Abstract
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(0)ΔΠ(par)ΔA and E(s∗)-G(s∗)=A(0)ΔΠ(perp)ΔA. 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(0)ΔΠ(par)ΔA≅A(0)ΔΠ(perp)ΔA meaning E(s∗)≫G(s∗) and E(s∗)≅A(0)ΔΠ(par)ΔA≅A(0)ΔΠ(perp)ΔA. 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(0)ΔΠ(par)ΔA>A(0)ΔΠ(perp)ΔA 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.