Spherical LDH-Ag°-montmorillonite heterocoagulated system with a pH-dependent sol-gel structure for controlled accessibility of AgNPs immobilized on the clay lamellae.

Abstract

Aqueous suspensions of spherical ZnMgAl-layered double hydroxides [LDH(sph)] and antibacterial silver nanoparticles (AgNPs) deposited on the lamellae of montmorillonite were used for the synthesis of composites, which behave like coherent gels at low pH (≲4.5) and incoherent sols at higher pH (≳4.5). The composition of the composite was chosen as LDH(sph)/Ag°-montm. = 25:75 wt % in order to ensure a sol-gel transition that can also be characterized by viscometry. This pH-sensitive heterocoagulated system consisting of oppositely charged colloid particles was suitable for the release of antimicrobial AgNPs immobilized on the clay lamellae via a pH-controlled gel-sol transition. The heterocoagulation process was also characterized by surface charge titration measurements. Spherical LDH/Ag°-montmorillonite composite samples were identified by X-ray diffraction (XRD) measurements. The morphological properties of the composites were studied, and the presence of the heterocoagulated structure was confirmed by scanning electron microscopy (SEM). The nanoscale structure of the LDH(sph)-Ag°-montmorillonite composite obtained was also verified by small-angle X-ray scattering (SAXS), and the rheological characteristics were studied at various pH values. The viscosity and yield value of the composite decreased by an order of magnitude upon increasing the pH from 3.0 to 5.5. The sol-gel transition of the composite suspension was reversible in the previously mentioned pH range.