(141e) Surfactant-Nanoparticle Interactions: Order of Addition Matters

Interactions of nanoparticles and Langmuir monolayers of surfactants are important for several fields including food, cosmetics and pharmaceutical sciences. Several reports have shown that nanoparticles can disrupt the function of surfactant monolayers. However, results have been inconsistent and the underlying mechanisms are not fully understood. Considering the variability of results, it is of great importance to understand how the differences in methods are affecting the mechanisms of interaction. The present study focuses on interaction protocols and their effects on the mechanisms of interaction between carboxyl modified polystyrene nanoparticles (200 nm) and Langmuir monolayers of dipalmitoylphosphatidylcholine (DPPC).

Two interaction protocols were used to study nanoparticle effects on DPPC function: addition of nanoparticles after spreading the DPPC monolayer (subphase injection) and spreading DPPC on a subphase that already contains nanoparticles (DPPC addition). Surfactant function was probed by measuring the dynamic surface tension of the surfactant films during compression and expansion cycles using a computer-controlled Langmuir trough equipped with a Wilhelmy plate balance. A surface potentiometer was used to measure surface potential and provide information on surfactant packing density. Also, surfactant microstructure and topology were studied using fluorescence and atomic force microscopy.

The two interaction protocols resulted in significantly different surface pressure isotherms. Subphase injection resulted in higher surface pressures in the liquid expanded-liquid condensed coexistence phase but did not affect the maximum surface pressure. Subphase injection of nanoparticles reduced the size of the condensed surfactant phases without nanoparticle penetration to the air-water interface suggesting that electrostatic screening of surfactant dipoles by negatively charged nanoparticles is the mechanism of effect. On the contrary, DPPC addition resulted in premature monolayer collapse and lower maximum surface pressure in the following cycles. DPPC addition protocol did not show a significant electrostatic screening effect but resulted in an early rise in surfactant dipoles suggesting that space competition between nanoparticles and DPPC molecules has resulted in a more compact packing of DPPC molecules. These results show that interaction protocols greatly affect the mechanisms of interaction between nanoparticles and surfactant and are important in understanding mechanisms proposed in the literature.