Effect of nanoconfinement and pore geometry on point of zero charge in synthesized mesoporous siliceous materials
Summary¶
Point of zero charge (PZC) and surface acidity control adsorption, film stability, and electrostatic corrections in porous silica–water systems, yet published PZC values for nominally similar materials scatter widely in the literature. This JCIS Open article combines potentiometric titrations on synthetic mesoporous silicas (SBA-15, SBA-16, MCM-41) with ReaxFF molecular dynamics models of amorphous silica to ask whether nanoconfinement or pore geometry (cylindrical versus cage-like motifs) systematically shifts PZC once experimental protocols are controlled. Pore sizes around 4–13 nm bracket common mesopore regimes relevant to catalysis and subsurface shale-like confinement narratives. Electrolytes were pre-saturated with silica to reduce bulk dissolution artifacts during measurement. The study also contextualizes results against a broad literature compilation (>150 studies) to interpret scatter versus true physical effects.
Methods¶
Experimentally, the authors perform potentiometric titrations and extract PZC and pK descriptors for each archetype under silica-saturated fluid conditions, comparing across pore geometry and diameter as described in the article. Computationally, ReaxFF simulations estimate surface equilibrium constants / charging response for amorphous silica models, linking molecular-scale deprotonation/protonation behavior to macroscopic PZC trends. A meta-analysis aggregates published PZC measurements for porous and nonporous silicas to separate protocol-induced variability from material trends. Together, the workflow aims to couple reactive interface MD with careful wet chemistry on well-characterized powders.
ReaxFF MD (simulation block in the article): Engine: LAMMPS ReaxFF; amorphous silica slabs with explicit water/hydronium/hydroxide in 3D PBC; NVT-class integration with thermostat and ~fs timestep; ps–ns trajectories to sample proton transfer and surface pK-related populations; room-temperature (300 K) or the T in JCIS Open; barostat N/A if constant volume; hydrostatic pressure N/A; external electric field N/A; umbrella/replica exchange N/A; cutoffs and QEq-style electrostatics per Methods.
Findings¶
ReaxFF is used to rationalize the wide spread of PZC values reported for macroporous/nonporous silicas when laboratories differ in ionic strength, pretreatment, and equilibration history. For the mesoporous materials studied (SBA-15, SBA-16, MCM-41), the combined dataset does not resolve a PZC shift attributable to nano-confinement or pore geometry within the experimental and modeling resolution reported. The practical implication stated in the framing is that surface charging models in mesopores may not require an ad hoc confinement correction to PZC for these synthetic silicas—while still cautioning that natural geomaterials can differ due to impurities, clays, and roughness not captured in idealized powders. The study’s design—controlled synthesis plus silica-saturated electrolytes—aims to reduce artifacts that historically inflate PZC scatter in the literature, making the “no shift” conclusion a statement about well-controlled mesopores rather than all nanoporous media. Together, the experimental sweep across SBA-15, SBA-16, and MCM-41 and the ReaxFF interpretation of amorphous silica acidity aim to separate pore shape effects from surface chemistry effects that can masquerade as confinement in poorly controlled datasets. For geoscience readers, the manuscript’s caution is explicit: mesoporous synthetic silicas are not shale microstructures, but if PZC is stable across these well-defined pores, then confinement-first explanations for PZC shifts elsewhere merit extra scrutiny.
Limitations¶
Synthesis heterogeneity (defects, hydration layers, residual surfactant chemistry) can mask small shifts; natural shales and reservoir rocks require case-specific validation.
Relevance to group¶
Joint ReaxFF + experiment study with van Duin authorship on silica–water geochemical interfaces.