Probing anisotropic surface properties and interaction forces of chrysotile rods by atomic force microscopy and rheology

Our plain-English summary of Probing anisotropic surface properties and interaction forces of chrysotile rods by atomic force microscopy and rheology, by D. Yang, L. Xie, E. Bobicki, et al (Langmuir, 2014).

The study

• Authors: D. Yang, L. Xie, E. Bobicki, Z. Xu, Q. Liu, H. Zeng
• Journal: Langmuir
• Published: 2014;30(36):10809-17
• Cited by: 27 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Understanding the surface properties and interactions of nonspherical particles is of both fundamental and practical importance in the rheology of complex fluids in various engineering applications. In this work, natural chrysotile, a phyllosilicate composed of 1:1 stacked silica and brucite layers which coil into cylindrical structure, was chosen as a model rod-shaped particle. The interactions of chrysotile brucite-like basal or bilayered edge planes and a silicon nitride tip were measured using an atomic force microscope (AFM). The force-distance profiles were fitted using the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, which demonstrates anisotropic and pH-dependent surface charge properties of brucite-like basal plane and bilayered edge surface. The points of zero charge (PZC) of the basal and edge planes were estimated to be around pH 10-11 and 6-7, respectively. Rheology measurements of 7 vol % chrysotile (with an aspect ratio of 14.5) in 10 mM NaCl solution showed pH-dependent yield stress with a local maximum around pH 7-9, which falls between the two PZC values of the edge and basal planes of the rod particles. On the basis of the surface potentials of the edge and basal planes obtained from AFM measurements, theoretical analysis of the surface interactions of edge-edge, basal-edge, and basal-basal planes of the chrysotile rods suggests the yield stress maximum observed could be mainly attributed to the basal-edge attractions. Our results indicate that the anisotropic surface properties (e.g., charges) of chrysotile rods play an important role in the particle-particle interaction and rheological behavior, which also provides insight into the basic understanding of the colloidal interactions and rheology of nonspherical particles.

In plain English

This was a laboratory study in surface chemistry. It did not study people, health effects or disease. The authors, at the University of Alberta, used natural chrysotile from a mineral supplier as a model rod-shaped particle. They explained that chrysotile in nickel ores thickens mineral slurries and hinders nickel recovery. With an atomic force microscope, they measured forces between a probe tip and the two surfaces of chrysotile rods (the outer "basal" surface and the cut "edge") in a sodium chloride solution at pH 4, 6, 8 and 11. From these forces, using a theoretical model, they estimated each surface's electrical potential. The basal surface's potential changed from positive to negative at roughly pH 10 to 11, and the edge's at about pH 6 to 7. They also measured the yield stress of a 7% by volume suspension of ground chrysotile rods. This is the shear stress needed to make the suspension begin to flow. It was highest around pH 7 to 9. The authors proposed that attraction between oppositely charged basal and edge surfaces explains this peak. They concluded that adjusting pH can control how such particle suspensions behave.

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: atomic force microscopy force measurements and rheology (flow) measurements, combined with theoretical calculation of interaction energies
• Who was studied: No human or animal subjects. A natural chrysotile sample from Minerals Unlimited (Ridgecrest, CA), mostly chrysotile with minor calcite. For the flow tests the chrysotile was ground into rods averaging 825 nm long and 57 nm across, an aspect ratio of 14.5.
• Exposure: Not a health exposure study. Chrysotile surfaces and a 7 vol% (16.6 wt%) chrysotile suspension were tested in 10 mM NaCl at different pH values.
• Main results:
  • The point of zero charge of the chrysotile brucite-like basal plane was roughly between pH 10 and 11.
  • The fitted surface potentials of the chrysotile edge surface changed from positive to negative at about pH 6 to 7.
  • At pH 4 and 6, the fitted edge surface potentials were 36 mV and 5 mV respectively. At pH 11, the edge surface potential was -42 mV and the basal plane was -14 mV.
  • The theoretical force fittings agreed with the measured force curves with R2 ranging from 0.91 to 0.93.
  • In calculated interaction energies at pH 8, basal-edge interactions between chrysotile surfaces were attractive, while edge-edge and basal-basal interactions were repulsive at short range.
  • The shear yield stress of the 7 vol% chrysotile suspension depended on pH, with a local maximum around pH 7 to 9.
  • XRF analysis gave an Mg:Si atomic ratio of 1.25 for the sample, compared with an ideal value of 1.5.
• The authors' conclusion: The authors concluded that the surface interactions and flow behavior of suspensions of non-spherical, unevenly charged particles such as chrysotile rods can be manipulated by tuning pH. They proposed that a network held together by basal-edge attraction is likely responsible for the maximum yield stress at pH 7 to 9.
• Limitations the authors state:
  • At very short distances (under 2 to 3 nm), the fitted curves did not match the measurements. The authors attributed this possibly to other interactions such as hydration forces.
  • Some selective dissolution of chrysotile might occur at pH 11. The authors stated this would not affect the edge point of zero charge.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Chrysotile.

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

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