Aggregation of Elongated Colloids in Water

Our plain-English summary of Aggregation of Elongated Colloids in Water, by L. Wu, C.P. Ortiz, D.J. Jerolmack (Langmuir, 2017).

The study

• Authors: L. Wu, C.P. Ortiz, D.J. Jerolmack
• Journal: Langmuir
• Published: 2017;33(2):622-629
• Cited by: 16 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Colloidal aggregation is a canonical example of disordered growth far from equilibrium and has been extensively studied for the case of spherical monomers. Many particles encountered in industry and the environment are highly elongated; however, the control of particle shape on aggregation kinetics and structure is not well-known. Here, we explore this control in laboratory experiments that document aqueous diffusion and aggregation of two different elongated colloids: natural asbestos fibers and synthetic glass rods, with similar aspect ratios of about 5:1. We also perform control runs with glass spheres of similar size (∼1 μm). The aggregates assembled from the elongated particles are noncompact, with morphologies and growth rates that differ markedly from the classical aggregation dynamics observed for spherical monomers. The results for asbestos and glass rods are remarkably similar, demonstrating the primacy of shape over material properties-suggesting that our findings may be extended to other elongated colloids such as carbon nanotubes/fibers. This study may lead to enhanced prediction of the transport and fate of colloidal contaminants in the environment, which are strongly influenced by the growth and structure of aggregates.

In plain English

This laboratory experiment studied how tiny particles clump (aggregate) in water. It involved no people or animals. Under an optical microscope, the authors watched three particle types in water at 150 ppm and pH 3.6 to 8.0: natural chrysotile asbestos fibers, synthetic glass (silica) rods, and, for comparison, glass spheres. The chrysotile fibers had been treated with dilute acetic acid. The authors said this treatment could account for the fibers' altered surface charge, which to some extent explained their similarity to rods. Asbestos fibers and glass rods grew nearly identically. All three types formed fractal clusters, but fiber and rod clusters were sparser than the more compact sphere clusters. In representative experiments at pH 5.0, the fractal dimension (a compactness measure) was 1.71 for asbestos, 1.69 for rods and 1.83 for spheres, each plus or minus 0.01. Lower pH sped aggregation. A standard cluster-size model (Smoluchowski) roughly fit the spheres but was inadequate for the elongated particles. The authors concluded that shape, not material, was responsible for these differences. The authors noted their images captured only a two-dimensional slice of three-dimensional clusters.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory experiment: optical microscopy of particle aggregation in water in a sealed liquid cell
• Who was studied: Particles, not people: natural chrysotile asbestos fibers from a raw ore block (purity over 90%) from the El Dorado Mine, Salt River, Arizona; synthetic glass (silica) rods; and glass spheres as controls
• Exposure: Suspensions of each particle type at a fixed 150 ppm concentration, with pH varied from 3.6 to 8.0 in a subset of experiments. The chrysotile fibers had been treated with dilute acetic acid. They averaged 1.9 plus or minus 0.5 µm in diameter and 10 plus or minus 0.8 µm in length, with lengths up to 20 µm. Glass rods were 2 µm by 8 µm; glass spheres were 2.9 µm in diameter.
• Main results:
  • Over the range of pH tested, growth curves for chrysotile asbestos fibers and glass rods were nearly identical and distinct from those of glass spheres
  • The characteristic cluster size reached the same final value, about 4.3 µm, for asbestos fibers and glass rods
  • Lower pH values resulted in faster aggregation
  • Clusters from all three particle types were fractal (fractal dimension below 2), not solid, at the scale of the clusters
  • Clusters formed from the elongated particles were sparse and non-compact
  • The higher fractal dimension of the sphere clusters suggested more compact and globular shapes
  • In representative experiments at pH 5.0, the fractal dimension was 1.71 plus or minus 0.01 for asbestos and 1.69 plus or minus 0.01 for glass rods
  • At pH 5.0, the fractal dimension for glass spheres was higher, 1.83 plus or minus 0.01
  • The Smoluchowski model gave a decent approximation of cluster sizes for spheres but was inadequate for the elongated particles
  • For silica rods and silica spheres, measured aggregation was faster than DLVO theory predicted under unfavorable conditions. At the highest pH values, the difference was about a factor of 2 for spheres and up to 10 for rods.
  • The isoelectric point of the acetic-acid-treated chrysotile fibers was about 4 to 5. This is lower than values reported elsewhere for pristine chrysotile.
  • The authors attributed this lower value possibly to the deliberate acid-leaching of the chrysotile fibers, which turns the zeta potential from positive to negative
  • The authors said this result to some extent explains why the growth curves of chrysotile fibers and silica rods were similar
• The authors' conclusion: The authors concluded that the similarity between chrysotile fibers and glass rods shows that shape, not material, is responsible for the departures from classical aggregation behavior
• Limitations the authors state:
  • Image analysis examined only a two-dimensional slice near the bottom of the cell, so the three-dimensional structure of clusters was not captured
  • Observed dynamics may have been influenced slightly by sedimentation
  • Because growth saturated, the growth curves covered too limited a range to be fitted confidently to the classical model
  • The fractal dimensions were determined from planar images of three-dimensional objects, so only relative comparisons can be made
  • The method used to calculate zeta potential may overestimate it for elongated particles by up to 20%
  • The proposed explanation of greater surface-charge heterogeneity in elongated particles could not be confirmed

Topics

Medical terms in this study, explained in our Lexicon: Aspect Ratio (Length-to-diameter ratio).

Related studies

• How are the physical and chemical properties of chrysotile asbestos altered by a 10-year residence in water and up to 5 days in simulated stomach acid? — K. Seshan (1983)
• In Situ Liquid Cell Observations of Asbestos Fiber Diffusion in Water — L. Wu, C. Ortiz, et al. (2015)
• Morphological and chemical mechanisms of elongated mineral particle toxicities — A.E. Aust, P.M. Cook, et al. (2011)
• Origin and fate of nanoparticles in marine water - Preliminary results — B. Graca, A. Zgrundo, et al. (2018)
• Airborne fiber control in buildings during asbestos material removal by amended water methodology — R.N. Sawyer, A.N. Rohl, et al. (1985)
• The impact of asbestos cement pollution in irrigation water on physiological and germination characteristics of Trifolium pratense, Medicago sativa, and Solanum lycopersicum seeds — G.Z. Macher, A. Torma, et al. (2025)

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