Structural patterns of core-interlayer-shell (CIS) nanoparticles in Langmuir monolayers formed at the air-water interface
10-04-2018 12:58
Polymeric hard core/interlayer/soft shell (CIS) nanoparticles are spread at the air-water interface to form Langmuir monolayers. The isotherms of these spread nanoparticles are obtained using a Wilhelmy plate (WP) by monitoring the induced surface pressure (Π) as a function of the allowed area per particle (A) during compression and expansion of a monolayer. On the water surface, the morphology of the monolayer is characterized by means of Brewster angle microscope. Morphologies of monolayers transferred onto Si substrates are characterized by both bright field optical microscopy and atomic force microscopy. CIS nanoparticles show different patterns of looser and closed-packed regimes depending on their mutual interactions, at the air-water interface, at different allowed interdistances during compression and expansion of the monolayer. Our results can be explained based on the “dipolar character” model. This model predicts the presence of early long-range, compared with van der Waals, capillary attractive force between like charged floating nanospheres at fluid-fluid interfaces. This capillary force is raised from the distortion of the interface by the presence of the spread particles and it is laterally working against the electrostatic repulsive force. Upon compression, CIS nanoparticles are assembled anisotropically on the water surface as the entropy of the soft shells is balanced by the gained energy by the approaching hard cores. Fully closed-packed clusters of hexagonally ordered CIS nanoparticles are observed when the monolayer is fully compressed. With expansion, the packed CIS nanoparticles are reversibly aggregated due to the entropy of distorted soft chains while the long-range electrostatic repulsion helps in redistributing these nanoparticles on the whole water surface.