Surface relaxation in spread insoluble monolayers of cholesterol and dipalmitoyl lecithin

Joos, P.; Uffelen, M. Van; Serrien, G

doi:10.1016/0021-9797(92)90052-n

Cited by 34 publications

(21 citation statements)

References 15 publications

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“…For DPPC it could be shown that ε P possesses a minimum at f = 12.5 mHz. This coincides well with the relaxation frequencies k measured in stress relaxation experiments by Joos et al [51] with their first relaxation frequency k 1 = 13.0 ± 6.1 mHz and by Wüstneck et al [4] who found 14 mHz ≤ k ≤ 18 mHz. The generalized Maxwell model can be used to interpret this coincidence in the way that around the relaxation frequency the elasticity is minimized.…”

Section: Resultssupporting

confidence: 90%

“…For oscillations with a period t < τ i the respective spring/dashpot pair contributes more to the elastic response, for t > τ i it contributes more to the viscous response. Therefore, we interpret the coincidence of the minimum of ε P with the relaxation frequency reported by Joos et al [51] and Wüstneck et al [4] in the following manner: if harmonic oscillations are produced to measure ε P in the range of the relaxation frequency k 1 , the elastic response is minimized.…”

Section: Discussionmentioning

confidence: 54%

“…Based on stress-relaxation experiments, Joos et al [51] approximated the decay of film pressure in time and its approach to an equilibrium value after a stepwise compression with two independent frequency constants k 1 and k 2 :…”

Section: Discussionmentioning

confidence: 99%

“…According to Joos et al [51] a second relaxation time constant k 2 = 0.46 ± 0.24 mHz has to be taken into account which contributes much stronger to the relaxation process than k 1 owing to β = 0.25 ± 0.09. Unfortunately, we could not find any similar results in the literature reporting about a local minimum of the elasticity of a viscoelastic material from either theoretical or experimental studies.…”

Section: Discussionmentioning

confidence: 99%

“…Coupling of different factors contributing to molecular relaxation could then be the cause for a local increase of the apparent elasticity of the monolayer with decreasing f. However, an extension of the frequency range well below k 2 would be necessary in future experiments to set a sound basis for an interpretation of this effect. For cholesterol, [51] report k 1 = 538 ± 268 mHz, k 2 = 40.7 ± 26.7 mHz with the partition factor β = 0.27 ± 0.30. In our measurements ε C shows a slow monotonic increase of 14% (for the smallest amplitude a = 1%) with increasing f over the range considered (8 mHz ≤ f ≤ 25 mHz).…”

Section: Discussionmentioning

confidence: 99%

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

Vrânceanu

Winkler

Nirschl

et al. 2007

Colloids and Surfaces A: Physicochemical and Engineering Aspect

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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.

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Section: Resultssupporting

confidence: 90%

Section: Discussionmentioning

confidence: 54%

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

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