Insight into ions adsorption at the C-S-H gel-aqueous electrolyte interface: From atomic-scale mechanism to macroscopic phenomena

Ions adsorption at the solution-solid interface is ubiquitous, and accordingly, much interest has been generated to pursue an intrinsic understanding of fundamental yet elusive adsorption law. Herein, general mechanisms of the ion adsorption by C-S-H gel were given from the perspective of electrostatic force. By means of molecular dynamics, a thermal-equilibrated electrical double layer (EDL) with the Stern layer resembling parallel-plates capacitor can be naturally produced at the interface. Aqueous Na+ partly dehydrated and penetrated into the channel between silicate chains, while Cl- was electrostatically attracted and yet repulsed by the substrate and adsorbed as a fully hydrated species. Comprehensive interpretation was given in terms of adsorption/desorption dynamics, atomic arrangement, and structure stability, etc. Adsorption behavior of C-S-H was controlled by the electrostatic interaction between charged species, rather than ions/solid's hydration shell. Water molecules just played roles in filling the vacancy of ions/solid's coordinated sites. Simulated and experimental results linked the atomic-scale mechanism to macroscopic phenomena, providing a clear picture of ions adsorption at the interface and also a general interpretation to electric potential results.