particle number curves. For each filter, particle number, surface and volume removal as a function of particle size was determined and revealed to be an excellent tool to evaluate the filter plants under practical conditions. The number removal efficiency for small particles in the colloidal size range proved to be the most sensitive parameter and could be related to operating parameters such as flocculant dose and filtration rate. The results of the particle analysis were compared to quality parameters related to small particles such as micro-organism and heavy metal concentrations. A good agreement between the removal of small size particles (< 8 µm) and bacteria counts as well as copper was found in the different filter systems. The filters were finally classified according to their particle removal performance.

The Uptake of Colloidal Melanin from Seawater by Marine Bivalves

Abstract. A number of coastal organisms (e.g., bivalves) are known as important consumers of particulate organic carbon (POC) in the sea. In this paper we present new evidence that they are also important consumers of dissolved organic carbon (DOC) in the colloidal size range down to about 0.2 μm in diameter. Those colloids play an important role in the global flux of carbon in the seas.We compare the uptake of colloidal DOC by marine bivalves of the North Sea and of the western Indian ocean.To measure this uptake we made use of colloidal melanin as an alternative way to measure the uptake of colloidal DOC.

Size-Distribution Analysis of Sub-Micron Colloidal Particles in River Water

The particle size spectra of sub-micron colloidal particles collected from a river water, the Water of Leith in Dunedin, New Zealand, have been determined by transmission electron microscopy (TEM). Particles were isolated onto hydrophilic carbon films by ultracentrifugation at 300,000 g for 1 hr. Size spectra were found to conform to a power-law size distribution dn/dr = A r − s, behaviour also found for much larger-sized particles in natural waters and marine colloids. Values of the exponent s for unfiltered samples were in the range 2.3–2.9, consistent with Brownian aggregation as the principal mechanism maintaining the size spectrum. Comparison with spectra obtained from sampled prefiltered through Nuclepore 0.4µm filters showed clear evidence of particle removal down to 100–120 nm radius by filtration. Particles exhibited a range of electron densities under TEM, indicating a mixture of inorganic minerals and organic matter in the colloidal size range. Examples of fractal aggregates were also found.

Characterization of the submicrometre phase in surface waters. A review

The need for improved knowledge about the composition and behaviour of solid matter in the colloidal size range (1 nm to 1 µm) in natural waters has been identified by several authors in the literature. The increased interest in these colloids is due to their large impact on the transport, redistribution and bioavailability of numerous chemical compounds (such as trace metals, nutrients and organic pollutants). This paper presents a review of different analytical techniques available for determination of some of the properties of interest (e.g., colloid concentration, size distribution, mineral composition and surface properties). It is obvious that most methods suffer from limitations and that the only feasible way to overcome these problems is by using a combination of different techniques. Sampling in a Swedish lake, as an example, illustrates one promising combination including light scattering and gravimetric analysis for determination of colloid concentration, photon correlation spectroscopy and scanning electron microscopy for size distribution measurements and energy dispersive microscopy, powder X-ray diffraction and infrared spectroscopy in order to obtain information about mineral constituents and general surface properties. Size fractionations were performed by filtration and centrifugation to gain information regarding the distribution of metals and organic matter between dissolved and solid phases. The study indicated, for instance, a seasonality of the size distribution of the colloidal matter, with smallest colloids (range 120–340 nm) found in spring, while the colloid samples during summer and autumn have a size distribution in the range 280–700 nm. This result also illustrates the limitations when using filters with, e.g., a pore size of 0.45 µm for the distinction between particulate and dissolved substances.