The thermal conductivity of the amorphous phase of polyamide-6,6 is investigated by nonequilibrium molecular dynamics simulations. Two different algorithms are used, reverse nonequilibrium molecular dynamics and the dual-thermostat method. Particular attention is paid to the force field used. Four different models are tested, flexible and rigid bonds and all-atom and united-atom descriptions. They mainly differ in the number of high-frequency degrees of freedom retained. The calculated thermal conductivity depends systematically on the number of degrees of freedom of the model. This dependence is traced to the quantum nature of the fast molecular vibrations, which are incorrectly described by classical mechanics. The best agreement with experiment is achieved for a united-atom model with all bonds kept rigid. It could be shown that both the thermal conductivity and the heat capacity of a model show a similar (but not equal) dependence on its number of degrees of freedom. Hence, the computationally more convenient heat capacity can be used for force field optimization. A side result is the anisotropy of the thermal conductivity in stretched polyamide; heat conduction is faster parallel to the drawing direction than perpendicular to it.
The stabilizing effect of mannitol during the freeze-drying of proteins was studied using L-lactate dehydrogenase (LDH, rabbit muscle), beta-galactosidase (Escherichia coli) and L-asparaginase (Erwinia chrysanthemi) as model proteins. Crystallization of mannitol was studied by powder X-ray diffraction and differential scanning calorimetry (DSC), in relation to the stabilizing effect. All the enzymes were protected concentration-dependently by amorphous mannitol, but the stabilizing effect was decreased with an increase in mannitol crystallinity. The heat-treatment of frozen solutions above crystallization temperature prior to drying enhanced mannitol crystallization and LDH inactivation. The importance of maintaining excipients in an amorphous state during freeze-drying, previously reported for Aspergillus oryzae beta-galactosidase (K. Izutsu et al., Pharm. Res., 10, 1233 (1993)), was confirmed using three different enzymes.
The effects of amphiphilic excipients on the inactivation of lactate dehydrogenase (LDH) during freeze-thawing and freeze-drying were studied. Some amphiphilic excipients such as hydroxypropyl-beta-cyclodextrin (HP-beta-CD), CHAPS, polyethylene glycol (PEG) 3350, and sucrose fatty acid monoester prevented LDH inactivation during freeze-thawing and freeze-drying at a lower concentration than sugars and amino acids. Polyoxyethylene 9 lauryl ether and PEG 400 protected LDH during freeze-thawing but not during freeze-drying. The buffer concentration of the solution to be freeze-dried (10, 50, and 200 mM) affected the stabilizing effect of trehalose, but not that of HP-beta-CD. (c) 1994 John Wiley & Sons, Inc.
The crystallization of colloidal particles at an air-water interface is investigated by computer simulations. We numerically clarify the bond-orientational order parameter Phi(6) with different particle concentrations. In addition, the profiles of the pair-correlation function g(r), the orientational correlation function g(B)(r), and the static structure factor S(q) of this system are calculated. We find the two-stage melting of the colloidal crystal and the existence of the hexatic phase at intermediate concentration.
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