Crystal solution interface 353

Real polymer processes involved in polymer crystallization are those at the crystal-melt or crystal-solution interfaces and inevitably 3D in nature. Before attacking our final target, the simulation of polymer crystallization from the melt, we studied crystallization of a single chain in a vacuum adsorption and folding at the growth front. The polymer molecule we considered was the same as described above a completely flexible chain composed of 500 or 1000 CH2 beads. We consider crystallization in a vacuum or in an extremely poor solvent condition. Here we took the detailed interaction between the chain molecule and the substrate atoms through Eqs. 8-10. 

It is somewhat remarkable that surface solution only occurs at the edge of the crystal exposed to water-air interface and solution does not occur from the parts of the crystal immersed in the water. 

II aldolases FucA and RhuA from E. coli have been crystallized solution of their spatial structures confirmed a close similarity in their overall fold [14]. Both enzymes are homotetramers in which subunits are arranged in C4 symmetry. The active site is assembled in deep clefts at the interface between adjacent subunits, and the catalytic zinc ion is tightly coordinated by three His residues. From X-ray... 

When an ionic single crystal is immersed in solution, the surrounding solution becomes saturated with respect to the substrate ions, so, initially the system is at equilibrium and there is no net dissolution or growth. With the UME positioned close to the substrate, the tip potential is stepped from a value where no electrochemical reactions occur to one where the electrolysis of one type of the lattice ion occurs at a diffusion controlled rate. This process creates a local undersaturation at the crystal-solution interface, perturbs the interfacial equilibrium, and provides the driving force for the dissolution reaction. The perturbation mode can be employed to initiate, and quantitatively monitor, dissolution reactions, providing unequivocal information on the kinetics and mechanism of the process. 

Each pair of chambers is connected to the protein sample and one of 48 crystallization solution reservoirs. The chip has 480 integrated valves that are actuated through three separately addressable control lines. As a precaution, 48 safety valves are included at the solution inlets to avoid the unwanted loss of protein sample in the unlikely event of an interface valve failure. The remaining two lines simultaneously control all interface and containment valves. By virtue of this parallel architecture and the robustness of the BIM scheme, solutions of varying viscosity, surface tension, pH and ionic strength may be simultaneously metered and mixed at three different mixing ratios using only two hydraulic control lines. 

Once it has been ascertained that the hot solution is saturated with the compound just below the boiling point of the solvent, it is allowed to cool slowly to room temperature. Crystallization should begin immediately. If it does not, add a seed crystal or scratch the inside of the tube with a glass rod at the liquid-air interface. Crystallization must start on some nucleation center. A minute crystal of the desired compound saved from the crude material will suffice. If a seed crystal is not available, crystallization can be started on the rough surface of afresh scratch on the inside of the container. 

The rate of nucleation / is a probability process connected with the energy of formation of the critical cluster, AGo-ib which owing to the creation of the new crystal/solution and crystal/substrate interfaces, is always positive. The probability of a fluctuation connected with an increase of the Gibbs energy AG of a system is given in the case of nucleation, with AG = AGcdt, by... 

Where J is the number of nuclei formed per unit time per unit volume Nq is the number of molecules of the crystallizing phase in a unit volume v is the frequency of atomic or molecular transport at the nucleus—liquid interface u is the molecular volume of the crystallizing solute /ig is the interfacial energy per... 

The tip-generated interfacial undersaturation is governed by the interplay between mass transport in the tip/substrate gap and the dissolution kinetics. This concept is illustrated in Figures 15 and 16. Figure 15a and b shows the radial dependence of the steady-state concentration and flux at the crystal/solution interface for a first-order dissolution process characterized by K, = 1, 10, and 100. For rapid kinetics (K, = 100), the dissolution process is able to maintain the interfacial concentration close to the saturated value and only a small depletion in the concentration adjacent to the crystal is observed over a radial distance of about one electrode dimension. Under these conditions, diffusion in the z-direction dominates over radial diffusion. As the rate constant decreases, diffusion is able to compete with the interfacial kinetics and consequently the undersaturation at the crystal surface... 

The decrease in crystal activity at r , coupled with strongly hindered diffusion in solution in the SECM configuration, results in a rapid depletion in the local concentration of electroactive material in the tip-substrate gap and crucially at the crystal/solution interface. Eventually a value for the interfacial undersaturation is reached that exceeds the threshold required to spontaneously nucleate fresh dissolution sites from the perfect surface and dissolution is initiated again at rcrit. This is reflected in the sudden surge of current. To model this simply, the entire surface was allowed to become active once the undersaturation at any point on the surface exceeded a critical value (50). Thus, the following condition applied, in conjunction with Eqs. (31) and (33) ... 

The mechanism by which the impurity incorporation increases with growth rate was first thoroughly investigated by J.A. Burton et al. (1953), and later by Wilson (1978a) and Rosenberger (1986). These workers postulated that partial rejection of impurities at the crystal-solution interface (i.e., < 1) cause the concentration... 

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