Dynamic adsorption, surface tension, and direct probing of surfactants, lipids, and proteins at the air -water interface
Abstract
The adsorption and tension behavior of several aqueous solutions and dispersions of surfactants, lipids, and proteins were studied for understanding the key mechanism of producing low tensions and for their relevance in lung surfactant replacement drug formulations. Sodium myristate was studied because it is the first known low-surface tension system which is a single-phase solution. The protonation of sodium myristate to form myristic acid and acid-soaps affects strongly the solution, phase, and tension behavior. With no added NaOH, the minimum equilibrium tension is 22 mN/m. At pulsating area, the dynamic tension at minimum area can be well below 10 mN/m and as low as 1 mN/m for concentrations ranging from 2 to 6 mM. The monolayer for the low-tension solutions was probed directly with infrared reflection absorption spectroscopy (IRRAS) and shown to be a mixture of myristate and myristic acid at nearly close-packed density. The adsorption and tension behavior of aqueous dipalmitoylphosphatidylcholine (DPPC) dispersions, bovine serum albumin (BSA) solutions, and mixtures of them were studied. DPPC is the main component of lung surfactant, and serum proteins are known to be capable of inhibiting the function of lung surfactant. The tension behavior of DPPC depends strongly on the preparation protocols. Sonication of DPPC above the main gelto-liquid-crystal phase transition temperature to break the large liposomes into smaller vesicles can greatly improve the adsorption rate and tension-reduction ability of DPPC dispersions. Whereas surface tension and ellipsometry are only affected by the monolayer, IRRAS results establish for the first time the hypothesis that DPPC migrates to the surface in particulate form, and the particulates remain attached to the monolayer. The extra material in the surface-associated reservoir influences the dynamic tension during area oscillation. In the presence of BSA, the mechanism of DPPC surface film formation is blocked, since the protein adsorbs first and remains on the surface controlling the surface tension. However, when DPPC is introduced via a spread monolayer mechanism, DPPC expels partly or completely the adsorbed BSA monolayer and then controls the tension. The results have important implications in understanding the function of lipids and proteins and for designing effective formulations.
Degree
Ph.D.
Advisors
Franses, Purdue University.
Subject Area
Chemical engineering|Pharmaceuticals|Biomedical research
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