This study utilizes a Surface Force Apparatus (SFA) to directly probe the interactions and structuring of model geocolloids (monodisperse silica nanoparticles, SNPs) confined between symmetric mica surfaces under varying salinity (KCl) and particle concentration. Complementary interfacial rheological measurements assess the impact of confinement and interparticle forces on dispersion dynamics. In electrolyte-free water, long-range electrostatic double-layer repulsion, stemming from dissociated silanol groups and their associated counterion clouds, dominates, leading to particle exclusion from the narrowing gap, consistent with classical DLVO theory predictions. Increasing KCl concentration screens surface charges, reducing the Debye length (κ-1) and allowing short-range van der Waals attraction to induce aggregation and entrapment within the confined film, evidenced by stepwise increases in the measured hard-wall separation distance possibly reflecting layers of aggregated particles stabilized by residual hydration forces. Interfacial rheology reveals a confinement-induced transition from Newtonian fluid behavior to shear-thinning with a measurable yield stress, indicative of structured network formation within the confined dispersion due to increased particle density and interparticle interactions. To mimic contaminant-modified geocolloids, SNPs were functionalized with chlorotrimethylsilane (CTMS) to create Janus nanoparticles (JNPs) with varying hydrophilic-lipophilic balance (HLB). JNP interactions are dictated by the anisotropic distribution of polar (silanol) and non-polar (alkylsilane) surface chemistries, leading to competing intermolecular forces: electrostatic repulsion (from silica regions), van der Waals attraction (universal), short-range hydration forces (near hydrophilic patches), and hydrophobic interactions (between alkylated regions). SFA measurements show intriguing JNP aggregation behavior compared to SNPs, influenced by HLB and salinity, reflecting the modified balance of forces and potentially patchy or anisotropic interparticle attraction. These structural changes manifest as significantly altered rheological signatures.
This work demonstrates how confinement, electrolyte (counterion effects), particle concentration, and surface chemistry modifications (introducing hydrophobic interactions) synergistically control the governing intermolecular forces, aggregation state, confined structure, and resultant rheological dynamics of geocolloid dispersions, offering critical insights into predicting contaminant fate and transport in nanoconfined geological environments.