Influence of Subphase Ionic Composition on the Interfacial Morphology of Nanoclay Films Under 2D confinement

Laponite, a disc-shaped clay with uneven charge distribution, exhibits gelation behavior strongly influenced by the ionic strength and pH of the suspension. These parameters govern the charge on particle edges and faces, thereby modulating electrostatic interactions and colloidal stability. This study investigates the adsorption of nanoclay particles onto a positively charged 1,2-dimyristoyl-3-trimethylammonium-propane (DMTAP) lipid monolayer at the air–water interface under varying subphase ionic conditions. Surface pressure–area isotherms of nanoclay-lipid films on water, saline, and basic subphases were measured using the Langmuir technique. The basic subphase (pH 10) promotes enhanced adsorption via strong electrostatic attraction, while saline (1 mM NaCl; pH ≈ 7) yields moderate adsorption due to partial charge screening, and water (pH ≈ 7) shows minimal interaction. Atomic Force Microscopy (AFM) micrographs reveal increasing nanoclay density with concentration and distinct morphological transitions governed by subphase composition. Water-based films appear uniform, whereas saline and basic subphases produce heterogeneous structures, most pronounced under basic conditions. Out-of-plane X-Ray Reflectometry (XRR) indicates enhanced surface roughness in basic films, corroborating lateral aggregation observed in AFM. These findings highlight the critical role of subphase composition in modulating interfacial properties and clustering behavior in lipid-nanoclay systems under 2D confinements.