(564e) Dispersing Nano- and Micro-Sized Portlandite Particulates Via Electrosteric Exclusion at Short Screening Lengths
AIChE Annual Meeting
2020
2020 Virtual AIChE Annual Meeting
Particle Technology Forum
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Friday, November 20, 2020 - 9:00am to 9:15am
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 particleâs isoelectric point (IEP). We show that the high native ionic strength (pH â 12.6, IEP: pH â 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 suspensionâs 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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