Molecular conformation and dynamics of amphiphiles monolayers at the air/water interface.

摘要

Surface-active amphiphilic molecules form monolayers at the air/water interface (A/W). Structure and dynamics of cholesterol-mixed DLPC monolayers are studied with a full atomic MD simulation for four different systems; 0%, 10% distributed, 20% distributed, and 20% clustered cholesterol/DLPC monolayer systems. The head group conformation of DLPC does not change with the presence of cholesterol, while the tail groups are extended and organized in contacts with cholesterol. The hydroxyl group of cholesterol has a stronger hydrogen bonding with the phosphate group over any other group of DLPC. Lateral diffusion of DLPC with 20% cholesterol is reduced from that of pure lipid monolayers. Mean lateral diffusion coefficient of cholesterol is greater than that of DLPC. Abnormality of the mean square displacement at short time lapse is analyzed in terms of fractal and spectral dimensions. The spectral dimension of DLPC monolayer is found as 1.3, while the geometric fractal dimension is 2.2.; Dynamics of an isolated polymer chain are investigated with MD simulation on the 2D solid surface. The dynamics of a polymer chain on the fixed surface can be explained with the Zimm model in 2D. Surprisingly, the Rouse dynamics is recovered for a polymer chain on the energy-dissipating surface, which may be most closely representing the experimental situations.; The surface viscoelasticities of polymers and lipids at A/W are examined using the surface light scattering technique, with pH and temperature as the independent variables. The viscoelastic behavior of a class of polyester seems to be elasticity dominant at any pHs. The surface viscosity of monolayers of a class of alternating copolymer, poly(α-olefin-co-maleic acid), PXcMA, gets enhanced with increasing pH. All the systems (PXcMA, triblockcopolymers, and phospholipids systems) investigated for the thermal effect show that the temperature increment induces a marginal decrease of the surface viscosity.; Molecular organization of poly(dimethyIsiloxane) (PDMS) in spread monolayers at A/W is studied with the aid of the vibrational sum frequency spectroscopy. It is found that methyl groups in PMDS chains are completely disordered in the dilute regime (A). It is proposed that PDMS chains form horizontal folding triple layers than helices in the first collapsed regime (C).