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

    Year: 2023

    Journal: Journal of Colloid and Interface Science, Volume 629, 01/01/2023

    Authors: Ciutara, Clara O.; Barman, Sourav; Iasella, Steven; Huang, Boxun; Zasadzinski, Joseph A.

    Keywords: β-lactoglobulin; Interfacial rheology; Dilatational modulus; DPPC; Interfacial dilatational rheology; Interfacial shear rheology; Lysolipids; Phospholipids; Soluble surfactant

    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, ΔΠpar and perpendicular ΔΠperp to the barriers in a Langmuir trough. Experimental Applying area oscillations, A0+ ΔAeiωt, in a rectangular Langmuir trough induces changes in surface pressure, ΔΠpar and ΔΠperp for monolayers of soluble palmitoyl-lysophosphatidylcholine (LysoPC), insoluble dipalmitoylphosphatidylcholine (DPPC), and the protein β-lactoglobulin to evaluate Es∗+Gs∗=A0ΔΠparΔA and Es∗-Gs∗=A0ΔΠperpΔA. Gs∗ was independently measured with a double-wall ring apparatus (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, A0ΔΠparΔA≅ A0ΔΠperpΔA meaning Es∗≫Gs∗ and Es∗≅A0ΔΠparΔA≅ A0ΔΠperpΔA. 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 β-lactoglobulin films, A0ΔΠparΔA> A0ΔΠperpΔA and Es∗ and Gs∗ were in qualitative agreement with independent CPM and DWR measurements. For β-lactoglobulin, both Es∗ and Gs∗ varied with film age and history on the trough, suggesting the evolution of the protein structure.
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