Critical dynamics and kinetics of elementary reactions of 2,6-dimethylpyridine–water

The Effect of Alkali Halides on the Critical Exponents of the 2,6-Dimethylpyridine-Water System

The diffusion coefficients and shear viscosities of 2,6-dimethylpyridine-water mixtures of critical composition have been measured without and with small amounts of alkali halides added. The data have been analyzed in terms of power law behavior. Deviations from power law behavior indicate a coupling between the critical fluctuations in the local concentrations and the formation of mesoscopic molecular aggregates. The critical exponent of the fluctuation correlation length, the shear viscosity exponent, and the critical exponent of the relaxation rate of fluctuations have been evaluated to show noticeable influences from the salts. The correlation length exponent indicates a suppression of the critical fluctuations, whereas the viscosity exponent rather points at the activation of some extra fluctuations. No clear evidence has been obtained that the added salts affect the critical behavior, and thus cause the opposed effects in the exponents, directly by the long-range ionic fields. Alternatively, the ions may have an influence on the aggregate formation which in turn could modify the critical fluctuations either by reducing the region of true critical exponents or by affecting the critical behavior due to the presence as well as the formation and disintegration kinetics of the multimolecular structures.

Critical behavior of nanoparticle-containing binary liquid mixtures

Simultaneous measurements of small-angle neutron scattering and dynamic light scattering have been performed on a binary mixture of partially miscible liquids, 2,6-dimethylpyridine and water. At critical composition the temperature dependence of the correlation length of fluctuations in composition is strongly affected by the addition of nanoparticles of a triblock copolymer polyethylene oxide-polypropylene oxide-polyethylene oxide. A crossover between Ising-type critical behavior and mean-field critical behavior is observed when the correlation length is equal to the size of the nanoparticles.

On the physical basis of the amino acid polar requirement

Understanding how codons became associated with their specific amino acids is fundamental to deriving a theory for the origin of the genetic code. Carl Woese and coworkers designed a series of experiments to test associations between amino acids and nucleobases that may have played a role in establishing the genetic code. Through these experiments it was found that a property of amino acids called the polar requirement (PR) is correlated with the organization of the codon table. No other property of amino acids has been found that correlates with the codon table as well as PR, indicating that PR is uniquely related to the modern genetic code. Using molecular dynamics simulations of amino acids in solutions of water and dimethylpyridine used to experimentally measure PR, we show that variations in the partitioning between the two phases as described by radial distribution functions correlate well with the measured PRs. Partition coefficients based on probability densities of the amino acids in each phase have the linear behavior with base concentration as suggested by PR experiments.