Dilatational and shear rheology of soluble and insoluble monolayers with a Langmuir trough.

HYPOTHESIS

The surface dilatational and shear moduli of surfactant and protein interfacial layers can be derived from surface pressures measured with a Wilhelmy plate parallel, ΔΠ and perpendicular ΔΠ to the barriers in a Langmuir trough.

EXPERIMENTAL

Applying area oscillations, A+ ΔAe, in a rectangular Langmuir trough induces changes in surface pressure, ΔΠ and ΔΠ for monolayers of soluble palmitoyl-lysophosphatidylcholine (LysoPC), insoluble dipalmitoylphosphatidylcholine (DPPC), and the protein β-lactoglobulin to evaluate E+G=AΔΠΔA and E-G=AΔΠΔA. G was independently measured with a double-wall ring apparatus (DWR) and E by area oscillations of hemispherical bubbles in a capillary pressure microtensiometer (CPM) and the results were compared to the trough measurements.

FINDINGS

For LysoPC and DPPC, AΔΠΔA≅AΔΠΔA meaning E≫G and E≅AΔΠΔA≅AΔΠΔA. Trough values for E were quantitatively similar to CPM when corrected for interfacial curvature. DWR showed G was 4 orders of magnitude smaller than E for both LysoPC and DPPC. For β-lactoglobulin films, AΔΠΔA>AΔΠΔA and E and G were in qualitative agreement with independent CPM and DWR measurements. For β-lactoglobulin, both E and G varied with film age and history on the trough, suggesting the evolution of the protein structure.