Adsorption of 尾-Casein鈥揝urfactant Mixed Layers at the Air鈥揥ater Interface Evaluated by Interfacial Rheology

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• 作者：Armando Maestro ; Csaba Kotsmar ; Aliyar Javadi ; Reinhard Miller ; Francisco Ortega ; Ram贸n G. Rubio
• 刊名：The Journal of Physical Chemistry B
• 出版年：2012
• 出版时间：April 26, 2012
• 年：2012
• 卷：116
• 期：16
• 页码：4898-4907
• 全文大小：511K
• 年卷期：v.116,no.16(April 26, 2012)
• ISSN：1520-5207

文摘

This work presents a detailed study of the dilational viscoelastic moduli of the adsorption layers of the milk protein 尾-casein (BCS) and a surfactant at the liquid/air interface, over a broad frequency range. Two complementary techniques have been used: a drop profile tensiometry technique and an excited capillary wave method, ECW. Two different surfactants were studied: the nonionic dodecyldimethylphosphine oxide (C₁₂DMPO) and the cationic dodecyltrimethylammonium bromide (DoTAB). The interfacial dilational elasticity and viscosity are very sensitive to the composition of protein鈥搒urfactant mixed adsorption layers at the air/water interface. Two different dynamic processes have been observed for the two systems studied, whose characteristic frequencies are close to 0.01 and 100 Hz. In both systems, the surface elasticity was found to show a maximum when plotted versus the surfactant concentration. However, at frequencies above 50 Hz the surface elasticity of BCS + C₁₂DMPO is higher than the one of the aqueous BCS solution over most of the surfactant concentration range, whereas for the BCS + DoTAB it is smaller for high surfactant concentrations and higher at low concentrations. The BCS鈥搒urfactant interaction modifies the BCS random coil structure via electrostatic and/or hydrophobic interactions, leading to a competitive adsorption of the BCS鈥搒urfactant complexes with the free, unbound surfactant molecules. Increasing the surfactant concentration decreases the adsorbed proteins. However, the BCS molecules are rather strongly bound to the interface due to their large adsorption energy. The results have been fitted to the model proposed by C. Kotsmar et al. ( J. Phys. Chem. B 2009, 113, 103). Even though the model describes well the concentration dependence of the limiting elasticity, it does not properly describe its frequency dependence.