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, DPpar and perpendicular DPperp to the barriers in a Langmuir trough.Experimental: Applying area oscillations, A0+ DAeixt; in a rectangular Langmuir trough induces changes in surface pressure, DPpar and DPperp for monolayers of soluble palmitoyl-lysophosphatidylcholine (LysoPC), insoluble dipalmitoylphosphatidylcholine (DPPC), and the protein b-lactoglobulin to evaluate Es* + Gs* _ A0DPpar DA and Es* -Gs* _ A0DPperp DA.Gs* was independently measured with a double-wall ring appara-tus (DWR) and Es* 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, A0DPpar DA = A0DPperp DA meaning Es* & drangbrac; Gs* and Es* = A0DPpar DA = A0DPperp DA : Trough values for Es* were quantitatively similar to CPM when corrected for interfacial curvature. DWR showed G* was 4 orders of magnitude smaller than Es* for both LysoPC and DPPC. For b-lactoglobulin films, A0DPpar DA > A0DPperp DA and Es* and Gs* were in qualitative agreement with independent CPM and DWR measurements. For b-lactoglobulin, both Es* and Gs* varied with film age and history on the trough, suggesting the evolution of the protein structure.(c) 2022 Elsevier Inc. All rights reserved.