Kiyoshi Arakawa

The asymptotic behavior of the site-site Ornstein-Zernike equation and RISM-2 approximations for polyatomic molecular fluids

It is shown that the asymptotic properties of the reference interaction site model (RISM)-2 type integral equations for polyatomic molecular fluids are quite similar to those of simple fluids. The exact asymptotic form of the new site-site direct correlation function C αγ ( r ), which is defined by the site-site Ornstein-Zernike equation, is given by using the exact asymptotic form of the site-site total correlation function. The result is C αγ ( r )≃- U (1, 2)/ k B T for nonpolar fluids composed of homonuclear diatomic molecules as well as heteronuclear diatomic molecules, where U (1, 2) is an intermolecular potential, k B the Boltzmann constant and T the absolute temperature. The asymptotic form of C αγ ( r ) given by the RISM-2 approximations is found to be in accordance with the exact result. Thus, the RISM-2 theory is concluded to have no theoretical inconsistency like that in the RISM-1 theory.

Statistical Thermodynamics of Liquid Alkali Alloys

A statistical thermodynamic calculation of liquid Na–K alloys using a generalized van der Waals theory is performed. The theory combining the Lebowitz equation for a hard-sphere mixture with an energy term showing the V -1/3 -dependence ( V : volume) proves to be essentially successful to explain various thermodynamic properties (volume, enthalpy, entropy, concentration fluctuations, etc.) of the liquid alloys. The physical parameters (energy parameters, packing fraction, etc.) used in the calculation are determined by solving the equation of state directly. Structure factors calculated are found to reproduce well observed data. Some critical discussions about other theories are made.

Molecular Dynamics Study of Dense Fluids

A computer experiment has been performed to investigate dynamical properties of a fluid system composed of 864 particles interacting through a modified Lennard-Jones-type potential. The velocity autocorrelation functions are computed and discussed in comparison with the Enskog predictions. The correlation functions are found to be very similar to those of the effective hard sphere fluids after the lapse of about 8 successive collisions. The slow power-law decaying behavior in the time correlation function is observed for a fluid composed of particles interacting with a harshly repulsive core and an attractive potential.

The pH Dependence of the Rheological Properties of Hydrogels of Agar-agar

寒天ゲルがpHに対してどのように影響されるかを調べることを目的として,鎖式応力緩和計を用いて応力緩和実験を行なった。寒天濃度は2.46±0.03wt%で,温度50℃で測定したpH2.4～9.8の範囲の試料ゲルについて,約25.0～6.5℃の温度範囲にわたり応力緩和曲線を求めた。これらの緩和曲線を3個のMaxwell模型を並列にした力学模型で解析した。その結果,寒天ゲルの主要三次元構造に対応する弾性定数E1および緩和時間τ1は,pHが3.8から9.8までほとんど一定値を示した。しかし7pHが3.2になるとE1の滅少が表われ,pH2.4で急激に低下した。τ1はE1にくらべてその滅少は緩慢である。一方,アルカリ側ではpH9.8までほとんど一定値を示した。寒天ゲルのゲル形成能力は,寒天分子鎖の溶媒和性と結晶性の一定の均衡関係に依拠するとされているが,以上の結果によれば上記pH範囲ではそれがほとんど変化せず,すなわち架橋点の数密度がpH4～10までの範囲ではほとんど変化ないと考えられる。また,pH3.2～9.8までの寒天ゲルから求めた見かけの活性化エネルギーの平均は約5.4kcal/molであった。この値は既報の寒天水溶液ゲルの広範囲の濃度から求めた見かけの活性化エネルギーとほぼ同じ値である。