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

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_par and perpendicular ΔΠ_perp to the barriers in a Langmuir trough.

Applying area oscillations, A₀+ ΔAe^iωt, in a rectangular Langmuir trough induces changes in surface pressure, ΔΠ_par and ΔΠ_perp for monolayers of soluble palmitoyl-lysophosphatidylcholine (LysoPC), insoluble dipalmitoylphosphatidylcholine (DPPC), and the protein β-lactoglobulin to evaluate E^s∗+G^s∗=A₀ΔΠ_parΔA and E^s∗-G^s∗=A₀ΔΠ_perpΔA. G^s∗ was independently measured with a double-wall ring apparatus (DWR) and E^s∗ by area oscillations of hemispherical bubbles in a capillary pressure microtensiometer (CPM) and the results were compared to the trough measurements.

For LysoPC and DPPC, A₀ΔΠ_parΔA≅A₀ΔΠ_perpΔA meaning E^s∗≫G^s∗ and E^s∗≅A₀ΔΠ_parΔA≅A₀ΔΠ_perpΔA. Trough values for E^s∗ were quantitatively similar to CPM when corrected for interfacial curvature. DWR showed G^∗ was 4 orders of magnitude smaller than E^s∗ for both LysoPC and DPPC. For β-lactoglobulin films, A₀ΔΠ_parΔA>A₀ΔΠ_perpΔA and E^s∗ and G^s∗ were in qualitative agreement with independent CPM and DWR measurements. For β-lactoglobulin, both E^s∗ and G^s∗ varied with film age and history on the trough, suggesting the evolution of the protein structure.