DPPC-DOPC Langmuir monolayers modified by hydrophilic silica nanoparticles: Phase behaviour, structure and rheology

Langmuir monolayers of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and mixtures of DOPC with 1,2-dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC), at a ratio of 37:63 in weight, spread on pure water and on silica nanoparticle dispersions, have been investigated using a combination of thermodynamic, surface rheology, BAM and AFM diagnostics. The compression surface pressure isotherms were determined in a Langmuir trough as well as the surface pressure response to harmonic area variation of the monolayer. Composite layers were obtained at selected thermodynamic states by transfer from the fluid interface to solid substrates and then analysed by AFM diagnostics. Aim of this study was to evaluate the effect of the incorporation of silica nanoparticles on the phase behaviour and structural properties of these monolayers. In fact, as shown in previous works on similar lipid systems, the hydrophilic silica nanoparticles dispersed in the sub-phase are transferred into the monolayer due to the interaction with lipid molecules which makes them partially hydrophobic. The results here obtained indicate that the appreciable influence of silica nanoparticles, previously observed for DPPC alone, is also important for DOPC and DOPC-DPPC mixture. Moreover, as confirmed by the AFM results on the deposited layers, these effects are mainly due to the disruption of the molecular packing and to the modification of the miscibility between the two lipid components.