Tsuneo Okubo

Ordered structure in dilute solutions of highly charged polymer lattices as studied by microscopy. I. Interparticle distance as a function of latex concentration

The coexistence of ordered and disordered regions has been observed in dilute solutions of highly charged monodisperse latex particles, a copolymer of styrene and styrenesulfonate, by using a metallurgical microscope. Unlike latex particles of low charge numbers, the interparticle distances (2Dexp) in the hexagonal ordered regions are smaller than the calculated average values for a face‐centered‐cubic distribution (2D0,h) at low polymer concentrations (0.2–2 vol %). However, at high concentrations, up to 12%, the 2Dexp value approaches 2D0,h. The results are consistent with a ‘‘two‐state structure’’ containing ordered and disordered regions and indicate the existence of ‘‘attractive’’ interactions between the charged solutes. The findings are also consistent with those for other species of high charge densities such as synthetic macroions and simple ions.

‘‘Ordered’’ structure in dilute solutions of sodium polystyrenesulfonates as studied by small‐angle x‐ray scattering

The small angle x‐ray scattering measurements were performed for aqueous solutions of sodium polystyrenesulfonates having relatively narrow molecular weight distributions. As was observed for other synthetic macroions, polynucleotide and proteins, a single, broad peak was observed. The scattering vector at the peak position (Sm) was shifted toward larger values with increasing polymer concentration and toward lower values with increasing salt concentration, which confirmed earlier observations with polyacrylate and poly‐L‐lysine. The molecular weight dependence of the scattering behavior, which was earlier observed, was confirmed to be true for samples with Mw of 74 000, 18 000, and 4600. The mixture of two fractions with different Mw’s gave a scattering curve which was again different from the composite curve obtained with the parent curves before mixing. A similar situation was observed for the mixture of polystyrenesulfonate and polyacrylate. Thus, it was concluded that the observed single peak indicat...

Ordered solution structure of a monodispersed polystyrene latex as studied by the reflection spectrum method

Ordered lattice structures, face-centred cubic (f.c.c.) and body-centred cubic (b.c.c.) lattices, are systematically analysed by the reflection spectrum measurements of monodispersed polystyrene latices in deionized solution. The nearest-neighbour interparticle distance (2Dexp) is determined from the wavelength at the reflection peaks. 2Dexp decreases with increasing latex concentration and is smaller than the calculated average values for the f.c.c. or b.c.c. distribution (2D0). The results are consistent with a ‘two-state structure’ which has ordered and disordered regions. The f.c.c. and b.c.c. lattices are stable at high and low concentrations, respectively. The b.c.c. structure often appears at a high solution temperature and in the presence of a small amount of foreign salt. The size of the ordered region is estimated from the width of the reflection pattern and also from microscopic photographs. The number of crystal layers is estimated to be several hundred and depends on latex concentration and solution temperature.

Determination of the effective charge numbers of colloidal spheres by conductance measurements

Abstract The effective charge number of colloidal spheres was determined for monodisperse polystyrene spheres and colloidal silica (8–1022 nm in diameter and 0.3–171 μC/cm 2 in charge density) by conductance measurements. The fraction of free macroions and/or gegenions (β) in deionized suspension is close to but smaller than unity for spheres having several and several tens analytical valency (Z). The β decreases with increasing Z. The relation log β = 2.61 − 0.87 log Z, which has been obtained from electrophoretic mobility measurements, holds for spheres of Z > 3000. The charge renormalization theory by Alexander et al. is supported.

Microscopic observation of ordered colloids in sedimentation equilibrium and important role of Debye‐screening length. I. Heavy and monodispersed polystyrene type spheres (specific gravity=1.50) in aqueous and aqueous methanol suspensions

A metallurgical microscope is used to directly observe the ordered structures of heavy and monodispersed polystyrene type spheres (specific gravity= 1.50) in sedimentation equilibrium. The interparticle distances (2Dexp) increase with time after the latex sample is set into the cell, and reach an equilibrium value after seven days. 2Dexp continues to decrease as the amount of spheres fed increases and from these data Young’s modulus of the ordered lattices is estimated to be ∼100 dyn cm−2. The hexagonal ordering is observable not only in the deionized state but also in the presence of sodium hydroxide (10−6–10−3 M), and 2Dexp decreases with increasing alkali concentration. The results show that electrostatic interparticle repulsion and the elongated Debye‐screening length around the spheres are essential for the ordering.

On the electrophoretic mobility of isolated colloidal spheres

We studied the electrophoretic mobility μ of highly charged colloidal spheres in very dilute low salt aqueous suspension. We combined experiments on individual particles and ensemble averaged measurements. In both cases μ was observed to be independent of particle size and surface chemistry. Corresponding effective charges Zμ*, however, scaled with the ratio of particle size to Bjerrum length λB: Zμ* = Aa/λB with a coefficient . Our results are discussed in comparison to other charge determination experiments and charge renormalization theory and with respect to the issue of charge polydispersity.

Colloidal crystallization of monodisperse and polymer-modified colloidal silica in organic solvents

Abstract The formation of colloidal crystals from polymer-modified silica in organic solvent was studied. The colloidal crystallization of poly(maleic anhydride-styrene)-, polystyrene- and poly(methyl methacrylate)-modified silica took place in organic solvents, which were good solvents for the grafted polymer. It is, however, observed that there is a matching or a mismatching between the grafted polymer and solvent for the crystallization. The critical volume fraction (φo) of the silica particles for the colloidal crystallization depended on the modified polymer, the extent of grafting, and molecular weight of the polymer. From the fact that φo in the crystallization of poly(malic anhydride-styrene)-modified silica in acetonitrile decreased with the addition of a salt, NH4PF6, the crystallization occurred in the organic solvents due to the electrostatic repulsion between the particles, as well as those in aqueous solution. The neighboring inter-sphere spaces agreed well with the calculated values, which were obtained by the assumption of the close packing under electrostatic repulsion between the particles. It is suggested that the grafted polymer contributes to the enhancement of the elecrostatic interaction through electric double layer between the particles in the colloidal crystallization in organic solvent.

Importance of electrical double layers in structural and diffusional properties of deionized colloidal suspensions

Abstract An important role of electrical double layers in the structural, theological, and diffusional properties of colloidal suspensions, especially in the deionized state, is discussed. Formation of giant colloidal single crystals, which are brilliantly colored and very beautiful, is due to the electrostatic intersphere repulsion and to the highly expanded electrical double layers surrounding colloidal spheres. The phase diagram, rigidity and viscosity of the colloidal crystals are nicely explained with the contribution of the electrical double layers. The translational and rotational diffusion coefficients of colloidal particles are quite sensitive to the ionic concentration of the suspension, which is also explained by the thinning of the electrical double layers with increasing ionic concentration. Furthermore, diffusive modes in the colloidal crystals and liquids analyzed by dynamic light scattering measurements are consistent with the important contribution of the electrical double layers.

Surface tension of synthetic polyelectrolyte solutions at the air-water interface

Abstract Surface tensions, γ, of synthetic polyelectrolytes are studied systematically at the air-water interface by the Wilhelmy method. The polyelectrolytes used are anionic ones: (1) polyacrylic acid and its sodium and tetraalkylammonium (alkyl = methyl, ethyl, n-propyl, and n-butyl) salts, (2) polymethacrylic acid and its sodium salt, (3) sodium poly(ethylene sulfonate), and (4) sodium and tetraalkylammonium salts of poly(styrene sulfonic acid). Cationic macroions are (5) copolymer of dimethyldiallylammonium chloride and sulfur dioxide, (6) poly-4-vinyl-N-alkylpyridinium halide, (7) copolymer of 4-vinyl-N-benzylpyridinium chloride (95 mole%) and 4-vinyl-N-n-cethylpyridinium bromide (5 mole%), (8) 3-3, 6-3, 6-6, and 6–10 type ionen polymers, and (9) quaternized polyethylenimine with methyl groups. In addition, (10) low-molecular-weight analogs of the methylated polyethylenimine and the bolaform-type electrolytes are studied. The surface activity of vinyl-type synthetic polyelectrolytes is low due to the fact that their conformations are inadequate to separate their groups sterically to hydrophilic and hydrophobic parts at the air-water interface. The γ values of the tetraalkylammonium salts are, however, low due to the arrangement of hydrophobic gegenions at the interface assisted by macroions. For ionen-type polymers, γ is low. This is because a folded conformation is formed rather easily at the interface. For many polyelectrolytes, γ begins to decrease above the critical concentration, m ∗ , at which the lateral and attractive intermacroion interaction occurs at the interface, and the two-dimensional orientation forms. Below m ∗ , the surface activity is negligible. The excess concentrations at the interface are evaluated using Manning theory for a polyelectrolyte solution.

Determination of the number of effective charges of colloidal particles by transference measurements

The number of effective charges of colloidal particles was determined for polystyrene‐based polymer latices by transference measurement. The fraction of free counterions (f) was surprisingly low and appeared to decrease with the analytical valency; the f values were about 0.1 for a latex having 1.7×104 SO3H groups per particle. A more highly charged latex (with 7.2×105 sulfonate groups) had f values of about 0.04. These values are definitely smaller than those reported for soluble polyelectrolytes (about 0.4).