The effect of pH on surface dilatational and shear properties of phospholipid monolayers

We have studied the surface dilatational (surface dilatational modulus, E, its elastic, Ed, and viscous, Ev, components, and loss angle tangent, tan θ) and surface shear (elastic, G′, and viscous components, η) characteristics of dipalmitoyl phosphatidylcholine (DPPC) and dioleoyl phosphatidylcholine (DOPC) monolayers, at 20 °C and at pH 5, 7, and 9. The stress response to compression–expansion sinusoidal deformation of the interface was measured in a modified Wilhelmy-type trough with two oscillating barriers and in a ring trough (with an isotropic area deformation), as a function of deformation amplitude and frequency. Measurements of surface shear rheology were carried out by using an automatic Du Noüy ring oscillatory apparatus. The monolayer structure and, especially, the conditions at which the monolayer collapses, determine the viscoelastic behaviour under dilatational and shear deformations of the monolayer and the linear response of the stress to area deformation. The non-linear viscoelastic behaviour of the interface has been associated with the phospholipid monolayer collapse. It was found that the dilatational modulus is not only determined by the interactions between spread molecules (which depend on the surface pressure or surface density), but that the structure of the spread monolayer also plays an important role. Surface shear rheology is more sensible to the effect of pH on surface rheological parameters of phospholipid monolayers than surface dilatational rheology. The surface shear characteristics for DPPC and DOPC monolayers were higher at pH 5, where attractive van der Waals interactions between phospholipid molecules take place, and were lower at pH 9, where electrostatic repulsions between phospholipid molecules weakened the monolayer structure. The surface dilatational and shear moduli were also higher for DPPC than for DOPC at every pH.