Dispersing nanosized portlandite particulates via electrosteric exclusion at short screening lengths

Published

March 16, 2020

Author(s)

Jason Timmons, Iman Mehdipour, Shang Gao, Hakan Atahan, Narayanan Neithalath, Mathieu Bauchy, Edward Garboczi, Samanvaya Srivastava, Gaurav Sant

Abstract

In spite of their high surface charge (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 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, 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 suspension. As a result, comb- like dispersants that stretch into the solvent reduce the suspension's yield stress by nearly 5x at similar levels of adsorption as compared to linear dispersants, thus enhancing critical solid loading (i.e., at which jamming occurs) by 1.4x. Significantly, the behavior of the different dispersants studied 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 have strong charge screening behavior.

Citation

Soft Matter

Volume

16

Pub Type

Journals

Keywords

suspension rheology, aggregation, polymer adsorption, polyelectrolyte dispersant

Citation

Timmons, J. , Mehdipour, I. , Gao, S. , Atahan, H. , Neithalath, N. , Bauchy, M. , Garboczi, E. , Srivastava, S. and Sant, G. (2020), Dispersing nanosized portlandite particulates via electrosteric exclusion at short screening lengths, Soft Matter (Accessed December 2, 2024)