Surface rheology of monolayers of phospholipids and cholesterol measured with axisymmetric drop shape analysis

Abstract

Surface rheology of monolayers of a saturated phospholipid (dipalmitoylphosphatidylcholine, DPPC), an unsaturated phospholipid (dioleoylphosphatidylcholine, DOPC) and cholesterol is studied with axisymmetric drop shape analysis at the argon/water interface. Measurement techniques for lipids are described in detail. Profile analysis tensiometry (PAT) is used to determine the film pressure Π, surface elasticity and surface dilational viscosity of monolayers upon sinusoidal oscillations of the drop surface for various amplitudes a and frequencies f to assess their dependence on these dynamic parameters. It is shown that surface dilational viscosity strongly depends on the frequency and decreases by a factor 2-5 with increasing f in the considered range. Dilational viscosity is higher the more the monolayer approaches a relaxed state. Thus, the molecular interactions are stronger in the relaxed than in the stressed state. Surface elasticity is much less dependent on dynamic conditions. For DPPC a minimum of the dynamic surface elasticity is found for f = 12.5 mHz (at Π = 17.5 mN m −1 ) which coincides well with the relaxation frequencies measured in stress relaxation experiments. The dynamic surface elasticity of DPPC exhibits a plateau in the range 13.5 mN m −1 ≤ Π ≤ 27 mN m −1 in good coincidence with the phase boundaries of the coexistence region of micron-sized liquid crystalline domains surrounded by a fluid monolayer phase. In equilibrium measurements (Π/A-isotherms) a plateau of the film pressure is seen at the lower bound and a break at the upper bound of the coexistence region. Film pressure/area isotherms produced by PAT and a Langmuir film balance closely coincide as is shown in a comparison to literature values. However, the surface elasticities measured dynamically with oscillating surfaces widely deviate from those derived from isotherms in the case of DPPC and cholesterol, whereas for DOPC very good agreement can be found.

Surface Tension Measurements with the Drop Profile Analysis Tensiometry—Consideration of the Surfactant Mass Balance in a Single Drop

“…There are various types of applications in many technological fields, as explained in [2][3][4][5][6][7]. The pioneering work in the field was made by Neumann and his team [8][9][10][11], so this methodology is now one of the routine methods for measuring the dynamic and static surface and interfacial tensions of solutions.…”

Asymmetric or symmetric bilayer formation during oblique drop impact depends on rheological properties of saturated and unsaturated lipid monolayers

“…1A and B show the surface dilational viscosity g and the surface elasticity e, of DPPC and DOPC and their mixtures with 60 mol% cholesterol. The rheological properties were determined with a profile analysis tensiometer (PAT) from Sinterface (Berlin, Germany) [41] as described in [33,34]. The main principle of PAT is to determine the surface tension of a liquid from the shape of a pendent drop.…”

“…Therefore, the rheological properties of the lipid mixtures used for drop impact experiments were studied [33,34] and compared with results from literature [35][36][37][38][39][40]. Rheological measurements show the effect of film pressure and cholesterol content on surface elasticity and dilational viscosity of saturated and unsaturated phospholipid monolayers.…”

The importance of aspect ratio in profile analysis tensiometry

“…During the last 20 years, the experimental setups have become more sophisticated with the use of digital cameras and computers for the determination of interface profiles from the digital images [2,3], and the determination of surface tension by the best fit of the experimental profiles with the numerical solution of the Young-Laplace equation [4][5][6][7][8]. A recent development includes oscillating the drop or bubble volume at low frequency [9] for determining the surface viscoelastic parameters of surfactant adsorption layers [10][11][12][13][14] and cyclic dynamic surface tension [15]. However, there are still a number of hidden pitfalls associated with profile analysis tensiometry.…”