Structural morphology of nanoclay films at the air–water interface under varying ionic compositions of the subphase medium

Abstract

LAPONITE®, a disc-shaped clay with uneven charge distribution, demonstrates gelation behavior that is strongly influenced by the ionic strength and pH of the suspension. These factors affect the charge on the edges and faces of the clay particles, thereby impacting electrostatic interactions and colloidal stability. In this study we investigate the adsorption behavior of the nanoclay particles on the positively charged lipid layer at the air–water interface under varying subphase ionic compositions. The Π–A compression isotherms of the nanoclay layer adsorbed on the lipid monolayer, on various subphases (water, saline, and basic), were obtained using the Langmuir method. Compression isotherms reveal that the basic subphase (pH 10) enhances nanoclay adsorption through strong electrostatic interactions between negatively charged LAPONITE® particles and the positively charged DMTAP monolayer. Films from the saline subphase (1 mM NaCl; pH ∼ 7) show moderate adsorption due to salt-induced charge neutralization while the water subphase (pH ∼ 7) exhibits minimal particle–surface interaction. The in-plane surface morphology and out-of-plane structural characteristics of nanoclay films were characterized ex situ by Atomic Force Microscopy (AFM) and X-Ray Reflectivity (XRR) respectively. AFM analysis reveals increasing nanoclay density with concentration across all subphases, with distinct morphological transitions influenced by subphase conditions. Films formed on pure water remain relatively uniform, while saline and basic subphases yield increasingly heterogeneous structures at higher nanoclay concentrations, with maximum heterogeneity observed in films from basic subphase. XRR unravels increased thickness and roughness in basic solution-based films, validating lateral aggregation observed in AFM micrographs. These findings highlight the role of subphase composition in modulating interfacial properties and clustering behavior in lipid–nanoclay systems.