Takuhiro Nishio

The cooperative binding of large ligands to a one-dimensional lattice: the steric hindrance effect.

The cooperative binding of monomeric ligands to a long lattice of a linear polymer with complete or partial steric hindrance is treated using a matrix method. Results and typical calculations of the model are represented. Non-saturated cooperative binding as well as two-step (biphasic) binding isotherms can be interpreted by the steric hindrance model. This is applicable to the analysis of the binding of surfactants to polymer. The usefulness and the limitation also are discussed.

Effect of salts on the conductance of polylectrolyte solutions

Abstract Electrical conductance of aliphatic x , y -ionenes as well as some acidic polyelectrolytes such as poly(styrene sulfonic acid) (PSS) and poly(acrylic acid) (PAA) in solution was measured with special attention to the effect of salt added. The conductance measurements exhibited a marked departure from the additivity in the presence of salt, contrary to the case of counterion activity. Namely, the conductance of the ionene solution was always lower than the sum of those of salt-free ionene solution and corresponding salt solution. The relative departure was almost unchanged when the polyion concentration was lowered to 5×10 −5 N l −1 , suggesting that this phenomenon is interpreted as an intramolecular process. The deviation was less pronounced in case of ionene-fluoride, a hydrophilic counterion, as well as other polyelectrolytes tested such as potassium salts of PSS and PAA. The deviation reduced by changing degree of neutralization by addition of OH − , which is understood as partial replacement of the counterion by OH − . These results are discussed in terms of chain flexibility as well as counterion specificity.

Effects of ion size and valence on ion distribution in mixed counterion systems of rodlike polyelectrolyte solution. I. Mixed-size counterion systems with same valence

The influence of the counterion size is investigated in the mixed counterion systems of the salt-free polyelectrolyte solution in a cylindrical cell model. The mixtures of two species of the counterion having the same valence and different size are simulated systematically by means of the Monte Carlo method in the primitive model of the rodlike polyelectrolyte solution. The results of the free fractions and the selectivity coefficients are compared with the numerical solutions of the Poisson–Boltzmann equation. The observed differences between both methods are explained in terms of the ion–ion correlations and the effect of finite ion size.

Monte Carlo studies on potentiometric titration of poly(glutamic acid).

The potentiometric titration of poly(glutamic acid) with special attention to its helix-coil transition is investigated in terms of the previously developed Monte Carlo method. The simulations of the potentiometric titration are carried out for helical and coiled form of the peptide, separately. A cylindrical rod with spherical ionizable groups is adopted as each conformational model of poly(glutamic acid) molecule. A spherical charge with a hard core potential is assumed as a mobile hydrated ion. The helix-coil transition curves are analyzed by the Zimm-Bragg theory. A satisfactory agreement is achieved for the titration curves with the experimental data in most cases. The significance and the limitations of the simulation method are discussed.

Monte Carlo studies on potentiometric titration of (carboxymethyl)cellulose

Monte Carlo simulations of the potentiometric titration are carried out for (carboxymethyl)cellulose in aqueous salt solutions by a previously developed method. A nearly elliptic cylinder with spherical ionizable groups is assumed as model of (carboxymethyl)cellulose molecule. The spherical charges with a hard core potential are adopted as mobile hydrated ions. A fairly satisfactory agreement of the titration curves with the experimental data is achieved by using reasonable molecular dimensions. Dependence of the calculated titration profiles on the molecular model and the characteristics of the system are discussed.

Monte Carlo simulations on potentiometric titration of cylindrical polyelectrolytes: Introduction of a method and its application to model systems without added salt

Abstract A novel method of the Monte Carlo simulation on the potentiometric titration behavior is developed for an aqueous salt-free solution of rod-like polyelectrolytes in a cylindrical cell system. The trials are attempted in an iterative loop for the ionization and deionization of each ionizable group on the polymer rod as well as for the random motion of small ions within the cell free volume by using the Monte Carlo criterion. In the simulation, pH value of the aqueous solution is assumed as a fixed parameter that determines the excess energy with ionization process. The interaction between charged species is modeled as a combination of a point charge on an impenetrable rod surface and a charged hard sphere with several sizes. The averaged degree of ionization, the ion distribution around the polyion and other thermodynamic quantities are finally obtained as those of the equilibrium state by repeating iteration after the convergence is fulfilled. The results are compared with the analytical solutions of the Poisson-Boltzmann (PB) equation for the uniformly charged cylindrical polyelectrolytes. The counterion distribution agrees with each other, whereas the titration curve is deviated from the PB results. The deviation is interpreted as the difference in the electrostatic free energies between both models. The characteristics as well as the limitations of the present method are also discussed.

Development of an equation for potentiometric titration of polyelectrolytes using a periodic lattice model: A numerical analysis of interactions among ionizable groups

A numerical method is presented for analysing the potentiometric titration behavior of linear polyelectrolytes. A polyelectrolyte molecule is treated as a one-dimensional lattice containing a large number of lattice points, each of which has an identical ionizable group. In this method, the polyelectrolyte model lattice is divided into identical repeating unit cell systems with a finite number of ionizable groups to calculate the thermodynamic partition function of the system of polyelectrolyte solution. The electrostatic interaction between ionized groups adopted in the present study is given by the Debye-Hückel type screened Coulombic potential. The titration behavior as well as several thermodynamic quantities is derived by making a canonical ensemble summation of all states in a unit cell system under an appropriate periodic boundary condition. This method serves as a model of the two-step ionization often appearing in polyions with strong neighboring interactions such as homo- and copolymers of maleic acid. Several characteristics of the titration behavior of these polyelectrolytes are well reproduced by using a lattice model with pairwise intervals, including the effects of hydrogen bond formation and change in dielectric constant of the medium around an ionizable group. In addition, this method is valid for a more detailed analysis of the titration behavior of polyelectrolytes with various kinds of arrangements of charged groups.

Dielectric studies on synthetic and natural polyelectrolytes

Abstract Several characteristics of the dielectric properties of aqueous polyelectrolyte solutions are reviewed on the basis of our theoretical and experimental studies on these materials. Samples used were ionic homopolymers, copolymers of maleic acid, and natural polysaccharides, all of which were flexible linear polymers having a number of ionizable groups. Measurements were made by using an ordinary bridge method or a pseudo-random noise spectrometer in a frequency region between 10 Hz and 1 to 5 MHz. These polyelectrolyte solutions exhibit quite a large dielectric constant, e.g., several times larger than the value of water below 100 Hz. The dispersion profile consists of two regions, one in the low frequency region below 10 kHz (LFD), and the other in the high frequency region near 1 MHz(HFD). The results are consistent with the counterion fluctuation theory proposed previously. Namely, according to the theory, the dielectric polarization is related to the fluctuation of the bound ions and its dispe...