(564e) Dispersing Nano- and Micro-Sized Portlandite Particulates Via Electrosteric Exclusion at Short Screening Lengths | AIChE

(564e) Dispersing Nano- and Micro-Sized Portlandite Particulates Via Electrosteric Exclusion at Short Screening Lengths


Bhagavathi Kandy, S. - Presenter, University of California-Los Angeles
Timmons, J., University of California Los Angeles
Mehdipour, I., Laboratory for the Chemistry of Construction Materials (LC2), UCLA
Atahan, H., Laboratory for the Chemistry of Construction Materials
Neithalath, N., Arizona State University
Bauchy, M., Laboratory for the Physics of Amorphous and Inorganic Solids
Garboczi, E., Applied Chemicals and Materials Division, Material Measurement Laboratory, National Institute of Standards and Technology
Sant, G., University of California, Los Angeles
In spite of their high surface charge (zeta potential ζ = + 34 mV), aqueous suspensions of portlandite (calcium hydroxide: Ca(OH)2) exhibit a strong tendency to aggregate, and thereby present unstable suspensions. While a variety of commercial dispersants seek to modify the suspension stability and rheology (e.g., yield stress, viscosity), it remains unclear how the performance of electrostatically and/or electrosterically based additives is affected in aqueous environments having either a high ionic strength and/or a pH close to the particle’s isoelectric point (IEP). We show that the high native ionic strength (pH ≈ 12.6, IEP: pH ≈ 13) of saturated portlandite suspensions strongly screens electrostatic forces (Debye length: κ-1= 1.2 nm). As a result, Coulombic repulsion alone is insufficient to mitigate particle aggregation and affect rheology. However, a longer-range geometrical particle-particle exclusion that arises from electrosteric hindrance caused by the introduction of comb polyelectrolyte dispersants is very effective at altering the rheological properties and fractal structuring of suspensions. As a result, comb-like dispersants that stretch into the solvent reduce the suspension’s yield stress by 5x at similar levels of adsorption as compared to linear dispersants, thus enhancing the critical solid loading (i.e., at which jamming occurs) by 1.4x. Significantly, the behavior of diverse dispersants is found to be inherently related to the thickness of the adsorbed polymer layer on particle surfaces. These outcomes inform the design of dispersants for concentrated suspensions that present strong charge screening behavior.


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