Water interface, molecular motion

Zimdars D, Dadap J I, Eisenthal K B and Heinz T F 1999 Anisotropic orientational motion of molecular adsorbates at the air-water interface J. Chem. Phys. 103 3425-33... 

When comparing Eqs. 19-1 and 19-3, the reader may remember the discussion in Chapter 18 on the two models of random motion. In fact, these equations have their counterparts in Eqs. 18-6 and 18-4. If the exact nature of the physical processes acting at the bottleneck boundary is not known, the transfer model (Eqs. 18-4 or 19-3) which is characterized by a single parameter, that is, the transfer velocity vb, is the more appropriate (or more honest ) one. In contrast, the model which started from Fick s first law (Eq. 19-1) contains more information since Eq. 19-4 lets us conclude that the ratio of the exchange velocities of two different substances at the same boundary is equal to the ratio of the diffusivities in the bottleneck since both substances encounter the same thickness 5. Obviously, the bottleneck model will serve as one candidate for describing the air-water interface (see Chapter 20). However, it will turn out that observed transfer velocities are usually not proportional to molecular diffusivity. This demonstrates that sometimes the simpler and less ambitious model is more appropriate. 

It is well known that both nanometre and nanosecond-picosecond resolutions at an interface can be achieved by total internal reflection (TIR) fluorescence spectroscopy. Unlike steady-state fluorescence spectroscopy, fluorescence dynamics is highly sensitive to microscopic environments, so that time-resolved TIR fluorometry at water/oil interfaces is worth exploring to obtain a clearer picture of the interfacial phenomena [1]. One of the interesting targets to be studied is the characteristics of dynamic motions of a molecule adsorbed on a water/oil interface. Dynamic molecular motions at a liquid/liquid interface are considered to be influenced by subtle changes in the chemical/physical properties of the interface, particularly in a nanosecond-picosecond time regime. Therefore, time-resolved spectroscopy is expected to be useful to study the nature of a water/oil interface. 

A schematic representation of the boundary layers for momentum, heat and mass near the air—water interface. The velocity of the water and the size of eddies in the water decrease as the air—water interface is approached. The larger eddies have greater velocity, which is indicated here by the length of the arrow in the eddy. Because random molecular motions of momentum, heat and mass are characterized by molecular diffusion coefficients of different magnitude (0.01 cm s for momentum, 0.001 cm s for heat and lO cm s for mass), there are three different distances from the wall where molecular motions become as important as eddy motions for transport. The scales are called the viscous (momentum), thermal (heat) and diffusive (molecular) boundary layers near the interface. 

Molecular Motion of Surfactant Molecules at the Air—Water Interface... 

Block equations can be constructed that describe the macroscopic magnetization of the spin system including the relaxation mechanism involving spin exchange caused by molecular collisions. These equations were solved as a function of the exchange frequency, wex, for the nitroxide free radical case. Spectra were computed by assuming the degrees of motional freedom observed with the labeled androstane or cholestane molecules localized at the air-water interface. 

The effect and interrelationship between primary (segmental backbone) and secondary (side chain) molecular motions on thrombogenesis, independent of morphological order/dis-order, crystallinity, and/or associated water, were elucidated using an amorphous hydrophobic polymer of poly[(trifluoro-ethoxy) (fluoroalkoxy)phosphazene]. The results indicated that for an amorphous hydrophobic polymer, thrombogenesis was sensitive, and depended on the degrees and types of primary and secondary molecular motions at the polymer interface. 

An anisotropy decay more typical of proteins is shown by phospholipase A2. This enzyme catalyzes the hydrolysis of phospholipids and is active mostly when located at a lipid-water interface. Phospholipase Ai has a single tryptophan resichie (trp-3), which serves as the intrinsic probe. The anisotropy decay is clearly more complex than a single exponential. At long times, the correlation time is 6.5 ns, consistent with overall rotational diffusion. However, in comparison with LADH, there is a dramatic decrease in anisotropy at short times (Figure 11.14), The correlation time of the fast component is less than 50 ps, and this motion accounts for one-(hird of the total anisotropy. Independent tryptophan motions have been observed in a large number of proteins " and have been predicted by molecular dynamics calculations. Fast components in the anisotropy decay are also observed for labeled pro teins. Hence, segmental motions of intrinsic and extrinsic fluorophores appear to be a common feature of proteins. 

As a pioneering model, one kind of molecular machine, steroid cyclophane, was embedded at the air-water interface as a dynamic interface, to demonstrate capture and release of a target guest molecule through macroscopic mechanical motions (Fig. 2.2.9) [23]. The steroid cyclophane machine possesses a cyclic core consisting of a l,6,20,25-tetraaza[6.1.6.1]... 

PFMA-C2, (c) PFMA-C4, and (d) PFMA-C thin films in the dried and hydrated states. The F13/C13 and O13/C13 values represent the relative magnitude of F and O concentrations at the surface. In the dried state, the Fj /Cig and Ojj/Cjj values completely agreed with the theoretical values, which are calculated from the chemical stmcture of PFMA-Cy. In the pseudo-hydrated state, the Fij/Cfs and Oij/Cis values did not change at below T. On the other hand, the Fi /Cis values decreased and the Ois/Cjs values increased at above T. The low values of Fjj/Cis and the high values of Oig/Cfs are attributable to the surface reorganization and the exposure of carbonyl groups to the water interface. The results also support the relationship between the surface molecular motion and the wetting behavior of PFMA-Cj, thin films. 

The QM/MM Hamiltonian can be used to cany out Molecular Dynamics simulations of a complex system. In the case of liquid interfaces, the simulation box contains the solute and solvent molecules and one must apply appropriate periodic boundary conditions. Typically, for air-water interface simulations, we use a cubic box with periodic boundary conditions in the X and Y directions, whereas for liquid/liquid interfaces, we use a rectangle cuboid interface with periodic boundary conditions in the three directions. An example of simulation box for a liquid-liquid interface is illustrated in Fig. 11.1. The solute s wave function is computed on the fly at each time step of the simulation using the terms in the whole Hamiltonian that explicitly depend on the solute s electronic coordinates (the Born-Oppenheimer approximation is assumed in this model). To accelerate the convergence of the wavefunction calculation, the initial guess in the SCF iterative procedure is taken from the previous step in the simulation, or better, using an extrapolated density matrix from the last three or four steps [39]. The forces acting on QM nuclei and on MM centers are evaluated analytically, and the classical equations of motion are solved to obtain a set of new atomic positions and velocities. 

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