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 par and perpendicular ΔΠ perp to the barriers in a Langmuir trough.

EXPERIMENTAL: Applying area oscillations, A 0+ 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∗=A 0ΔΠ parΔA and E s∗-G s∗=A 0ΔΠ 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.

FINDINGS: For LysoPC and DPPC, A 0ΔΠ parΔA≅A 0ΔΠ perpΔA meaning E s∗≫G s∗ and E s∗≅A 0ΔΠ parΔA≅A 0ΔΠ 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, A 0ΔΠ parΔA>A 0ΔΠ 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.