Colloidal Forces from Molecular Simulations: Effect of Colloid Size, Interparticle Distance and Salinity
Room E006
Abstract
Colloidal interactions within saturated porous media have attracted increasing attention in recent decades due to the remarkable ability of colloids to capture and transfer non-aqueous phases. This capability allows them to control the physico-chemical characteristics of the solution and affect the poromechanical properties of the media. Understanding and pre-dicting colloidal behaviour within geological formations requires first studying inter-particle couplings at the nanometer level. The underlying mechanisms of colloidal interactions have been well described using the Derjaguin-Landau-Vervey-Overbeek (DLVO) theory, accounting for Van der Waals forces and osmotic repulsion across a broad range of spatio-temporal scales (1,2). Nevertheless, this approach has proven to be inadequate at interparticle separations below 3 nm, since phenomena such as electrostatic attraction in electrical double layers, contact and hydration repulsion dominate hydrodynamic interactions (3). While the nature of colloidal couplings at such ultra-small scales plays a key role in geological processes, such as enhanced oil recovery and groundwater remediation, it remains poorly investigated.
In this work, an umbrella sampling molecular dynamics technique is applied to characterize the potential of mean force landscapes of a pair of spherical silica nanoparticles immersed in various aqueous solutions with a particular focus on sub – 3 nm interparticle distances. Initially, we show the substantial impact of ultra-thin fluid layers on PMF fluctuations, highlighting the significance of non-DLVO forces at these minimal separations. Notably, our findings reveal that identical silica colloids exhibit an attraction at their minimum separation distance. This attraction intensifies with increased salinity, challenging the conventional screening effect expected at higher ionic strengths. Furthermore, we suppose that larger colloids may manifest a stronger interparticle attraction, thereby complexifying the investigation of colloidal interaction dynamics.
These findings point to the limitations of DLVO theory in accurately capturing colloidal interactions at close proximities and under variable salinity conditions. They underscore the need for further investigation to advance the understanding of colloidal phenomena within porous structures.