Junction dynamics experimental data

Interface between two liquid solvents — Two liquid solvents can be miscible (e.g., water and ethanol) partially miscible (e.g., water and propylene carbonate), or immiscible (e.g., water and nitrobenzene). Mutual miscibility of the two solvents is connected with the energy of interaction between the solvent molecules, which also determines the width of the phase boundary where the composition varies (Figure) [i]. Molecular dynamic simulation [ii], neutron reflection [iii], vibrational sum frequency spectroscopy [iv], and synchrotron X-ray reflectivity [v] studies have demonstrated that the width of the boundary between two immiscible solvents comprises a contribution from thermally excited capillary waves and intrinsic interfacial structure. Computer calculations and experimental data support the view that the interface between two solvents of very low miscibility is molecularly sharp but with rough protrusions of one solvent into the other (capillary waves), while increasing solvent miscibility leads to the formation of a mixed solvent layer (Figure). In the presence of an electrolyte in both solvent phases, an electrical potential difference can be established at the interface. In the case of two electrolytes with different but constant composition and dissolved in the same solvent, a liquid junction potential is temporarily formed. Equilibrium partition of ions at the - interface between two immiscible electrolyte solutions gives rise to the ion transfer potential, or to the distribution potential, which can be described by the equivalent two-phase Nernst relationship. See also - ion transfer at liquid-liquid interfaces. 

A particularly important application of molecular dynamics, often in conjunction with the simulated annealing method, is in the refinement of X-ray and NMR data to determine the three-dimensional structures of large biological molecules such as proteins. The aim of such refinement is to determine the conformation (or conformations) that best explain the experimental data. A modified form of molecular dynamics called restrained molecular dynamics is usually used in which additional terms, called penalty junctions, are added to the potential energy function. These extra terms have the effect of penalising conformations... 

FIGURE 5.19 Plot of the coupling parameter of junction dynamics nj determined from experimental data by Shi et al. (Shi et al., 1993) for four polymer networks with different molecular weights between crosslinks, Mq. Solid inverted triangles and open circles are from NMR data taken using cross (CP) and direct (DP) polarization, respectively. The solid square is for a swollen sample with Me = 650. The lines are drawn to guide the eyes. 

The combination of careful chemical synthesis with NSE and SANS experiments sheds some light on the fast relaxation processes observed in the collective dynamics of block copolymers melts. The results reveal the existence of an important driving force acting on the junction points at and even well above the ODT. Modelling the surface forces by an expression for the surface tension, it was possible to describe the NSE spectra consistently. The experimental surface tension agrees reasonably well with the Helfand predictions, which are strictly valid only in the strong-segregation hmit. Beyond that, these data are a first example for NSE experiments on the interface dynamics in a bulk polymer system. 

The implications of individual neuron dynamics on neuronal network synchronization is evident. In Fig. 7.9 (from Schneider et al., unpublished data) this is demonstrated with network simulations (10 x 10 neurons) of nearest neighbor gap-junction coupling. It is illustrated in quite a simple form which, in a similar way, can also be experimentally used with the local mean field potential (LFP). In the simulations LFP simply is the mean potential value of all neurons. In the nonsynchronized state LFP shows tiny, random fluctuations. In the completely in-phase synchronized states the spikes should peak out to their full height... 

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