Thermodynamic and Structural Properties of Phospholipid Langmuir Monolayers on Hydrosol Surfaces

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Abstract
Measurements of Langmuir pressure/area isotherms, rheology, grazing incidence X-ray diffraction (GIXD), and grazing incidence diffuse X-ray scattering out of the specular plane (GIXOS) have been used to investigate the influence of a hydrosol containing charged mineral nanoparticles on the thermodynamic and structural properties of a DPPC monolayer. The mineral adsorption layer that is formed via electrostatic interaction underneath the lipid layer alters the thermodynamic properties of the phospholipid monolayer in terms of maximal achievable compression, compressibility, and phase behavior. Modifications appear in the latter case as a coolinglike effect. Rheology measurements of the bulk viscoelastic properties revealed a stabilizing effect of the transient bulk network on the surfactant layer. The lipid chain lattice is found to be reorganized and adapted to the internal atomic structure of the mineral particles. A model for the superposition of Bragg rods from the lipid chains and the minerals is applied to separate these scattering contributions. In the vicinity of the mineral particles, the (21̄) reflection for DPPC on a liquid substrate was found, indicating strongly suppressed fluctuations at the surface. An estimation of the Debye−Waller factor associated with the lipid layer organization is used to quantify the damping of fluctuations within the lipid matrix due to the rigidifying and stabilizing effect of the mineral particles.

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